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

Abstract

N-Acetylchitooligosaccharides, fragments of a main backbone polymer of fungal ceil wall, elicit defense responses including phytoalexin production in suspension-cultured rice cells. The purified oligosaccharide triggers rapid, transient membrane depolarization. Ion fluxes induced by the oligosaccharides were analyzed by using ion-selective electrodes. Treatment of the cells with the oligosaccharides induced transient efflux of K+ and influx of H+ immediately after the elicitation. To monitor the pH values of the cytoplasm and the vacuoles noninvasively under a physiological condition, in vivo 31P-nuclear magnetic resonance spectroscopy was applied to the cells to which oxygenated growth medium was perfused continuously. The cytoplasmic pH showed significant transient decrease, correspondingly. Only the N-acetylchitooligosaccharides with a degree of polymerization higher than 5 were active, whereas deacetylated chitosan oligomers caused no effect. Less than 1 nM of N-acetylchitoheptaose was sufficient to induce rapid flux of ions. Such strict structural requirements for the induction of ion fluxes were similar to those of specific binding to the putative plasma membrane receptor as well as a series of signaling events specifically induced by the oligosaccharides, suggesting the involvement of transient changes in cytoplasmic ion concentration in oligosaccharide signaling for defense responses.