The amino acid permease <i>SlAAP6</i> contributes to tomato growth and salt tolerance by mediating branched-chain amino acid transport

Branched-chain amino acids (BCAAs) are essential amino acids in tomato (<i>Solanum lycopersicum</i>) required for protein synthesis, which also modulate growth and abiotic stress responses. To date, little is known about their uptake and transport in tomato especially under abiotic stress. Here, the tomato <i>amino acid permease 6</i> (<i>SlAAP6</i>) gene was identified as an amino acid transporter that restored mutant yeast cell growth on media with a variety of amino acids, including BCAAs. Overexpression of <i>SlAAP6</i> (<i>SlAAP6-OE</i>) in tomato raised the BCAA content and elevated the fresh weight, while <i>SlAAP6</i> knockouts (<i>slaap6</i>) showed reduced levels of neutral and basic amino acids in seedling tissues and lower total free amino acid distribution to shoots. In comparison to wild type and <i>slaap6</i> mutants, <i>SlAAP6-OE</i> alleviated root limited growth by elevated BCAA transport and upregulated the expression of root-growth-related genes by increasing BCAAs <i>in vivo</i>. As SlAAP6 serves as a positive regulator for BCAA abundance, <i>SlAAP6-OE</i> lines showed greater salinity tolerance, while <i>slaap6</i> mutants exhibited increased salt sensitivity. The salt tolerance of <i>SlAAP6-OE</i> plants was further enhanced by the application of exogenous BCAAs. In addition, BCAA supplementation reduced the accumulation of H₂O₂ in root under salt stress conditions. Based on these findings, SlAAP6-mediated uptake and transport of BCAAs facilitated growth and salt tolerance in tomato. By characterizing this key amino acid transporter, this study provides a novel approach to simultaneously enhance tomato nutritional quality, growth and development, and stress resistance through genetic improvement.