The Production of Hormones in Higher Plants

Abstract

SUMMARY Although much is known about the effects of plant hormones and their role in the control of growth and differentiation, little is known about the way in which hormone production is itself controlled or about the cellular sites of hormone synthesis. The literature on hormone production is discussed in this review in an attempt to shed some light on these problems. The natural auxin of plants, indol‐3yl‐acetic acid (IAA) is produced by a wide variety of living organisms. In animals, fungi and bacteria it is formed as a minor by‐product of tryptophan degradation. The pathways of its production involve either the transamination or the decarboxylation of tryptophan. The transaminase route is the more important. In higher plants auxin is also produced as a minor breakdown product of trypto phan, largely via transamination. In some species decarboxylation may occur but is of minor importance. Tryptophan can also be degraded by spontaneous reaction with oxidation products of certain phenols. The unspecific nature of the enzymes involved in IAA production and the probable importance of spontaneous, nonenzymic reactions in the degradation of tryp to phan make it unlikely that auxin production from tryptophan can be regulated with any precision at the enzymic level. The limiting factor for auxin production is the availability of tryptophan, which in most cells is present in insufficient quantities for its degradation to occur to a significant extent. Tryptophan levels are, however, considerably elevated in cells in which net protein breakdown is taking place as a result of autolysis. An indole compound, glucobrassicin, occurs in Brassica and a number of other genera. It breaks down readily to form a variety of products including indole aceto‐nitrile, which can give rise to IAA. There is, however, no evidence to indicate that glucobrassicin is a precursor of auxin in vivo. Conjugates of IAA, e.g. IAA‐aspartic acid and IAA‐glucose, are formed when IAA is supplied in unphysiologically high amounts to plant tissues. These and other IAA conjugates occur naturally in developing seeds and fruits. There is no persuasive evidence for the natural occurrence of IAA‐protein complexes. Tissues autolysing during prolonged extraction with ether produce IAA from tryptophan released by proteolysis. IAA is produced in considerable quantities by autolysing tissues in vitro. During the senescence of leaves proteolysis results in elevated levels of trypto phan. Large amounts of auxin are produced by senescent leaves. Coleoptile tips have a vicarious auxin economy which depends on a supply of IAA, IAA esters and other compounds closely related to IAA from the seed. These move acropetally in the xylem and accumulate at the coleoptile tip. The production of auxin in coleoptile tips involves the hydrolysis of IAA esters and the conversion of labile, as yet unidentified compounds, to IAA. There is no evidence for the de novo synthesis of IAA in coleoptiles. Practically all the other sites of auxin production are sites of both meristematic ...

Manipulating Broad-Spectrum Disease Resistance by Suppressing Pathogen-Induced Auxin Accumulation in Rice

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TheTRANSPORT INHIBITOR RESPONSE2Gene Is Required for Auxin Synthesis and Diverse Aspects of Plant Development

“…Subsequently, an IPA decarboxylase converts IPA to indole-3-acetaldehyde, and indole-3-acetaldehyde is oxidized to IAA. The IPA pathway is considered a major IAA biosynthetic pathway in plants, since potential intermediates have been isolated from different species (Sheldrake, 1973;Cooney and Nonhebel, 1991;Koga, 1995;Tam and Normanly, 1998). In addition, Trp transamination activity has been found in many plants (Gamborg, 1965;Forest and Wightman, 1972;Truelsen, 1973).…”

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