Appendix 4 For Taiz's Plant Physiology and Development
Appendix 4 For Taiz's Plant Physiology and Development
Appendix 4 For Taiz's Plant Physiology and Development
Secondary
Metabolites
CO2
Photosynthesis
Tricarboxylic Acetyl-CoA
acid cycle
Aliphatic
amino acids
Aromatic
amino acids
Nitrogen-containing
secondary products
Phenolic
Terpenes
compounds
O O H O
O
C
CH3 C S CoA CH3 C C
H C OH
OH
3 Acetyl-CoA (C2) CH2O P
Pyruvate (C3)
Glyceraldehyde
3-phosphate (C3)
OH O
OH H3C OH
CH2 O P P
O P P O P P
Isopentenyl diphosphate (IPP, C5) Dimethyallyl diphosphate Isopentenyl diphosphate (IPP, C5)
(DMAPP, C5)
Isoprene (C5)
CH2 O P P CH2 O P P
Phenolic Compounds
H C
The phytoecdysones, first isolated from the common Plants produce a large
3 variety
CH — CHof secondary
CH2 compounds
fern (Polypodium vulgare), are a group of plant steroids that H 2C group: a hydroxyl functional group
that contain a phenol
have the same basic structure as insect molting hormones on an aromatic ring:
(FIGURE A4.5B). Ingestion of phytoecdysones by insects
disrupts molting and other developmental processes, often OH
with lethal consequences. In addition, phytoecdysones
C6
(A) Azadirachtin, a limonoid
CH3OC HO C6 C3
CH3 O CH3
O O
OH
H3C C O
O
CH3 O
C6 C1
O
CH3CO O OH
FIGURE A4.5 Structure of two tri- CH3OC O
terpenes, azadirachtin (A) and C6 C3 C6
α-ecdysone (B), that serve as pow-
erful insecticides. Azadirachtin (B) α-Ecdysone, an insect molting hormone
affects more than 200 species of OH
insects and can be considered a
CH3
natural insecticide. α-Ecdysone, H3C
OH
a plant-derived steroidal prohor- CH3 CH3
mone of the insect molting hor-
mone 20-hydroxyecdysone, can CH3
cause irregular molting in insect HO
herbivores. (A, photo of neem OH
leaves © RN Photos/istockphoto; HO
B, photo of Polypodium vulgare
leaves, © blickwinkel/Alamy.) O
These substances are classified as phenolic compounds, or phosphate pathway into the three aromatic amino acids:
phenolics. Plant phenolics are a chemically heterogeneous phenylalanine, tyrosine, and tryptophan. One of the path-
group of nearly 10,000 individual compounds: Some are way intermediates is shikimic acid, which has given its
soluble only in organic solvents, some are water-soluble name to this whole sequence of reactions. The well-known
carboxylic acids and glycosides, and others are large, insol- broad-spectrum herbicide glyphosate (available commer-
uble polymers. cially as Roundup) kills plants by blocking a step in this
In keeping with their chemical diversity, phenolics pathway (see Appendix 1). The shikimic acid pathway
play a variety of roles in the plant. Many serve as defenses is present in plants, fungi, and bacteria but is not found
against herbivores and pathogens. Others function in in animals. Animals have no way to synthesize aromatic
mechanical support, in attracting pollinators and fruit amino acids—phenylalanine, tyrosine, and tryptophan—
dispersers, in absorbing harmful ultraviolet radiation, or which are therefore essential nutrients in animal diets.
in reducing the growth of nearby competing plants. After The most abundant classes of phenolic secondary com-
giving a brief account of phenolic biosynthesis, we will pounds in plants are derived from phenylalanine via the
discuss three principal groups of phenolic compounds and elimination of an ammonia molecule to form cinnamic
what is known about their roles in the plant. acid (FIGURE A4.7). This reaction is catalyzed by phenyl-
alanine ammonia lyase (PAL), perhaps the most studied
enzyme in plant secondary metabolism. PAL is situated at
Phenylalanine is an intermediate in the biosynthesis
a branch point between primary and secondary metabo-
of most plant phenolics lism, so the reaction it catalyzes is an important regulatory
Plant phenolics are synthesized by several different routes step in the formation of many phenolic compounds.
