An increase in phosphoinositide-specific phospholipase C activity precedes induction of C4 phosphoenolpyruvate carboxylase phosphorylation in illuminated and NH4Cl-treated protoplasts from Digitaria sanguinalis

A Ca2+-dependent phospoinositide-specific phospholipase C (PI-PLC) activity has been characterized in the microsomal fraction of Digitaria sanguinalis mesophyll cell protoplasts. Microsomal PI-PLC was found to be inhibited in vitro by a mammalian anti-PLC-delta 1 antibody and by the aminosteroide U-73122, tin inhibitor of PI-PLC activity in animal cells. In Western blot experiments, the antibody recognized an 85 kDa protein in both microsomal protein extracts from mesophyll protoplasts and rat brain protein extracts containing the authentic enzyme. The involvement of the microsomal PI-PLC in the light-dependent transduction pathway leading to the phosphorylation of C-4 phosphoenolpyruvate carboxylase (PEPC) was investigated in D, sanguinalis protoplasts. A transient increase in the PI-PLC reaction product inositol-1,4,5 trisphosphate (Ins(1,4,5)P-3) was observed in situ during early induction of the C-4 PEPC phosphorylation cascade. U-73122, but not the inactive analogue U-73343, efficiently blocked the transient accumulation of Ins(1,4,5)P-3, and both the increase in C-4 PEPC kinase activity and C-4 PEPC phosphorylation in illuminated and weak base-treated protoplasts. Taken together, these data suggest that PI-PLC-based signalling is a committed step in the cascade controlling the regulation of C-4 PEPC phosphorylation in C-4 leaves.