The mechanistic basis of sodium exclusion in Puccinellia tenuiflora under conditions of salinity and potassium deprivation

Soil salinity is a significant threat to global agriculture. Understanding salt exclusion mechanisms in halophyte species may be instrumental in improving salt tolerance in crops. Puccinellia tenuiflora is a typical salt-excluding halophytic grass often found in potassium-deprived saline soils. Our previous work showed that P. tenuiflora possesses stronger selectivity for K+ than for Na+ ; however, the mechanistic basis of this phenomenon remained elusive. Here, P. tenuiflora PutHKT1;5 was cloned and the functions of PutHKT1;5 and PutSOS1 were characterized using heterologous expression systems. Yeast assays showed that PutHKT1;5 possessed Na+ transporting capacity and was highly selective for Na+ over K+ . PutSOS1 was located at the plasma membrane and operated as a Na+ /K+ exchanger, with much stronger Na+ extrusion capacity than its homolog from Arabidopsis. PutHKT2;1 mediated high-affinity K+ and Na+ uptake and its expression levels were upregulated by mild salinity and K+ deprivation. Salinity-induced changes of root PutHKT1;5 and PutHKT1;4 transcript levels matched the expression pattern of root PutSOS1, which was consistent with root Na+ efflux. The transcript levels of root PutHKT2;1 and PutAKT1 were downregulated by salinity. Taken together, these findings demonstrate that the functional activity of PutHKT1;5 and PutSOS1 in P. tenuiflora roots is fine-tuned under saline conditions as well as by operation of other ion transporters/channel (PutHKT1;4, PutHKT2;1, and PutAKT1). This leads to the coordination of radial Na+ and K+ transport processes, their loading to the xylem, or Na+ retrieval and extrusion under conditions of mild salinity and/or K+ deprivation.