and thus constitute a heterogeneous group from a meta- The activity of PAL is increased by environmental fac-
bolic point of view. Two basic pathways are involved: the tors such as low nutrient levels, light (through its effect on
shikimic acid pathway and the malonic acid pathway phytochromes), and fungal infection. The point of control
(FIGURE A4.6). The shikimic acid pathway participates appears to be the initiation of transcription. Fungal inva-
in the biosynthesis of most plant phenolics. The malonic sion, for example, triggers the transcription of messenger
acid pathway, although an important source of phenolic RNA that codes for PAL, thus increasing the amount of
secondary products in fungi and bacteria, is of less signifi- PAL in the plant, which then stimulates the synthesis of
cance in higher plants. phenolic compounds. The regulation of PAL activity in
The shikimic acid pathway converts simple carbohy- many plant species is made more complex by the exis-
drate precursors derived from glycolysis and the pentose tence of multiple PAL-encoding genes, some of which are
Erythrose Phosphoenolpyruvic
4-phosphate acid (from glycolysis)
(from pentose
phosphate pathway)
Shikimic acid
Acetyl-CoA
pathway
Hydrolyzable
tannins
[C 6 C3 ][ C 6 ]
C1 [C 6 C3 C6
] Miscellaneous
phenolics
Simple phenolics Flavonoids
[ C6
]
C3 n [ C6 C3 C6
]n
Lignin Condensed tannins
FIGURE A4.6 Plant phenolics are synthesized in several basic arrangement of carbon skeletons: C6 indicates a
different ways. In higher plants, most phenolics are de- benzene ring, and C3 is a three-carbon chain. More detail
rived at least in part from phenylalanine, a product of the on the pathway from phenylalanine onward is given in
shikimic acid pathway. Formulas in brackets indicate the Figure 13.7.
A4-8 APPENDIX 4
COOH
expressed only Hin3Cspecific tissues or only under Benzoic acid
CH — CH2 — CH3 derivatives (Figure A.8C)
certain environmental
H3C conditions (Logemann et
al. 1995). trans-Cinnamic acid
Reactions subsequent to that catalyzed by
H3C
PAL lead to the addition
CH — CHof more
CH2 hydroxyl
H C
groups and other substituents. The metabolites
2
COOH Caffeic acid
trans-cinnamic acid, p-coumaric acid, and their and other simple
derivatives are simple phenolic compounds phenylpropanoids
OH HO (Figure A.8A)
called phenylpropanoids because they contain
p-Coumaric acid
a benzene ring:
CoA-SH
H3C Coumarins (Figure A.8B)
CH — CH2 — CH3
H3C C6
COSCoA
Lignin precursors
and a three-carbon
H3C
side chain.
