Potassium transporter OsHAK18 mediates potassium and sodium circulation and sugar translocation in rice

High-affinity potassium (K⁺) transporter (HAK)/K⁺ uptake permease (KUP)/K⁺ transporter (KT) have been identified in all genome-sequenced terrestrial plants. They play an important role in K⁺ acquisition and translocation and in enhancing salt tolerance. Here, we report that plasma membrane–located OsHAK18 functions in K⁺ and sodium (Na⁺) circulation and sugar translocation in rice (Oryza sativa). OsHAK18 was expressed mainly, though not exclusively, in vascular tissues and particularly in the phloem. Knockout (KO) of OsHAK18 reduced K⁺ concentration in phloem sap and roots but increased K⁺ accumulation in the shoot of both ‘Nipponbare’ and ‘Zhonghua11’ cultivars, while overexpression (OX) of OsHAK18 driven by its endogenous promoter increased K⁺ concentration in phloem sap and roots and promoted Na⁺ retrieval from the shoot to the root under salt stress. Split-root experimental analysis of rubidium (Rb⁺) uptake and circulation indicated that OsHAK18-OX promoted Rb⁺ translocation from the shoot to the root. In addition, OsHAK18-KO increased while OsHAK18-OX reduced soluble sugar content in the shoot and oppositely affected the sugar concentration in the phloem and its content in the root. Moreover, OsHAK18-OX dramatically increased grain yield and physiological K⁺ utilization efficiency. Our results suggest that—unlike other OsHAKs analyzed heretofore—OsHAK18 is critical for K⁺ and Na⁺ recirculation from the shoot to the root and enhances the source-to-sink translocation of photo-assimilates.