UNIT 3 POLLINATION AND

FERTILIZATION
Structure Page No.

Introduction
Objectives
Pollination
3.2.1 Types of Pollination
3.2.2 Self- Vs Cmss-Pollination
Fertilization
3.3.1 Pollen-Stigma Interaction
3.3.2 Pollen Gemination-Events on Stigma and in Style
3.3.3 Pollen Tube Growth in vitro
3.3.4 Syngamy and Triple Fusion
Incompatibility
3.4.1 Intraspecific Incompatibility
3.4.2 Interspecific Incompatibility
3.4.3 Biological Significance of Incompatibility
3.4.4 Methods to Overcome Incompatibility
Apomixis
3.5.1. The Recurrent Type
3.5.2 Non-recurrent Type
3.5.3 Endosperm Development in Apomicts
3.5.4 Anthers of Apomicts
3.5.5 Causes of Apomixis
3.5.6 Parthenogenesis
3.5.7 Significance of Apomivis
summary
Terminal Questions
Answers
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3.1 INTRODUCTION

Thi :eproductive smchlre of angiosperms including the details of formation of male and
l~malegametophytes were considered in the earlier units. You m y recall @at as a
result of meiosis, haploid pollen grains and egg cell are formed. After the formation of
the gametophytes, the next two essential steps of sexual reproduction, i.e., pollination
and fertilisation take place. As a result zygote is formed, which eventually develops into
the embryo. Before fertilisation can occur, the pollen must be transferred from the
stamen to the stigma of the carpel. The transfer of pollen is known as pollination and it
may be accomplished by a number of agencies such as wind, water or animals. Double
fertilisation, a process unique to flowering plants, follows pollination. In this unit you
will become familiar with the types of pollination, some of the important adaptations
exhibited by plants for successful pollination, details of the structural features of the
pistil, pollen-pistil interaction, double fertilisation, incompatibility and apomixis.

Objectives
After studying this unit you should be able to:
explain the process of sexual reproduction in flowering plants;
describe the methods that the plants adapt to disperse the pollen grains for effective
pollinatiow,
interpret as to why in certaiq- ases hybridization fails;
defrne the methods to overcome incompatibility;
analyse how apomixis operates to ensure survival in certain plants;
 e relate the control mechanism that plants have developed to avoid indiscriminate Polllnatlan and FertIUzatlon I
sexwl reproduction.

3.2 POLLINATION

Pollination refers to the transfer of pollen from dehiscing anthers to the pistil. Unlike
animals, plants cannot move to their mates fur sexual reproduction. Hence, they need
some external &vice or agency for the transfer of pollen grains from the male parent to
the stigma of the female parent. Exceptionally, in Vallisnena, an aquatic plant, complete
male flower may be transported to the female flowers, The physical (wind and water)
and biological (insects, birds and bats) agencies promote cross-pollination. The
&hiscene of anthers and the transfer of pollen are the prime requirements of
pollination.
Anther dehiscence: Anther dehiscence simply means the release of pollen graims from
dry and mature anthers. It involves the rupturing of anther wall due to the mechanical
pressure &veloped by the fibrous thickenings of endothecial cells along the stomium
L
(the area where mechanical layer does not differentiate). If endothecium is lacking, the
mechanical role is passed on to epidermal cells. In most of the angiosperms the stomium .
is a narrow strip along the entire length of the anther lobe. It may, however, also be
restricted to a lid or valve (Berberidaceae) or pores (Solanurn, Cassia, Polygala).
Pollen Transfer: The transfer of pollen can be autogamous (self-pollination) in which
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the pollen grains of an anther reach the stigma of the same flower. In cross-pollination,
the pollen of one plant reach the pistil of some other plant of the same species. If the
pollination occurs between two flowers of the same plant it is termed geitonogamy and ,
if it is between two flowers on different plants it is xenogamy.

3.2,l Types of Pollination

Self-Pollination
I
1 Self-pollination refers to the transfer of pollen grains from the anther to the stigma of
I the same flower. In chasmogamous flowers the mature anthers and stigma are exposed
to pollinating agents. In cleistogamous flowers fertilization is accomplished without
I exposing the sex organs (cleistogamy) to the abnosphere.
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I
Cotnmelina benghalensis produces both chasmogamous (aerial) and cleistogamous
I (underground) flowers. Conversion of chasmogamous flowers to cleistogamous type
depends on environmentd conditions, such as temperature.
Cross-pollination

In this kind of pollination the pollen &om anther of one individual is transferred to the
stigma of another individual of the same species. The process is mediated by physical or
biological agents thdnclu& wind, water, insect, birds or mammals. Whereas, cross-
pollination is obligatory in unisexual flowers, the bisexual flowers may have adaptations
that prevent self-pollination. These adaptations include: self sterility, dichogarny,
herkogamy and heterostyly. These have been discussed in Subsection 3.2.2. We shall
now take up some of the common agents that mediate cross-pollination.
(a) Anemophily: It is also commonly referred to as wind pollination, i.e., the pollen
grains are carried through wind currents. To ensure good pollination the
anemophilous plants produce astronomical number of small, dry, light and smooth
pollen grains that are released preferably on warm and dry days. Flowers in such
plants are unisexual with reduced sepals and petals so as to effectively position the
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long and feathery stigma for pollen interception. Stamens have long filaments and
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are exposed to facilitate convenient pollen dispersal, Palms, grasses, millets,
I bamboos are common examples.
(b) Hydrophily: All hydrophytes are not necessarily pollinated hy water. In fact most
k"--t 9,geton or
of the aquatic plant. are anemophilous, e.g., Myrioph~~llr~rn,
entomophilous, e.g.. Alisma, Nymphaea. Like arre~nophilot~st z - tb floral envelops
i
Plant Development-I are highly reduced or absent in hydrophilous plants. Hydrophily may involve
underwater pollination referred to as hyphydrophily, e.g., Ceratophyllum, Majus,
Zostera.

A unique example of this type is Zostera mariana (a submerged marine perennial)

in which pollen grains are long (up to 250pm) and needle-like resembling pollen
tubes. Because of the specific gravity of these pollen they freely float at any depth,
and when they come in contact with the stigma they coil around it.

In some taxa ephydrophily operates. In these plants pollination is brought about at

the surface of water. The classical example is the submerged dioecious plant,
Vallisneria. The male and female flowers are produced under water but on maturity
the males get detached from the stalk and float on the surface while the female
flowers attached to thin, spirally coiled, long, slender stalks are brought to the
surface at the time of pollination. Pollination is achieved through water currents
when male flowers come in contact with pistillate flowers. After pollination the
flowers are dragged down to the bottom by the recoiling of the stalk. The fruits
thus develop under water.

(c) Entomophily: It involves insects to carry the pollen to achieve pollination. Salvia
exhibits a specialised 'tumapipe' floral mechanism that signifies classic adaptation
for bee pollination. In Salvia the corolla is bilipped and the stamens are attached to
corolla tube. Only one-half of each anther is fertile, the other half being sterile joins
together to form a sterile plate of tissue placed above the lower lip at the mouth of
the flower. However, the fertile part lies under the hood of the upper lip of the
corolla. When a bee visits the flower for nec'tar it pushes against the sterile plate
which consequently brings down the fertile anthers on its back dusting it with
pollen. When the bee visits another flower, the forked stigmas picks up the pollen
from the back of the insect.

Plants that are pollinated by insects often have predominently yellow or blue petals.
Among insects bees and butterflies do not perceive colour in the same manner as
the humans. They are able to see in the ultraviolet range of the electromagnetic
spectrum, an area that is invisible to human eye. They see blue and yellow flowers
differently than humans. Red appears black to them. Consequently, flowers that are
pollinated by insects are not usually red. Many insect-pollinated flowers have
dramatic ultra-violet markings, that are invisible to us but direct the insect to the
flower, where pollen or nectar may be located. Insects have a well developed sense
of smell.

Some flowers have developed "fly-trap mechanism" for their pollination, by emitting
unpleasant odours, e.g., Raflesia (rotten meat), Arum (human excreta), and
Aristolochia (decaying tobacco and humus).
In the orchid, Ophrys speculum a highly specialized type of pollination has evolved.
It is pollinated by :hairy wasp, Colpa amea. As the appearance and smell of
female wasp matches with that of orchid flowers, the male wasp mistakes t'he flower
as its female partner and carries our pseudo-copulation. In this process the transfer
of pollinia from one flower to another takes place.

An obligate symbiotic relationship has been observed between a moth Tageticula

and the plant Yucca. The moth cannot complete its life cycle without the association
of Yucca flower and in turn Yucca has no other pollinator. The female moth lays
her eggs in the ovary. Neither would be able to reproduce successfully without the
other. In case, one species were to become extinct, the other would also become
extinct eventually.

(d) Omithophily: In tropical areas, the birds dominate over insects as important
pollinators. The most common among them are humming-birds, sun-birds and
honey-eaters. Flowers pollinated by birds are usually red, orange or yellow. Birds
see well in this region of visible light. Birds do not have a strong sense of smell;
conseiguently, bird-pollinated flowers usually lack much scent. Characteristic features
of ornithophilous flowers are their tubular (Nicotiana glauca), cup shaped
(Callisternon), or urn shaped (some members of Ericaceae) form, bright colour,
excess of pollen and nectar. As humming-birds (they occur only in The New World)
 are able to extract nectar while hovering over the flower, they do not need any Pollination and Fedhation
heavy platform to land when they visit pendant flowers for pollination. In contrast,
sun-birds (old world inhabitants) can perch in any position and suck out nectar
even if they visit erect flowers.
(e) Cheiropterophily: Pollination brought about by bats is called cheiropterophily. Bats
which feed at night and do not see very well, are frequent pollinators in the tropics.
Bat-pollinated flowers have dusky, dull-coloured petals. The flowers of these plants
produce a strong scent, usually of fermented fruit. Bats are attracted to the flowers
by the scent and they lap up the nectar. As they move from flower to flower, pollen
is transferred. To facilitate the visit of bats, the flowers in cheiropterophilous plants
are borne singly or in clusters quite away from the branches and foliage. A bat
clasps the flower with its claws and during nectar lapping its back becomes dusted
with pollen grains. Examples pollinated by bats include the sausage tree (Kigelia
pinnata), Baobab tree (Adansonia digitata).

