Salt stress, with Na+ being the most dominant harmful ion, is a significant environmental constraint on crop growth and yield worldwide. The plant Bile Acid Sodium Symporter (BASS) family encodes a class of sodium/solute symporters found on the chloroplast envelope. However, the role of BASS family members in tomato salt stress response is uncertain. We found SlBASS4, a chloroplast envelope-located transporter in tomato (Solanum lycopersicum L.), and explored its role in salt stress response. High salinity activated the SlBASS4 gene, which in turn positively regulated tomato salt tolerance. Under salt stress, SlBASS4 overexpression (OE) lines outperformed wild-type (WT) plants, with increased fresh weight, more chlorophyll and osmolyte, improved antioxidative enzyme activity, and lower reactive oxygen species (ROS) accumulation. In contrast, the performance of RNAi lines of SlBASS4 was the inverse. Following salt treatment, the chloroplasts of OE lines collected less Na+, protecting the photosynthetic apparatus from Na+ toxicity, but the photosynthetic apparatus of RNAi lines was damaged due to excess Na+. The western blot results indicated that SlBASS4 may sustain the content of D1 protein levels during salt stress. Furthermore, SlBASS4 upregulated the expression of genes encoding sodium–potassium ion transporters. In conclusion, SlBASS4 positively regulates salt tolerance in tomatoes via modulating ion homeostasis, accumulating osmolyte, and scavenging ROS.

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