Rapid Changes in Polyphosphoinositide Metabolism in Pea in Response to Fungal Signals

Effects of the elicitor and/or suppressor from Mycosphaerella pinodes on polyphosphoinositide metabolism (PI metabolism) in pea were examined both in vivo and in vitro. The elicitor induced a rapid and biphasic increase in levels of phosphatidylinositol-4,5-bisphosphate (PtdInsP2) and inositol 1,4,5-trisphosphate (IP3) in epicotyl tissues that was apparent within 15 min. A transient increase in levels of PtdInsP2 and IP3 was detected immediately in elicitor-treated plasma membranes. However, the concomitant presence of suppressor with elicitor resulted in inhibition of these increases both in vivo and in vitro. These findings suggest that the elicitor rapidly activates phosphatidylinositol kinase, phosphatidylinositol-4-monophosphate kinase and phospholipase C, which are involved in PI metabolism, whereas the suppressor markedly inhibits these enzymes. Neomycin, a known inhibitor of phospholipase C, blocked the elicitor-induced accumulation both of IP3 and pisatin and it also induced local susceptibility in pea tissues that resembled that of the fungal suppressor. From these results, it appears that rapid changes in PI metabolism are indispensable in the signal transduction related to defense responses of pea plants.