Simple phenolic compounds
CH — CH CH are widespread HO
H2C C6 C3 2 p-Coumaroyl-CoA
in vascular plants and appear to function in
3 Malonyl-CoA molecules
various capacities. Their structures include the
following:
C6 OH
C Chalcone synthase
1
• Simple phenylpropanoids,
H3C such as trans- OH
cinnamic acid, CH — CH2 —acid,
p-coumaric CH3 and their
H3C
derivatives, such as caffeic acid, which have HO OH
C
C66
a basic phenylpropanoid C3carbonC6 skeleton
(FIGURE A4.8A
H3C ) OH
CH — CH CH2
H2C OH O
C6 C3 HO O
Chalcones
OH
• Phenylpropanoid lactones
OH (cyclic esters)
called coumarins,C which
C1
also have a OH O
6 HO O
phenylpropanoid carbon skeleton (FIGURE
Flavones
A4.8B) C 6
• Benzoic acid derivatives, which have a carbon HO
C6 phenylpropanoids
C3 C6 OH O O
skeleton formed from by
the cleavage of a two-carbon fragment from Flavanones
C6
the side chain ( FIGURE CA4.8C
3 ) (see also
Figure A4.7): OH OH O
OH
HO O Isoflavones (isoflavonoids)
C6 C1
OH OH
As with many other secondary products, plants
OH O
can elaborate on the basic
C6 carbon
C3 skeletons
C6 of HO O
Dihydroflavonols
these simple phenolic compounds to make more
complex products.5/E Taiz/Zeiger
Plant Physiology OH
Now that
Sinauer the biosynthetic pathways leading
Associates
Morales Studio Anthocyanins OH O
to most widespread phenolic compounds have
Figure 13.00 in-text Date 02-04-10 (Figure A.10A)
been determined, researchers have turned their Condensed tannins Flavonols
attention to studying how those pathways are (Figure A.12A)
SECONDARY METABOLITES A4-9
Lignin plays both primary and secondary roles in plants. From shikimic acid
Its precise structure is not known because it is difficult to pathway via phenylalanine
extract from plants, in which it is covalently bound to cel- [C 6 ]
C3
lulose and other polysaccharides of the cell wall. 3′
Lignin is generally formed from three different phenyl- 2′ 4′
From malonic B
propanoid alcohols: coniferyl, coumaryl, and sinapyl, all of acid pathway 8 1 1′
7 O 5′
which are synthesized from phenylalanine via various cin-
namic acid derivatives. The phenylpropanoid alcohols are [ ]
C6 A C 2 6′
6 3
joined into a polymer through the action of enzymes that 5 4
generate free-radical intermediates. The proportions of the
three phenylpropanoid alcohols in lignin vary among spe- The three-carbon bridge
cies, plant organs, and even layers of a single cell wall. In Basic flavonoid skeleton
the polymer, there are often multiple C—C and C—O—C
bonds in each phenylpropanoid alcohol unit, resulting in a FIGURE A4.9 Basic flavonoid carbon skeleton. Flavo-
complex structure that branches in three dimensions. noids are synthesized from products of the shikimic acid
Unlike the monomeric units of polymers such as and malonic acid pathways. Flavonoids contain 15 car-
bons in the basic molecular skeleton provided by two
starch, rubber, or cellulose, those of lignin do not appear
aromatic rings and one 3-carbon bridge. Positions of
to be linked in a simple, repeating way. However, recent carbons on the flavonoid ring system are numbered as
research suggests that a guiding protein may bind the indi- shown.
vidual units during lignin biosynthesis, giving rise to a
scaffold that then directs the formation of a large, repeating
unit (Davin and Lewis 2000; Hatfield and Vermerris 2001). Flavonoids are classified primarily on the basis of the
Lignin is found in the cell walls of various cell types that degree of oxidation of the three-carbon bridge. We will dis-
make up supporting and conducting tissues, notably the cuss four of these groups here: the anthocyanins, the fla-
tracheids and vessel elements of the xylem. It is deposited vones, the flavonols, and the isoflavones (see Figure A4.7).
chiefly in the thickened secondary wall, but may also be The basic flavonoid carbon skeleton may have numer-
present in the primary wall and middle lamella in close ous substituents. Hydroxyl groups are usually present at
contact with the celluloses and hemicelluloses already positions 3, 5, and 7, but they may also be found at other
present. The mechanical rigidity of lignin strengthens positions. Sugars are very common as well; in fact, the
stems and vascular tissue, allowing upward growth and majority of flavonoids exist naturally as glycosides.
H3C
permitting water andCH minerals
— CH2 —toCHbe3 conducted through Whereas both hydroxyl groups and sugars increase the
H3C
the xylem under negative pressure without collapse of the water solubility of flavonoids, other substituents, such as
tissue. Because lignin is such a key component of water methyl ethers or modified isopentyl units, make flavonoids
transport tissue, H
the
3C ability to synthesize lignin must have lipophilic (hydrophobic). Different types of flavonoids per-
CH — CH CH2
been one of the H most important adaptations permitting form very different functions in the plant, including pig-
2C
primitive plants to colonize dry land. mentation and defense.
