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 syn thesis in tomato leaves. We show here that systemin elicits a transien t depolarization of the tomato mesophyll cell membrane. Furthermore it triggers a transient decrease in the external pH of the mesophyll tis sue which is followed by a sustained pH increase. In the presence of f usicoccin (which has been shown to antagonize the synthesis of protein ase inhibitors) the depolarization and transient H+ efflux are attenua ted 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 transduct ion.