Plasma-membrane electrical responses to salt and osmotic gradients contradict radiotracer kinetics, and reveal Na+-transport dynamics in rice (Oryza sativa L.)

Main conclusionA systematic analysis of NaCl-dependent, plasma-membrane depolarization () in rice roots calls into question the current leading model of rapid membrane cycling of Na+ under salt stress.To investigate the character and mechanisms of Na+ influx into roots, Na+-dependent changes in plasma-membrane electrical potentials () were measured in root cells of intact rice (Oryza sativa L., cv. Pokkali) seedlings. As external sodium concentrations ([Na+](ext)) were increased in a step gradient from 0 to 100mM, membrane potentials depolarized in a saturable manner, fitting a Michaelis-Menten model and contradicting the linear (non-saturating) models developed from radiotracer studies. Clear differences in saturation patterns were found between plants grown under low- and high-nutrient (LN and HN) conditions, with LN plants showing greater depolarization and higher affinity for Na+ (i.e., higher V-max and lower K-m) than HN plants. In addition, counterion effects on were pronounced in LN plants (with decreasing in the order: Cl->SO42->HPO2-), but not seen in HN plants. When effects of osmotic strength, Cl- influx, K+ efflux, and H+-ATPase activity on were accounted for, resultant K-m and V-max values suggested that a single, dominant Na+-transport mechanism was operating under each nutritional condition, with K-m values of 1.2 and 16mM for LN and HN plants, respectively. Comparing saturating patterns of depolarization to linear patterns of Na-24(+) radiotracer influx leads to the conclusion that electrophysiological and tracer methods do not report the same phenomena and that the current model of rapid transmembrane sodium cycling may require revision.