The Recretohalophyte Tamarix TrSOS1 Gene Confers Enhanced Salt Tolerance to Transgenic Hairy Root Composite Cotton Seedlings Exhibiting Virus-Induced Gene Silencing of GhSOS1

Abstract

The <i>salt overly sensitive 1</i> (<i>SOS1</i>) gene encodes the plasma membrane Na⁺/H⁺ antiporter, SOS1, that is mainly responsible for extruding Na⁺ from the cytoplasm and reducing the Na⁺ content in plants under salt stress and is considered a vital determinant in conferring salt tolerance to the plant. However, studies on the salt tolerance function of the <i>TrSOS1</i> gene of recretohalophytes, such as <i>Tamarix</i>, are limited. In this work, the effects of salt stress on cotton seedlings transformed with tobacco-rattle-virus-based virus-induced gene silencing (VIGS) of the endogenous <i>GhSOS1</i> gene, or <i>Agrobacterium rhizogenes</i> strain K599-mediated <i>TrSOS1</i>-transgenic hairy root composite cotton plants exhibiting VIGS of <i>GhSOS1</i> were first investigated. Then, with <i>Arabidopsis thaliana AtSOS1</i> as a reference, differences in the complementation effect of <i>TrSOS1</i> or <i>GhSOS1</i> in a yeast mutant were compared under salt treatment. Results showed that compared to empty-vector-transformed plants, <i>GhSOS1</i>-VIGS-transformed cotton plants were more sensitive to salt stress and had reduced growth, insufficient root vigor, and increased Na⁺ content and Na⁺/K⁺ ratio in roots, stems, and leaves. Overexpression of <i>TrSOS1</i> enhanced the salt tolerance of hairy root composite cotton seedlings exhibiting <i>GhSOS1</i>-VIGS by maintaining higher root vigor and leaf relative water content (RWC), and lower Na⁺ content and Na⁺/K⁺ ratio in roots, stems, and leaves. Transformations of <i>TrSOS1</i>, <i>GhSOS1</i>, or <i>AtSOS1</i> into yeast NHA1 (Na⁺/H⁺ antiporter 1) mutant reduced cellular Na⁺ content and Na⁺/K⁺ ratio, increased K⁺ level under salt stress, and had good growth complementation in saline conditions. In particular, the ability of <i>TrSOS1</i> or <i>GhSOS1</i> to complement the yeast mutant was better than that of <i>AtSOS1</i>. This may indicate that <i>TrSOS1</i> is an effective substitute and confers enhanced salt tolerance to transgenic hairy root composite cotton seedlings, and even the <i>SOS1</i> gene from salt-tolerant <i>Tamarix</i> or cotton may have higher efficiency than salt-sensitive <i>Arabidopsis</i> in regulating Na⁺ efflux, maintaining Na⁺ and K⁺ homeostasis, and therefore contributing to stronger salt tolerance.