PLASMA-MEMBRANE ATPASE ACTIVITY DURING PEPINO (SOLANUM-MURICATUM) RIPENING

Plasma membrane-enriched samples were extracted from pepino fruit (cv. El Camino) by phase partitioning. H+-ATPase (EC 3.6.1.35) activity in these samples increased during late fruit development (immediately be fore the onset of ripening) and western blotting confirmed there was a n increase in enzyme abundance at this time. H+-ATPase activity decrea sed during early ripening and then increased again in the final phase of ripening. Immunolocalisation showed the plasma membrane H+-ATPase w as most abundant in the outer cell layers of the fruit, which are cons idered to have a major role in determining fruit texture. Fruit soften ing was not accelerated by harvest and there was no stimulation of H+- ATPase activity by harvest. An in vitro tensile test using fruit rings showed tissue softening proceeded faster at low apoplastic pH (4.5) t han pH 6.5; and tissue buffered at pH 6.5 softened less than unbuffere d rings. Erythrosin B, an inhibitor of the plasma membrane H+-ATPase, also retarded softening in vitro. These data suggest that plasma membr ane H+-ATPase activity may contribute to the onset of pepino softening through a reduction in apoplastic pH.