Plant Physiology 5/E Taiz/Zeiger
Besides providing mechanical support, lignin has sig- Sinauer Associates
nificant protective functions inOH plants. Its physical tough- Morales Studio
Anthocyanins
Figure 13.09 areDate
colored flavonoids
11-16-09 that
ness deters herbivory, and its chemical durability makes it
attract animals
relatively indigestible. By bonding to cellulose and protein,
lignin also reduces the digestibility of those substances. The colored pigments of plants provide visual cues that
C6
Lignification blocks the growth of pathogens and is a com- help to attract pollinators and seed dispersers. These pig-
mon response to infection or wounding. ments are of two principal types: carotenoids and flavo-
noids. Carotenoids, as we have already seen, are yellow,
C6 C3 orange, and red terpenoid compounds that also serve as
There are four major groups of flavonoids accessory pigments in photosynthesis (see Chapter 7). The
The flavonoids are one of the largest classes of plant phe- flavonoids also include a wide range of colored substances.
nolics. The basic carbonCskeleton
C1
of a flavonoid contains The most widespread group of pigmented flavonoids is
6
15 carbons arranged in two aromatic rings connected by a the anthocyanins, which are responsible for most of the
three-carbon bridge: red, pink, purple, and blue colors observed in flowers and
fruits.
C6 C3 C6 Anthocyanins are glycosides that can have various sugars
at position 3 (FIGURE A4.10A) and sometimes elsewhere.
This structure results from two separate biosynthetic path- Without their sugars, anthocyanins are known as anthocy-
ways: the shikimic acid pathway and the malonic acid
pathway (FIGURE A4.9).
SECONDARY METABOLITES A4-11
Anthocyanidin
FIGURE A4.10 The structures of anthocyanins (A) and an-
Isoflavonoids have widespread
thocyanidins (B). The colors of anthocyanidins depend pharmacological activity
in part on the substituents attached to ring B (see Table
The isoflavones (isoflavonoids) are a group of flavonoids in
13.1). An increase in the number of hydroxyl groups shifts
absorption to a longer wavelength and gives a bluer which the position of one aromatic ring (ring B) is shifted
color. Replacement of a hydroxyl group with a methoxyl (see Figure A4.7). Isoflavonoids, which are found mostly in
group (—OCH3) shifts absorption to a slightly shorter legumes, have several different biological activities. Some,
wavelength, resulting in a redder color. such as rotenone, can be used effectively as insecticides,
A4-12 APPENDIX 4
(A) (B)
FIGURE A4.11 Black-eyed Susan (Rudbeckia sp.) as seen found in the inner parts of the rays, but not in the tips.
by humans (A) and as it might appear to honeybees (B). The distribution of flavonols in the rays creates a “bull’s-
(A) To humans, the flowers have yellow rays and a brown eye” pattern visible to honeybees, which presumably
central disc. (B) To bees, the tips of the rays appear “light helps them locate pollen and nectar. Special lighting was
yellow,” the inner portion of the rays “dark yellow,” and used to simulate the spectral sensitivity of the honeybee
the central disc “black.” UV-absorbing flavonols are visual system. (Courtesy of Thomas Eisner.)
pesticides (e.g., as rat poison), and piscicides (fish poisons). Hydrolyzable tannins are heterogeneous polymers
Other isoflavones have anti-estrogenic effects; for example, containing phenolic acids, especially gallic acid, and sim-
sheep grazing on clover rich in isoflavonoids often suf- ple sugars (FIGURE A4.12B). They are smaller than con-
fer from infertility. The ring system of isoflavones has a densed tannins and may be hydrolyzed more easily; only
three-dimensional structure similar to that of steroids (see dilute acid is needed. Most tannins have molecular masses
Figure A4.5B), allowing these substances to bind to estro- between 600 and 3000 Da.
gen receptors. Isoflavones may also be responsible for the Tannins are general toxins that can reduce the growth
anticancer benefits of foods prepared from soybeans. and survival of many herbivores when added to their
In the past few years, isoflavones have become best diets. In addition, tannins act as feeding repellents to a
known for their role as phytoalexins, antimicrobial com- great variety of animals. Mammals such as cattle, deer,
pounds synthesized in response to bacterial or fungal and apes characteristically avoid plants or parts of plants
infection that help limit the spread of the invading patho- with high tannin contents. Unripe fruits, for instance, fre-
gen. Phytoalexins are discussed in more detail later in this quently have very high tannin levels, which deter feed-
Plant Physiology 5/E Taiz/Zeiger
chapter.
