The substrate specificity of tryptophan (Trp) decarboxylase (TDC) for Trp and tyrosine (Tyr) decarboxylase (TYDC) for Tyr was used to modify the in vivo pools of these amino acids in transgenic tobacco. Expression of TDC and TYDC was shown to deplete the levels of Trp and Tyr, respectively, during seedling development. The creation of artificial metabolic sinks for Trp and Tyr also drastically affected the levels of phenylalanine, as well as those of the non-aromatic amino acids methionine, valine, and leucine. Transgenic seedlings also displayed a root-curling phenotype that directly correlated with the depletion of the Trp pool. Nontransformed control seedlings could be induced to display this phenotype after treatment with inhibitors of auxin translocation such as 2,3,5-triiodobenzoic acid or N-1-naphthylphthalamic acid. The depletion of aromatic amino acids was also correlated with increases in the activities of the shikimate and phenylpropanoid pathways in older, light-treated transgenic seedlings expressing TDC, TYDC, or both. These results provide in vivo confirmation that aromatic amino acids exert regulatory feedback control over carbon flux through the shikimate pathway, as well as affecting pathways outside of aromatic amino acid biosynthesis.

The wound-activated biosynthesis of phytoalexin hydroxycinnamic acid amides of tyramine was compared in untransformed and transgenic tobacco (Nicotiana tabacum) lines that express tryptophan decarboxylase (TDC), tyrosine decarboxylase (TYDC), or both activities. Transgenic in vitro-grown tobacco lines expressing TDC activity accumulated high levels of tryptamine but not hydroxycinnamic amides of tryptamine. In contrast, transgenic tobacco lines expressing TYDC accumulated tyramine as well as p-coumaroyltyramine and feruloyltyramine. The MeOH-soluble and cell wall fractions showed higher concentrations of wound-inducible p-coumaroyltyramine and feruloyltyramine, especially at and around wound sites, in TYDC and TDC 3 TYDC tobacco lines compared to wild-type or TDC lines. All the enzymes involved in the biosynthesis of hydroxycinnamic acid amides of tyramine were found to be similarly wound inducible in all tobacco genotypes investigated. These results provide experimental evidence that, under some circumstances, TYDC activity can exert a rate-limiting control over the carbon flux allocated to the biosynthesis of hydroxycinnamic acid amides of tyramine.The importance of tyramine during suberization of wounded potato (Solanum tuberosum) tubers is well established. Borg-Olivier and Monties (1993) used alkaline hydrolysis to fractionate and characterize the MeOH-extractable components of wound periderm in healing potato tuber discs. Fourteen days postwounding, their results established that tyramine accounts for 23% of total phenylpropanoids and Tyr-derived metabolites occurring in the MeOH-extractable free residue, whereas tyramine was undetected in the control nonwounded potato tuber discs. The presence of tyramine in suberizing potato periderm has also been corroborated by in situ noninvasive solid-state 13 C-NMR spectroscopy that allows the characterization of the polyaromatic domain of suberin in its native state within the plant cell wall matrix (Bernards et al., 1995;Bernards and Lewis, 1998).Phytoalexin hydroxycinnamic acid amides of tyramine (PCAAT), which are formed by the conjugation of tyramine with cinnamoyl-CoA thioesters (Negrel and Javelle, 1997), represent a major class of tyraminederived metabolites. The biosynthesis of PCAAT is elicited by wounding (Pearce et al., 1998;Ishihara et al., 2000) or pathogen inoculation (Keller et al., 1996;Muhlenbeck et al., 1996;Schmidt et al., 1998;Newman et al., 2001) in many species (Martin-Tanguy et al., 1996), especially in solanaceous plants (Clarke, 1982;Keller et al., 1996;Muhlenbeck et al., 1996). PCAAT can be further integrated into cell walls via a peroxidase-mediated process (Negrel and Lherminier, 1987;Keller et al., 1996) that yields monoor dicovalent ether bonds between cinnamoyl or tyramine moieties of PCAAT and the plant phenolic cell wall matrix (Lapierre et al., 1996). Although some PCAAT possess cytotoxic properties (Yamamoto et al., 1991;Park and Schoene, 2002), it is generally speculated that their major function in plants is to reinforce cell wa...

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Plants of the Asteraceae and Hypericaceae possess secondary compounds that induce photooxidation in insect herbivores that consume them. One of the well-established modes of action of these substances is peroxidation of membrane lipids. Some herbivores counteract these defences by avoidance of light and tissues rich in phototoxins or the ability to detoxify these secondary substances. The cytochrome P-450 polysubstrate monooxygenase systems involved, the metabolic products, and a new putative toxin pump have been described. Dietary antioxidants (β-carotene, vitamin E, ascorbate) are additional defences against phototoxicity. They reduce mortality in herbivores exposed to phototoxins and some specialist herbivores have high constitutive levels. Adapted specialist insects also have higher constitutive levels of superoxide dismutase (SOD) and respond to phototoxins in their diet by the induction of catalase (CAT), glutathione reductase (GR), and increased levels of reduced glutathione (GSH). Artificial inhibition of the enzymes SOD and CAT had little effect on phototoxicity but inhibition of GSH synthesis in herbivores enhanced photooxidative effects of administered phototoxins on lipid peroxidation. While insects have many mechanisms to overcome plant photooxidants, the Asteraceae appear to have adopted a strategy of counterattack. We suggest and provide preliminary evidence that a second group of secondary substances, the sesquiterpene lactones, occurring in the Asteraceae can attack key antioxidant defences to synergise phototoxins. © 1995 Wiley-Liss, Inc.