Nomenclature for HKT transporters, key determinants of plant salinity tolerance

Salinity tolerance in many plants is inversely related to the extent of Na+ accumulation in the shoot, notably in the major cereals such as wheat and rice [ 1 Tester M. Davenport R. Na+ tolerance and Na+ transport in higher plants. Ann. Bot. (Lond.). 2003; 91: 503-527 Crossref PubMed Scopus (2471) Google Scholar ]. In Arabidopsis and rice, there is evidence indicating a central role for members of the HKT gene family of Na+ and Na+/K+ transporters in controlling Na+ accumulation [ 2 Mäser P. et al. Altered shoot/root Na+ distribution and bifurcating salt sensitivity in Arabidopsis by genetic disruption of the Na+ transporter AtHKT1. FEBS Lett. 2002; 531: 157-161 Abstract Full Text Full Text PDF PubMed Scopus (315) Google Scholar , 3 Berthomieu P. et al. Functional analysis of AtHKT1 in Arabidopsis shows that Na+ recirculation by the phloem is crucial for salt tolerance. EMBO J. 2003; 22: 2004-2014 Crossref PubMed Scopus (472) Google Scholar , 4 Rus A. et al. AtHKT1 facilitates Na+ homeostasis and K+ nutrition in planta. Plant Physiol. 2004; 136: 2500-2511 Crossref PubMed Scopus (278) Google Scholar , 5 Ren Z.-H. et al. A rice quantitative trait locus for salt tolerance encodes a sodium transporter. Nat. Genet. 2005; 37: 1141-1146 Crossref PubMed Scopus (1082) Google Scholar , 6 Sunarpi et al. Enhanced salt tolerance mediated by AtHKT1 transporter-induced Na+ unloading from xylem parenchyma cells. Plant J. 2005; 44: 928-938 Crossref PubMed Scopus (473) Google Scholar ] and, thus, in determining salinity tolerance. However, in heterologous systems, whereas the wheat TaHKT1 protein transports both Na+ and K+, AtHKT1 is more Na+-specific [ 7 Mäser P. et al. Glycine residues in potassium channel-like selectivity filters determine potassium selectivity in four-loop-per-subunit HKT transporters from plants. Proc. Natl. Acad. Sci. U. S. A. 2002; 99: 6428-6433 Crossref PubMed Scopus (228) Google Scholar , 8 Uozumi N. et al. The Arabidopsis HKT1 gene homolog mediates inward Na+ currents in Xenopus laevis oocytes and Na+ uptake in Saccharomyces cerevisiae. Plant Physiol. 2000; 122: 1249-1259 Crossref PubMed Scopus (390) Google Scholar ] and their sequences are not particularly closely related. Recent studies suggest that members of the HKT gene family in rice and Arabidopsis are expressed in xylem parenchyma cells and protect leaves from salinity stress by removing sodium from the xylem sap [ 5 Ren Z.-H. et al. A rice quantitative trait locus for salt tolerance encodes a sodium transporter. Nat. Genet. 2005; 37: 1141-1146 Crossref PubMed Scopus (1082) Google Scholar , 6 Sunarpi et al. Enhanced salt tolerance mediated by AtHKT1 transporter-induced Na+ unloading from xylem parenchyma cells. Plant J. 2005; 44: 928-938 Crossref PubMed Scopus (473) Google Scholar ]. Given the wealth of sequences becoming available and the potential for confusion inherent in the current nomenclature, it is timely to propose an internationally agreed nomenclature for the family.