Sinauer Associates ing on the fruits until their seeds are mature enough for
Morales Studio dispersal.
Figure 13.11 Date 11-16-09 Although crop plants generally produce fewer second-
Tannins deter feeding by herbivores ary metabolites, there are exceptions. Humans often prefer
A second category of plant phenolic polymers with defen- a certain level of astringency in tannin-containing foods,
sive properties, besides lignin, is the tannins. The term tan- such as apples, blackberries, tea, and grapes. The tannins
nin was first used to describe compounds that could con- in red wine have been shown to block the formation of
vert raw animal hides into leather in the process known endothelin-1, a signaling molecule that makes blood ves-
as tanning. Tannins bind the collagen proteins of animal sels constrict (Corder et al. 2001). This effect of wine tan-
hides, thereby increasing their resistance to heat, water, nins may account for the often-touted health benefits of
and microbes. red wine, especially the reduction in the risk of heart dis-
There are two categories of tannins: condensed and ease associated with moderate red wine consumption. In
hydrolyzable. Condensed tannins are compounds formed recent years, however, another phenolic compound, the
by the polymerization of flavonoid units (FIGURE A4.12A). stilbene phenylpropanoid resveratrol, has also been identi-
They are common constituents of woody plants. Because fied as a health benefit factor in red wine.
condensed tannins can often be hydrolyzed into anthocy- Moderate amounts of specific tannins may have health
anidins by treatment with strong acids, they are sometimes benefits for humans, but the defensive properties of most
called pro-anthocyanidins. tannins are due to their toxicity, which is generally attrib-
SECONDARY METABOLITES A4-13
(A) Condensed tannin FIGURE A4.12 Structure of two salivary proteins with a very high proline
OH types of tannins. (A) The general content (25–45%) that have a high affin-
B structure of a condensed tannin, ity for tannins. Secretion of these proteins,
HO where n is usually 1 to 10. There
O
OH which is induced by ingestion of food
A C may also be a third hydroxyl group
on ring B. (B) The hydrolyzable tan- with a high tannin content, greatly dimin-
OH
OH nin from sumac (Rhus semialata) ishes the toxic effects of tannins (Butler
OH consists of glucose and eight mol- 1989). The large number of proline resi-
HO O ecules of gallic acid. dues gives these proteins a very flexible,
OH
n open conformation and a high degree of
OH hydrophobicity, which facilitate binding
OH
OH
to tannins.
HO O
Plant tannins also serve as defenses
OH against microorganisms. For example,
the nonliving heartwood of many trees
OH
contains high concentrations of tannins,
OH
which help prevent fungal and bacterial
decay.
(B) Hydrolyzable tannin
O OH OH
O
CH2O C OH C OH
O OH
Nitrogen-Containing
O
O
OH O C OH
Compounds
HO O
H A large variety of plant secondary metab-
O O C OH OH
HO C olites have nitrogen as part of their struc-
O H
OH ture. Included in this category are such
HO HO CO H
well-known antiherbivore defenses as
H O
HO O alkaloids and cyanogenic glycosides,
O
C O which are of considerable interest because
of their toxicity to humans as well as their
C O medicinal properties. Most nitrogenous
Gallic acid
HO OH secondary metabolites are synthesized
OH
from common amino acids.
