A Magnesium Transporter OsMGT1 Plays a Critical Role in Salt Tolerance in Rice

Abstract

Salt stress is one of the major factors limiting rice (<i>Oryza sativa</i>) production globally. Although several transporters involved in salt tolerance have been identified in rice, the mechanisms regulating their transport activity are still poorly understood. Here, we show evidence that a rice Mg transporter OsMGT1 is required for salt tolerance probably by regulating transport activity of OsHKT1;5, a key transporter for the removal of Na⁺ from the xylem sap at the root mature zone. Knockout of <i>OsMGT1</i> did not affect total Na uptake, but increased Na concentration in the shoots and xylem sap, resulting in a significant increase in salt sensitivity at low external Mg²⁺ concentration (20-200 μm). However, such differences were abolished at a higher Mg²⁺ concentration (2 mm), although the total Na uptake was not altered. <i>OsMGT1</i> was expressed in both the roots and shoots, but only that in the roots was moderately up-regulated by salt stress. Spatial expression analysis revealed that <i>OsMGT1</i> was expressed in all root cells of the root tips but was highly expressed in the pericycle of root mature zone. <i>OsMGT1</i> was also expressed in the phloem region of basal node, leaf blade, and sheath. When expressed in <i>Xenopus laevis</i> oocytes, the transport activity of OsHKT1;5 was enhanced by elevating external Mg²⁺ concentration. Furthermore, knockout of <i>OsHKT1;5</i> in <i>osmgt1</i> mutant background did not further increase its salt sensitivity. Taken together, our results suggest that Mg²⁺ transported by OsMGT1 in the root mature zone is required for enhancing OsHKT1;5 activity, thereby restricting Na accumulation to the shoots.