Overexpression of a plasmalemma Na/H antiporter from the halophyte Nitraria sibirica enhances the salt tolerance of transgenic poplar.

The plasmalemma Na/H antiporter Salt Overly Sensitive 1 (SOS1) is responsible for the efflux of Na from the cytoplasm, an important determinant of salt resistance in plants. In this study, an ortholog of SOS1, referred to as NsSOS1, was cloned from Nitraria sibirica, a typical halophyte that grows in deserts and saline-alkaline land, and its expression and function in regulating the salt tolerance of forest trees were evaluated. The expression level of NsSOS1 was higher in leaves than in roots and stems of N. sibirica, and its expression was upregulated under salt stress. Histochemical staining showed that β-glucuronidase (GUS) driven by the NsSOS1 promoter was strongly induced by abiotic stresses and phytohormones including salt, drought, low temperature, gibberellin, and methyl jasmonate, suggesting that NsSOS1 is involved in the regulation of multiple signaling pathways. Transgenic 84 K poplar (Populus alba × P. glandulosa) overexpressing NsSOS1 showed improvements in survival rate, root biomass, plant height, relative water levels, chlorophyll and proline levels, and antioxidant enzyme activities versus non-transgenic poplar (NT) under salt stress. Transgenic poplars accumulated less Na and more K in roots, stems, and leaves, which had a lower Na/K ratio compared to NT under salt stress. These results indicate that NsSOS1-mediated Na efflux confers salt tolerance to transgenic poplars, which show more efficient photosynthesis, better scavenging of reactive oxygen species, and improved osmotic adjustment under salt stress. Transcriptome analysis of transgenic poplars confirmed that NsSOS1 not only mediates Na efflux but is also involved in the regulation of multiple metabolic pathways. The results provide insight into the regulatory mechanisms of NsSOS1 and suggest that it could be used to improve the salt tolerance of forest trees.