HO OH In this section we will examine the
OH structures and biological properties of
various nitrogen-containing secondary
uted to their ability to bind proteins nonspecifically. It has metabolites, including alkaloids, cyanogenic glycosides,
long been thought that plant tannins bind proteins in the glucosinolates, and nonprotein amino acids.
guts of herbivores by forming hydrogen bonds between
their hydroxyl groups and electronegative sites on the
Alkaloids have dramatic physiological
proteins (FIGURE A4.13A). More recent evidence indicates
effects on animals
that tannins and other phenolics can also bind to dietary
protein in a covalent fashion (FIGURE A4.13B). The foliage The alkaloids are a large family of more than 15,000 nitro-
of many plants contains enzymes that oxidize phenolics gen-containing secondary metabolites. They are found in
to their corresponding quinone forms in the guts of her- approximately 20% of vascular plant species. The nitrogen
bivores (Felton et al. 1989). Quinones are highly reactive atom in these compounds is usually part of a heterocyclic
electrophilic molecules that readily react with the nucleo- ring, a ring that contains both nitrogen and carbon atoms.
philic —NH2 and —SH groups of proteins (Appel 1993). As a group, alkaloids are best known for their striking
By whatever mechanism protein–tannin binding occurs, pharmacological effects on vertebrate animals.
Plant Physiology 5/E Taiz/Zeiger
this process
Sinauer has a negative effect on herbivore nutrition.
Associates As their name would suggest, most alkaloids are alka-
TanninsStudio
Morales can inactivate herbivore digestive enzymes and line. At the pH values commonly found in the cytosol (pH
Figure 13.12 Date 11-16-09
create complex aggregates of tannins and plant proteins 7.2) or the vacuole (pH 5–6), the nitrogen atom is proton-
that are difficult to digest. ated; hence alkaloids are positively charged and are gener-
Herbivores that habitually feed on tannin-rich plant ally water soluble.
material appear to possess some interesting adaptations Alkaloids are usually synthesized from one of a few
to remove tannins from their digestive systems. For exam- common amino acids—in particular, lysine, tyrosine, or
ple, some mammals, such as rodents and rabbits, produce
(A) Hydrogen bonding between tannin and protein FIGURE A4.13 Proposed mechanisms for the interaction
of tannins with proteins. (A) Hydrogen bonds may form
between the phenolic hydroxyl groups of the tannin
d+ d− and electronegative sites on the protein. (B) Phenolic
O H N Protein hydroxyl groups may bind covalently to proteins follow-
H2 ing activation by oxidative enzymes, such as polyphenol
Tannin
oxidase.
(B) Covalent bonding to protein after oxidation
Tannin in phenol form tryptophan. However, the carbon skeleton of some alka-
OH loids contains a component derived from the terpene path-
Polyphenol oxidase
way. TABLE 13.2 lists the major alkaloid types and their
amino acid precursors. Several different types, including
nicotine and its relatives (FIGURE A4.14), are derived from
ornithine, an intermediate in arginine biosynthesis. The B
Tannin in quinone form
vitamin nicotinic acid (niacin) is a precursor of the pyridine
O
(six-membered) ring of this alkaloid; the pyrrolidine (five-
H2N Protein membered) ring of nicotine arises from ornithine (FIGURE
A4.15). Nicotinic acid is also a constituent of NAD+ and
NADP+, which serve as electron carriers in metabolism.
