plant hormones

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.

 

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).