Control of intracellular Na⁺ and K⁺ concentrations in plant tissues under saline conditions is crucial for salt tolerance. In this study, two Na⁺ ‐selective class I‐HKT transporters, HKT1;1 and HKT1;2, from tomato were found to be closely linked in proximity to the position with the maximum LOD score for major QTLs in chromosome 7 (at a distance of 35 kb), thus contributing to stem and leaf Na⁺ and K⁺ in two populations of tomato RILs derived from two salt‐tolerant lines from wild tomato species S. pimpinellifolium and S. cheesmaniae. This result, together with their patterns of expression and phenotypic effects on a pair of near isogenic lines (NILs) differing in terms of HKT1‐like alleles, support the hypothesis that HKT1;1 and/or HKT1;2 genes underlies a major tomato QTL for shoot Na⁺ / K⁺ .

The function of HKT1 in roots is controversial. We tackled this controversy by studying Na⁺ uptake in barley (Hordeum vulgare) roots, cloning the HvHKT1 gene, and expressing the HvHKT1 cDNA in yeast (Saccharomyces cerevisiae) cells. High-affinity Na⁺ uptake was not detected in plants growing at high K⁺ but appeared soon after exposing the plants to a K⁺-free medium. It was a uniport, insensitive to external K⁺ at the beginning of K⁺ starvation and inhibitable by K⁺ several hours later. The expression of HvHKT1 in yeast was Na⁺ (or K⁺) uniport, Na⁺-K⁺ symport, or a mix of both, depending on the construct from which the transporter was expressed. The Na⁺ uniport function was insensitive to external K⁺ and mimicked the Na⁺ uptake carried out by the roots at the beginning of K⁺ starvation. The K⁺ uniport function only took place in yeast cells that were completely K⁺ starved and disappeared when internal K⁺ increased, which makes it unlikely that HvHKT1 mediates K⁺ uptake in roots. Mutation of the first in-frame AUG codon of HvHKT1 to CUC changed the uniport function into symport. The expression of the symport from either mutants or constructs keeping the first in-frame AUG took place only in K⁺-starved cells, while the uniport was expressed in all conditions. We discuss here that the symport occurs only in heterologous expression. It is most likely related to the K⁺ inhibitable Na⁺ uptake process of roots that heterologous systems fail to reproduce.

TRK1 and TRK2 encode proteins involved in K+ uptake in Saccharomyces cerevisiae. A kinetic study of Rb+ influx in trk1 TRK2, trk1 TRK2D, and trk1 trk2 mutants reveals that TRK2 shows moderate affinity for Rb+. K(+)-starved trk1 delta TRK2 cells show a low-affinity component accounting for almost the total Vmax of the influx and a moderate-affinity component exhibiting a very low Vmax. Overexpression of TRK2 in trk1 delta TRK2D cells increases the Vmax of the moderate-affinity component, and this component disappears in trk1 delta trk2 delta cells. In contrast, the low-affinity component of Rb+ influx in trk1 delta TRK2 cells is not affected by mutations in TRK2. Consistent with the different levels of activity of the moderate-affinity Rb+ influx, trk1 delta TRK2 cells grow slowly in micromolar K+, trk1 delta TRK2D cells grow rapidly, and trk1 delta trk2 delta cells fail to grow. The existence of a unique K+ uptake system composed of several proteins is also discussed.