Leaf wounding and the
wound signaling
peptide systemin induce expression of
wound response genes while the fungal toxin
fusicoccin (FC) induces expression of pathogenesis-related genes. Consistent with their functional differences, FC and
systemin regulate the extracellular pH in opposite ways, with
systemin inducing an alkalinization and FC an acidification response. Here we show that
systemin, wounding and FC activate the same
mitogen-activated protein kinases (MAPKs; MPKs) MPK1 and 2 in tomato (Lycopersicon esculentum) leaves and L. peruvianum
suspension-cultured cells. Wounding and FC activated an additional MAPK, MPK3. Pronounced differences were observed with regard to MAPK activation kinetics. FC induced prolonged, and
systemin transient activity of the MAPKs. This shows that functionally different elicitors engage the same signaling components, yet induce signal-specific activation dynamics. A comparative analysis of pH effects and MAPK activity in response to specific treatments revealed that the kinetics of pH changes and MAPK activation did not correlate. Simultaneous application of FC and
systemin did not lead to immediate pH changes but resulted in rapid increases in MAPK activity. Furthermore, changes in extracellular pH could be induced without concomitant MAPK activation by exchanging
conditioned medium with fresh medium. This shows that changes in the extracellular pH are neither required nor sufficient for MAPK activation, suggesting that signaling pathways involving MAPKs and extracellular pH changes operate in parallel and are not part of the same linear pathway.