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 ¹⁵NO₃^- 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 ¹⁵NO₃^- 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.