3.2.2 Self- vs Cross-Pollination

A major advantage of self-pollination is its certainty. Continued self-pollination over
many generations, however, results in weaker progeny. This is referred to as inbreeding
! depression. From the evolutionary point of view, self-pollination is a disadvantage as
I 'there is no scope of genetic recombination.
Cross-pollination brings pollen grains from other plants which are genetically different.
Genetic heterogeneity is advantageous for the plant in many respects. The offspring are
more vigourous and better adapted for survival even under adverse environmental
conditions. Thus cross-pollinated species show wider distribution when compared to self-
pollinated species. Thus cross-pollination is favourable for evolution. The main
disadvantage of cross-pollination is its uncertainty. It also involves considerable
expenditure of resources by the plants as they have to produce an enormous amount of
pollen, as compared to self-pollinated plants, to compensate for wastage. Further, when
the pollinating agent is an animal, the plant should also provide adequate rewards for
the pollinating agent in the form of pollen or nectar. These disadvantages are offset by
the advantages mentioned above.
Because of the specific benefits of cross-pollination, flowering plants have evolved many
I
devices to prevent self-pollination and to encourage cross-pollination. The most common
ones are discussed below.
(a) Dichogamy: In many species the anthers and the stigma come to maturity at
different times. That is, the dehiscence of anthers and the receptivity of the stigma
of a flower do not coincide. In the sunflower plant, the anther dehisces before the
stigma becomes receptive and thus self-pollination cannot occur. This condition is
called protandry. In Mirabilis, and Magnolia the stigma becomes receptive before
the anthers dehisce. This condition is called protogyny.
(b) Herkogamy: Some species show structural adaptations to prevent pollen grains
from coming into contact with the stigma of the same flower. In many herkogamous
species the relative position of the anthers and the stigma is such that self-
pollination cannot occur. For example, the stigma in many plants projects beyond
the level of anthers and as a result the pollen of the same flower cannot land on the
stigma. Similarly the pollinia (pollen in sacs) of orchids and Calotropis cannot reach
the stigma of the same flower.
(c) Self-sterility: In many species, self-pollination does not result in fertilization. This
is because pollen germination on the stigma or the growth of pollen tubes in the
f stigma or style is inhibited. For effective fertilization, pollen has to come from
! another plant. Self-sterility is widespread in flowering plants. It is estimated that
about half the total number of species of flowering plants exhibit this phenomenon.
I It is genetically controlled and is considered a primitive character. It seems to have
evolved very early in the evolution of flowering plants as an effective mechanism
for outbreeding.
(d) Dicliny: In these species flowers are unisexual. Male and female flowers are borne
either on the same plant (e.g., many cucurbits). This condition is referred to as
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I Plant Development-I monoecious. When male and female flowers are borne on different plants (eg., date
palm, mulberry, cannabis) the condition is called dioceous. Since pollination in
these, including the monoecious plants, involves two different flowers, it is
considered as cross-pollination.

Put a tick mark on the correct word given in the bracket.

(a) The (autogamouslallogamous) condition involves transfer of pollen from the anther
to the stigma of the same flower.
(b) '(Geimogamy/Xenogamy) refers to cross-pollination involving flowers on different
plants.
(c) In (chasmogamouslcleistogamous)flowers, both pollination and fertilization take
place within the unopened flower.
(d) (CrossISelf) pollination is obligatory in ~~nisexual
flowers.
(e) The (anemophilouslornithophilous)flowers, produce a copious amount of
pollen grains that are small, smooth, dry and light and their stigmas are long and
feathery.
(f) The species that undergo (sdf/cross) pollination show wider ecological distribution,
as they are better adapted to adverse environmental conditions.
I

3.3 FERTILIZATION

The ultimate aim of pollination is to lead to successful fertilization through the fusion
of male and female gametes. In flowering plants, thae are a number of barriers which
must be overcome. The barriers start immediately after pollination with pollen-stigma
interaction.

3.3.1 Pollen Stigma Interaction

The Stigma
After landing on the stigma pollen grain germinates, and produces a pollen tube
that carries the male gametes. The stigma has been classified into two principal
types depending on the presence or absence of stigmatic exudate at the time of
pollination: (i) the wet stigma is covered by a sticky secretion, e.g., Aegle m a m l o s
and Petunia hybrida, and (ii) the dry stigma lacks any secretion, e.g., cotton. EM
observations indicate that the exudate is secreted by the ER and it is extruded by
exocytosis. The diagrammatic representation of the mechanism involved is shown in
Figure 3.1. In some plants such as Lilium, the stigma is non-secretory. The exudate
which is present on stigma is, in fact, secreted by the stigmatic papillae and those
present in the stylar canal emanate from the style. Figure 3.2 represents dbgmmmatic
sketch of a papilla.
Wet Stigma: Petunia shows several randomly distributed 2-celled papillae on its
surface. In a developing stigma, the epidermis is covered by a continuous, thin cuticle
and the subepidennal cells are densely cytoplasmic, without any intercellular spaces. In
a mature stigma, the cells of the subepidennal zone elongate to form a secretory zone
with large schimgenous cavities filled with a lipoidal secretion. The secretory zone is
delimited from the basal part of the stigma by a storage zone. The cells of the stigma
contain numerous' amyloplasts and a large amount of lipid globules which gradually
coalesce and migrate to the peripheral part of the cytoplasm and eventually out of the
cell. The lipoidal exudate accumulates between the cell wall and the cuticle. In the
secretory zone the exudate fills the large intercellular, schizogenous cavities. At the time
of anthesis, the epidermis becomes disorganized, the cuticle is discarded in the fonn of
flakes and the accumulated exudate spreads over the entire surface of stigma. A very
thin layer of water is also trapped.
 ER Vesicle
Exudate

Plasmalemma

Incorporation of Vesicle
$. into the vacuole

Holocrine
excretion

r Fig. 3.1: Postulated secretion pathways in stigmatic papillae of Aptenia (After I(risten
et PI.1979).
I
The exudate is a highly viscous, retiactive and adhesive substance. It appears in the
form of tiny droplets due to high surface tension and is a complex mixture of lipids and
phenolic compounds. The lipid compound protects the stigma from desiccation, and
regulates the availability of water to pllen. The phenolic compounds occur as esters or
glycosides, and protect the stigma from insects and other pests. Enzymes diffusing out
of pollen grains on the stigma probably release free sugars from phenolic glycosides
which then provide proper osmotic conditions. Reducing sugars (glucose. hctose, and
sucrose) are also present in the stigmatic exudate.
... ..
Pellicle

Discontinuities

Plasmalemma

Stigma Papilla Papillar wall

Fig. 32: S t i g ~ ~ ~Papilla

~ t i c (Af'tcr Shivanna, 1977).

Dry Stigma: The Cotton (Gossypium hirsutum) stigma is covered with long unicellular
hairs. At the time of pollination, the stigmatic hairs show a distinct and continuous
cuticle which is closely pressed to the thin wall. The pellicle that represents extracellular
proteins are present on the dry stigma. In addition, the stigmatic surface also carries
lipids and phenolic substances. Immediately below are several layers of a thin-walled
pamchymatous tissue witb large intercellular spaces. The size of the intercellulaf spaces
graduaUy decreases until no such space is present and the cell wall thickens with a
heavy pectin content. This tissue provides a connecting link witb the transmitting tissue.
The Style
The style has been distinguished into two types: (i) in open styles a stylar canal is
present which is lined with a well-developed glandular epidermis (monocotyledons), (ii)
in closed styles a compact core of transmitting tissue is present (dicotyledons, especially
in Gamopetalae). A correlation between the type of style with that of composition of the
stigmatic exudate has been observed. The solid style exudate is usually rich in
polysaccbarides, lipids and proteins whereas, open style exudates only have
polysaccharides.
Open Style: Aegle, Fritiliuria, Lilium spp. have variable number of stylar canals
depending on the number of carpels. The epidermal cells of stylar canal divide actively
~ h l~cv-cnt-I
t and become papillate in aaopetal suecession. In Lilium each cell contain 1-5 nuclei
which later fuse. The stylar canal thus becomes lined with highly glandular and
secretory c e b which are dome-shapedwith a thick outer tangential wall (canal cells).
The wall towards the canal is smooth but is highly convoluted towards the interior of
the cells. In Citrus the inner tangential wall of canal cells is thick and made up of
fibrUar homogeneous and granular nonhomogeneous matedial.
The canal cells have a large nucleus and often become multinucleate. Cytoplasm is rid
in organelles such as mitochondria, dictyosomes, free ribosomes or polysomes, smooth
and rough ER and occasional amyloplasts. It is believed that a major portion of the
sectetion product is transported to the canal cells fkom the neighbouring parenchyma
cells through the nummus plasmodesrnatal connections.
The golgi apparatus of canal cells of Lilium regale and Lilium devidii secrete a non-
cellulosic and amorphous polysaccharide containing mucilage during the bud stage. This
is easily transported to the outer walls of canal cells. In L longijbrum, the secretion
products of the canal cells are retained with the help 'of a thin and continuous layer of
cuticle until after pollination (Fig. 3.3 a*). The stigmatic papillae of Lilium lack the
characteristic seaetmy zone seen in canal cells and the stigmatic exudate is known to
appear before pollination. The stigmatic exudate, may, therefore be a secretion product
of the canal ceUs transported through the intexcellular spaces. The stylar exudate is
p r o d u d in two phases in Lycopersicon; the first contains carbohydrate and the second

Fig. 33: The structure of hollow atyk. a) Longlrcdion, note contbruow styhr canal. b. c)
Transcction, the secretion product accumulatta between the cudck and the canal
cells (b); the cuticle L later disrupted (c).

