Rice potassium transporter OsHAK18 mediates phloem K+loading and redistribution

ABSTRACT High-Affinity K + transporters/K + Uptake Permeases/K + Transporters (HAK/KUP/KT) are important pathways mediating K + transport across cell membrane, which function in maintaining K + homeostasis during plant growth and stress response. An increasing number of studies have shown that HAK/KUP/KT transporters play important roles in potassium uptake and root-to-shoot translocation. However, whether some HAK/KUP/KT transporters mediate K + redistribution in phloem remains unknown. In this study, we revealed that a phloem-localized HAK/KUP/KT transporter, OsHAK18 operated as a typical KUP/HAK/KT transporter mediating cell K + uptake when expressed in yeast, E. coli and Arabidopsis . It was localized at plasma membrane. Disruption of OsHAK18 rendered rice seedlings insensitive to low-K + stress. Compared with WT, the oshak18 mutants accumulated more K + in shoots but less K + in roots, leading to a higher shoot/root ratio of K + per plant. Although disruption of OsHAK18 doesn’t affect root K + uptake and K + level in xylem sap, it significantly decreases phloem K + concentration and inhibits root-to-shoot-to-root K + translocation in split-root assay. These results reveal that OsHAK18 mediates phloem K + loading and redistribution, whose disruption is favor of shoot K + retention under low-K + stress. Our findings not only reveal a unique function of rice HAK/KUP/KT family member, but also provide a promising strategy to improve rice tolerance under K + deficiency.