ApHKT1 confers salinity tolerance in Apocynum by restraining the intake of Na+/K+in root tissues

Apocynum, exhibiting tolerance to severe salt stress, provides important materials for textile, medicine and biofuels. However, the underlying mechanism of Apocynum adapting to salt stresses remains poorly understood. In this study, physiological methods combined with bioinformatics tools were employed to insight the mechanism of Apocynum responding to salt stresses. Three representative species, A. hendersonii, A. venetum and tetraploid of A. venetum and NaCl treatments ranges from 50 mM to 400 mM were compared to reveal a comprehensive profile of the plants under salt stresses. As a crucial phenotypic characteristic for selecting plants that exhibit resilience to salinity and drought stress, the root-to-shoot ration of Apocynum was increased significantly while the growth of Apocynum was markedly inhibited as the degree of salt stress intensifying. The stomatal apertures on the leaf epidermis of Apocynum significantly narrowed in response to salt stress, and the chlorophyll content exhibited an overall declining trend. Salt stress notably elevated the Na⁺ and K⁺ content in the roots, stems, and leaves of Apocynum, with a significant decrease in the K⁺/Na⁺ ratio, while A. hendersonii showing the greatest change in this ratio. Phenotypic analysis indicated that A. hendersonii possessed the strongest salt tolerance among the species. ApHKT1, the highly conservative protein in the three species which primarily expressed in roots were hypothesized to adapt to salt stress by regulating the transportation of Na⁺ and K⁺. Although the content of Na⁺ and K⁺were increased in stem and leaf, there was no significant accumulation of Na⁺ and K⁺in root tissues. Expression pattern analysis found that ApHKT1 were significantly down-regulated under the rising salt stress in the root. These results suggested that Apocynum mainly take the strategy of reducing of ApHKT1 expression and the Na⁺/K⁺ intake to maintain the ion balance under salt stress.