Degradation of oil bodies isolated from cotyledons during germination of rapeseed seedlings

Abstract

The mobilization of the storage oil in the cotyledons of rapeseed seedlings (Brassica napus L.) starts about 24 h after imbibition. This was demonstrated by both the decrease of the total amount of triacyl-glycerols and the increase in lipase activity. The apparent rate of the degradation of triacylglycerols in vitro (lipase activity) clearly exceeded that of the in vivo breakdown of storage oils during germination. In addition, for the first time a low level of lipase activity was detected in extracts from dry seeds.. Studies were carried out on isolated oil bodies still surrounded by a proteinaceous coat (half unit membrane), including both the autolytic breakdown and the lipolytic degradation by partially purified lipase isolated from cotyledons of 4-day-old seedlings. Native oil bodies purified from cotyledons of rapeseeds started to show autolysis between day 1 and day 2 of germination, with optimum activity at pH 5.0, while optimum activity of the cytoplasmic (<soluble>) lipase was at pH 7.0 with native oil bodies as substrate. Lipase prepared from 4-day-old rapeseed seedlings hydrolyzes the TAG of the native oil bodies completely. However, such lipase catalyzed degradation 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. Evidendy the inhibition of the action of lipase is caused by the <half unit membrane> surrounding such oil bodies. These data cast doubt on the role of the prominent oil body proteins (oleosins) in anchoring lipase. It is proposed that the interaction of lipase with the surface of the oil bodies relies on negatively charged constituents of the oil body coat, which are different from the oleosins. In addition, since in early germination the activity of acylester acyl-hydrolase rises prior to lipase activity it should be assumed to be the initial enzyme in storage oil mobilization in germinating rapeseeds.