The wheat cDNA LCT1 generates hypersensitivity to sodium in a salt-sensitive yeast strain

Salinity affects large areas of agricultural land, and all major crop speci es are intolerant to high levels of sodium ions. The principal route for Na + uptake into plant cells remains to be identified. Non-selective ion chann els and high-affinity potassium transporters have emerged as potential path ways for Na+ entry. A third candidate for Nat transport into plant cells is a low-affinity cation transporter represented by the wheat protein LCT1, w hich is known to be permeable for a wide range of cations when expressed in yeast (Saccharomyces cerevisiae). To investigate the role of LCT1 in salt tolerance we have used the yeast strain G19, which is disrupted in the gene s encoding Na+ export pumps and as a result displays salt sensitivity compa rable with wheat. After transformation with LCT1, G19 cells became hypersen sitive to NaCl. We show that LCT1 expression results in a strong decrease o f intracellular K+/Na+ ratio in G19 cells due to the combined effect of enh anced Na+ accumulation and loss of intracellular K+. Na+ uptake through LCT 1 was inhibited by K+ and Ca2+ at high concentrations and the addition of t hese ions rescued growth of LCT1-transformed G19 on saline medium. LCT1 was also shown to mediate the uptake of Li+ and Csf. Expression of two mutant LCT1 cDNAs with N-terminal truncations resulted in decreased Ca2+ uptake an d increased Na+ tolerance compared with expression of the full-length LCT1. Our findings strongly suggest that LCT1 represents a molecular link betwee n Ca2+ and Na+ uptake into plant cells.