SlNRT1.5 transporter and the SlSKOR K+ channel jointly contribute to K+ translocation in tomato plants

Accumulation of K⁺ in shoots is largely dependent on K⁺ transport via the xylem and has important implications not only for K⁺ nutrition but also for stress tolerance. In tomato plants, the K⁺ channel SlSKOR contributed to K⁺ translocation but the decrease in the shoot K⁺ content in slskor mutants was only ∼15 %, indicating that additional K⁺ transport systems operated in the tomato stele. Here, we studied the physiological roles of the transporter SlNRT1.5 in tomato plants, whose homolog in Arabidopsis, AtNRT1.5, contributed to xylem K⁺ load. By using heterologous expression of SlNRT1.5 in Xenopus oocytes and a slnrt1.5 knock-out mutant, we have gained insights into its role in shoot K⁺ nutrition. Expression of SlNRT1.5 in Xenopus oocytes resulted in K⁺ efflux, similar to that mediated by AtNRT1.5, which could indicate that SlNRT1.5 operates as a K⁺ transport system. Plants lacking slnrt1.5 accumulated less K⁺ in shoots than WT plants under low external pH (4.5), and low supply of K⁺ (0.05 mM) and N (0.5 mM). Interestingly, slnrt1.5 plants accumulated less Na⁺ and Cl^- in shoots than WT plants. Further analyses on slskor slnrt1.5 double mutant plants revealed an overlapping role of SlSKOR and SlNRT1.5 in shoot K⁺ accumulation. Double mutants showed a 40 % decrease in shoot K⁺ content in comparison with slskor and slnrt1.5 single mutants. Altogether, this study showed that SlNRT1.5 and SlSKOR are major players in shoot K⁺ accumulation in tomato plants.