EFFECTS OF ERWINIA-AMYLOVORA HARPIN ON TOBACCO LEAF CELL-MEMBRANES ARE RELATED TO LEAF NECROSIS AND ELECTROLYTE LEAKAGE AND DISTINCT FROM PERTURBATIONS CAUSED BY INOCULATED E-AMYLOVORA

We have characterized the membrane potential response of cells in toba cco leaves to externally applied or injected harpin from Erwinia amylo vora (Ea321) and linked it to subsequent electrolyte leakage and event ual necrosis. We similarly examined depolarization in response to inje cted Ea321 cells, and mutant bacteria that are defective in harpin pro duction. Injected and applied harpin depolarize only the H+-ATPase-dep endent component of membrane potential, in contrast to injected wild-t ype Ea321, which greatly depolarizes both pump and diffusion potential s. The functional capacity of the H+-ATPase component is also much les s impaired in harpin- than Ea321-injected leaves. Furthermore, harpin production does not account for the entire set of differences in respo nse to injected wild-type and mutant bacteria. Membrane depolarization in response to applied harpin and necrosis induced by injected harpin are similarly concentration-dependent and directly related. Parallel inhibition of harpin-caused membrane depolarization, electrolyte leaka ge and necrosis by the Ca2+ channel blocker, La3+, or the protein kina se inhibitor, K252a, links these events and suggests that prolonged Ca 2+ entry and phosphorylation underlie them. Although harpin similarly depolarizes leaf and suspension cell potentials, there is a substantia l difference: leaf cell membranes are less sensitive and repolarize. I ncreased repolarization upon harpin removal points to probable reversi ble association with the plant cell. Dependence of repolarization upon order of harpin/fusicoccin addition suggests separate modes or sites of action. (C) 1998 Academic Press.