CRISPR/Cas9-mediated editing of SlALS1 enhances fruit sweetness in tomato

Fruit sweetness is a primary determinant of consumer preference and market value in tomato (Solanum lycopersicum), yet increasing sugar content without negatively affecting plant performance remains a major challenge in breeding programs. In this study, we targeted Acetolactate synthase 1 (SlALS1), a key enzyme in branched-chain amino acids (BCAAs) biosynthesis, to investigate whether modulation of primary metabolism could enhance fruit sugar accumulation. Tomato contains two ALS homologs, SlALS1 and SlALS2. Using CRISPR/Cas9-mediated genome editing, we generated SlALS1 knock-out lines in the Micro-Tom background. Loss of SlALS1 function led to a significant reduction in the levels of valine, leucine, and isoleucine in fruit tissues, confirming disruption of the BCAA biosynthetic pathway. Notably, soluble sugar content was markedly increased in the mutant lines, with glucose and fructose levels elevated by approximately 1.3-fold compared with wild-type fruits, resulting in a significantly higher Brix index. Metabolic profiling indicates that reduced BCAA biosynthesis may influence carbon allocation toward soluble sugar accumulation in fruit tissues, revealing an association between amino acid metabolism and carbohydrate accumulation during fruit development. Our findings demonstrate that precise editing of a central metabolic enzyme can enhance tomato fruit sweetness and suggest that SlALS1 may represent a promising target for genome editing aimed at improving fruit quality traits. However, further evaluation of agronomic traits, including plant growth and fruit yield, will be necessary to assess its potential for practical breeding applications.