Rice OsHKT2;1 transporter mediates large Na+ influx component into K+-starved roots for growth

Excessive accumulation of sodium in plants causes toxicity. No mutation that greatly diminishes sodium (Naþ) influx into plant roots has been isolated. The OsHKT2;1 (previously named OsHKT1) transporter from rice functions as a relatively Naþ-selective transporter in heterologous expression systems, but the in vivo function of OsHKT2;1 remains unknown. Here, we analyzed transposon-insertion rice lines disrupted in OsHKT2;1. Interestingly, three independent oshkt2;1-null alleles exhibited significantly reduced growth compared with wildtype plants under low Naþ and Kþ starvation conditions. The mutant alleles accumulated less Naþ, but not less Kþ, in roots and shoots. OsHKT2;1 was mainly expressed in the cortex and endodermis of roots. 22Naþ tracer influx experiments revealed that Naþ influx into oshkt2;1-null roots was dramatically reduced compared with wild-type plants. A rapid repression of OsHKT2;1-mediated Naþ influx and mRNA reduction were found when wild-type plants were exposed to 30mM NaCl. These analyses demonstrate that Naþ can enhance growth of rice under Kþ starvation conditions, and that OsHKT2;1 is the central transporter for nutritional Naþ uptake into Kþ-starved rice roots.