FACTORS, AFFECTING IN-VITRO ACTIVATION OF POTATO PHOSPHOLIPASE-A(2)

In potato infected with an incompatible race of Phytophthora infestans, the late blight fungus, or treated with fungal elicitor hyphal wall components (HWC), there is possible involvement of phospholipase A2 (PLA2) in induction of resistance reactions. Activation of soluble PLA2 in a partially purified PLA2 preparation in vitro required Ca2+, Mg2+, diacylglycerol (DG), ATP and calmodulin. The optimal activity of PLA2 in the presence of Ca2+ was at a pH of 8.5. CaCl2 or DG stimulated the activity directly. Increased activity with Ca2+, Mg2+, DG and ATP together suggests the involvement of a protein kinase in PLA2 activation. The activity of PLA2 was inhibited by staurosporine, a protein kinase inhibitor, and ophiobolin, a calmodulin inhibitor, in the presence of exogeneous Ca2+ and calmodulin. Because staurosporine, but not ophiobolin, prevented activation by exogeneous Ca2+, Mg2+, DG and ATP, inhibition appears to result from an indirect effect of calmodulin on PLA2. The activity of PLA2 measured in the presence of Ca2+, Mg2+, DG and ATP increased to peak at 2-3 h following treatment of potato tuber discs with HWC. Modulation of PLA2 activity by Ca2+, DG and protein kinase(s) seems to be involved in signal transduction during the initiation of defence responses in potato tubers.