DEGRADATION OF OIL BODIES ISOLATED FROM COTYLEDONS DURING GERMINATIONOF RAPESEED SEEDLINGS

The mobilization of the storage oil in the cotyledons of rapeseed seed lings (Brassica napus L.) starts about 24 h after imbibition. This was demonstrated by both the decrease of the total amount of triacylglyce rols and the increase in lipase activity. The apparent rate of the deg radation of triacylglycerols in vitro (lipase activity) clearly exceed ed that of the in vivo breakdown of storage oils during germination. I n addition, for the first time a low level of lipase activity was dete cted in extracts from dry seeds. Studies were carried out on isolated oil bodies still surrounded by a proteinaceous coat (half unit membran e), including both the autolytic breakdown and the lipolytic degradati on by partially purified lipase isolated from cotyledons of 4-day-old seedlings. Native oil bodies purified from cotyledons of rapeseeds sta rted to show autolysis between day 1 and day 2 of germination, with op timum activity at pH 5.0, while optimum activity of the cytoplasmic (< soluble>) lipase was at pH 7.0 with native oil bodies as substrate. Li pase prepared from 4-day-old rapeseed seedlings hydrolyzes the TAG of the native oil bodies completely. However, such lipase catalyzed degra dation was found only with oil bodies isolated from seedlings at-least 2 days old, but not with oil bodies obtained from dry seeds or 1-day- old seedlings. Evidently the inhibition of the action of lipase is cau sed by the chaff unit membrane, surrounding such oil bodies. These dat a cast doubt on the role of the prominent oil body proteins (oleosins) in anchoring lipase. It is proposed that the interaction of lipase wi th the surface of the oil bodies relies on negatively charged constitu ents of the oil body coat, which are different from the oleosins. In a ddition, since in early germination the activity of acylester acylhydr olase rises prior to lipase activity it should be assumed to be the in itial enzyme in storage oil mobilization in germinating rapeseeds.