Plant hormones:
Auxin: indole-3-acetic acid (IAA) derived from
indole , stimulate cell elongation.
Gibberellins: derivative of gibberellane,
induce elongating growth of internodes.
Zeatin: a cytokinin is a prenylated adenine
and stimulates cell-division.
Abscisic acid: formed from carotenoids,
regulate water balance, induce seed dormancy.
Ethylene and
jasmonic acid: derivative of fatty acids, enhance senescence.
Brassinosteroids: functions in regulation of
cell development.
Peptide hormones: regulate plant development.
And in addition to jasmonic acid and salicylic acid play a key role in pathogen
defense.
(A) Auxin stimulate
shoot elongation growth:
Charles Darwin and Francis noted growing plant seedlings
bend towards sunlight. Frits Went isolated from tip of oat seedlings, a growth
stimulating substance auxin, later IAA. Other substances with auxin properties
are phenylacetic acid. Synthetic auxin: 2,4- dichlorophenoxyacetic acid (2,4-
D, Rohm and Haas) is used as herbicide. It results in disordered morphogenesis
and increased synthesis of ethylene, thus leading to premature senescence of
leaves. 2,4-D is a selective herbicide that destroys dicot plants. Monocots are
insensitive to it because they eliminates the herbicide by degradation. For
this reason, 2,4-D is used for combating weeds in cereal crops. As agent
orange, it was used in Vietnam war to defoliate forests.
During early embryogenesis, auxin governs the formation of
main axis of polarity, with shoot meristem at the top and root at opposite
pole. Auxin generally influence cell division and cell differentiation. IAA
promotes elongating growth of cells. Highest IAA conc. are found in main growth
zone of shoot, primarily at tip of shoots, from there it is transported to
cells by an energy-dependent polar transport by efflux or influx carriers of
plasma membrane. Auxin is known to induce or repress specific set of genes. IAA
stimulates cell division in cambium, enhance apical dominance by suppression of
lateral bud growth and control embryo development. IAA prevents the formation
of abscission layer for leaves and fruits and is antagonist to ethylene. On the
other hand, high IAA conc. can induce the synthesis of ethylene. Auxin induces
the formation of fruits. Seeds produce IAA only after fertilization. IAA
prevent formation of seeds resulting in seedless eggplant but being four times
larger. IAA is synthesized from tryptophan by three different pathways:
(B) gibberellin
regulate stem elongation:
Infection of rice by fungus results in formation of extremely
tall plants that fall over and bear no seed. This disease was called foolish
seedling. Eiichi Kurozawa isolated a substance from this fungus that induces
unnatural growth and called it gibberellin. Derived from hydrocarbon ent-
gibberellane, they are intermediate or by-product of biosynthetic pathway. The
most important gibberellin is GA1- synthesized from isoprenoid geranylgeranyl
pyrophosphate. Gibberellin stimulate shoot elonagtion, it induces rosette
plants (spinach or lettuce) to shoot up for formation of flowers and also
regulate flowering. It terminates seed dormancy, by softening of seed coat and
facilitate seed germination by expression of genes for enzymes (amylase). Its
synthesis is controlled by light via phytochrome. It is important for production
of seedless grapes, it causes extension of cell and also parthenocarpy, malting
of barley for beer brewing, it is added to induce formation of amylase.
Inhibitors of GA are retardants e.g., cycocel, BASF- decrease the growth of
stalks. It influence gene expression and reduces the action of repressor
protein, DELLA proteins, which supress growth. Green revolution: increase the
yield of cereal crops dwarf wheat lines, reduced stalk growth. Due to mutation
in gene encoding transcription factor of GA signal transduction chain.
(C) cytokinin (CK)
stimulate cell-division:
Zeatin is the most common cytokinin. It is the derivative of
adenine. N-group is linked with hydroxylated isoprene in trans-position.
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| zeatin |
CK increases
sprouting of lateral buds. CK override apical dominance. They are antagonist of
auxin IAA. CK retards senescence and thus counteract ethylene. Larvae of
butterfly (Stigmella) excrete CK to prevent senescence of that leave on which
they are feeding. Some bacteria produce auxin and CK to induce unrestricted
cell-division, which results in tumor formation in plants. E.g., crown gall
induced by Agrobacterium tumefacien.
