Syringolin-mediated activation of the Pir7b esterase gene in rice cells issuppressed by phosphatase inhibitors

Inoculation of rice plants (Oryza sativa) with the nonhost pathogen Pseudom onas syringae pv, syringae leads to the activation of defense-related genes and ultimately to induced resistance against the rice blast fungus Pyricul aria oryzae, One of the molecular determinants of P. syringae pv, syringae that is recognized by the plant cells and evokes these defense responses is syringolin A, an elicitor that is secreted by the bacteria under appropria te conditions. In order to investigate signal transduction events elicited by syringolin A, the response of cultured rice cells to syringolin A applic ation was analyzed. Cultured rice cells were able to sense syringolin A at concentrations in the nanomolar range as observed by the transient accumula tion of Pir7b esterase transcripts. Syringolin A-mediated Pir7b transcript accumulation was inhibited by cyclohexamide, indicating that de novo protei n synthesis was required. Calyculin and okadaic acid, two protein phosphata se inhibitors, blocked Pir7b gene induction, whereas the serine/threonine p rotein kinase inhibitors staurosporine and K-252a had no effect on Pir7b tr anscript levels. Actin transcript levels were essentially not affected by i nhibitor treatments over the experimental time span. These results imply th at dephosphorylation of a phosphoprotein is an important step in the syring olin A-triggered signal transduction pathway.