Biosynthesis and Methods| Plant Hormones
Indole-3-acetic acid (IAA) is structurally related to the amino acid tryptophan, and plants convert tryptophan to IAA by several pathways. The tryptophan biosynthetic pathway in plants is shown in Figure A3.2. Much of what we know about the biosynthesis of IAA has been learned from studies of Arabidopsis mutants.
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The indole-3-pyruvic acid (IPyA) pathway (Figure A3.3) is the principal IAA biosynthetic pathway in plants (Ljung 2013). In Arabidopsis, IPyA is formed from tryptophan by tryptophan aminotransferase (TAA1, TAR). IPyA is then converted to IAA by the YUCCA flavin monooxygenases (Dai et al. 2013).
Figure A3.3 Tryptophan-dependent pathways of IAA biosynthesis in Arabidopsis. Dashed arrows indicate that neither a gene nor an enzyme activity has been identified in Arabidopsis. TRP, tryptophan; IAM, indole-3-acetamide; IPyA, indole-3-pyruvic acid; IAOx, indole-3-acetaldoxime; IG, indole-3-methylglucosinolate; TRM, tryptamine; IAN, indole-3-acetonitrile. (After Normanly 2010.)
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Figure A3.4 Ethylene biosynthetic pathway and the Yang cycle. The amino acid methionine is the precursor of ethylene. The rate-limiting step in the pathway is usually the conversion of AdoMet to ACC, which is catalyzed by the enzyme ACC synthase. The last step in the pathway, the conversion of ACC to ethylene, requires oxygen and is catalyzed by the enzyme ACC oxidase. The CH3—S group of methionine is recycled via the Yang cycle and thus conserved for continued synthesis. Besides being converted to ethylene, ACC can be conjugated to N-malonyl ACC. AOA, aminooxyacetic acid; AVG, aminoethoxy-vinylglycine. (After McKeon et al. 1995.)
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MacMillan J and Suter PJ (1958) The occurrence of gibberellin A1 in higher plants – isolation from the seed of runner bean (Phaseolus multiflorus). Naturwissenschaften 45(2): 46.
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In general, when ACC is supplied exogenously to plant tissues, ethylene production increases substantially. This observation indicates that the synthesis of ACC is usually the biosynthetic step that limits ethylene production in plant tissues. Exceptions include tissues with high rates of ethylene synthesis, such as ripening fruits (see below).
Plant Physiology, Sixth Edition
Phillips AL (2004) Genetic and transgenic approaches to improving crop performance. In: Davies PJ (ed.) Plant Hormones Biosynthesis, Signal Transduction, Action! pp. 582–609. Dordrecht: Kluwer Academic Publishers.
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ACC synthase is a member of a subfamily of carbon-sulfur lyases encoded by a multigene family that is differentially regulated by various inducers of ethylene biosynthesis. In tomato, for example, there are at least ten ACC synthase genes, different subsets of which are induced by auxin, wounding, and/or fruit ripening. The Arabidopsis genome contains nine ACC synthase genes. An analysis of purified proteins encoded by eight of these genes revealed a diversity of kinetic properties (for example, various affinities for the substrate AdoMet), suggesting that these isoforms might be optimized for different roles in various tissues and cell types (Yamagami et al. 2003). The deduced crystal structure of ACC synthase from both apple and tomato reveals a dimeric protein with shared active sites, similar to aminotransferases (Capitani et al. 1999; Huai et al. 2001).