ISHS Acta Horticulturae 381: International Symposium on Natural Phenols in Plant Resistance
SYNTHESIS AND DEGRADATION OF PHYTOALEXINS IN ALFALFA |
| Authors: | R. Edwards, A.D. Parry, A.C.E. Gregory, S.A. Tiller, T.J. Daniell |
| Abstract:
When alfalfa (Medicago sativa L.) cell cultures or plants are exposed to biotic or abiotic elicitors they rapidly accumulate the antifungal isoflavonoid phytoalexin medicarpin. In elicited cell cultures this accumulation primarily results from de novo synthesis from phenylalanine and is associated with a large-scale disruption in metabolism. We have recently determined that elicitation is associated with a large increase in transmethylation reactions and that the majority of the increase in methylating capacity is directed into the production of medicarpin and related methylated products. A considerable proportion of the newly synthesised phytoalexin is selectively exported from the cell into the medium. After medicarpin has accumulated to maximal levels the phytoalexin rapidly disappears from both the elicited cells and the medium. A major route of detoxification of medicarpin is it's 3-O-glucosylation followed by 6"-O-malonylation. The malonylated glucosides of medicarpin and its precursor formononetin are major metabolites in alfalfa root and cell cultures and are synthesised by specific glucosyl- and malonyl-transferases. However these conjugates are not end-point metabolites but undergo turnover following their hydrolysis by malonylesterases and a specific glucosidase. Radiolabelling experiments confirm that these conjugates can be a significant source of phytoalexins in eliciting plant tissue. In contrast to glycosylation little is known about the alternative routes of | |
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