Differences in shoot Na+ accumulation between two tomato species are due to differences in ion affinity of HKT1;2

HKT1 has been shown to be essential in Na⁺ homeostasis in plants. In this paper, we report the analysis of Na⁺ accumulation in different plant organs of two tomato species with contrasting salt tolerances: Solanum lycopersicum and Solanum pennellii. Furthermore, we relate these differences in Na⁺ accumulation between the two species to the differences in HKT1;2 transport kinetics and HKT1;2 expression. S. lycopersicum showed “Na⁺ excluder” behaviour, whereas S. pennellii showed “Na⁺ includer” behaviour. SlHKT1;2 expression, in contrast to SpHKT1;2 expression showed a significant effect of NaCl treatment, especially stems had a high increase in SlHKT1;2 expression. SlHKT1;2 promoter-GUS reporter gene analysis showed that SlHKT1;2 is expressed in the vasculature surrounding the roots and shoots of transformed Arabidopsis plants. In this paper, we present HKT1;2 protein sequences of both tomato species and provide evidence that both SlHKT1;2 and SpHKT1;2 are Na⁺ transporters. Our kinetic studies showed that SpHKT1;2, in comparison with SlHKT1;2, had a lower affinity for Na⁺. This low affinity of SpHKT1;2 correlated with higher xylem Na⁺ and higher accumulation of Na⁺ in stems and leaves of S. pennellii. Our findings demonstrate the importance of the understanding of transport characteristics of HKT1;2 transporters to improve the understanding of Na⁺ homeostasis in plants.