Abstract：
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.
摘要：
土壤盐分显著限制了水稻的生产力,但我们对过量的钠（Na+）在生殖阶段是如何输送到谷物的知之甚少。这里,我们对水稻进化枝IVHAK/KUP/KT转运蛋白亚家族成员OsHAK4进行了功能表征。OsHAK4定位于质膜并表现出Na+的内流转运活性，但不是K+。对器官和生长阶段依赖性表达模式的分析表明，在营养生长阶段检测到非常低的OsHAK4表达水平，但是它在最上面的节点I中高度表达，花梗,Rachis是在生殖阶段被发现的.免疫染色显示OsHAK4位于淋巴结I的韧皮部区域，花梗,还有Rachis.OsHAK4的敲除在营养阶段不影响生长和Na积累。然而,在生殖阶段，hak4突变体在花梗中积累了更高的Na+，Rachis,外壳,和糙米相比，野生型水稻。元素成像显示，突变体在花梗韧皮部区域的Na积累较高。这些结果表明，OsHAK4在从上部结节的韧皮部中回收Na中起着至关重要的作用，花梗,还有Rachis,从而防止Na在水稻生殖阶段分配到谷物中。