Pistils of Lilium longimrum secrete large quantities of an exudatc which accumulates on

the surface of the stigma in the f m of droplets. The stylar canal is also f111ed with this
secretion which is an aqueous solution of high molecular weight protein-containing
polysaccharides-galactose, arabinose, shamnose, glucmonic acid, galacturonic acid, and
monosaccharides. The composition is similar to plant gum exudates. The polysaccharide
gum exudates play an important role in sealing of wounds and it is quite likely that the
large amount of acidic polysaccharides found in the exudate of Lilium may be involved
in protecting the w e pollen tubes during their growth, in addition to providing a
..source of carbohydrate residues for pollen tube wall biosynthesis.
Closed (Solid) Style: Cotton shows an epidermis with stomata, a cortex of thin-walled
parenchyma with several vascular bundles and ~trandsof transmitting tissue. The cells of
transmitting tissue have thin uausverse walls but lateral walls are thick and consist of
several distinct and concentric layers. The innermost wall layer 1, is composed of pectic
substances and hemicellulose, surrounding this is wall layer 2 which is darker, thinner
and similar to wall layer I -in composition with a large hemicellulose content. Wall layer
3 is loosely textured rich in pectin substances and contains small amounts of
noncellulosic plysaccharides and cellulose but poor in hemicellulose. Wall layer 4 is
represented by the middle lamella region and is primarily pectic in nature. Small
amounts of protein is also present in layers 3.4 (3 also contains masses of small
vesicles). The cells of transmitting tissue contain many mitochondria and active vesicles
forming dictyosomes. The plastids are large with numerous amyloplasts, polysomes and
abundant rough ER.Transmittinp tissue cells have a spherical or slightly ellipsoidal
vacuole. Nuclei are large and frequently lobed indicating their active metabolic state.
EM studies of transmitting tissue in Petunia, Lycopersicon and Nicotiana and some .
other taxa show that the cells in general have thin walls traversed with plasmodemum.
There are hardly any cell divisinrls during the growth of transnsmitt~ngtissue from the Pollination and Fertillmtinn
very young stage; cell elongation, however, does take place. As seen in transections, the I

cells are circular and separated from one anot!er and u e s~~rrourided by intercelPd!ar
substance of different electron density. It is more coniplex than middle lsme!la and
comparable to the secretion fluid of stylar canai (Fig, i.4 a c) It conlams prrjtcin In
Lycopersicon. Along the transmitting tissue only carbohydrate, peroxidasc and acid
phosohatase is detectable.

(8) !b)
(C,

Fig. 3.4: Diagrammatic representation of solid style. a) Longisection. b, e) Transmitting

tissues in Iongitudinal (b), and transation Cs), Note the plasmodwrnatal
connections.

3.2.2 Pollen Germination-Events on Stigma and in Style

As you have learnt, the stigma provides approprizte cor~ditionsfor the retention and
germination of pollen grains and the subsequent growth of pollen tube (<Fig.,3.5).
Receptivity of the stigma is generally limited to a short period (before and after
anthesis) and varies from species to species. The stigma supports pollen adhesion,
hydration and germination.

Rg. 3.5: E.S. pistil showing gmPen hbc

passage and entry i n t ~the ovule,
Adhesion is accomplished by various means and is determined hy the stickiness of
pollen and stigma, exine ornamentation, composi!ion of pellicie, amount of surface-coat
substances, electrostatic forces and more importantly specificity between the two
parents. Pollen hydration is achieved by the moisture provided on the stigmatic surface.
On a dry stigma, hydration is gradual. Hydration triggcrs the release of pollen-wall
proteins and subsequent interaction (compatiDllity/inco;npatit~ility)between the two
parents.
The events that follow pollination are depicted below:
Pollination
1
r - adhesion

Intra- Inter-specific
specific barriers barriers
tube growth through style
1
entry of pollen tube in female gametophyte
In plants having,dry stigma with solid style, enzyme cutinase present in the pollen tube
digests the stigmatic cuticle at the point of contact. The tube penetrates the
pectocellulosic wall of the papillae and then traverses through the intercellular spaces of
the stigma and style. Finally, it travels through the intercellular matrix of the
transmitting tissue. In plants with dry stigma and hollow style, the cuticle over the
papillae is continuous throughout the stylar canal. The pollen tube grows through the
subcuticular mucilage. In plants with wet stigma and closed style the pollen tube enters
the intercellular matrix of the stigma before making its way into the style. In plants
with hollow style, the pollen tube grows on the surface of the stigma and then enters the
stylar canal.
The extra-cellular proteins contributed by tapetum localised in the pollen grain wall
(sporophytic) contribute to pollen germination, penetration of pollen tube and its early
growth. Other fractions are involved in recognitiod responses which control inter-and
intra-specific incompatibility. Gametophytic proteins are injected into the intine from
microspore cytoplasm and are probably involved in' the germination and early nutrition
of the grain and in gametophytically controlled incompatibility systems. Sporophytic
enzymes leach out within seconds after pollination but gametophytic enzymes are slower
to move and are detected after several minutes.
In the mustard family, the stigmatic papillae are completely covered by a cuticle layer
and no exudate is produced. Pollen grains breakdown the cuticle enzymatically and
come in direct contact with the stigmatic papillae. Pollen grains are then able to absorb
water from the turgid cells of the stigmatic surface and germinate readily. The walls of
the stigmatic papillae of Brassica nigra consist of an outer layex of cuticle, a thin
intermediate pectic-layer and an inner layer of pectin and cellulose. After cross-
pollination the pollen tubes penetrate the cuticle and the intermediate pectic layer and
actually grow in between the cellulosic lamellae of the innermost pectin-cellulose layer
by dissolving only pectic constituents of the wall. In B. okracea, the stigmatic papillae
are covered by an additional waxy layer and only after piercing this layer that the pollen
comes in contact with the cuticle. Enzymes for the breakdown of cutin and pectin have
been demonstrated in pollen grains.
The Passage of Pollen Tube
In cotton, the pollen produces a tube within an hour which grows on the surface of the
stigmatic hairs, and then between the cells of the stigma at the bases of hairs and
beyond. The cytoplasm of the stikmatic hair degenerates; no exudate is secreted. The
tube continues growth through the intercellular spaces of the thin-walled cells of
transmitting tissue. After reaching the thick-walled cells of the main strand, it actually
grows through wall layer 3. It has been reported that pollen tube of Petunia grows
within the compact matrix of the middle lamella of the transmitting tissue by
enzymatically creating a pipe-like path in front.
By the increased dictyosome activity the cells become thicker. Callose is deposited in
the pit fields on the transmitting tissue after the passage of pollen tube. The pollen tube
passage probably changes the permeability of cells and callose is formed as a wound
i.esponse and as a reaction against cell leakage. Once the pollen germinates and the
pollen tube has penetrated the stigmatic tissue, the path of the pollen tube through the
rest of the stigma and style appears to be determined by the nature and structure of the Po11laation and Fertilization
cell walls and the morphology and distribution of tbe transmitting tissue.
The nutritive role of the transmitting tissue was recognised early. Pollen tubes of Lilium,
Petunia and Oenothera are shown to draw nourishment (sugar and amino acids) from
the stylar tissue. Growth of tubes through style causes an increased inflow of
carbohydrates into the pistils. In Aegle marmelos the cells surrounding the stylar canals
show an optimal concentration of starch just before pollination, subsequently as the
starch is digested, the canal cells and the basal portions of the stigmatic papillae show
reducing sugars which also disappear within 3 days after pollination. Disappearance of
stylar starch has also been observed in Fritillaria, Zephyranthes and Pavonia.
Metabolism of Pollen Tubes
Pollen grains contain auxins, and gibberellins which are known to be involved in post-
pollination enlargement of the ovary and the development of the fruit. Pollen from
unrelated species, nonviable pollen or even pollen extracts can prevent abscission and
cause swelling of the ovary and formation of near-nonnal but seedless fruits. The initial
and small amounts of auxins such as IAA or other auxin-like substances and GA
supplied by the germinating pollen to the pistil serve to initiate some minimal growth
and metabolic processes as a result of which enzymes liberate additional amounts of
auxi? from the tissue of the style and the ovary, e.g., Nicotiana tabacum. The auxin
released initiates growth also in the fertilised ovules which later produce appreciable
amounts of auxins and gibberellins in the endosperm, and auxins and cytokinins in the
developing embryo. Thus, the initial supply of auxins and gibberellins from germinating
pollen not only triggers the development of the fruit but is also responsible for the
subsequent release and production of additional amounts of plant growth regulators in
the pistil tissues.
Respiration: In the unpollinated pistils of Hippeastrum hybridum very high 0,tension
exists from stigma down through most of the style. During pollen tube growth a marked
drop in 0,tension takes place in the region of style containing the tips bf pollen tube
and this drop in tension moves down progressively with the growth of the pollen tubes.
However, after the passage of the tube, the original high level is restored, though not
completely. It appears that the tube grows aerobically through the stigmatic and stylar
tissues and only in the lower most region of the style and ovary that the tube confronts
anaerobic condition.
Pollen Tube Structure

The pollen tube in the stigma is filled with cytoplasm containing numerous mitochondria
and dictyosomes. The number of dictyosome cisternae is reduced in the tubes. Large
vesicles associated with dictyosome seem to be incorporated in the tube wall. Abundant
ER and polysomes which are either in free form or attached to ER can also be seen.
The pollen tube wall in the stigma and style show two distinct regions: the outer part of
the wall (PAS positive), and the inner portion which is thicker, more homogeneous
(much less reactive to PAS), and rich in callose. The dense cytoplasm contains vesicles
of various sizes, ER, ribosomes, and a few poorly-developed plastids with swollen outer
membranes. Dictyosomes are quite numerous with 4 or 5 cisternae, and produce
vesicles. The vesicles appear to fuse with the plasma membrane of the pollen tube. A
very large population of small,spherical vesicles are scattered throughout the pollen tube
cytoplasm (Fig. 3.6).
The ER in the pollen grains and during early growth of the pollen tube has extended
cisternae and apparently serves as a storage site for proteins. As the pollen tube grows
down the style, the ER shows the common variety of narrow cistenlae indidting that
protein present is being gradually utilized during tube gowth. The ultrastructure .of the
distal region of the pollen tube and the wide variety of cell organelles are indicative of
active carbohydrate and protein metabolism. The part of the tube imrneuditely behind
the tip region shows less dense cytoplasm and more dispersed organelles. The more
mature parts of pollen tube contain only a thin layer of cytoplasm closely appressed to
the wall and a large vacuole occupies the rest of the space. Plugs of the wall material,
mostly callose serve to separate the older parts of the pollen tube from the growing
distill region. The plugs originate as rings on the inner side of the wall and grow
inwards like the closing of an iris di@hram.
Plant Development-1

Vesic:es o f golgi origin

Fig. 3.6: Representative fine-structural diagram of a growing pollen tube

(adapted from Iwanami et aB,1988).

