Rice OsHAK18 mediates shoot-to-root circulation of potassium and sodium and source-to-sink transport of soluble sugar

High-affinity potassium (K+) transporters HAK/KUP/KT have been identified in all genome-sequenced terrestrial plants. They play important role in K+ acquisition, translocation and enhancing salt tolerance. Here we report that the plasma-membrane-located OsHAK18 functions in K+ and sodium (Na+) circulation and sugar translocation in rice. OsHAK18 is expressed mainly, though not exclusively, in vascular tissues and particularly in the phloem. Knockout (KO) of OsHAK18 reduced K+ concentration in phloem sap and in the root 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 in roots, and promoted Na+ retrieval from shoot to root under salt stress. Split-root experimental analysis of rubidium (Rb+) uptake and circulation indicated that OsHAK18-OX promoted Rb+ translocation from shoot to root. In addition, OsHAK18-KO increased while OsHAK18-OX reduced soluble sugar content in the shoot and affected oppositely the sugar concentration in the phloem and its content in the root. Moreover, OsHAK18-OX increased dramatically grain yield and physiological K+ utilization efficiency. Taken together, our results suggest that - unlike other OsHAKs analyzed heretofore - OsHAK18 is critical to K+ and Na+ re-circulation from shoot to root and enhances the source-to-sink translocation of a photo-assimilate. One sentence summaryRice potassium transporter OsHAK18 mediates shoot-to-root circulation of potassium and sodium and source-to-sink translocation of soluble sugar which improves potassium use efficiency and grain yield.