The positive effect of salinity on nitrate uptake in Suaeda salsa

Nitrate plays both nutritional and osmotic roles in the salt tolerance of halophytes. However, how halophytes take up NO3- under saline conditions is still not well understood. Seedlings of Suaeda salsa L. were treated with 0, 200 and 500 mM NaCl under 0.5 mM NO3--N with or without Na3VO4 (the inhibitor of plasma membrane H+-ATPase) for 24 h. Salinity treatment of 200 mM NaCl up-regulated the gene expression of nitrate transporter 2.1 (SsNRT2.1) in the roots, increased the root net influx of H+ and NO3- and (NO3-)-N-15 accumulation in the leaves and roots. The expression of SsNRT2.1 at 200 mM NaCl with Na3VO4 was much higher than that without supplying Na3VO4, and the opposite trend was found in (NO3-)-N-15 accumulation in the leaves and roots. Supplying Na3VO4 had no significant effect on the net H+ flux, but induced a net NO3- efflux in the roots at 200 mM NaCl. Salinity may directly activate the expression of SsNRT2.1 and promote NO3 uptake via the increment of pumping H+ by PM H+-ATPase in S. salsa, which may explain why certain halophytes can absorb and accumulate high concentration of NO3- under low NO3- and high salinity conditions.