Pollen Tube Growth

There are significant differences in the fine structure of the tips of pollen tubes in
compatible and incompatible pistils such as incLilium. Tubes growing in compatible
pistils show deep embayments but no compartments (tubes in incompatible pistil have a
compartmented cap). The compatible tubes undergo a transition from autotrophic
nutrition (characterized by compartmented cap of golgi derived vesicles) to heterotrophic
condition in which the secretion product from the stylar canals enter the pollen tubes
through the deep embayments (tubes growing in incompatible pistils are unable to make
this transition and thus are unable to continue growth due to the exhaustion of
endogenous food reserves).
The pollen tube wall is composed mainly of poiysacchakdes. Since the pollen grain
carries limited food reserves and a relatively large amount of new pollen wall material is
synthesized during growth, it has been assumed that at least a part of the substance
needsd for pollen tube wall is contributed from the polysaccharides present in the pistil
tissues. It has been shown that the polysaccharide component of the stigmatic exudate of
L. longiflorum is incorporated into the cytoplasm of the growing pollen tubes and later a
specific fraction of the incorporated exudate is extensively metabolised before being
utilized for pollen tube wall biosynthesis.

3.2.3 Pollen Tube Growth in Vitro

Pollen tubes of a few species can be grown in culture. Lily pollen tubes grow to a
maximum length of only about one cm in vitro whereas the lily pistil is 10 times longer.
Obviously then, the pistil provides appropriate conditions for tube growth. Also, a
constant nutrient medium is employed in culture experiments. The environment of the
tube may be changing as it grows through the pistil. Many amino acids and hormones
have been reported to stinnuiate the pollen tube growth in vitro.
High relative humidity is the most essential requiyement for pollen germination. Other
factors important for in vitro pollen germination are:
1. Carbohydrate-sugars control osmotic pressure and serve as respiratory substrates.
Sucrose is most effective.
2. Boron-most pollen are deficient in boron content and this is made up by its
presence in stigma and style. Boron reduces bursting of pollen tube and helps in the
 translocation of sugars. It also has direct or indirect effect on enzymatic steps that Pollination and Fertilization
are involved in the biosynthesis of carbohydrates.
3. Calcium-population effect is mediated through Ca++ions. The growth of the tube
is more vigorous and they are more straight and rigid. Permeability of the tubes is
also controlled. Calcium antagonises the inhibitory effects of certain heavy ions.
Calcium effect is dependent on the presence of a suitable osmotic milieu, 0, and
borate. It is enhanced by a methyl donor and other inorganic cations especially
Mg++.K+,Na+ and H+.
4. Enzymes--cellulase, pectinase and callase are present in pollen grains. They
increase the rate of tube elongation when presellt. in the medium.
5. Plant hormones-tube growth is promoted by auxins and gibberellins.
6. Germination Medium
Sucrose 100 mgll
H3B0, 100 mgll
ca(No3 2 300 mgll
MgSO, 200 mgll
KN03 100 mgn

Media containing raffinose often yield better growth. Cobalt, Zn and other minerals
have occasionally been reported to stimulate pollen tube growth. Calcium was found
to be insignificant in easter lily. Growth of lily pollen in vitro is stimulated by
Co" . It is accumulated in the pistils from where the growing tubes can apparently
accumulate it, Co++activates an aminoacylase found in pollen.
7. Physical factors-temperature (20-30" C).
Fine Structure of Pollen Tubes Grown in vitro: The growth in pollen tubes is
exclusively restricted to the tip. Cytochemical analysis reveals the pollen tip zone to
be rich in RNA and protein. This zone has numerous vesicles and an elaborate
network of smooth membranes. The vesicles appear to rise from the ends of
dictyosome cisternae. They coalesce with one another and ultimately contribute their
membrane and contents to the compartmented cap covering the growth zone at the
tip. The cap and the vesicles contain pectin and the RNA resides in the smooth
membra~es.In the region behind the tip the tube contains those organelles present
in pollen before germination, as well as numerous amyloplasts. The tube wall is thin
(tubes growing in pistil have complex wall). Cellulose is the primary wall
component. The cytoplasm of pollen tubes growing in vitro lack microtubules. EM
shows that the generative cell is surrounded by its own distinct wall and the
cytoplasm is different from the grain proper. It contains a small number of poorly
developed organelles which possess little storage material.
Thus, the pollen chemistry and growth studies provide the following general
conclusions:

1. Metabolic pathways in pollen are those common to most non-green tissues;

2. The overall composition and balance of pollen chemical constituents vary with
species, plant nutrient level and environment during development;
3. Enzymes or some chemical constituents rapidly diffuse out of pollen;
4. Chemicals diffusing out of pollen or pollen surface can interact with the pistil
tissues;
5. Tube growth can*bemodified by certain chemicals;
6. Tube extension occurs by the addition of pectin and hemicellulose via addition of
vesiculated membrane-like components at the tip, cellulose is probably added after
the initial tube membrane is formed;
7. Decreased pollen viability after dehiscence is generally related to enzyme activities
metabolising, endogenous substrate.
Plant ~evelopment-I 3.3.4 Syngamy and Triple Fusion

After traversing through the stylar region, the ultimate destination of the pollen tube is
to reach the female gametophyte and release the male gametes that can ensure
fertilization. In angiosperms, double fertiuzation is an important feature which involves
fusion of one male gamete with the egg (syngamy) to from the zygote, the progenitor of
next generation and the other male gamete fuses with the fusion product of the polar
nuclei (sedondary nucleus),resulting in triple fusion (haploid male gamete + two haploid
polar nuclei = 3n), the primary endosperm nucleus.
Entry of Pollen Tube into the Embryo Sac: The pollen tube enters the embryo sac
through the filiform apparatus of ong of the synergids. Generally, one of the synergids
degenerates before the entry of pollen tube and the tube invariably enters through such
synergid. In Plumbago, where synergids are absent and the egg has the filiform
apparatus, the tube enters directly into the egg. In several taxa, however, both the
synergids remain healthy until the entry of pollen tube and the one that receives the
pollen tube starts degenerating. It has been proposed that the degeneration of synergid
and also of pollen tube cytoplasm is essential to prevent male gametes from a rejection
or antigen-antihy type of reaction.
The choice of the male gamete involved in syngamy and triple fusion has long been a
subject of speculation. Embryologists were curious to know which of the two male
gametes participated in syngamy. Transmission electron microscopy and scanning
electron microseopy have revealed that in certain plants the two male garnates are
unequal in size and in the number of plastids and mitochonoria they contain. The plastid
rich sperm seems to be preferentially involved in syngamy and the one with poor
plastids fuses with the secondary nucleus 3.7). However recent studies indicate that
heteromorphic sperms are not seem in other plants.
.I
Polar
I
nuclei
I
I

I
Sperm
nuclei

Probable mode of
Entry of pollen tube Discharge of pollen tube gamete transfer

Fig. 3.7: Diagrammatic representation of fertilisation (After Jensen, 1973).

Syngamy: The pollen tube grows to a very limited extent in the synergid. It releases the
contents either through a terminal or a subterminal pore. The contents include the two
male gametes alongwith the accompanying cytoplasm, some reserve nutrients and
perhaps vegetative nucleus. One of the sperms enters the egg and the other to the central
cell. The distance that the male gamete has to travel to come in contact with the egg or
secondary nucleus is insignif~cant.
The male gamete comes in contact with the plasma membrane of the egg that forms a
bridge through which the male gamete enters. Nuclear fusion is initiated by the joining
of the outer membrane of the two nuclei. The inner membrane also fuses at localized
areas forming a small bridge between the two nuclei. Enlargement of bridge causes
eventual fusion of the male and egg nuclei (syngamy). Depending upon the stage of
the male gamete at the time of nuclear fusion, three types have been recognized.
(a) in pre-mitotic type-the male nucleus fuses with the egg before reaching the
mitotic interphase, (b) in post-mitotic type the male nucleus undergoes interphase while
in contact with the egg and the fusion is postponed until the initiation of the first
mitosis in both, and (c) in intermediate cases, the fusion occurs when male nucleus is
still at interphase. Generally, the male cytoplasm does not take part in fertilization.
However, there are plants in which biparental inheritance of plastids has been Pollination and Fertilization
demonstrated.
Triple Fusion: The fusion process between the other male gamete and the secondary
nucleus follows the same pattern as syngamy. In most plants, the polar nuclei are only
partially fused when the male gamete approaches. The fusion of the male gamete with
one of the polar nuclei completes the fusion process of all the three nuclei (triple
fusion). Interestingly, initiation of syngamy starts before triple fusion, but triple fusion is
completed first.
SAQ 2

Fill in the blank spaces with appropriate word(s).

a) The growth of the pollen tube is confined to the ..................region.

b) The enzymes namely .....................................and .................. that are
present in the pollen grains, aid in pollen tube growth in the style.

c) The stigmas which secrete exudates are called .................. stigmas, e.g.,
.................. and those which do not are called .................. stigmas, e.g.,

d) The stigmatic exudates are extruded from a stigmatic cell by ...................

e) The .................. component of stigmatic exudate protects it from desiccation and
also regulates the available water, whereas, the .................. components protect it
from pathogens.

f) The exudates in .................. styles are rich in polysaccharides, lipids and proteins
whereas those of .................. style have mostly polysaccharides.

g) The ....................................are associated with open styles and the

..................................are associated with the solid styles.
h) The .................. enzymes are leached out within seconds after pollination,
whereas the .................. enzymes are slower'to come out, and they can be
detected several minutes later.

i) The .................. and ..................present in the growing pollen tubes are

involved in post-pollination enlargement of ovary and the development of fruit.

j) In angiosperms, the two products of double fertilization are: one, the ..................
and second, the ...................

