Potassium transporter OsHAK18 mediates shoot-to-root circulation of potassium and sodium and source-to-sink translocation of soluble sugar in rice

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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 summary Rice 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.