Role of HKT1 in Thellungiella salsuginea, a model extremophile plant

Abstract

Maintenance of the cytosolic Na(+)/K(+) ratio under saline conditions is crucial for plants. HKT-type Na(+) transporters play a key role in keeping low cytosolic Na(+) concentrations thus retaining a low Na(+)/K(+) ratio, that reduces Na(+) toxicity and causing high salinity stress tolerance. Two HKT-type transporters, AtHKT1 from Arabidopsis and TsHKT1;2 from Thellungiella salsuginea, that share high DNA and protein sequence identities, are distinguished by fundamentally different ion selection and salinity stress behavior. On the level of transcription, TsHKT1;2 is dramatically induced upon salt stress, whereas AtHKT1 is downregulated. TsHKT1;2-RNAi lines show severe potassium deficiency and are also sensitive to high [Na(+)]. We have validated the ability of the TsHKT1;2 protein to act as an efficient K(+) transporter in the presence of high [Na(+)] by expression in yeast cells. K(+) specificity is based on amino acid differences in the pore of the transporter protein relative to AtHKT1.

Keywords: HKT1; Na+/K+ ratio; Thellungiella salsuginea; halophyte; salt stress.

Figure 1.TsHKT1;2-RNAi lines are sensitive to high salinity. Six-week-old soil grown plants were treated with 300 mM NaCl for 2 wk. RNAi lines were more sensitive to salt stress as compared with wild type.

Figure 2. Ionic selectivity of AtHKT1 and TsHKT1;2. AtHKT1, TsHKT1;2. Empty vector (negative control) and AtKAT1 (Potassium channel as a positive control) were expressed in a sodium sensitive yeast strain AXT3K. AtHKT1 expressing cells were sensitive to high concentrations of NaCl and failed to grow, whereas TsHKT1;2 expressing cells grew similar to cells expressing the genuine potassium channel AtKAT1.

Figure 3. Hypothetical model explaining the role of TsHKT1;2 under salt stress. (A) During salinity stress TsHKT1;2 is transcriptionally as well as at protein level, upregulated leading to increased uptake of K⁺ which maintain a high [K⁺/Na⁺] to balance the inescapable entry of some [Na⁺]. (B) In contrast, AtHKT1 is downregulated at transcriptional level upon salt stress. Moreover, AtHKT1 has very low affinity for K⁺ and fails to maintain a balance between Na⁺ and K⁺.