Salt stress is one of the major environmental factors limiting crop productivity. Although physiological and molecular characterization of salt stress response in plants has been the focus for many years, research on transporters for sodium ion (Na+) uptake, translocation and accumulation in plants, particularly in food crops like rice is limited. In this study, we functionally identified an uncharacterized sodium ion transporter named OsNHAD which encodes a putative Na+ ⁄ H+ antiporter in rice. Homology search shows its close relation to the Arabidopsis Na+/H+ antiporter AtNHD1 with 72.74% identity of amino acids. OsNHAD transcripts mainly express in leaves and are induced by Na+ stress. Confocal laser scanning microscopy analysis of OsNHAD::GFP fusion in tobacco leaves shows that OsNHAD resides in the chloroplast envelop. Knock-down of OsNHAD by RNA interference led to increased rice sensitivity to Na+, manifested by stunted plant growth, enhanced cellular damage, reduced PSII activity and changed chloroplast morphology. Mutation of OsNHAD also resulted in accumulation of more Na+ in chloroplasts and in shoots as well, suggesting that OsNHAD is involved in mediating efflux and detoxification of Na+ but does not affect K+ accumulation in plant cells. Complementation test reveals that OsNHAD was able to functionally restore the Arabidopsis mutant atnhd1-1 growth phenotype. These results suggest that OsNHAD possibly mediates homeostasis of sodium ions in the subcellular compartments and tissues of the plants when challenged to salt stress.

中文翻译：

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OsNHAD 是水稻（Oryza sativa）抗盐胁迫所需的叶绿体膜转运蛋白

盐胁迫是限制作物生产力的主要环境因素之一。尽管多年来植物盐胁迫反应的生理和分子特征一直是关注的焦点，但对钠离子 (Na+) 吸收、转运和积累在植物中，特别是在水稻等粮食作物中的转运蛋白的研究是有限的。在这项研究中，我们在功能上鉴定了一种名为 OsNHAD 的未表征的钠离子转运蛋白，它编码水稻中推定的 Na+ ⁄ H+ 逆向转运蛋白。同源性搜索显示其与拟南芥 Na+/H+ 逆向转运蛋白 AtNHD1 密切相关，具有 72.74% 的氨基酸同一性。OsNHAD 转录物主要在叶片中表达并受 Na+ 胁迫诱导。对烟叶中 OsNHAD::GFP 融合的共聚焦激光扫描显微镜分析表明 OsNHAD 存在于叶绿体包膜中。通过 RNA 干扰敲除 OsNHAD 导致水稻对 Na+ 的敏感性增加，表现为植物生长受阻、细胞损伤增强、PSII 活性降低和叶绿体形态改变。OsNHAD 的突变还导致更多的 Na+ 在叶绿体和芽中积累，表明 OsNHAD 参与介导 Na+ 的外排和解毒，但不影响植物细胞中 K+ 的积累。互补测试表明 OsNHAD 能够在功能上恢复拟南芥突变体 atnhd1-1 生长表型。这些结果表明，当受到盐胁迫时，OsNHAD 可能介导植物亚细胞区室和组织中钠离子的稳态。降低 PSII 活性并改变叶绿体形态。OsNHAD 的突变还导致更多的 Na+ 在叶绿体和芽中积累，表明 OsNHAD 参与介导 Na+ 的外排和解毒，但不影响植物细胞中 K+ 的积累。互补测试表明 OsNHAD 能够在功能上恢复拟南芥突变体 atnhd1-1 生长表型。这些结果表明，当受到盐胁迫时，OsNHAD 可能介导植物亚细胞区室和组织中钠离子的稳态。降低 PSII 活性并改变叶绿体形态。OsNHAD 的突变还导致更多的 Na+ 在叶绿体和芽中积累，表明 OsNHAD 参与介导 Na+ 的外排和解毒，但不影响植物细胞中 K+ 的积累。互补测试表明 OsNHAD 能够在功能上恢复拟南芥突变体 atnhd1-1 生长表型。这些结果表明，当受到盐胁迫时，OsNHAD 可能介导植物亚细胞区室和组织中钠离子的稳态。表明 OsNHAD 参与介导 Na+ 的外排和解毒，但不影响植物细胞中 K+ 的积累。互补测试表明 OsNHAD 能够在功能上恢复拟南芥突变体 atnhd1-1 生长表型。这些结果表明，当受到盐胁迫时，OsNHAD 可能介导植物亚细胞区室和组织中钠离子的稳态。表明 OsNHAD 参与介导 Na+ 的外排和解毒，但不影响植物细胞中 K+ 的积累。互补测试表明 OsNHAD 能够在功能上恢复拟南芥突变体 atnhd1-1 生长表型。这些结果表明，当受到盐胁迫时，OsNHAD 可能介导植物亚细胞区室和组织中钠离子的稳态。