3.4 INCOMPATIBILITY

Plants growing under natural conditions have a preference for their mating partners. The
stigma of the female parent receives all kinds of pollen. However, the choice of pollen
of the desired parent that would accomplish fertilization is finally determined by both,
pistil and the pollen. This gametophyte (pollen) and sporophyte (pistil) interaction leads
to recognition (acceptance or rejection) of mating partners. In angiosperms, the female
gamete is located in the ovule present in the ovary. Therefore, pollen grains carry male
gametes through pollen tubes that travel from stigma to ovary to affect fertilization. The
pollen tube has to traverse through the tissues of the stigma and style. A situation in
which fertile pollen fails to accomplish fertilization process so that a viable embryolseed
is not formed is sexual incompatibility. Self-incompatibility means the inability of the
plant producing functional gametes to set seed upon self-pollination. It may operate
between the individuals of the s w e species-intraspeciflc or self-incompatibility or of
different species-interspecific incompatibility.
I
Plant Development-I 3.4.1 Intraspeeific Incompatibility

The majority of flowering plants are fertilized successfully only by the pollen of .
other plmts. Various floral adaptations have evolved to prevent self-pollination. These
include, dichogamy, herkogamy and unisexuality about which you have studied in
Subsection 3.2.2. On the basis of morphology done, self-incompatibility can be
categorized into:
Heteromorphic Types: Plants of the same species produce flowers that differ in
morphology. This involves two (distyly) or three (tristyly) morphologically distinct types
of flowers showing similar breeding behaviow (mating type) within a species. The
difference in the mating types lies in the position of stigma and anthers (heterostyly).
Distyly is under the control of a single gene complex with two alleles, S (for short
style-dominant) and s (for long style-recessive).Long style individuals are, therefore,
homozygous recessive (ss) and shott style are heterozygous (Ss). This ensures
approximately equal ratio of short and long-styled individuals from compatible crosses.
Tristyly refers to three floral morphs with individuals having either long, mid- or short-
styled flowers. Each type ha5 stamens of two heights corresponding to the height of
stigma in the other two forms. Successful pollination results only between forms having
stigma and stamens of the same height (Fig. 3.8). Two genes, M and S with two alleles
each control tristyly. Gene S is epistatic to M. Homozygous recessive for both the genes
(ssmm) determine long style, mid-style have ssMM or ssMm and short-styled flowers
carry the genes SsMm or Ssmm or SsMM. In both di- and Uimorphic forms sporophytic
incompatibility operates.

DIMORPHIC SYSTEM TRIMORPHIC SYSTEM

Fig. 3.8: Heterornorphic intraspecirrc incompatibility.

The S-gene has been suggested to be a super gene complex with several linked genes.
It is supposed to have at least six (may he more) closely-linked genes which determine
length of style (gene G ) , surface of stigma (S), pollen incompatibility (I'), stylar
incompatibility (I"), pollen size, and/or shape (P), and stamen height (A). Being closely
linked they are inherited together but because of crossing over, can be separated. In
addition heteromorphic incompatibility can be further characterized by-(a)
sporophytically-determined incompatibility reaction, b) inhibited growth of incompatible
pollen tube in the style, and c) expression of dominance between alleles of the
incompatibility genes in both, pollen and style.
It has been proposed that physiological incompatibility is superimposed by
heteromorphism. The two supplement each other in controlling unwanted matings.
Rejection through morphological variations is nlechanicd and relates to interspecific
incompatibility. The physiological control is similar to homomorphic mechanism.
Homomorphic Types: It is characterized by morphologically indistinguishable mating
types within a species. A proper breeding is required for their recognition. This kind of
incompatibility operates in more than 250 genera belonging to 71 families of
angiosperms. It operates through multiple alleles (as many as 45 alleles are already
reported from 500 plants) of S-gene. Pollen grains with one S-dlele common with one
or both S-allele/s of pistil is incompatible, It is therefore conceivable that on pollen side PalU~tlolra d FertibtJon
the haploid gametophyte controls incompatibility. The S-allele m y express a
relationship of dominance or independence in pollen andlor pistil.

Basis of Self-Incompatibility: Very few m a have been worked ou: to explore the
genetics of inwmpatibihty. In many taxa it is controlled by c,ne gene, in Poaceae by
two genes (S and Z) each with many alleles and in Chenopuhaceae, Brassicaceae and
Ranunculaceae by 3 or more genes.

East and Mangelsbrf in 1925 proposed "Opposition S-alleles" hypothesis, about the
genetic control of self-incompatibility. According to this hypothesis, a single gene, the
S-gene with several alleles controls the incompatible reactions. Pollen grains having S-
allele common to any one of the two alleles present in the plstil will not be functional
on that particular pistil. For example, a plant having S, and S, alleles in its sporophytic
cell, including the pistil (Fig.3.9). two types of pollen one half baving S, and the other
half with S, allele will be produced during microsporogenesis. Neither of these pollen
types (S, or S,) will be functional m this plant because in the stylar cells also S, and S,
alleles are present. However, if this plant were lo be pollinated with pollen from a plant
of S,, S, genotype, pollen grains carrying oniy S, allele would be able to bring a b u t
fertilization. Tbe other half with S, d e l e would be nonfunctional. One hundred percent
pollen grains would be functional from a plant having S, S, on the pistil of a S, S,
plant as none of the alleles is common between these two plants. In all such cases it is
the S-allele of the pollen or male gametophyte which controls the incompatible reactions
(Gametophytic self inwmpatibility-CjSI).

No germination

Gametophytic Sporophytic

Fig. 39: The operation of gamebphytie and spo~ophyti~ incompatibility. a h c S, and S, pollea are
inhibited in both types of incumpatibilitk. S: S, plants pollinated with S,S, (not shown in .
figure) will function in the gametophytic system because S, pollen have a matching allele in
the pltil but S, pollen is inhibited. h sporophytic system the S allele may express
independently, or show dominance of one over the other in polkn a?d!or pistil. When S,
and S, bave independent action or S, is dominirat in pllen b o a are inhibited. However, S,
is dominaet over S,, both are functional. S, S, alleles in the pistil are considered to have
independent action.

However, in SSI-systems (sporophytic self !ncompatible) the pollen behaviour is same

for all irrespective of the S-allele they carry. For example a plant carrying S,, S, alleles
would be completely incompatible tn plants carrying S , SZ,S, S, or SZS4, SZS, and SO
on but a plant carrying S, S, or S, S, and so on would show one hundred per cent
compatibility (see Fig.3.9).

Based on the difference in time of S-gene action, the two types of incompatibility are
further explained. Because S-gene is activated befo~cthe mpletion of meiosis in
sporophytic systems, products of bsth the geses a e ificorprated in d1 the mimspores.
Nonetheless at last in some plants, S-allele specific subsmms are prd~lcedlin the
tapetum and then mcorporated into pollen exine. Thus bath u~apeth!:a3 @ken
components are involved in controlling sprophytjc incompatibihry i 8 gametophytis
 Plant Development-I systems since S-gene action is delayed, two microspores receive the product of one
I S-allele and the other two microspores receive another S-allele.
Studies have been carried out to assess the biochemical nature of the incompatibility
factors. Antisera produced in rabbits against pollen extracts of some plants precipitate in
the presence of diffusates of pollen. In certain plants S-allele specific antigens are
identical in pollen and pistil.

Through imrnunodiffusion tests it has been possible to demonstrate the presence

of a unique protein for each S-allele in Brassica. S-allele specific antigens could also
be detected from diffusates of intact stigma. Findings related to the nature of self-
incompatibility proteins in relation to phenotypic expression of self-incompatibility in F,
and F, of cross S, S, x S,, S,, provides interesting insight. All F, progenies showed
protein concentration intermediate to those of parents, whereas F, individuals showed
high, intermediate and low levels of S-protein in the ratio of 4:7:4. The results indicate
that the quantity of S-protein is directly proportional to the intensity of the
incompatibility reaction.

Barriers to Fertilisation: Incompatibility can occur any where from pollination to

syngamy and consequently obstructing fertilisation. The pollen fails to germinate or the
pollen tube is inhibited to penetrate the stigma. The progamic barriers to fertilisation
may be on the stigma or in the style at any level from pollen germination to the
discharge of male gamete in the vicinity of egg. On the other hand, syngamy barriers
include the inhibition of the entry of pollen tube to the ovary, ovule or within embryo
sac. At the stage of pollination. the barriers to fertilisation are of morphological and
ecological types, whereas, at progamic phase and syngamy physiological barriers occur.

Physiology and Biochemistry of Incompatibility: On the basis of recognition and

rejection reaction, the type of incompatible process can be distinguished.
Recognition reaction: The compatibility of pollen grains is decided at the molecular
level by the pistil. In SSI systems acceptance or rejection of a pollen grain is decided on
the stigmatic surface. Certain GSI systems also operate recognition reaction on stigma
though normally it occurs in the style.

Rejection reaction: For rejection reaction the physiological and biochemical processes
are set in the pistil by the recognition reaction specific to the type of pollen that lands
on stigma. In contrast to systems where inhibition can occur on stigma itself thus
preventing germination of pollen or its entry into style, in GSI system it may occur in
style leading to either inhibition of pollen tube growth or its bursting.
The pollen wall and its protein content plays an important role in pollen stigma
interaction. Through cytochemical studies it has been observed that the pollen wall is
perforated by protoplasmic strands giving the appearance of a living physiological
structure playing a very responsible role in the process of interchange between the
pollen grain and the substrate.

The incorporation of proteinaceous substances in the exine and the intine have been
demonstrated by EM studies of the pollen wall during development. A good amount of
mobile proteins of the pollen grains is S-gene specific. The recognition proteins of
incompatibility are held extra-cellularly in the pollen wall. In GSI plants, these proteins
are present in the intine while in SSI plants they occur in the exine.

The Exine Layer: It consists of two layers, inner (nexine) which is continuous and an
outer (sexine) the sculptured one. The outer layer has ornamentations comprising
rod-like bacula showing terminal expansion which sometimes fuses to form a roof like
tectum perforated by micropores. During pollen development, the tapetum releases a
mixture called tryphine, consisting of carotenoid lipid droplets and fibrogranular
proteins into the thecal cavity. These fibrogranular proteins are enclosed in membrane
bound cisternae. In tectate pollen,:the releaskd substances of tapetum become
incorporated on the surface of pollen grains where cistirnae membrane ruptures and
protein released through micropores enter the tectum and accumulate in the interspaces
of bacula. Pollenkitt is the pigmented lipid fraction left on the surface of the tectum.
Unlike intine, the exine held proteins exhibit only one enzymatic activity. The hydration
of pollen is described to be stirnulatory in releasing the proteins held in the pollen wall
 layers. The calculated time taken by the exine held proteins of sporophytic origin to Pollination and Fertilization
pass out is 30 seconds while the intine of gametophytic origin takes a few minutes. This
time difference in the protein release has made it easy to collect both samples
separately. In SSI systems therefore the rejection reaction is faster than in GSI system.

