HORMONAL-CONTROL OF LIPASE ACTIVITY IN OILSEED RAPE GERMINANTS

In oilseeds, storage lipids provide the respiratory fuel for seedling growth. The enzyme responsible for their initial hydrolysis is lipase (triacylglycerol acylhydrolase; EC 3.1.1.3). We investigated the possi bility that lipase is regulated by gibberellins (GAs). In four oilseed rape cultivars of Brassica napus and B. rapa, seed imbibition in 10(- 6) to 10(-3) M GA(3) increased lipase activity 1.5- to 7-fold over con trol levels. Conversely, imbibition in 10(-7) to 10(-5) M abscisic aci d or 10(-6) to 10(-4) M paclobutrazol, an inhibitor of GA biosynthesis , markedly decreased lipase activity. While lipase activity in B. napu s cv. Parkland increased during the first 5 days following imbibition, concentrations of endogenous GA(1), GA(8) and GA(19) (as measured by GC-selected ion monitoring using [H-2(2)]GA internal standards) were r elatively constant and GA(20), a precursor of GA(1), decreased. Levels of endogenous GA(3) were apparently variable. Thus, lipase activity w as not correlated with GA(I) concentration, but the inverse correlatio n with GA(20) concentration suggests that GA turnover could be positiv ely correlated with lipase activity. Lipase activity was also examined in three genotypes of rapid cycling B. rapa that vary in endogenous G A content: rosette, a GA-deficient dwarf, a normal line and elongated internode, a tall mutant with high GA content. The three genotypes sho wed similar patterns of lipase activity during the first 4 days follow ing imbibition and the subcellular distribution of lipase activity was also similar in the three geno-types. Although GA may be involved in the regulation of lipase in oilseed rape germinants, it is not the sol e regulatory factor.