Limiting cytosolic Na+ confers salt tolerance to rice cells in culture: a two‐photon microscopy study of SBFI‐loaded cells

Abstract

Rice (Oryza sativa L.), a staple food in Asia, is very sensitive to soil salinity. However, intraspecific variations exist, with the coastal cultivar Pokkali tolerating even brackish water. This study explores cellular mechanisms that contribute to salt tolerance in rice. It is widely accepted that limiting cytosolic Na+ should improve the survival of plants subjected to saline stress. However, an understanding of the mechanisms by which Na+ levels are controlled in relatively tolerant cultivars requires monitoring cytosolic Na+ non‐invasively and in real time, which is technically challenging. We have used two‐photon excitation for the ratiometric estimation of cytosolic Na+ in cultured cells using sodium‐binding benzofuran isophthalate. Pokkali cells maintained low cytosolic Na+ (approximately 25 mM), and a viability of over 85% under high salinity , while Jaya cells were unable to maintain low cytosolic Na+ and suffered decreased viability even at moderate saline stress. Here we show that the permeability of the Pokkali plasma membrane to Na+ is significantly lower than that of Jaya, to the extent that it is comparable with permeabilities reported for halophytes. Pokkali effectively sequesters Na+ in intracellular compartments utilizing a Ca2+‐regulated transport system(s). Together these cellular mechanisms allow Pokkali to maintain low cytosolic Na+ up to a stress of 250 mM NaCl. The findings demonstrate that differences in survival between these contrasting varieties of rice are mainly because of differences in membrane transport mechanisms and thus have significance in crop improvement.