Overexpression of a Plasma Membrane Bound Na ⁺/H ⁺ Antiporter-Like Protein ( SbNHXLP) Confers Salt Tolerance and Improves Fruit Yield in Tomato by Maintaining Ion Homeostasis

A Na ⁺/H ⁺ antiporter-like protein (NHXLP) was isolated from Sorghum bicolor L. ( SbNHXLP) and validated by overexpressing in tomato for salt tolerance. Homozygous T ₂ transgenic lines when evaluated for salt tolerance, accumulated low Na ⁺ and displayed enhanced salt tolerance compared to wild-type plants (WT). This is consistent with the amiloride binding assay of the protein. Transgenics exhibited higher accumulation of proline, K ⁺, Ca ²⁺, improved cambial conductivity, higher PSII, and antioxidative enzyme activities than WT. Fluorescence imaging results revealed lower Na ⁺ and higher Ca ²⁺ levels in transgenic roots. Co-immunoprecipitation experiments demonstrate that SbNHXLP interacts with a Solanum lycopersicum cation proton antiporter protein2 ( SlCHX2). qRT-PCR results showed upregulation of SbNHXLP and SlCHX2 upon treatment with 200 mM NaCl and 100 mM potassium nitrate. SlCHX2 is known to be involved in K ⁺ acquisition, and the interaction between these two proteins might help to accumulate more K ⁺ ions, and thus maintain ion homeostasis. These results strongly suggest that plasma membrane bound SbNHXLP involves in Na ⁺ exclusion, maintains ion homeostasis in transgenics in comparison with WT and alleviates NaCl stress.