Induction of Lipid and Oleosin Biosynthesis by (+)-Abscisic Acid and Its Metabolites in Microspore-Derived Embryos of Brassica napus L. cv Reston: Biological Responses in the Presence of 8′-Hydroxyabscisic Acid

Microspore-derived (MD) embryos of Brassica napus L. cv Reston were used to test the effects of (+)-abscisic acid ([(+)-ABA]) and its metabolites, 8′-hydroxyabscisic acid (8′-OH ABA) and (-)-phaseic acid (PA), on the accumulation of very long-chain monounsaturated fatty acids (VLCMFAs) and induction of genes encoding a 19-kD oleosin protein and a Δ15 desaturase during embryogenesis. Developing early- to mid-cotyledonary MD embryos at 16 to 19 d in culture were treated with 10 μM hormone/metabolite for 4 d. At various times during incubation, embryos and medium were analyzed to determine levels of hormone/metabolite, VLCMFAs, and oleosin or Δ15 desaturase transcripts. The VLCMFAs, 20:1 and 22:1, primarily in triacylglycerols, increased by 200% after 72 h in the presence of (+)-ABA and 8′-OH ABA relative to the control. In contrast, treatment with PA for 72 h had little effect (20% increase) on the level of VLCMFAs. The first 24 to 72 h of (+)-ABA treatment were critical in the induction of VLCMFA biosynthesis, with 8′-OH ABA lagging slightly behind (+)-ABA in promoting this response. The accumulation of VLCMFAs was positively correlated with an increase in elongase activity. (+)-ABA and its 8′-OH ABA metabolite induced the accumulation of a 19-kD oleosin transcript within 2 to 4 h in culture. In addition, both (+)-ABA and 8′-OH ABA induced the same level of Δ15 desaturase transcript by 8 h. PA had no effect on the induction of either oleosin or Δ15 desaturase transcripts. To our knowledge, this is the first report of the biological activity of 8′-OH ABA and of stimulatory effects of (+)-ABA and 8′-OH ABA on lipid and oleosin biosynthesis.