Preferential phospholipase A2 activity on the oil bodies in cotyledons during seed germination in Helianthus annuus L. cv. Morden

Oil body mobilization during seed germination in oilseed plants is expected to involve controlled degradation of oil body membrane proteins prior to or accompanying the mobilization of triacylglycerols (TAGs). Present findings demonstrate that phosphatidylcholine (PC), a constituent of the oil body membranes, starts getting depleted with a concomitant accumulation of its degradation product—lysophosphatidylcholine (LPC), during seedling development in dark-grown sunflower (Helianthus annuus L. cv. Morden) seedling cotyledons. An increase in the LPC content was observed between 2 and 5 d of seedling development. A simultaneous enhancement of phospholipase A2 (PLA2) activity was also observed in the total soluble protein fraction, indicating its possible role in the degradation of PC, leading to the formation of LPC. Evidence obtained both from in vitro and in vivo experiments have indicated enhanced PLA2 activity in association with oil bodies in seedling cotyledons. Enzyme activity was further localized by in vivo assay using a PLA2-specific, fluorogenic substrate: PED6. Intracellular PED6-binding sites fluoresced bright green upon excitation at 505 nm (em 507 nm). Oil body surface begins to exhibit preferential PLA2 activity in protoplasts from 2 d old seedling cotyledons onwards. The enzyme activity enhanced further on the surface of oil bodies in 4–5 d old seedling cotyledons, coinciding with the phase of maximal lipolysis. These observations suggest the involvement of PLA2 in causing changes in the oil body membrane phospholipids, thereby making them susceptible to subsequent lipolytic action during seed germination.