HN Protein
The role of alkaloids in plants has been a subject of spec-
Covalent bond ulation for at least a century. Alkaloids were once thought
to be nitrogenous wastes (analogous to urea and uric acid
OH in animals), nitrogen storage compounds, or growth regu-
Tannin linked to protein lators, but there is little evidence to support any of these
functions. Most alkaloids are now believed to function as
defenses against herbivores, especially mammals, because
TABLE 13.2 N
Major types of alkaloids,
N their amino acid precursors, and well-known examples of each type
Alkaloid class N
Structure Biosynthetic precursor Examples Human uses
N
Pyrrolidine N
Ornithine (aspartate) Nicotine Stimulant, depressant,
N
N tranquilizer
NN
N
Tropane Ornithine Atropine Prevention of intestinal spasms,
N
antidote to other poisons,
N
N dilation of pupils for
N
N
examination
N
Cocaine Stimulant of the central nervous
N
system, local anesthetic
Piperidine N Lysine (or acetate) Coniine Poison (paralyzes motor
N
N
N neurons)
NN
N
Pyrrolizidine
Plant Physiology 5/E Taiz/Zeiger
Ornithine Retrorsine None
N
Sinauer Associates
Morales Studio NN
N
Figure 13.13
Quinolizidine Date 02-04-10
N Lysine Lupinine Restoration of heart rhythm
N
N
N
Isoquinoline N
N
Tyrosine Codeine Analgesic (pain relief), treatment
N
of coughs
N N
Morphine Analgesic
N
Indole N Tryptophan Psilocybin Hallucinogen
N Reserpine Treatment of hypertension,
N N treatment of psychoses
N N Strychnine Rat poison, treatment of eye
N disorders
N
N
N
N
SECONDARY METABOLITES A4-15
with fungal symbionts often grow faster and are better 2. In the second step the resulting hydrolysis product,
defended against insect and mammalian herbivores than called an α-hydroxynitrile or cyanohydrin, can
those without symbionts. Unfortunately, certain grasses decompose spontaneously at a low rate to liberate
with symbionts, such as tall fescue, are important pasture HCN. This second step can be accelerated by the lytic
grasses that may become toxic to livestock when their alka- enzyme hydroxynitrile lyase.
loid content is too high. Efforts are under way to breed Cyanogenic glycosides are not normally broken down
tall fescue with alkaloid levels that are not poisonous to in the intact plant because the glycoside and the degra-
livestock but still provide protection against insects . dative enzymes are spatially separated in different cellu-
lar compartments or in different tissues. In sorghum, for
example, the cyanogenic glycoside dhurrin is present in
Cyanogenic glycosides release the poison
the vacuoles of epidermal cells, while the hydrolytic and
hydrogen cyanide lytic enzymes are found in the mesophyll (Poulton 1990).
Various nitrogenous protective compounds other than Under ordinary conditions this compartmentalization
alkaloids are found in plants. Two groups of these sub- prevents decomposition of the glycoside. When the leaf is
stances—cyanogenic glycosides and glucosinolates—are damaged, however, as during herbivore feeding, the cell
not themselves toxic but are readily broken down to give contents of different tissues mix, and HCN forms. Cyano-
off poisons, some of which are volatile, when the plant is genic glycosides are widely distributed in the plant king-
crushed. Cyanogenic glycosides release the well-known dom and are found in many legumes, grasses, and species
poisonous gas hydrogen cyanide (HCN). of the rose family.
The breakdown of cyanogenic glycosides in plants is a Considerable evidence indicates that cyanogenic glyco-
two-step enzymatic process. Species that make cyanogenic sides have a protective function in certain plants. HCN is
Plant Physiology 5/E Taiz/Zeiger
glycosides also make the enzymes necessarySinauerto hydrolyze
Associates a fast-acting toxin that inhibits metalloproteins such as the
the sugar and liberate HCN: Morales Studio iron-containing cytochrome oxidase, a key enzyme of mito-
Figure 13.16 Date 02-04-10
chondrial respiration. The presence of cyanogenic glycosides
1. In the first step the sugar is cleaved by a glycosidase,
a hydrolytic enzyme that separates sugars from other deters feeding by insects and other herbivores such as snails
molecules to which they are linked (FIGURE A4.17). and slugs. As with other classes of secondary metabolites,
however, some herbivores have adapted to feed on cyano-
genic plants and can tolerate large doses of HCN.
The tubers of cassava (Manihot esculenta), a high-car- without ill effects. For adapted herbivores such as the cab-
bohydrate staple food in many tropical countries, contain bage butterfly, glucosinolates serve as stimulants for adult
high levels of cyanogenic glycosides. Traditional processing feeding and egg laying, and the isothiocyanates produced
methods, such as grating, grinding, soaking, and drying, after glucosinolate hydrolysis act as volatile attractants
lead to the removal or degradation of a large fraction of the (Renwick et al. 1992). In addition, the caterpillars can redi-
cyanogenic glycosides present in cassava tubers. However, rect the glucosinolate hydrolysis reaction to the production
chronic cyanide poisoning leading to partial paralysis of the of the less toxic nitriles (Wittstock et al. 2004).
