Boerjan et al. (1995) describe superroot, a recessive mutation in Arabidopsis, which confers auxin overproduction. All 13 mutants with a rooty phenotype (including superroot1 (sur1)) are allelic and recessive. The Rty (Sur1) gene, has been cloned by T-DNA tagging and shown to encode a protein similar to tyrosine aminotransferases, which implies that an aminotransferase normally acts to limit free IAA accumulation. The RTY aminotransferase might act to enhance IAA (or IAA conjugate) degradation or to limit IAA synthesis (Bartel, 1997). It is possible that the tryptophan-independent pathway of IAA biosynthesis involves the conversion of a tryptophan precursor (e.g. indole-3-glycerol phosphate (IGP)?) to indole-3-pyruvic acid which is then normally actively catabolized to tryptophan by the tryptophan (RTY) aminotransferase [EC 2.6.1.27 or EC 2.6.1.28]. In Arabidopsis a specific isoform of aldehyde oxidase [EC 1.2.3.1] (AO1) is several-fold increased above wildtype in seedlings of an auxin-overproducing mutant, superoot1 (sur1). AO1 has preference for indole-3-acetaldehdye and indole-3-aldehyde as substrates (Seo et al, 1998).

Tryptamine provides the indole unit of monoterpenoid-indole and derived alkaloids, many of which are psychoactive. The terpenoid indole alkaloids are derived from strictosidine, which is formed from tryptamine and secologanin in the reaction catalyzed by strictosidine synthase, encoded by the Str gene in Catharanthus roseus (Whitmer et al, 1998). Serotonin is a hydroxy-tryptamine derivative and is found in nature as a component of stings and venom including stinging hairs of nettle (Urtica dioica) (Smith et al, 1979).

Auxins induce tryptophan decarboxylase [EC 4.1.1.28] in radicles of Catharanthus seedlings (Aerts et al, 1992). Tryptophan decarboxylase has been cloned from Catharanthus roseus, and overexpressed in tobacco; it confers high tryptamine levels and resistance to whitefly (Thomas et al, 1995). However, this may also result in increased susceptibility to certain diseases in potato (Yao et al, 1995).

Aerts RJ, Alarco A-M, De Luca V 1992 Auxins induce tryptophan decarboxylase activity in radicles of Catharanthus seedlings. Plant Physiol. 100: 1014-1019.

Bartel B 1997 Auxin biosynthesis. Annu. Rev. Plant Physiol. Plant Mol. Biol. 48: 51-66.

Boerjan W, Cervera M-T, Delarue M, Beeckman T, Dewitte W, Bellini C, Caboche M, Van Onckelen H, Van Montagu M, Inze D 1995 superroot, a recessive mutation in Arabidopsis, confers auxin overproduction. The Plant Cell 7: 1405-1419.

Seo M, Akaba S, Oritani T, Delarue M, Bellini C, Caboche M, Koshiba T 1998 Higher activity of an aldehyde oxidase in the auxin-overproducing superoot1 mutant of Arabidopsis thaliana. Plant Physiol. 116: 687-693.

Smith TA, Bagni N, Fracassini DS 1979 The formation of amines and their derivatives in plants. In (EJ Hewitt, CV Cutting eds) "Nitrogen Assimilation of Plants", Academic Press, NY, pp 557-570.

Thomas JC, Adams DG, Nessler CL, Brown JK, Bohnert HJ 1995 Tryptophan decarboxylase, tryptamine, and reproduction of the whitefly. Plant Physiol. 109: 717-720.

Whitmer S, Canel C, Hallard D, Goncalves C, Verpoorte R 1998 Influence of precursor availability on alkaloid accumulation by transgenic cell line of Catharanthus roseus. Plant Physiol. 116: 853-857.

Yao K, De Luca V, Brisson N 1995 Creation of a metabolic sink for tryptophan alters the phenylpropanoid pathway and the susceptibility of potato to Phytophthora infestans. Plant Cell 7: 1787-1799.