Ethylene Signal Is Transduced via Protein Phosphorylation Events in Plants.

A plethora of abiotic and biotic environmental stresses exert their influence on plants via the gaseous hormone ethylene. In addition, aspects of plant development and climacteric fruit ripening are regulated by ethylene. Sensitivity to ethylene is presumably mediated by a specific 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 the plant cell. Ethylene application induced very rapid and transient protein 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 inhibitors blocked the response induced by an ethylene-dependent elicitor, [alpha]-AB. Reciprocally, application of okadaic acid, a specific inhibitor of phosphatases type 1 and type 2A, enhanced total protein phosphorylation and by itself elicited the accumulation of PR proteins. In the presence of H-7 and K-252a, PR protein accumulation induced by okadaic acid was blocked. In contrast to the action of ethylene and [alpha]-AB, xylanase elicits the accumulation of PR protein by an ethylene-independent pathway. Xylanase-induced PR protein accumulation was not affected by H-7 and K-252a. The results indicate that responsiveness to ethylene in leaves is transduced via putative phosphorylated intermediates that are regulated by specific kinases and phosphatases.