INHIBITION OF STEROL BIOSYNTHESIS DURING ELICITOR-INDUCED ACCUMULATION OF FURANOCOUMARINS IN PARSLEY CELL-SUSPENSION CULTURES

Addition of a cell wall extract from Phytophthora megasperma f. sp. glycinea to parsley cell suspension cultures was found to trigger a rapid and marked inhibition of sterol biosynthesis as demonstrated by feeding experiments with tracer amounts of radioactive sodium acetate over short periods of time. Such an inhibition was detected as soon as 7 hr after elicitor addition, i.e. just before the furanocoumarin accumulation in the culture medium. It resulted in a dramatic decrease in the radioactivity incorporated into the main Delta(5)-sterols (24-methylcholesterol, sitosterol and stigmasterol) as well as into their precursors, squalene, 4,4-dimethyl- and 4 alpha-methyl-sterols. In elicitor-treated cells, the bulk of the residual radioactivity was recovered into the 4,4-dimethylsterol pool, suggesting a block at the level of the C-4 demethylation of 24-methylenecycloartanol to give cycloeucalenol. The elicitor addition was also found to induce an important change in the sterol profile of parsely cells consisting of an 8-fold increase in the ratio of stigmasterol to sitosterol after 80 hr. Measurements of the squalene synthase activity in microsomal fractions from control and elicitor-treated cell cultures clearly showed a rapid decline of this enzyme activity in response to the elicitor addition, which might be responsible, at least in part, for the inhibition of the overall sterol pathway.