The bile acid sodium symporter SlBASS4 enhances tomato salt tolerance

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 <i>SlBASS4</i>, a chloroplast envelope-located transporter in tomato (<i>Solanum lycopersicum</i> L.), and explored its role in salt stress response. High salinity activated the <i>SlBASS4</i> gene, which in turn positively regulated tomato salt tolerance. Under salt stress, <i>SlBASS4</i> 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 <i>SlBASS4</i> 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 <i>SlBASS4</i> may sustain the content of D1 protein levels during salt stress. Furthermore, <i>SlBASS4</i> upregulated the expression of genes encoding sodium-potassium ion transporters. In conclusion, <i>SlBASS4</i> positively regulates salt tolerance in tomatoes via modulating ion homeostasis, accumulating osmolyte, and scavenging ROS.