H2O2 and cytosolic Ca2+signals triggered by the PM H plus -coupled transport system mediate K+/Na+ homeostasis in NaCl-stressed Populus euphratica cells

Using confocal microscopy, X-ray microanalysis and the scanning ion-selective electrode technique, we investigated the signalling of H2O2, cytosolic Ca2+ ([Ca2+](cyt)) and the PM H+-coupled transport system in K+/Na+ homeostasis control in NaCl-stressed calluses of Populus euphratica. An obvious Na+/H+ antiport was seen in salinized cells; however, NaCl stress caused a net K+ efflux, because of the salt-induced membrane depolarization. H2O2 levels, regulated upwards by salinity, contributed to ionic homeostasis, because H2O2 restrictions by DPI or DMTU caused enhanced K+ efflux and decreased Na+/H+ antiport activity. NaCl induced a net Ca2+ influx and a subsequent rise of [Ca2+](cyt), which is involved in H2O2-mediated K+/Na+ homeostasis in salinized P. euphratica cells. When callus cells were pretreated with inhibitors of the Na+/H+ antiport system, the NaCl-induced elevation of H2O2 and [Ca2+](cyt) was correspondingly restricted, leading to a greater K+ efflux and a more pronounced reduction in Na+/H+ antiport activity. Results suggest that the PM H+-coupled transport system mediates H+ translocation and triggers the stress signalling of H2O2 and Ca2+, which results in a K+/Na+ homeostasis via mediations of K+ channels and the Na+/H+ antiport system in the PM of NaCl-stressed cells. Accordingly, a salt stress signalling pathway of P. euphratica cells is proposed.