ROLE OF THE RING METHYL-GROUPS IN ABSCISIC-ACID ACTIVITY IN ERUCIC-ACID ACCUMULATION IN OILSEED RAPE (BRASSICA-NAPUS L.)

Modification of the structure of abscisic acid (ABA) has been reported to result in modification of its physiologic activity. In this study we tested the effect of removing methyl groups from the ring and of ch irality of ABA on activity in microspore-derived embryos of oilseed ra pe (Brassica napus L.). The natural (+)-ABA molecule induced growth in hibition and an increase in the amount of erucic acid accumulated in t he oil at medium concentrations less than 1 mu M. (-)-ABA showed simil ar effects. Removing the 7'-methyl group resulted in a dramatic decrea se in activity: (+)-7'-demethyl-ABA retained some activity as a growth inhibitor; a 10-100 mu M concentration of this compound was needed fo r a response, and (-)-7'-demethyl-ABA was almost completely inactive. Similar effects were observed with regard to elongase activity, which catalyzes erucic acid sar biosynthesis from oleic acid. Removal of the 8'- and 9'-methyl groups resulted in a more complex response. These c ompounds all showed intermediate activity; for growth inhibition, the presence of the 9'-methyl was the more important determinant, whereas chirality dominated the response on erucic acid accumulation, with the (+)-enantiomers being more active.