Zeatin is formed from AMP and dimethyallyl-pyrophosphate.
The isoprene unit is transferred by CK synthase to the N-group of AMP and is
then hydroxylated. CK synthesis takes place in meristematic tissues. CK
receptors are dimeric histidine kinases. Upon binding of CK, the two histidine
kinases phosphoryl their histidine residues reciprocally by
auto-phosphorylation.
(D) Abscisic acid
control water balance:
It causes abscission of leaves and fruits. An important
function is induction of dormancy of seeds and buds. AA induces with nitric
oxide (NO) the closure of stomata. It prevents vivipary (seed embryo from
germinating before seeds mature). Mutants have witling of leaves and fruits
like in tomatoes. AA is product of isoprenoid metabolism. Synthesis of AA
proceeds via oxidation of violaxanthin. AA synthesis occurs in leaves and roots
and transported by xylem vessels from roots to leaves. It causes alteration in
metabolism by influencing gene expression. It involves G-proteins, protein
kinases, phosphatase and messenger substances like cyclic ADP-ribose (cADPR).
This cause discharge of Ca2+ ions and inactivation of ion channels in stomata.
(E) Ethylene makes
fruit ripen:
It is involved in induction of senescence by (I) degradation
of leaf material is initiated (II) proteins are degraded to amino acids (III)
ions are withdrawn from senescense leaves. Ethylene induces defense reaction
after infection by fungi or wounded plants. It stimulates abscission of fruit,
ethylene functions in fruit ripening: breakdown of chlorophyll and synthesis of
other pigments e.g., apple – green to red, fruit softening due to breakdown of
cell walls by cellulase and pectinase. S-adenosylmethionine is precursor for
ethylene. ACC (aminocyclopropane carboxylate) synthase and ACC oxidase catalyze
the reaction.
(F) Steroid and peptide
hormones:
Brassinosteroids like brassinolide synthesized from
campesterol.it regulate plant development, stimulate shoot growth, folding of
leaves and differentiation of xylem. Retard root growth and formation of anthocyan. First isolated from
pollen. Developmental defects in mutants are dwarf growth, reduced apical
dominance and lowered fertility.
(G) Systemin induces
defense against herbivores:
It is of 18 amino acids, binds to receptor like kinase
(RLK). It generated jasmonic acid, a signal in the transcription activation of defense-related
genes. Accumulation of proteinase inhibitors, respond to insect attack because
they impair their digestion.
(H) phytosulfokines
regulate cell proliferation:
Mixture of two small polypeptides named phytosulfokines
(PSKI and II) containing two tyr residue which both –OH are esterified with
phosphate. They have an important effect on dedifferentiation of cells.
(I) PHYTOALEXINS:
H2O2 is involved in lignification process and thus play role
in solidification of cell wall as defense against pathogens. Formation of NO, a
radical. It is messenger formed by oxidation of arginine, catalyzed by NO
synthase. NO is important messenger in hormonal and defense responses : high
NO- high Ca2+ - signal cascade. It induce opening of stomata, initiation of
programmed cell death and formation of phytoalexins and synthesis of salicylic
acid. SA induces beta-1,3- glucanase which digest cell wall of fungi and
lipoxygenase- which synthesis jasmonic acid.
JA and its methylester as well as its precursor,
12-oxo-phtodienoic acid (OPDA) play role in defense reaction. As response to
fungal infection, JA induces the synthesis of phenylammonium lyase (PAL),
chalcone synthase (CHS)- flavonoid synthesis. JA induces plants to produce
proteinase inhibitors. As a response to mechanical stress, JA induces increased
growth in thickness of stem or tendrils to give plants higher stability. JA
regulate the development of pollen. Mutant plants which are unable to synthesis
JA cannot produce functioning pollen and hence, sterile male. JA like auxin and
GB uses the ubiquitin pathway to control
gene expression through protein degradation. JA functions as systemic wound
signal (an attack by herbicides- initiates defense response not only in wounded
leaves but also in more distant parts –systemic response).
