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THE PLANT CELL, Vol 7, Issue 11 1787-1799, Copyright © 1995 by American Society of Plant Biologists
Creation of a Metabolic Sink for Tryptophan Alters the Phenylpropanoid Pathway and the Susceptibility of Potato to Phytophthora infestans
K. Yao, V. De Luca and N. Brisson
Department of Biochemistry, Universite de Montreal, Montreal, Quebec, H3C 3J7 Canada
The creation of artificial metabolic sinks in plants by genetic engineering
of key branch points may have serious consequences for the metabolic
pathways being modified. The introduction into potato of a gene encoding
tryptophan decarboxylase (TDC) isolated from Catharanthus roseus
drastically altered the balance of key substrate and product pools involved
in the shikimate and phenylpropanoid pathways. Transgenic potato tubers
expressing the TDC gene accumulated tryptamine, the immediate
decarboxylation product of the TDC reaction. The redirection of tryptophan
into tryptamine also resulted in a dramatic decrease in the levels of
tryptophan, phenylalanine, and phenylalanine-derived phenolic compounds in
transgenic tubers compared with nontransformed controls. In particular,
wound-induced accumulation of chlorogenic acid, the major soluble phenolic
ester in potato tubers, was found to be two- to threefold lower in
transgenic tubers. Thus, the synthesis of polyphenolic compounds, such as
lignin, was reduced due to the limited availability of phenolic monomers.
Treatment of tuber discs with arachidonic acid, an elicitor of the defense
response, led to a dramatic accumulation of soluble and cell wall-bound
phenolics in tubers of untransformed potato plants but not in transgenic
tubers. The transgenic tubers were also more susceptible to infection after
inoculation with zoospores of Phytophthora infestans, which could be
attributed to the modified cell wall of these plants. This study provides
strong evidence that the synthesis and accumulation of phenolic compounds,
including lignin, could be regulated by altering substrate availability
through the introduction of a single gene outside the pathway involved in
substrate supply. This study also indicates that phenolics, such as
chlorogenic acid, play a critical role in defense responses of plants to
fungal attack.
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