Plant Growth Regulators (2014)

Plant Growth Regulators
AKA Plant Hormones

• Plant Growth Regulators - control growth,
development and movement
 Internal and external signals that regulate plant growth are mediated, at
least in part, by plant growth-regulating substances, or hormones (from
the Greek word hormaein, meaning "to excite").
 Plant hormones differ from animal hormones in that:
 No evidence that the fundamental actions of plant and animal
hormones are the same.

Plant Growth Regulator
 Unlike animal hormones, plant hormones are not made in tissues
specialized for hormone production. (e.g., sex hormones made in the
gonads, human growth hormone - pituitary gland)
 Unlike animal hormones, plant hormones do not have definite target
areas (e.g., auxins can stimulate adventitious root development in a
cut shoot, or shoot elongation or apical dominance, or differentiation
of vascular tissue, etc.).
 plant growth regulators are necessary for, but do not control, many
aspects of plant growth and development. - better name is growth
regulator.
 the effect on plant physiology is dependent on the amount of
hormone present and tissue sensitivity to the plant growth regulator
Plant Growth Regulator
 Substances produced in small quantities by a plant, and
then transported elsewhere for use
have capacity to stimulate and/or inhibit physiological
processes
 at least five major plant hormones or plant growth
regulators:
auxins, cytokinins, gibberellins, ethylene and abscisic
acid
General plant hormones
 Auxins (cell elongation)
 Gibberellins (cell elongation + cell division -
translated into growth)
 Cytokinins (cell division + inhibits senescence)
 Abscisic acid (abscission of leaves and fruits +
dormancy induction of buds and
seeds)
 Ethylene (promotes senescence, epinasty, and
fruit ripening)
EARLY EXPERIMENTS ON PHOTROPISM SHOWED THAT A STIMULUS (LIGHT)
RELEASED CHEMICALS THAT INFLUENCED GROWTH
Auxin
• Auxin increases the plasticity of plant cell walls
and is involved in stem elongation.
• Arpad Paál (1919) - Asymmetrical placement of
cut tips on coleoptiles resulted in a bending of
the coleoptile away from the side onto which
the tips were placed (response mimicked the
response seen in phototropism).
• Frits Went (1926) determined auxin enhanced
cell elongation
Auxin
Demonstration of transported chemical
Auxin
 Discovered as substance associated with
phototropic response.
 Occurs in very low concentrations.
 Isolated from human urine, (40mg 33 gals-1)
 In coleoptiles (1g 20,000 tons-1)
 Differential response depending on dose.
Auxin
Auxin
• Auxin promotes activity of the vascular
cambium and vascular tissues.
– plays key role in fruit development
• Cell Elongation: Acid growth hypothesis
– auxin works by causing responsive cells to actively
transport hydrogen ions from the cytoplasm into
the cell wall space
Auxin
• Synthetic auxins
widely used in agriculture and horticulture
prevent leaf abscission
prevent fruit drop
promote flowering and fruiting
control weeds
Additional responses to auxin
 abscission - loss of leaves
 flower initiation
 sex determination
 fruit development
 apical dominance
Apical Dominance
Apical Dominance
Apical Dominance
 Apical dominance is
disrupted in some plants by
removing the shoot tip,
causing the plant to become
bushy.
 Lateral branch growth are
inhibited near the shoot
apex, but less so farther
from the tip.
Gibberellin
Gibberellin
• Gibberellins are named after the fungus
Gibberella fujikuroi which causes rice plants to
grow abnormally tall.
– synthesized in apical portions of stems and roots
– important effects on stem elongation
– in some cases, hastens seed germination
Gibberellin
• Cell elongation.
• GA induces cellular division and cellular elongation; auxin induces
cellular elongation alone.
• GA-stimulated elongation does not involve the cell wall acidification
characteristic of auxin-induced elongation
• Breaking of dormancy in buds and seeds.
• Seed Germination - Especially in cereal grasses, like barley. Not
necessarily as critical in dicot seeds.
• Promotion of flowering.
• Transport is non-polar, bidirectional producing general responses.
Gibberellin
• Fruit Formation -
"Thompson Seedless"
grapes grown in California
are treated with GA to
increase size and decrease
packing.
Gibberellin
Mobilization of reserves
Cytokinins
Cytokinins
• In 1964, the first naturally occurring cytokinin was
isolated from corn called zeatin. Zeatin and zeatin
riboside are found in coconut milk. All cytokinins
(artificial or natural) are chemically similar to
adenine.
• Cytokinins move nonpolarly in xylem, phloem, and

parenchyma cells.
• Cytokinins are found in angiosperms, gymnosperms,
mosses, and ferns. In angiosperms, cytokinins are
produced in the roots, seeds, fruits, and young
leaves
Function of Cytokinins
 Promotes cell division.
 Morphogenesis.
 Lateral bud development.
 Delay of senescence.
• Cytokinins, in combination with auxin,
stimulate cell division and differentiation.
– most cytokinin produced in root apical meristems
and transported throughout plant
• inhibit formation of lateral roots
– auxins promote their formation
Cytokinins
Cytokinins
Organogenesis: Cytokinins and auxin affect
organogenesis
High cytokinin/auxin ratios favor the
formation of shoots
Low cytokinin/auxin ratios favor the formation
of roots.
Abscisic acid
In 1940s, scientists started searching for hormones that would
inhibit growth and development, what Hemberg called dormins.
In the early 1960s, Philip Wareing confirmed that application of
a dormin to a bud would induce dormancy.
F.T. Addicott discovered that this substance stimulated abscission
of cotton fruit. he named this substance abscisin. (Subsequent
research showed that ethylene and not abscisin controls
abscission).
Abscisin is made from carotenoids and moves nonpolarly
through plant tissue.
Functions of abscisic acid
 General growth inhibitor.
 Causes stomatal closure.
 Produced in response to stress.
• Abscisic acid is produced chiefly in mature
green leaves and in fruits.
– suppresses bud growth and promotes leaf
senescence
– also plays important role in controlling stomatal
opening and closing
Discovery of ethylene
 In the 1800s, it was recognized that street lights that
burned gas, could cause neighboring plants to develop
short, thick stems and cause the leaves to fall off. In 1901,
Dimitry Neljubow identified that a byproduct of gas
combustion was ethylene gas and that this gas could affect
plant growth.
 In R. Gane showed that this same gas was naturally
produced by plants and that it caused faster ripening of
many fruits.
 Synthesis of ethylene is inhibited by carbon dioxide and
requires oxygen.
Ethylene
H H
\ /
C = C
/ \
H H
Functions of ethylene
 Gaseous in form and rapidly diffusing.
 Gas produced by one plant will affect nearby plants.
 Fruit ripening.
 Epinasty – downward curvature of leaves.
 Encourages senescence and abscission.
 Initiation of stem elongation and bud development.
 Flowering - Ethylene inhibits flowering in most species, but
promotes it in a few plants such as pineapple, bromeliads, and
mango.
 Sex Expression - Cucumber buds treated with ethylene become carpellate (female)
flowers, whereas those treated with gibberellins become staminate (male) flowers.
HOW PLANTS RESPOND TO
ENVIRONMENTAL STIMULI
• Tropisms - plant growth toward or away from
a stimulus such as light or gravity.
• Nastic Movements - response to

environmental stimuli that are independent
of the direction of the stimulus. Pre-
determined response.
The End

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Plant Growth Regulators

AKA Plant Hormones

• Cytokinins move nonpolarly in xylem, phloem, and

• Nastic Movements - response to

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