Phospholipase C activation during elicitation of the oxidative burst in cultured plant cells

Although phospholipase C hydrolysis of polyphosphoinositides constitutes one of the major second messenger pathways in animal cells, its participation in signal transduction in higher plants has not been established. To determine whether activation of phosphatidylinositol-directed phospholipase C might be involved in signaling the elicitor-induced oxidative burst in plants, suspension-cultured soybean cells were treated with two stimulants of the H2O2 burst and examined for polyphosphoinositide turnover. Both polygalacturonic acid elicitor and the G protein activator, mastoparan, promoted a transient increase in inositol 1,4,5-trisphosphate (IP3) content that exceeded basal IP3 levels (0.9 +/- 0.4 pmol of IP3/10(6) cells, n = 28) by 2.6- and 7-fold, respectively. In each case, intracellular IP3 content reached a maximum at 1 min post-stimulation and declined to near basal levels during the subsequent 5-10 min. Neomycin sulfate, an inhibitor of polyphosphoinositide hydrolysis, blocked the IP3 transient, and Mas-17, an inactive analogue of mastoparan, induced no change in IP3. Thin layer chromatography of lipid extracts of the soybean cells corroborated the above results by revealing a rapid decrease in phosphatidyl-inositol monophosphate and phosphatidylinositol 4,5-bisphosphate following polygalacturonic acid elicitor and mastoparan (but not Mas-17) stimulation. Since the rise in IP3 preceded H2O2 production and since neomycin sulfate inhibited the appearance of both, we hypothesize that phospholipase C activation might constitute one pathway by which elicitors trigger the soybean oxidative burst.