The Intine Layer: As soon as the tetrads release the microspores, the inner layer of
the pollen wall (intine) is formed. Proteinaceous lamellae are embedded in the matrix of
the intine, concentrating around the germpore. The proteins incoporated into the intine
are contributed by the cytoplasm of the gametophyte, i.e., pollen cytoplasm. In members
of the Malvaceae and several other plants, the proteinaceous lamellae are dispersed in
the intine without coming in contact with the exine or pollen cytoplasm.

Sporophytic and Gametophytic Self-Incompatibility: Besides the categories based on

morphology, self incompatibility can be further classified into sporophytic or
gametophytic types depending on the origin of factors that determine the mating types
on the pollen side. i) Gametophytic self-incompatibility (GSI) is governed by the
genotype of pollen (male gametophyte) itself, e.g., Poaceae, Liliaceae, Solanaceae,
Fabaceae, and Cornmelinaceae. ii) Sporophytic self-incompatibility (SSI) the genotype
of the sporophytic tissue which donates the pollen controls the incompatibility process,
e.g., Asteraceae, Brassicaceae and Convolvulaceae. Other characteristics of GSI and SSI
systems are described in Table 3.1 (See p. 68) and 3.2. A correlation has been
established between the type of self-incompatibility, pollen cytology, and the site of
inhibition. Generally, species that shed pollen at the Zcelled stage are credited with
gametophytic incompatibility (inhibition occurring in style) while those shedding 3-celled
pollen show sporophytic incompatibility (zone of inhibition on stigma). Additionally,
taxa having dry type of stigma are associated with sporophytic incompatibility whereas
those with wet stigma show gametophytic incompatibility. There are a few exceptions to
these correlations.

Table 3.2: The incompatibility factors, and the site of action

Pollen Pistil Inhibition site

Gametophytic intine wall and/or pollen surface of stigma andlor surface of stigma,
systems cytoplasm transmitting transmitting zone
zone of the stigma and of the stigma.
upper region of the style upper region of the style

pollen cytoplasm stylar canal secretion stylar canal

I

Sprophytic exine layer and surface of stigma surface of stigma

systems probably pollen
cytoplasm

a
Sporophytic Incompatibility: The recognition and rejection reactions in a SSI system
occur on the stigma surface, placing barriers for pollen germination or penetration of

I
I
pollen tube into the stigma. Thus the pollen grains either fail to germinate or the small
tube they put out is inhibited by callose deposition at the tip (Fig. 3.10). The papillae at
the stigma also develop a lenticular callose plug within 10 min. after pollination. Infact,
in incompatible cases, inhibition of varying degrees operates at every level starting from
pollen adhesion, hydration, germination and tube entry into the stigma.

In some plants stigma is covered with a cuticular layer such that the incompatible pollen
tube fails to penetrate. Various studies have proved the necessity of enzyme cutinase in
eroding the cuticle. This enzyme is activated only by the compatible cases.

Incompatible stigmas may lack c~~ious'exudate, but dry stigmas have a hydrated layer
called pellicle over the cuticle. This pellicle consists of a lipid layer where a mosaic of
proteins floats. The pellicle probably originates by the protrusion on the surface of the
papillae through discontinuities in the cuticle. As soon as pollen is received by the
stigmatic surface the papillate cells exude moisture. The incompatibility is the result of
interaction between protein fractions of exine and stigma.

In incompatible pollination a number of irregularities observed in the Rehaviour of male

gameto~hvteare: (1) the mllen does not germinate (2) if it germinates, the pollen tube
I
I

I
PlantDevelopment-I
 does not grow (3) the inhibition of one tube leads to the emergence of another tube Poublim Pad Pelalzauon
(4) tip of the pollen tube swells like an appressorium (5) the most distinguishing
response for incompatibility is the development of a d o s i c plug between the plasma
membrane and pectocellulosic layer of the stigmatic papihe, just below the point of
contact with the pollen. Likewise a plug also develops at the tip of pollen tube.
Consequently, growth of the pollen tube ceases.

tube

Fig. 3.10: Stigma interaction in sporophytic system. a) compatible, b) bcompatlble; the

deposition of callose plug between the plasmalemma and the cell wall b the
stigmatic papilla (After Shivanna, 1982).

The inhibition reaction between pollen and stigma is extremely localised and does not
interfere with the growth of other pollen grains lying on the same stigma. Detection of
incompatible pollen can be made by the callose deposition at the rejection site. It is thus
obvlous that in incompatible reactions the exine borne proteins are involved. Sucb
proteins when isolated and applied directly on stigma also respond similarly, as do intact
pollen. Fragments of anther tapeturn also behave likewise when placed on stigma
surface.

I The pistillate factors in this type of incompatibility are therefore localized only on the
I surface of the stigma. Other distinguishing features are: (a) presence of S-allele specific
proteins on the stigmatic surface alone,(b) isolation of characteristic specific proteins
from the stigmatic leachates, and (c) overcoming incompatibility by organic solvent
treatment of stigma.
Expesimental evidences show that recognition factors are synthesised in the stigma
I
during its maturation. Bud pollination serves as one of the possible methods to
overcome self-incompatibility as S-allele specific proteins are absent or present in
insimcant amounts, at this stage. This indicates that factors that inhibit gennination
and tube growth are built up gradually in the pistil.
Gametophytlc IncompatibUlty: In GSI systems callose deposition is not evident on the
stigma but is very conspicuous in the pollen tube. Sometimes the d o s e deposition
occurs even in the gennpore, inhibiting pollen germination. In the Poaceae, the
rejection reaction is completed within a few minutes after pollination and subsequent
rejection or acceptance takes 10 minutes.
with the exception of Poaceae and Oenothera, the recognitiontrejection reaction in GSI
system occurs after the pollen tube has grown about two third the length of the style.
Genetically the recognition factor on the male side has been attributed to the male
gametopbyte, the proteins incorporated during pollen wall development in the intine are
involved. Since in GSI systems the rejection reaction is comparatively delayed, is also
possible that the synthesis or activation, and release of the recognition substances are
I also delayed and they occur in the pollen tube. In some species the tubes show reduced
level of carbohydrates and starch.

3.4.2 Interspecific Incompatibility

This type of incompatibility is characterised by the prevention of fusion of gametes
between members of different species. The incompatibility factors also operate after
fertilization and cause eventual breakdown and failure of embryo development. The
factors that prevent seed set following interspecific crosses have not been worked out.
 Plant Development-I It is however apparent that rejection reaction may operate at any level. In Petunia the
I morphological abnormalities of the pollen tube are similar in both self and inter-generic
I
incompatibility. The inhibition may result from unilateral incompatibility, or incongruity
or a passive rejection may operate (see Table 3.3).
I
Interspecific incompatibility involves non-proteinamus substances besides the stigma
surface proteins. Phenolics and carbohydrates also seem to play an important role. It is
believed that more than one gene at different loci controls the interspecific
incompatibility. In crosses between incompatible species either fertilisation does not
occur or the hybrid embryo aborts due to either inadequate development of endospenn
or lack of support from endosperm.
Unilateral Incompatibility: This relates to incompatibility that operates in one direction.
The most common way to ascertain unilateral incompatibility is to perform crosses
between a self-incompatible species and a self-compatible species. The cross is
successful only when the self-compatible species serves as the female parent. This kind
of incompatibility also operates between species of different genera. Unilateral
incompatibility can be explained on the following basis:
-
Table 3.3: Interspeciflc incompatibility Passive rejection and possible
mechanisms.
Pollinaton

No morphological
complementation

Pollen adhesion

Osmotic pressure differences

Co-adaptation in supply
and demand absent

Hydration

Absence of gemination
factors
Hydration of pollen
affected

Gemination

Absence of activatorsl
precursors of pollen
cutinases

Pollen tube entry into the

stigma (dry stigma)

I
Tube growth through
the style
Pollen tube fails to
utilize nutrients
length of style of the
two parents different

3.4.3 Biological Significance of Incompatibility

Both interspecific and inmpecific incompatibility determine the degree of inbreeding
and outbreeding of plants. Self-incompatibility is the natural way to circumvent the
extensive selfing that promotes homozygous individuals and eventual inbreeding
depression (accumulation of lethal recessives).
Sexual incompatibility also serves as a hindrance in crop improvement programmes. PollI~Eloaand FertiUzPElon
Until the technique of androgenic haploidy was evolved, continuous self pollination was
the only way to obtain homozygous individuals. Even now where haploid production is
not possible through anther culture, a successful approach can only be through self-
pollination. Even interspecific incompatibility prevents distant hybridization. Therefore,
various methods have now been adopted to overcome the inter-and intxa-specific
incompatibility.

3.4.4 Methods to Overcome Incompatibility

It has been possible to facilitate the germination of incompatible pollen by extracting
pollen wall proteins from compatible pollen and supplying these during interspecific
crosses. The pellicle, which lies over the cutinised wall of the stigmatic papillae is
functionally important in the capture and hydration of pollen grains and may also be the
site of the recognition reactions.
In Oenothera stylar grafting experiments led to pollen tube growth in incompatible
styles. Treatment with Ca* resulted in pollen tube growth in incompatible style to some
extent in Oenothera. Emasculation of immature flowers in Petunia reduced pistil
elongation and weakened the incompatibility barrier. In general relatively high
temperature has been found to inactivate the incompatibility mechanism and the site of
activation has been the style. Self-incompatibility in Lilium could be overcome by an
interaction of temperature and 1% solution of naphthaleneacetamide-in lanolin. Heating
of intact and detached style in water at 50' for 6 min before pollination could also
overcome incompatibility. The stigmas of some plants with sporophytic incompatibility
have a cuticular layer which serves as an incompatibility barrier. Pollen of such plants
possess a cutinase enzyme system for destroying the cuticular layer. More specifk
methods useful in overcoming self-incompatibility are mentioned below.
1. Use of mentor Pollen: There are various ways to overcome incompatibility by the
use of mentor pollen. In a mixture of mentor (compatible) pollen and incompatible
pollen the recognition protein from compatible pollen masks the inhibition reaction
and allows incompatible pollen to germinate and penetrate the stigma. Mentor pollen
seem to provide a pollen growth promoting (PGS) or regulating substance which
permits incompatible pollen to sustain tube growth. The other mechanism proposed
is that it involves the signals which stimulate mentor pollen tube to provide
substances critical for sustained growth of ovules, ovary and other fruit tissues..
2. Bud Pollination: In Petunia uxillaris early self-pollination of buds leads to normal
fertilization.
3. Stub Pollination: Incompatibility can also be overcome by either removing the
stigma or the upper part of style, if the inhibition factor lies on stigma. Sometimes
length of the style also hinders the process. To overcome self-incompatibility in
Zpomoea trichocarpa, the stigmatic lobes or a part of the style are cut off to make
an easy way for pollination. Stylar length in Nicotiana tabacum is greater than that
in N. rustica and N. debney. By removing a major portion of style from N. tabacum
and smearing its cut surface with agar sucrose medium suitable for germination, it is
possible to achieve fertilization with N. rustica and N. debney pollen.

