Systemin transiently depolarizes the tomato mesophyll cell membrane and antagonizes fusicoccin-induced extracellular acidification of mesophyll tissue

The plant polypeptide signal systemin induces proteinase inhibitor synthesis in tomato leaves. We show here that systemin elicits a transient depolarization of the tomato mesophyll cell membrane. Furthermore it triggers a transient decrease in the external pH of the mesophyll tissue which is followed by a sustained pH increase. In the presence of fusicoccin (which has been shown to antagonize the synthesis of proteinase inhibitors) the depolarization and transient H+ efflux are attenuated whereas the slower phase of the sustained electroneutral H+ influx persists. These results suggest that systemin-induced changes in ion transport play a role in the early phases of systemin signal transduction.