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Chapter 17

Biosynthesis of Hormones
and Elicitor Molecules

 

A battery of endogenous hormones regulate plant growth and responses to environmental stimuli, including gibberellins, abscisic acid, cytokinins, the auxin indole-3-acetic acid, ethylene, brassinosteroids, polyamines, jasmonic acid, and salicylic acid. Most of these compounds are present in plant tissues in low concentrations (nanograms per gram fresh weight) and pool sizes, and are tightly controlled by various biosynthetic, catabolic, and conjugation pathways.
      GAs promote seed germination, stem elongation, flowering, and cone production and retard leaf and fruit senescence. They also induce de novo synthesis of numerous enzymes, including -amylase, in the aleurone layer of barley. The first steps in the synthesis of GAs involve the production of isopentenyl diphosphate by the pyruvate/ glyceraldehyde pathway and its conversion to geranylgeranyl diphosphate by the terpenoid pathway. The enzymes catalyzing the synthesis of geranylgeranyl diphosphate and its conversion to entkaurene, which is the first committed step in GA biosynthesis, are plastid-localized terpene cyclases. ent-Kaurene is oxidized to bioactive C19-GAs by way of GA12-aldehyde in a series of steps catalyzed by cytochrome-P450 monooxygenases located on the ER and by cytosolic -ketoglutarate-dependent dioxygenases. The molecular cloning of several of the genes encoding GA biosynthesis enzymes has provided information on feedback regulation of the bioactive C19-GA pools. The bioactive C19-GAs can be deactivated by various reactions, including 2ß-hydroxylation, glycosylation, and the formation of 2-keto derivatives.
      ABA, a C15 compound, is associated with desiccation tolerance, suppression of vivipary, and the closure of stomata induced by water stress. Like GAs, ABA is a product of the terpenoid pathway. In plants it is not produced directly from a C15 intermediate but by a circuitous route in which 9’-cis-C40 compounds undergo oxidative cleavage to yield a C15 intermediate, xanthoxin, that is converted to ABA by way of ABA-aldehyde. ABA is metabolized to phaseic acid, dihy-drophaseic acid, and dihydrophaseic acid glucoside. ABA-deficient mutants typically exhibit a wilty or viviparous phenotype. Although many such mutants are known, many of the genes encoding ABA biosynthetic enzymes have yet to be cloned.
      Cytokinins, in conjugation with auxin, promote cell division and determine cell differentiation. They also are associated with the senescence of plant organs, apical dominance, and stomata opening. The first step in the synthesis of cytokinins is the isopente-nylation of 5’-AMP. The resulting [9R-5’P]iP is modified by the trans-hydroxylation of the isopentenyl chain, dephosphorylation, or deribosylation, singly or in combination, to form such cytokinins as [9R]iP, iP, [9R]Z, and Z (see Table 17.1 for full names), of which Z exhibits the highest biological activity. Cytokinins are metabolized by the stereospecific hydrogenation of the side chain, removal of the isopentenyl side chain (catalyzed by cytokinin oxidase), and conjugation reactions. Conjugation steps include glycosylation of the side chain hydroxyl group and N-glucosylation or N-alanylation of the purine ring.
      IAA affects apical dominance, tropisms, shoot elongation, the induction of cambial cell division, and root initiation. Synthesized from L-tryptophan (by way of indole-3- pyruvic acid and indole-3-acetaldehyde), IAA can be released by hydrolysis of IAA glucosyl conjugates. In some species, including Arabidopsis, L-tryptophan is converted to IAA by way of indole-3-acetaldoxime and indole-3-acetonitrile; a tryptophan-independent route to IAA also exists. IAA is deactivated by addition of a 2-keto group and formation of aspartyl and N-glucosyl conjugates. Several Arabidopsis IAA homeostasis mutants, displaying a variety of phenotypes, have been isolated. Transgenic tobacco plants expressing bacterial IAA biosynthesis genes contain increased concentrations of free IAA and IAA conjugates; the abnormal phenotype shows pronounced apical dominance, dwarfism, increased adventitious root formation, excess lignification, leaf epinasty, and abnormal flower production. The dwarfism is an indirect consequence of the greater rates of ethylene synthesis in the tissues that overproduce IAA.
      Ethylene causes abnormal growth of etiolated seedlings and has an impact on shoot and root growth, flower development, senescence and abscission of flowers and leaves, and ripening of fruit. In planta, ethylene is synthesized from SAM by way of ACC. A gene encoding ACC oxidase has been cloned and expressed in antisense orientation in tomato. The transgenic fruits have 95% less ethylene production, are resistant to overripening, and can be stored at room temperature for long periods while remaining edible.
      BRs are essential factors for cell and stem elongation, unrolling of grass leaves, bending of grass leaves at the sheath/blade joints, xylogenesis, and ethylene production. BR biosynthesis and sensitivity mutants show dwarfism and, when grown in the dark, share some characteristics with light-grown plants. Brassinolide, the most biologically active and ubiquitous BR, is synthesized from campesterol. First, campesterol is hydrogenated to campestanol, which is converted to castasterone by the repeated oxidation/ hydroxylation of the side chain and A and B rings. Further B ring oxidation of castasterone yields BL. Several genes involved in BR/sterol synthesis have been cloned. BR deactivation reactions can include epimerization of A ring hydroxyls, glucosylation, esterification, modification, cleavage of the side chain, and glucosylation of the 23-OH group.
      Polyamines stimulate many reactions involved in the syntheses of DNA, RNA, and protein. The diverse physiological responses elicited by polyamines include cell division, tuber formation, root initiation, embryogenesis, flower development, and fruit ripening. Putrescine, spermidine, and spermine are synthesized from L-arginine and L-ornithine. Synthesis of spermidine and spermine requires an aminopropyl group derived from SAM, and there may be competition between the ethylene and polyamine biosynthesis pathways when concentrations of SAM are limited. The primary (terminal) amines of polyamines are oxidized by diamine oxidases, the secondary amines by polyamine oxidases. Polyamines occur both as free amines and as amide conjugates of hydroxycinnamates such as p-coumaric acid, ferrulic acid, and caffeic acid.
(–)-JA is associated with disease resistance, inhibits seed and pollen germination and seedling growth, and induces fruit ripening and abscision of flowers. It is synthesized from -linolenic acid, a membrane-derived C18 polyunsaturated fatty acid. The first specific step in the (–)-JA biosynthesis pathway, the conversion of 13(S)-hydroperoxylinolenic acid to 12,13(S)-epoxylinolenic acid, is catalyzed by allene oxide synthase. Wounding a leaf results in increased AOS activity and accumulation of (–)-JA.   (–)-JA is metabolized to hydroxylated products and amino acid and glycosylated conjugates.
      SA, synthesized from transcinnamic acid by way of a side branch of the phenylpropanoid pathway, is involved in thermogenesis in lilies and pathogen resistance in tobacco and other species. Until recently, benzoic acid was thought to be the immediate precursor of SA, but increasing evidence is supporting an alternative, as yet undefined, route to SA that does not involve benzoic acid. SA is metabolized to SA glucoside and 2,5-dihydroxybenzoic acid glucosyl ester.


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