TsHKT1;2, a HKT1 Homolog from the Extremophile Arabidopsis Relative Thellungiella salsuginea, Shows K+ Specificity in the Presence of NaCl    

Abstract

Cellular Na+/K+ ratio is a crucial parameter determining plant salinity stress resistance. We tested the function of plasma membrane Na+/K+ cotransporters in the High-affinity K+ Transporter (HKT) family from the halophytic Arabidopsis (Arabidopsis thaliana) relative Thellungiella salsuginea. T. salsuginea contains at least two HKT genes. TsHKT1;1 is expressed at very low levels, while the abundant TsHKT1;2 is transcriptionally strongly up-regulated by salt stress. TsHKT-based RNA interference in T. salsuginea resulted in Na+ sensitivity and K+ deficiency. The athkt1 mutant lines overexpressing TsHKT1;2 proved less sensitive to Na+ and showed less K+ deficiency than lines overexpressing AtHKT1. TsHKT1;2 ectopically expressed in yeast mutants lacking Na+ or K+ transporters revealed strong K+ transporter activity and selectivity for K+ over Na+. Altering two amino acid residues in TsHKT1;2 to mimic the AtHKT1 sequence resulted in enhanced sodium uptake and loss of the TsHKT1;2 intrinsic K+ transporter activity. We consider the maintenance of K+ uptake through TsHKT1;2 under salt stress an important component supporting the halophytic lifestyle of T. salsuginea.

Life at the extreme: lessons from the genome

“…In some cases, they have been inferred from the primary sequences of well-characterized genes, such as the 37-amino-acid stretch in L -myo-inositol-1-phosphate synthase, which distinguishes the salt-tolerant wild rice ( Porteresia coarctata ) from domesticated rice ( Oryza sativa ) [11], or the single-amino-acid variation in AtHKT1;1 (which encodes the high-affinity K + transporter 1) that distinguishes coastal from inland clines of Arabidopsis [12]. In other cases, they have been implicated by the constitutively higher expression - in the absence of stress - of genes that are induced by stress in Arabidopsis , as in the resurrection plant Craterostigma plantagineum [13], the salt-tolerant poplar Populus euphratica [14], or the Arabidopsis relatives T. parvula and Thellungiella salsuginea (formerly T. halophila ) [15-17]. …”

“…HKT1 exists as tandem duplicated copies in both Thellungiella species [10,17]. One copy encodes new protein functionality and also has an expression pattern different from that of the Arabidopsis counterpart [17].…”

A Single Amino-Acid Substitution in the Sodium Transporter HKT1 Associated with Plant Salt Tolerance

“…The genome of Thellungiella salsuginea (Ts), a halophytic relative of Arabidopsis, includes three copies of HKT1-type genes in a tandem array (Wu et al, 2012). Of the three TsHKT1 homologs, only TsHKT1;2 is greatly induced at the transcript level following salt stress (Ali et al, 2012), while the TsHKT1;1 transcript is downregulated the same way as in AtHKT1 under salt stress (Oh et al, 2010b;Wu et al, 2012). When expressed in yeast cells, TsHKT1;2 shows competence for K + transport whereas TsHKT1;1 is Na + -selective (Ali et al, 2012).…”

“…Of the three TsHKT1 homologs, only TsHKT1;2 is greatly induced at the transcript level following salt stress (Ali et al, 2012), while the TsHKT1;1 transcript is downregulated the same way as in AtHKT1 under salt stress (Oh et al, 2010b;Wu et al, 2012). When expressed in yeast cells, TsHKT1;2 shows competence for K + transport whereas TsHKT1;1 is Na + -selective (Ali et al, 2012). Yet another Arabidopsis halophytic close relative is T. parvula (Tp; now Schrenkiella parvula), whose genome sequence has been determined; it contains two HKT1 genes, TpHKT1;1 and TpHKT1;2 (Dassanayake et al, 2011).…”

“…Alignment of amino-acid sequences of known HKTs with the yeast K + transporter ScTRK1 provided, to our knowledge, new clues about structural and functional differences between Na + -and K + -selective HKT variants (Ali et al, 2012). Both TsHKT1;2 and TpHKT1;2 contain conserved Asp residues in their second pore-loop domains respectively) as well as in the adjacent transmembrane domain (Asp-238 and Asp-236, respectively).…”