CREATION OF A METABOLIC SINK FOR TRYPTOPHAN ALTERS THE PHENYLPROPANOID PATHWAY AND THE SUSCEPTIBILITY OF POTATO TO PHYTOPHTHORA-INFESTANS

The creation of artificial metabolic sinks in plants by genetic engine ering of key branch points may have serious consequences for the metab olic pathways being modified, The introduction into potato of a gene e ncoding tryptophan decarboxylase (TDC) isolated from Catharanthus rose us drastically altered the balance of key substrate and product pools involved in the shikimate and phenylpropanoid pathways, Transgenic pot ato tubers expressing the TDC gene accumulated tryptamine, the immedia te decarboxylation product of the TDC reaction, The redirection of try ptophan into tryptamine also resulted in a dramatic decrease in the le vels of tryptophan, phenylalanine, and phenylalanine-derived phenolic compounds in transgenic tubers compared with nontransformed controls, In particular, wound-induced accumulation of chlorogenic acid, the maj or soluble phenolic ester in potato tubers, was found to be two- to th reefold lower in transgenic tubers, Thus, the synthesis of polyphenoli c compounds, such as lignin, was reduced due to the limited availabili ty of phenolic monomers, Treatment of tuber discs with arachidonic aci d, an elicitor of the defense response, led to a dramatic accumulation of soluble and cell wall-bound phenolics in tubers of untransformed p otato 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 cel l wall of these plants, This study provides strong evidence that the s ynthesis and accumulation of phenolic compounds, including lignin, cou ld be regulated by altering substrate availability through the introdu ction of a single gene outside the pathway involved in substrate suppl y, This study also indicates that phenolics, such as chlorogenic acid, play a critical role in defense responses of plants to fungal attack.