A Two-Staged Model of Na⁺ Exclusion in Rice Explained by 3D Modeling of HKT Transporters and Alternative Splicing

The HKT family of Na+ and Na+/K+ transporters is implicated in plant salinity tolerance. Amongst these transporters, the cereal HKT1;4 and HKT1;5 are responsible for Na+ exclusion from photosynthetic tissues, a key mechanism for plant salinity tolerance. It has been suggested that Na+ is retrieved from the xylem transpiration stream either in the root or the leaf sheath, protecting the leaf blades from excessive Na+ accumulation. However, direct evidence for this scenario is scarce. Comparative modeling and evaluation of rice (Oryza sativa) HKT-transporters based on the recent crystal structure of the bacterial TrkH K+ transporter allowed to reconcile transcriptomic and physiological data. For OsHKT1;5, both transcript abundance and protein structural features within the selectivity filter could control shoot Na+ accumulation in a range of rice varieties. For OsHKT1;4, alternative splicing of transcript and the anatomical complexity of the sheath needed to be taken into account. Thus, Na+ accumulation in a specific leaf blade seems to be regulated by abundance of a correctly spliced OsHKT1;4 transcript in a corresponding sheath. Overall, allelic variation of leaf blade Na+ accumulation can be explained by a complex interplay of gene transcription, alternative splicing and protein structure.HKT转运蛋白家族涵盖钠离子转运体与钠-钾共转运体，该家族与植物耐盐性密切相关。在该家族的转运体中，谷类作物的HKT1;4与HKT1;5负责将钠离子排出光合组织，这是植物耐盐性的核心调控机制之一。已有研究提出，钠离子可在根系或叶鞘处从木质部蒸腾流中被回收，从而保护叶片避免过量钠离子积累。但目前关于该假说的直接实验证据仍较为匮乏。基于近期解析的细菌TrkH钾离子转运蛋白晶体结构，研究人员对水稻（Oryza sativa）HKT转运体开展比较建模与功能评估，成功整合了转录组学与生理学实验数据。针对OsHKT1;5，在多个水稻品种中，其转录本丰度与选择性滤器区域的蛋白质结构特征均可调控地上部钠离子积累水平。而对于OsHKT1;4，则需要考虑其转录本的可变剪接以及叶鞘的解剖结构复杂性。由此可见，特定叶片中的钠离子积累水平，似乎由对应叶鞘中正确剪接的OsHKT1;4转录本丰度所调控。综上，叶片钠离子积累的等位基因变异，可通过基因转录、可变剪接与蛋白质结构三者间的复杂互作得以解释。