limbs is still widespread in regions where cassava is a major Most of the recent research on glucosinolates in plant
food source because the traditional detoxification methods defense has concentrated on rape, or canola (Brassica napus),
employed to remove cyanogenic glycosides from cassava a major oilseed crop in both North America and Europe.
are not completely effective. In addition, many populations Plant breeders have tried to lower the glucosinolate levels
that consume cassava have poor nutrition, which aggra- of rapeseed so that the high-protein seed meal remaining
vates the effects of the cyanogenic glycosides. after oil extraction can be used as animal food. The first
Efforts are currently under way to reduce the cyano- low-glucosinolate varieties tested in the field were unable
genic glycoside content of cassava through both conven- to survive because of severe pest problems. However,
tional breeding and genetic engineering approaches. How- more recently developed varieties with low glucosinolate
ever, the complete elimination of cyanogenic glycosides levels in seeds but high glucosinolate levels in leaves are
may not be desirable, because these substances are prob- more resistant to pests and still provide a protein-rich seed
ably responsible for the pest resistance of stored cassava. residue for animal feeding.
Glucosinolates release volatile toxins Nonprotein amino acids are toxic to herbivores
A second class of plant glycosides, called the glucosino- Plants and animals incorporate the same 20 amino acids
lates, or mustard oil glycosides, break down to release into their proteins. However, many plants also contain
defensive substances. Found principally in the Brassicaceae unusual amino acids, called nonprotein amino acids, that
and related plant families, glucosinolates break down to are not incorporated into proteins. Instead, these amino
produce the compounds responsible for the smell and taste acids are present in the free form and act as defensive sub-
of vegetables such as cabbage, broccoli, and radishes. stances. Many nonprotein amino acids are very similar to
Glucosinolate breakdown is catalyzed by a hydrolytic common protein amino acids. Canavanine, for example,
enzyme, called a thioglucosidase or myrosinase, that cleaves is a close analog of arginine, and azetidine-2-carboxylic
glucose from its bond with the sulfur atom (FIGURE A4.18). acid has a structure very much like that of proline (FIGURE
The resulting aglycone—the nonsugar portion of the mole- A4.19).
cule—loses the sulfate and rearranges itself to give pungent Nonprotein amino acids exert their toxicity in various
and chemically reactive products, including isothiocyanates ways. Some block the synthesis or uptake of protein amino
and nitriles, depending on the conditions of hydrolysis. acids. Others, such as canavanine, can be mistakenly incor-
These defensive products function as toxins and herbivore porated into proteins. After ingestion by an herbivore,
repellents. Like cyanogenic glycosides, glucosinolates are canavanine is recognized by the enzyme that normally
stored in the intact plant separately from the enzymes that binds arginine to the arginine transfer RNA molecule, so
hydrolyze them, and they are brought into contact with it becomes incorporated into herbivore proteins in place
these enzymes only when the plant is crushed. of arginine. Canavanine is less basic than arginine and its
As with other secondary metabolites, certain animals incorporation usually results in a nonfunctional protein
are adapted to feed on glucosinolate-containing plants
R N C S
S Glucose Thioglucosidase SH Spontaneous
R C R C Isothiocyanate
– –
N O SO3 N O SO3
Glucose SO42– R C N
FIGURE A4.19 Nonprotein Nonprotein amino acid Protein amino acid analog
amino acids and their pro-
tein amino acid analogs. The HOOC CH CH2 CH2 O NH CH NH2 HOOC CH CH2 CH2 CH2 NH CH NH2
nonprotein amino acids are
not incorporated into pro- NH2 NH NH2 NH
teins, but are defensive com-
pounds found in free form Canavanine Arginine
in plant cells. Their activity
ranges from interference CH2 CH2 CH2
with the uptake of amino CH2 CH COOH
CH2 CH COOH
acids to the disruption of NH
NH
translation.
Azetidine-2-carboxylic acid Proline