4. Intra-ovarian Pollination: Incompatibility occurring on stigma or in the style can be

overcome by intra-ovarian pollination that refers to injection of pollen suspension
in the ovary and its subsequent germination inside the ovary.
In this technique ovary is first surface sterilized with ethanol. Then two holes one
for injecting pollen suspension into ovary and other for allowing ovarian air to eject
out are made. These holes are sealed with petroleum jelly and pollen are allowed to
germinate inside the ovary and bring about normal fertilization.

5. Test tube Pollination and Fertilization: The excised ovules or pistils are dusted with
pollen grains and allowed to grow on a nutrient medium. This technique favours
pollen germination followed by fertilization and development of seeds.

Self-incompatibility in Petu:ria hybrida and Brassica campestris can be broken by

placental pollination.
Plant Development-I Attempts have been made through placental pollination for interspecific, intergeneric
and interfamily crosses. Seeds with viable embryos develop in crosses between
Melandrium' album, M. rubrum, Viscaria vulgaris, Silene schafta and Nicotiana
alata.
Hybrid embryos also develop by pollinating the excised ovules of Nicotiana
tabacum with the pollen grains of Hyoscyamus niger.
6. Modification of Stigmatic Surface: Application of the lectin, concanavalin A, to
stigma is also helpful in overcoming incompatibility in Brassica. Likewise, prior
treatment of stigma with detergent (Tritox X-loo), and organic solvent (hexane)
helps to modify the incompatibility reaction. Pretreatment of stigma with NaCl
(15gIl) in Brassica campestris blocks the incompatibility reaction!
7. Heat Treatment of Style: In Lilium longiflorum pre-treatment of the style at 50" C
for 6 min. could prevent self incompatibility. In rye. even 30" C is effective in
inhibiting the incompatibilihy reaction.
8. Irradiation: Incompatibility in Lycopersicon peruvianunl could be overcome by
gamma rays. The higher seed-set per plant achieved by this method is ascribed to
prevention of flower abscission at early stage. Irradiation seems to be fruitful only
in plants where incompatibility is under the control of garnetophyte.
9. Chemical Treatment: When premature abscission is hhe barrier to compatibility,
application of p-chlorophenoxyacetic acid to the pedicel enhances the life span of
arrowroot flower.
Similarly, it is possible to overcome the premature flower abscission in Ipomoea
batatas by applying 100 ppm of 2,443 that helps early embryo development.
Application of p-chloromercuribenzoate and kinetin have proved useful in Liliuni
longiflorum and in Oenothera organensis respectively.
10. Increased CO, Level: In Brassica species, raising of atmospheric level of CO, by
100 fold, at 100%relative humidity helps to overcome self incompatibility. This
method is an important tooi to maintain the inbred parental lines of Brassica for the
production of S, hybrid seeds.
11. Parasexual Hybridization: As it involves the hybridization of somatic cells
(protoplast) to overcome sexual incompatibility this is known as "parasexual
hybridization".
The process involves three steps: 1. isolation of protoplasts 2. fusion of isolated
protoplasts and 3. culture of hybrid protoplast to regenerate whole plants.

SAQ 3
Which.of the following statements are not true?

(a) In the dimorphic and trimorphic forms sporophytic self incompatibility

operates.
(b) The syngamy barrier refers to any barrier on the stigma or in the style, in
other words, from pollen germination to the liberation of male gamete near the egg
cell.
(c) In the GSI plants, the recognition proteins are present in the intine, whereas in the
SSI plants, these occur in the exine.
(d) On pollen hydration, the proteins of gametophytic origin are first to leach out,
whereas the proteins of sporophytic origin take longer time.
(e) Sexual incompatibility may be between individuals of different species-
interspecific, or between the individuals of the same species-intraspecific.
(f) Interspecific incompatibility involves non-proteinaceous substances such as phenolics
and carbohydrates in addition to the stigma surface proteins.
(g) Parasexual hybridisation can be successfully used to overcome sexual
incompatibility.
 (h) In nature a balanced inbreeding and outbreeding of the plants is regulated by
intraspecific and interspecif~cincompatibility.

3.5 APOMIXIS

The formation of sporophyte from the gametophyte without sexual process signifies
apomixis. It relates to the replacement of alternation of a reduced gametophyte and an
unreduced sporophyte by an u~educedgametophyte and sporophyte (summarized in Fig.
3.11, Table 3.4). It occurs in relatively in about 35 families. The most common being
Asteraceae and Poaceae. Apomixis can be classified into two types, recurrent (in non-
reduced embryo sacs) and non-recurrent (in reduced embryo sacs). The la&r relates to
development of haploid embryos without the actual fusion of gametes.

. . -
Nuceller
cells
iI
Fig. 3.11: Diagrammatic representation of Apomixii types.

3.5.1 The Recurrent Type

In this type, euspory (seen in ncirmal cases) is replaced by aneuspory (Diplospory)
because of irregular meiosis. When the spore mother ceIl functions directly as the
embryo sac initial it is termed gonial apospory and when one or more of the somatic
cells of the nucellus or chalaza act as embryo sac initial it is called somatic apospory.
A. Aneuspory (Diplospory): Includes cases in which during spore fonnation the meiotic
process is irregular. Depending upon the extent of irregularity in meiosis in various
plants, aneuspory is further categorized into various types.
i. Datura type - the two unreduced nuclei undergo two mitotic'divisions to
produce an 8-nucleate embryo sac.
ii. Taraxacum type - first meiotic division ends in a restitution nucleus. Meiosis I1
produces unteduced cells. Ultimately an 8-nucleate embryo sac is formed. The
egg directly forms the embryo.
Plant Development-I iii. Ixeris type - it is like Taraxacum, but a single binucleate gynospore is formed.
Two divisions lead to the formation of an &nucleate embryo sac.
iv. Allium type - a premeiotic-endomitotic doubling makes meiotic prophase
to start with double chromosome number. The result is an unreduced embryo
sac.
Table 3.4: Schematic Representation of Apomixis .

Sexual

1
Anther

1
I
Archesporium

Sporogeneous
1
Diploid

-
Somatic cells
Apomictic,-

cells

1
Pollen mother
cells Meiosis

cell as cells
Pollen

3 gametophyte
(generative and
vegetative cells)

egg etc.
2 male gametes

zygote g

Pollination
NO Pollination

B. Gonial Apospory: There is no meiosis. The megaspore mother cell enlarges, a

small vacuole appears above and below the nucleus. The cell thus becomes
1-nucleate embryo sac. Three successive mitotic divisions result into an &nucleate
embryo sac. In Brachyocome, the polar nuclei function directly as the first
2 endosperm nuclei.
C. Somatic Apospory: Meiosis is apparently normal but megaspores play no part in the
embryo sac formation. Either during or soon after meiosis one or more sporophytic
cells (nucellar or chalazal) enlarge and invade the nucellar lobe destroying and
replacing the megaspore. After 3 successive nuclear divisions each such cell
becomes differentiated into an aposporic 2n embryo sac with normal organization.
Thus, garnetophytic generation is completely eliminated.
Organization in non-reduced embryo sacs
The fate of a nucleus in the embryo sac depends upon its position. Many irregularities
in the disposition of nuclei in the early polarization have been recorded. The mature
embryo sac otherwise shows a n o d organisation with two synergids, three antipodals,
an egg and proendospermic cell.
Embryogenesis in non-reduced embryo sacs Pollinotim and Fertillzatlon

Embryo development in these embryo sacs can take place as a consequence of

pollination. Those associated with pollination are called eugamous, semigarnous or
pseudogamous and those that develop without pollination stimulus are termed
parthenogenic.
Eugamy: Nonnal fertilization of the apomictic eggs takes place to produce the
zygote.
Sernigamy: The male gamete penetrates into the egg but does not fuse with the egg
nucleus. Both the nuclei multiply independently but the division of the male nucleus
stops early.
Unreduced Pseudogamy: The male gamete degenerates either inside or outside the
embryo sac. Thus, the egg develops without fusion taking place.

3.5.2 Non-recurrent Type

As mentioned earlier it refers to embryo development in reduced embryo sacs. The
mechanisms identified for such cases are:
i. Reduced Pseudogamy: The reduced egg develops in a pseudogamous, and
parthenogenic manner. The pollen tube enters nonnally but the male gamete fails to
fuse with the egg and disintegrates in the cytoplasm of egg.
ii. Reduced Parthenogenesis: Embryo development is accomplished by heat or cold
treatment.
iii. Androgenesis: The egg nucleus degenerates, the sperm nucleus functions in the
cytoplasm of the egg and produces the embryo.

3.5.3 Endosperm Development in Apomicts

Endosperm development is usually poor and without preceding the fusion of polar
nuclei, hence diploid.
3.5.4 Anthers of Apomicts
Meiosis is abnormal or there is total failure of meiosis. Formation of plasm&
microspore mother cells is reported. Polyads are also noticed. Usually, only 2
microspores of a tetrad are normal. The generative nucleus rarely divides. Thus pollen
remains at 2-celled stage.

