Differential regulation of MBP kinases by a glycoproptein elicitor and a polypeptide suppressor from Mycosphaerella pinodes in pea

A polypeptide fungal suppressor from a pea pathogen Mycosphaerella pinodes plays a key role in pathogenesis by suppressing elicitor-induced defense response(s) in pea (Pisum sativum L). In this study, we show that treatment of pea tissues with the polysaccharide elicitor secreted by M. pinodes results in rapid increased activation of two myelin basic protein (MBP)-dependent kinases p44 (≈44 kDa) and p48 (≈48 kDa) within 15-30 min upon elicitation. Interestingly, the suppressor inhibited the elicitor-induced activation of only p44 kinase. While the defense-inducing signalling molecules, chitosan and salicylic acid (SA) activated the p44 and p48 kinases, methyl jasmonate (MeJA) did not. The abiotic stress signals, abscisic acid (ABA), NaCl and wounding activated the p48 kinase alone. These results demonstrate that MAPKs are differentially activated in response to pathogen invasion and abiotic stress in pea. Furthermore, specific inhibition of elicitor-induced p44 kinase activation by a MAPKK inhibitor, PD098059 and protein kinase inhibitor, K252a correlated with the suppression of elicitor-induced phenylalanine ammonia lyase (PAL) gene expression, supporting a role for p44 in the elicitor-induced defense response(s) in pea. Inhibition of p44 by the phosphoinositide (PI) turnover inhibitor, neomycin (a fungal suppressor mimic), and potentiation of p44 by the diacylglycerol (DAG) kinase inhibitor, R59022 indicated that p44 may be acting downstream of (PI) metabolism. Taken together, our results indicate that suppressor of defense elicitation from M. pinodes acts through inhibition of a MAPK (p44), possibly through a PI signaling pathway, facilitating the establishment of basic compatibility during infection of pea.