OsHAK4 functions in retrieving sodium from the phloem at the reproductive stage of rice

Soil salinity significantly limits rice productivity, but it is poorly understood how excess sodium (Na⁺) is delivered to the grains at the reproductive stage. Here, we functionally characterized OsHAK4, a member of the clade IV HAK/KUP/KT transporter subfamily in rice. OsHAK4 was localized to the plasma membrane and exhibited influx transport activity for Na⁺, but not for K⁺. Analysis of organ- and growth stage-dependent expression patterns showed that very low expression levels of OsHAK4 were detected at the vegetative growth stage, but its high expression in uppermost node I, peduncle, and rachis was found at the reproductive stage. Immunostaining indicated OsHAK4 localization in the phloem region of node I, peduncle, and rachis. Knockout of OsHAK4 did not affect the growth and Na⁺ accumulation at the vegetative stage. However, at the reproductive stage, the hak4 mutants accumulated higher Na⁺ in the peduncle, rachis, husk, and brown rice compared to the wild-type rice. Element imaging revealed higher Na⁺ accumulation at the phloem region of the peduncle in the mutants. These results indicate that OsHAK4 plays a crucial role in retrieving Na⁺ from the phloem in the upper nodes, peduncle, and rachis, thereby preventing Na⁺ distribution to the grains at the reproductive stage of rice.