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

Abstract

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 ²²Na⁺ 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 ²²Na⁺ influx in WT, but reduced ²²Na⁺ 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.