Spatial and temporal changes in lipase activity sites during oil body mobilization in protoplasts from sunflower seedling cotyledons

Hydrolysis of triacylglycerols (TAGs) catalyzed by lipase (triacylglycerol acylhydrolase; EC 3.1.1.3) action, is the principal biochemical event during oil body mobilization in germinating oilseeds. Employing a fluorescence microscopic technique developed in the author's laboratory, a shift in the intracellular lipase activity has been demonstrated in the protoplasts of sunflower seedling cotyledons during seed germination. Lipase activity is primarily confined to protein storage vacuoles (PSVs) in 1 d old seedling cotyledons. At 2 d old stage, a relocalization of lipase activity begins and activity can be observed both on PSVs and oil bodies. At later stages of development (3-6 d), smaller PSVs coalesce into a large vegetative vacuole devoid of lipase activity. During this phase, lipase activity is confined to oil bodies only and maximum activity is detected in 4 d old seedlings, coinciding with maximum rate of lipolysis. Thus, present investigations on protoplasts from seedling cotyledons provide evidence for intracellular shift in lipase activity to sites of TAG hydrolysis (oil bodies) and also show a structural and functional reorganization of PSVs.