Transient Cytoplasmic pH Change and Ion Fluxes through the Plasma Memberan in Suspension-Cultured Rice Cells Triggered by N-Acetylchitooligosaccharide Elicitor

N-Acetylchitooligosaccharides, fragments of a main backbonepolymer of fungal ceil wall, elicit defense responses includingphytoalexin production in suspension-cultured rice cells. Thepurified oligosaccharide triggers rapid, transient membranedepolarization. Ion fluxes induced by the oligosaccharides wereanalyzed by using ion-selective electrodes. Treatment of thecells with the oligosaccharides induced transient efflux ofK⁺ and influx of H⁺ immediately after the elicitation. To monitorthe pH values of the cytoplasm and the vacuoles noninvasivelyunder a physiological condition, in vivo ³¹P-nuclear magneticresonance spectroscopy was applied to the cells to which oxygenatedgrowth medium was perfused continuously. The cytoplasmic pHshowed significant transient decrease, correspondingly. Onlythe N-acetylchitooligosaccharides with a degree of polymerizationhigher than 5 were active, whereas deacetylated chitosan oligomerscaused no effect. Less than 1 nM of N-acetylchitoheptaose wassufficient to induce rapid flux of ions. Such strict structuralrequirements for the induction of ion fluxes were similar tothose of specific binding to the putative plasma membrane receptoras well as a series of signaling events specifically inducedby the oligosaccharides, suggesting the involvement of transientchanges in cytoplasmic ion concentration in oligosaccharidesignaling for defense responses. (Received March 10, 1997; Accepted June 25, 1997)