EARLY SIGNALING EVENTS REQUIRED FOR INDUCTION OF PHYTOALEXIN ACCUMULATION IN PEA EPICOTYLS: PHOSPHOINOSITIDES AND THEIR ROLE INSIGNAL TRANSDUCTION

摘要

Glycoprotein elicitors from germination fluid of Mycosphaerella pinodes, a causal agent of Mycosphaerella blight on pea, immediately increased phosphatidylinositol 4,5-bisphosphate (PIP_2), followed by an transient increase in inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG) within a few minutes of the start of the elicitor treatment in pea epicotyls. However, the concomitant presence of suppressor from the fungus with the elicitor appreciably prevented the stimulation of polyphosphoinositide turnover and induction of the phytoalexin accumulation. Furthermore, an inhibitor of phospholipase C (PLC), neomycin suppressed the elicitor-induced increase in IP_3 and DAG, resulting in attenuation of the phytoalexin response in pea epicotyls, andalso conditioned the tissues to be susceptible even against an avirulent pathogen. These results suggest that activation of lipid kinases and the subsequent increase in PLC activity are part(s) of signal transduction pathway underlying the pea defense response. On the other hands, when epicotyl tissues were treated with a DAG kinase inhibitor (R59022), enhanced induction of the phytoalexin accumulation occurred which was induced by the elicitor. The marked induction was associated with a sustained accumulation of phenylalanine ammonia-lyase (PAL)- mRNA and the consequent increase in cellular PAL activity. These findings further suggest that inhibition of DAG phosphorylation leads to increased induction of the phytoalexin accumulation and support the hypothesis that DAG kinase may negatively regulate the signal transduction.