HKT sodium and potassium transporters in Arabidopsis thaliana and related halophyte species

Abstract

High salinity induces osmotic stress and often leads to sodium ion-specific toxicity, with inhibitory effects on physiological, biochemical and developmental pathways. To cope with increased Na⁺ in soil water, plants restrict influx, compartmentalize ions into vacuoles, export excess Na⁺ from the cell, and distribute ions between the aerial and root organs. In this review, we discuss our current understanding of how high-affinity K⁺ transporters (HKT) contribute to salinity tolerance, focusing on HKT1-like family members primarily involved in long-distance transport, and in the recent research in the model plant Arabidopsis and its halophytic counterparts of the Eutrema genus. Functional characterization of the salt overly sensitive (SOS) pathway and HKT1-type transporters in these species indicate that they utilize similar approaches to deal with salinity, regardless of their tolerance.

Fig. 1

Structure and classification of HKT proteins. (A) Basic structure of the HKT proteins. HKTs have eight transmembrane domains (M1_A–M2_D) and four pore‐loop domains (P_A–P_D). Serine (blue) and glycine (green) residues responsible for Na⁺/K⁺ selection are indicated by arrows in the first pore‐loop region, which is known as the selectivity filter position. N and C indicate N‐ and C‐terminus of HKT proteins. (B) HKT1 proteins are divided into two classes, classes I and II. Most members of class I possess a serine residue in the selectivity filter position, whereas most class II transporters contain a glycine residue at the same position.

Fig. 2

EsHKT1‐RNAi plants are sensitive to salt stress. E. salsuginea lines with knocked‐down expression of HKT1 (EsHKT1‐RNAi) are sensitive to high salinity. Four‐week‐old inert soil (porous soil) grown plants of the indicated genotypes without treatment (upper panel) or treated with 300 mM NaCl for 2 weeks (lower panel). Photographs were taken 2 weeks after salt stress. EsHKT1‐RNAi lines are more sensitive to salt stress as compared with vector control.

Fig. 3

Cation selectivity of wild type AtHKT1 and EsHKT1;2 and their corresponding mutants. Protein sequences of AtHKT1 and EsHKT1;2 in the second pore‐loop domain (PB). Arrow indicates the conserved amino acid. The models explain the diverse cation selectivity of AtHKT1 (left) and EsHKT1;2 (right) and their mutants. AtHKT1 is a Na⁺‐specific transporter in plants, yeast, and oocytes with very low K⁺ selectivity in E. coli. By contrast, EsHKT1;2 is a high‐affinity K⁺ and Na⁺ co‐transporter. Substitution of Asn (N211) in AtHKT1 or Asp (D207) in EsHKT1;2 change their cation selectivity. AtHKT1^N211D functions like EsHKT1;2, whereas EsHKT1;2^D207N functions as AtHKT1, which highlights the importance of asp in the pore‐loop domain.