(-ABSCISIC ACID METABOLISM, 3-KETOACYL-COENZYME-A SYNTHASE GENE-EXPRESSION, AND VERY-LONG-CHAIN MONOUNSATURATED FATTY-ACID BIOSYNTHESIS IN BRASSICA-NAPUS EMBRYOS())

Microspore-derived embryos of Brassica napus cv Reston were used to ex amine the effects of exogenous (+)-abscisic acid (ABA) and related com pounds on the accumulation of very-long-chain monounsaturated fatty ac ids (VLCMFAs), VLCMFA elongase complex activity, and induction of the 3-ketoacyl-coenzyme A synthase (KCS) gene encoding the condensing enzy me of the VLCMFA elongation system. Of the concentrations tested, (+)- ABA at 10 mu M showed the strongest effect. Maximum activity of the el ongase complex, observed 6 h after 10 mu M (+)-ABA treatment, was 60% higher than that of the untreated embryos at 24 h. The transcript of t he KCS gene was induced by 10 mu M (+)-ABA within 1 h and further incr eased up to 6 h. The VLCMFAs eicosenoic acid (20:1) and erucoic acid ( 22:1) increased by 1.5- to 2-fold in embryos treated with (+)-ABA for 72 h. Also, (+)-8'-methylene ABA, which is metabolized more slowly tha n ABA, had a stronger ABA-like effect on the KCS gene transcription, e longase complex activity (28% higher), and level of VLCMFAs (25-30% hi gher) than ABA. After 24 h approximately 60% of the added (+)-[H-3]ABA (10 mu M) was metabolized, yielding labeled phaseic and dihydrophasei c acid. This study demonstrates that (+)-ABA promotes VLCMFA biosynthe sis via increased expression of the KCS gene and that reducing ABA cat abolism would increase VLCMFAs in microspore-derived embryos.