Phosphatidic acid activates a wound-activated MAPK in Glycine max

Many plant species demonstrate a systemic increase in phosphatidic acid (PA ) levels after being wounded (Lee et al., 1997). To understand the role of PA in wound signal transduction, we investigated if PA can activate protein kinases in soybean (Glycine max L.). We found that a MAPK is activated in soybean seedlings in both wounded and neighboring unwounded leaves. The wou nd-activated soybean kinase is specifically recognized by an antibody again st the alfalfa MAPK, SIMK. When PA production is inhibited with n-butanol, an inhibitor of phospholipase D, the wound-induced activation of the MAPK i s suppressed, suggesting that an elevation in PA levels is essential for it s activation. Supporting this is the observation that exogenous PA activate s the MAPK in suspension-cultured soybean cells. Activation of the 49 kDa M APK occurs almost exclusively by PA, as other lipids are unable to or can o nly weakly activate the kinase. PA-induced activation of the MAPK is not a direct effect on the kinase but is mediated by upstream kinases. Our result s suggest that PA acts as a second messenger in wound-induced MAPK signalin g in plants.