A Sodium Transporter HvHKT1;1 Confers Salt Tolerance in Barley via Regulating Tissue and Cell Ion Homeostasis
2018
Han, Yong | Yin, Shuya | Huang, Lu | Wu, Xuelong | Zeng, Jianbin | Liu, Xiaohui | Qiu, Long | Munns, Rana | Chen, Zhong-Hua | Zhang, Guoping
Our previous studies showed that high salt tolerance in Tibetan wild barley accessions was associated with HvHKT1;1, a member of the high-affinity potassium transporter family. However, molecular mechanisms of HvHKT1;1 for salt tolerance and its roles in K⁺/Na⁺ homeostasis remain to be elucidated. Functional characterization of HvHKT1;1 was conducted in the present study. NaCl-induced transcripts of HvHKT1;1 were significantly higher in the roots of Tibetan wild barley XZ16 relative to other genotypes, being closely associated with its higher biomass and lower tissue Na⁺ content under salt stress. Heterologous expression of HvHKT1;1 in Saccharomyces cerevisiae (yeast) and Xenopus laevis oocytes showed that HvHKT1;1 had higher selectivity for Na⁺ over K⁺ and other monovalent cations. HvHKT1;1 was found to be localized at the cell plasma membrane of root stele and epidermis. Knock-down of HvHKT1;1 in barley led to higher Na⁺ accumulation in both roots and leaves, while overexpression of HvHKT1;1 in salt-sensitive Arabidopsis hkt1-4 and sos1-12 loss-of-function lines resulted in significantly less shoot and root Na⁺ accumulation. Additionally, microelectrode ion flux measurements and root elongation assay revealed that the transgenic Arabidopsis plants exhibited a remarkable capacity for regulation of Na⁺, K⁺, Ca²⁺ and H⁺ homeostasis under salt stress. These results indicate that HvHKT1;1 is critical in radial root Na⁺ transport, which eventually reduces shoot Na⁺ accumulation. Additionally, HvHKT1;1 may be indirectly involved in retention of K⁺ and Ca²⁺ in root cells, which also improves plant salt tolerance.
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