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SUMMARY: High‐affinity K+ transporters/K+ uptake permeases/K+ transporters (HAK/KUP/KT) are important pathways mediating K+ transport across cell membranes, 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 crucial roles in root K+ uptake and root‐to‐shoot translocation. However, whether HAK/KUP/KT transporters also function in phloem K+ translocation remain unclear. In this study, we revealed that a phloem‐localized rice HAK/KUP/KT transporter, OsHAK18, mediated cell K+ uptake when expressed in yeast, Escherichia coli and Arabidopsis. It was localized at the plasma membrane. Disruption of OsHAK18 rendered rice seedlings insensitive to low‐K+ (LK) stress. After LK stress, some WT leaves showed severe wilting and chlorosis, whereas the corresponding leaves of oshak18 mutant lines (a Tos17 insertion line and two CRISPR lines) remained green and unwilted. Compared with WT, the oshak18 mutants accumulated more K+ in shoots but less K+ in roots after LK stress, leading to a higher shoot/root ratio of K+ per plant. Disruption of OsHAK18 does not affect root K+ uptake and K+ level in xylem sap, but it significantly decreases phloem K+ concentration and inhibits root‐to‐shoot‐to‐root K+ (Rb+) translocation in split‐root assay. These results reveal that OsHAK18 mediates phloem K+ loading and redistribution, whose disruption is in favor of shoot K+ retention under LK stress. Our findings expand the understanding of HAK/KUP/KT transporters' functions and provide a promising strategy for improving rice tolerance to K+ deficiency. Significance Statement: HAK/KUP/KT transporters play important roles in K+ uptake and root‐to‐shoot translocation, but whether HAK/KUP/KT transporters function in phloem K+ translocation remain unclear. Here, we reported that a phloem‐localized rice HAK/KUP/KT transporter, OsHAK18, mediated phloem K+ loading and affected shoot‐to‐root K+ redistribution, which expands the understanding of HAK/KUP/KT transporters' functions and provides a potential strategy for improving rice tolerance to K+ deficiency. [ABSTRACT FROM AUTHOR]

Maintaining Na+/K+ homeostasis is a critical feature for plant survival under salt stress, which depends on the operation of Na+ and K+ transporters. Although some K+ transporters mediating root K+ uptake have been reported to be essential to the maintenance of Na+/K+ homeostasis, the effect of K+ long‐distance translocation via phloem on plant salt tolerance remains unclear. Here, we provide physiological and genetic evidence of the involvement of phloem‐localized OsAKT2 in rice salt tolerance. OsAKT2 is a K+ channel permeable to K+ but not to Na+. Under salt stress, a T‐DNA knock‐out mutant, osakt2 and two CRISPR lines showed a more sensitive phenotype and higher Na+ accumulation than wild type. They also contained more K+ in shoots but less K+ in roots, showing higher Na+/K+ ratios. Disruption of OsAKT2 decreases K+ concentration in phloem sap and inhibits shoot‐to‐root redistribution of K+. In addition, OsAKT2 also regulates the translocation of K+ and sucrose from old leaves to young leaves, and affects grain shape and yield. These results indicate that OsAKT2‐mediated K+ redistribution from shoots to roots contributes to maintenance of Na+/K+ homeostasis and inhibition of root Na+ uptake, providing novel insights into the roles of K+ transporters in plant salt tolerance. In Oryza sativa, a Shaker K+ channel, OsAKT2 mediates K+ redistribution from shoots to roots, contributing to maintenance of Na+/K+ homeostasis and inhibition of root Na+ uptake under salt stress. [ABSTRACT FROM AUTHOR]

High-affinity K + transporters/K + uptake permeases/K + transporters (HAK/KUP/KT) are important pathways mediating K + transport across cell membranes, 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 crucial roles in root K + uptake and root-to-shoot translocation. However, whether HAK/KUP/KT transporters also function in phloem K + translocation remain unclear. In this study, we revealed that a phloem-localized rice HAK/KUP/KT transporter, OsHAK18, mediated cell K + uptake when expressed in yeast, Escherichia coli and Arabidopsis. It was localized at the plasma membrane. Disruption of OsHAK18 rendered rice seedlings insensitive to low-K + (LK) stress. After LK stress, some WT leaves showed severe wilting and chlorosis, whereas the corresponding leaves of oshak18 mutant lines (a Tos17 insertion line and two CRISPR lines) remained green and unwilted. Compared with WT, the oshak18 mutants accumulated more K + in shoots but less K + in roots after LK stress, leading to a higher shoot/root ratio of K + per plant. Disruption of OsHAK18 does not affect root K + uptake and K + level in xylem sap, but it significantly decreases phloem K + concentration and inhibits root-to-shoot-to-root K + (Rb + ) translocation in split-root assay. These results reveal that OsHAK18 mediates phloem K + loading and redistribution, whose disruption is in favor of shoot K + retention under LK stress. Our findings expand the understanding of HAK/KUP/KT transporters' functions and provide a promising strategy for improving rice tolerance to K + deficiency., (© 2023 Society for Experimental Biology and John Wiley & Sons Ltd.)