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.