AtHKT1;1 and AtHAK5 mediate low-affinity Na+ uptake in Arabidopsis thaliana under mild salt stress

Salinity is a serious problem for agricultural production worldwide. Reducing Na+ influx is one of the key steps for controlling Na+ accumulation in plants and improving salt tolerance of crop plants. Researches on a number of species are now converging on HKT-type and KUP/HAK/KT type proteins, both of them are probable candidates of Na+ uptake into the root. To assess the contribution of AtHKT1;1 and AtHAK5 to low-affinity Na+ uptake in Arabidopsis thaliana, the Na-22(+) influx in A. thaliana wild type (WT) and hkt1;1 mutant (athkt1;1) with or without inhibitors (10 mM TEA(+) or 5 mM NH4 (+)) were investigated, in addition, the expression levels of AtHKT1;1 and AtHAK5 in plants exposed to different concentrations of NaCl, KCl or KCl plus NaCl were analyzed. Results showed that TEA(+) or NH4 (+) have no significant influence on Na-22(+) influx in WT, but reduced Na-22(+) influx by 42 and 46 %, respectively, in athkt1;1. Under 25 mM NaCl, 0.01 mM K+ facilitated higher net Na+ uptake rate in both WT and athkt1;1 than 2.5 mM K+. In addition, 0.01 mM K+ down-regulated AtHKT1;1 and up-regulated AtHAK5 in WT roots compared with 2.5 mM K+, and more interestingly, the transcript of AtHAK5 in athkt1;1 roots was always higher than that in WT roots during 48 h of 2.5 mM K+ plus 25 mM NaCl, and it increased continuously during 48 h of 0.01 mM K+ plus 25 mM NaCl. Therefore, it is proposed that AtHKT1;1 and AtHAK5 mediate low-affinity Na+ uptake, and both of them are regulated by external K+ concentrations. AtHKT1;1 might mediate low-affinity Na+ uptake under 2.5 mM K+, while 0.01 mM K+ might activate AtHAK5 and facilitate low-affinity Na+ uptake in WT. When AtHKT1;1 lost its function, AtHAK5 might mediate low-affinity Na+ uptake instead of AtHKT1;1 and this function becomes more important under low K+ condition.