NRT1.5/NPF7.3 Functions as a Proton-Coupled H⁺/K⁺ Antiporter for K⁺ Loading into the Xylem in Arabidopsis

Potassium and nitrogen are essential macronutrients for plant growth and have a positive impact on crop yield. Previous studies have indicated that the absorption and translocation of K⁺ and NO₃^- are correlated with each other in plants; however, the molecular mechanism that coordinates K⁺ and NO₃^- transport remains unknown. In this study, using a forward genetic approach, we isolated a low-K⁺-sensitive Arabidopsis thaliana mutant, lks2, that showed a leaf chlorosis phenotype under low-K⁺ conditions. LKS2 encodes the transporter NRT1.5/NPF7.3, a member of the NRT1/PTR (Nitrate Transporter 1/Peptide Transporter) family. The lks2/nrt1.5 mutants exhibit a remarkable defect in both K⁺ and NO₃^- translocation from root to shoot, especially under low-K⁺ conditions. This study demonstrates that LKS2 (NRT1.5) functions as a proton-coupled H⁺/K⁺ antiporter. Proton gradient can promote NRT1.5-mediated K⁺ release out of root parenchyma cells and facilitate K⁺ loading into the xylem. This study reveals that NRT1.5 plays a crucial role in K⁺ translocation from root to shoot and is also involved in the coordination of K⁺/NO₃^- distribution in plants.