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.