ETHYLENE SIGNAL IS TRANSDUCED VIA PROTEIN-PHOSPHORYLATION EVENTS IN PLANTS

A plethora of abiotic and biotic environmental stresses exert their in fluence on plants via the gaseous hormone ethylene. In addition, aspec ts of plant development and climacteric fruit ripening are regulated b y ethylene. Sensitivity to ethylene is presumably mediated by a specif ic ethylene receptor whose activation signal is then transduced via an unknown cascade pathway. We have used the plant pathogenesis response , exemplified by the induction of pathogenesis-related (PR) genes, as a paradigm to investigate ethylene-dependent signal transduction in th e plant cell. Ethylene application induced very rapid and transient pr otein phosphorylation in tobacco leaves. In the presence of the kinase inhibitors H-7 and K-252a, the transient rise in phosphorylation and the induced expression of PR genes were abolished. Similarly, these in hibitors blocked the response induced by an ethylene-dependent elicito r, alpha-AB. Reciprocally, application of okadaic acid, a specific inh ibitor of phosphatases type 1 and type 2A, enhanced total protein phos phorylation and by itself elicited the accumulation of PR proteins. In the presence of H-7 and K-252a, PR protein accumulation induced by ok adaic acid was blocked. In contrast to the action of ethylene and alph a-AB, xylanase elicits the accumulation of PR protein by an ethylene-i ndependent pathway. Xylanase-induced PR protein accumulation was not a ffected by H-7 and K-252a. The results indicate that responsiveness to ethylene in leaves is transduced via putative phosphorylated intermed iates that are regulated by specific kinases and phosphatases.