3.5.5 Causes of Apomixis

Apomictic species are generally hybrids or polyploids, as a consequence, there is
irregular meiosis. Apomixis appears to be controlled by a set of genes. The trait is
genetically inherited. The genes controlling sexual reproduction are non-allelic to those
of apomixis. Accordingly, any line of dissent carrying the genes for apomixis will
produce both types, apomicts as well as sexually reproducing plants.
It has been proposed that apomixis is governed by recessive genes. The three genes
(AABBCC) determine the breeding behaviour. In homozygous condition a forms
unreduced eggs, b prevents fertilization, and c promotes egg development without
fertilization. Thus, aaBBCC will have unreduced egg but cannot develop without
fertilization, AAbbCC produces reduced egg but no embryo development take place
because fertilization is prevented, and AABBcc will show normal sexual behaviour
because the gene C has no effect in the presence of A and B.
As a consequence of apomixis, genetic variability in such species is frozen as they have
the same genotypes as parents. However, facultative apomicts have an advantage as they
have retained both kinds of reproduction.
.
3.5.6 Parthenogenesis
The diploid egg produced in the embryo sacs during diplospory and apospory develops
into an embryo without fertilization, thus maintaining the sporophytic level of
chromosomes. This process of embryo development from an unfertilized egg is called
Plant Development-I parthenogenesis. The stimulus to form embryos may be pollination dependent. For
example, in grasses pseudogamy operates that involves pollination stimulus while in
apomictic taxa of Asteraceae and Rubiaceae no such stimulus is required.
Pseudogamy has been credited with: (i) supply of male nucleus for endosperm
development, (ii) activation of growth of ovule and ovary, and (iii) stimulation of
parthenogenesis.
Pollination even otherwise is reported to initiate the development of adventive embryos
of Citrus. Likewise, parthenogenetic development of embryo proceeds in apomictic
grasses but normal embryos result only when endosperm is also formed.
Parthencgenesis can be distinguished into reduced and unreduced, accordingly, the
developed embryo can be haploid or diploid, (see Fig. 3.11).

3.5.7 Significance of Apornixis

Apomixis offers the possibility of indefinite multiplication of specially favourable
biotypes without any variation due to segregation or recombination. In obligate apomicts
such as mangosteen (Garcinia mangostana) this advantage is enjoyed at the cost of l ~ n g
term evolutionary flexibility which is advanced chiefly through sexual reproduction.
However, in facultative apomicts or groups of plants where sexual and apomictic
members co-exist, the phenomenon is of special significance.
There is much interest currently to induce apomixis in important hybrids to fix the
vigour and to save the cost of hybrid production. There is already some success in
forage grasses.
SAQ 4
Part-1 Which of the following features characterise the recurrent type of apomixis?

a) formation of non-reduced embryo sacs due to irregular meiosis.

b) egg nucleus of urneduced embryo sac degenerates, and the sperm nucleus in the
cytoplasm of the egg forms the embryo.
c) megaspore mother cell acts as embryo sac initial.
d) on subjecting the unfertilised egg cell to heat treatment.
e) somatic cells of nucellus function as embryo sac initial.
Select the right answer from the choices given below.
i) a,b,c
ii) c,d,e
iii) a,c,e
iv) b,d,e.
Part-2 Which combination of the following features signify the non-recurrent
apomixis?

a) reduced egg nucleus degenerates and sperm nucleus function in the cytoplasm
of egg, to form the embryo.
b) development of reduced egg by pseudogamy, and in parthenogenic manner.
c) development of embryo from reduced egg cell by heavcold treatment.
d) the nucellar cell forms embryo.
e) the egg cell is diploid due to irregular meiosis.
Select the correct answer from the choices given below:
 iii) c,d,e Pollination and Fertillzatlon

iv) a,c,e

3.6 SUMMARY

In this unit you. have learnt that:

In sexually reproducing plants, pollination and fertilisation follow gametogenesis.
Pollination is the transfer of pollen from anthers to the stigma. Flowering plants
have evolved a number of strategies for pollination. A large number of plants
undergo self-pollination, others have evolved contrivances to ensure cross pollination
although their flowers are bisexual.
The events that follow pollination are: pollen adhesion, its hydration, germination,
pollen tube entry into the style, growth of pollen tube through the style, and into the
female gametophyte.
On reaching the female gametophyte, the two male gametes are released from the
pollen tube. One fuses with the egg (syngamy) to form the zygote and other fuses
with the nucleus of the central cell to form the primary endosperm nucleus (triple
fusion). This process termed 'double fertilization', is unique to angiosperms.
In nature, the stigma is exposed and can receive a latge variety of pollen grains,
but not all of them succeed in germinating, and effecting fertilisation. The plants
have different devices that allow the pollen of only the right mating type to
function normally, the others are discarded. One of the most exciting aspects of
reproduction is how plants register recognition at the cellular level. If a pistil
carrying functional 9 gamete(s) fails to set seeds following~pollinationwith viable
and fertile pollen, capable of bringing about fertilisation in another pistil, the two
are said to be incompatible and the phenomenon is known as sexual incompatibility.
Sexual incompatibility may be inter-or intraspecific. Morphological, genetical and
physiological mechanisms are involved on excersing avoidance of selfing and
providing cross pollination. It is also possible to overcome self incompatibility by
several surgical and chemical methods.
Apomixis is a type of reproduction in which an unfertilised egg develops into an
embryo. without sexual fusion. Apomixis may be of recurrent type or non-recurrent
type. Apomicts show irregular meiosis in their anthers, and have poorly developed
endospenn. Apomixis is controlled by a set of genes, and the trait is genetically
inherited.

TERMINAL OUESTIONS

1) What are the merits and demerits of self and cross-pollination? Present your answer
in tabular form.
2) List the common mechanisms devoloped by plants to prevent self-pollination, and
promote cross pollination write five lines about each.
3) Match the items given in key with the descriptions given below:

a) Anemophily
b) Hydrophily
C) Enbmphily
d) Ornithophily
e) Cheiropherophily

i) flowers with highly reduced perianth, slender pollen grains with low specific
gravity.
 ii) red or orange coloured, vessel-like flowers that produce large quantities of
pollen grains and nectar, found mostly in the tropical regions.
iii) flowers having peculiar strong odour, and are borne singly or in clusters quite
away from the branches and foliage.
iv) pollen grains small, light, smooth, produced in enonnous amounts by mostly the
. unisexual flowers with reduced perianth, Female flowers with long, feathery
stigmas.
v) flowers with showy corolla, often mmed, having odour, and nectar.
4) How does the open style differ from the closed style in terms of structure and the
nahlre of exudates?
5) What is a transmitting tissue? Where is it found? List its salient characters.
6) What are the factors that govern the adhesion of pollen grains on the stigmatic
surface?
7) Why is pollen hydration a crucial step in pollination?
8) Differentate between gametophytic and sporophytic incompatibility.
9) What structural changes do the open styles and the closed styles undergo as a
result of pollination?
10) What are the salient structural differences seen in the tips of the pollen tubes
growing in compatible and incompatible pistils?
11) What is meant by double fertilisation?
12) How does syngamy differ from triple fusion? What are the end products of the two
and their ploidy levels?
13) What are the major differences between the heteromorphic and homomorphic self
incompatibility?
14) Discuss the genetic basis of self-incompatibility.
15) The pollen wall and its protein contents play an important lole in pollen-stigma
interaction. Explain.
16) Why is rejection reaction faster in SBI systems than the GSI systems?
17) What are the features that enable you to ascertain whether it is sporophytic or
gametophytic self incompatibility.
19) List and write briefly about the important methods to ovewme incompatibility in
plants.
20) Differentiate between recurrent and non-recurrent type of apomixis.
21) Explain in genetic tenns the phenomenon of apomixis.
22) Of what ecological significance is apomixii over vegetative reproduction?

3.8 ANSWERS

Self-assessment Questions

1) a) autogamous
b) xenogamy
C) c1eistogamouS
d) cross
e) anemophilous
€) cross

2) a) tip
 b) cellulase, peztinase, callase P o U l ~ 1 mand FertlMPUon

c) wet, Petunia, dry, cotton

d) exocytosis I

e) lipid, phenolic
f) solid, open
g) canal cells, transmitting tissue
h) sporophytic, gametopbytic
i) auxins, gibberellins
j) zygote, primary endosperm nucleus

4) Part-1 (iii)
Part-2 (i)
Terminal Questions

1) You may refer to subsection 3i2.2.

2) See subsection 3.2.2.

4) Hint: Open styles lined by glandular and secaetory cells, exudates contain mainly
polysaccharides; closed style have a compact core of transmitting tissues, their
exudates contain lipids, proteins and polysaccharides.
5) See subsection 3.3.1
6) Hint: Stickiness of pollen and stigma, exine pattern, composition of pellicle, surface
coat substances, electrostatic forces and specificity between the two parents.

7) Hint: Hydration triggers the release of pollen-wall proteins which subsequently

govern the compatibilitylincompatibilityinteraction between the two parents.
8) You may refer to subsection 3.3.2.
9) See subsection 3.3.2.
10) See subsection 3.3.2, 'Pollen Tube Growth.'
11) Hint: fusion of one male gamete with egg, and the other one with the secondary
nucleus.
12) You may refer to subsection 3.3.4.
End products: syngamy-zygote and embryo, triple fusion primary endosperm
nucleus-endosperm.
Ploidy levels: zygote-2n; primary endosperm nucleus-3n.
13) See subsection 3.4.1.
Hint: Heteromorphic forms have morphologically different flowers. This kind of
incompatibility can be predicted by examining the different morphs. It operates at
the morphological and physiological levels.
Homomorphic-It consists of flowers that are morphologically indistinguishable.
This kind of incompatibility can be identified by proper breeding experiments. It
involves multiple alleles.
Plant Development-I 14) See subsection 3.4.1.
15) You may refer to subsection 3.4.1 - Physiology and Biochemistry of
incompatibility.
16) Hint: Usually, the proteins of sporophytic origin are situated in exine and these pass
out first while those of gametophytic origin situated in intine take a lo~gertime.
17) See Tables 3.1 and 3.2.
18) Hint: It prevents the formation of homozygous individuals through self-fertilisation.
These individuals have low survival rate. It helps in introducing genetic diversity to
enable them to adapt better in diverse ecological situations.
19) See subsection 3.4.4.
20) See subsections 3.5.1 and 3.5.2.
21) See subsection 3.5.5.
22) See section 3.5.