Role and Functional Differences of HKT1-Type Transporters in Plants under Salt Stress

Abstract

Abiotic stresses generally cause a series of morphological, biochemical and molecular changes that unfavorably affect plant growth and productivity. Among these stresses, soil salinity is a major threat that can seriously impair crop yield. To cope with the effects of high salinity on plants, it is important to understand the mechanisms that plants use to deal with it, including those activated in response to disturbed Na⁺ and K⁺ homeostasis at cellular and molecular levels. HKT1-type transporters are key determinants of Na⁺ and K⁺ homeostasis under salt stress and they contribute to reduce Na⁺-specific toxicity in plants. In this review, we provide a brief overview of the function of HKT1-type transporters and their importance in different plant species under salt stress. Comparison between HKT1 homologs in different plant species will shed light on different approaches plants may use to cope with salinity.

Keywords: HKT1; abiotic stresses; glycophytes; halophytes; high salinity.

Figure 1

Classification and structure of HKT1 proteins. (A) Structural analysis of HKT1 protein containing Ser or Gly residues in their conserved regions. Members of class-1 transporters carry a Ser in the selectivity filter position while on the same position class-2 transporters contain a Gly residue. P denotes pore-loop domain while N and C indicate N-terminus and C-terminus of HKT-proteins. (B) Members of class-1 HKT1 that carry a Ser residue transport Na⁺ while members of class-2 that carry a Gly residue can transport both Na⁺ as well as K⁺

Figure 2

Sequence comparison of HKT homologs from Arabidopsis, E. salsuginea and E. parvula. Amino acid sequences in the second pore loop region (PB) and the adjacent transmembrane domain (M2B) are aligned by clustalw (http://www.ebi.ac.uk/Tools/msa/clustalw2/). The conserved Gly residues in the PB region [49] are indicated by asterisks. The Asp residues specific for EsHKT1;2 (D207) and EpHKT1;2 (D205) are indicated by arrows.

Figure 3

EsHKT-RNAi plants are sensitive to low K⁺-limiting conditions. Wild type and knock-down lines of EsHKT1;2 (EsHKT1;2-RNAi) were grown on MS-medium for 10-days and then transferred to K⁺-deficient media with 0, 1 and 10 mM KCl (see Ali et al. 2012 for the detailed methodology) and allowed to grow for further 10-days. EsHKT1;2-RNAi lines were more sensitive to K⁺-limiting conditions as compared with the wild type Control. A gradual increase of K⁺ concentration greatly promotes the growth of wild type plants, whereas EsHKT1;2-RNAi lines were still sensitive. This result demonstrates the crucial role of EsHKT1;2 for K⁺-uptake.

Figure 4

EsHKT-RNAi plants are sensitive to salt stress. Wild type and knock-down lines of EsHKT1;2 (EsHKT-RNAi) were grown on MS-medium for 2-weeks and then transferred to inert soil (porous soil, see Ali et al. 2012 for details) and grown for further 3-weeks. Plants were then treated with 300 mM NaCl for another 3-weeks period, twice a week (control represents untreated plants). EsHKT-RNAi lines were more sensitive to salt stress as compared with wild type Control.

Figure 5

Functional properties of AtHKT1 and AtHKT1^N211D and differential selectivity for Na⁺ and K⁺ based on the Asn/Asp variance in the pore region. Wild type AtHKT1 is a sodium uniporter and does not confer salt stress tolerance. An altered version of AtHKT1 with a mutation of the Asn to Asp (AtHKT1^N211D) is also able to uptake potassium and confers salt stress tolerance. It has already been shown by Ali et al. 2016 that athkt1-1 plants complemented by AtHKT1^N211D showed higher tolerance to salt stress than lines complemented by the wild type AtHKT1. Thus, the introduction of Asp, replacing Asn, in HKT1-type transporters established altered cation selectivity and uptake dynamics.