Chloride modulates carbohydrate metabolism and ethylene synthesis in tomato fruits

Chloride ions (Cl^-) play a unique role in enhancing crop fruit quality, particularly through their effects on carbohydrate metabolism, although the underlying mechanism remains unclear. We optimized the Cl^- concentrations in tomato cultivation nutrient solutions via supplementation at varying levels and utilized an integrative approach combining biochemical assays, transcriptomics, and metabonomics to investigate how Cl^- modulates sugar metabolism in tomato fruits. Treatment with 3-5 mm Cl^- significantly improved plant biomass, photosynthesis, dry matter accumulation, and fruit quality without affecting yield or altering the dry matter distribution. Our findings indicate that Cl^- increased starch and sucrose contents in fruits during the cell enlargement stage and elevated glucose, fructose, chloride ions, potassium ions, magnesium ions, and manganese ions during the red-ripening stage. Physiological and correlation analyses revealed that Cl^- significantly enhanced the activity of α-amylase, sucrose phosphate synthase, and sucrose synthase. Additionally, principal component analysis confirmed that the red-ripening stage is a critical phase for the chloride regulation of fruit quality. Transcriptomic and targeted metabonomic analyses of mature fruits revealed that Cl^- predominantly affected gene expression related to starch and sucrose metabolism and ATP-binding cassette (ABC) transporters, and specifically impacted ethylene synthesis. This study provides the first insight into the mechanism by which chloride affects carbohydrate metabolism and ethylene production in tomato fruits, and offers a comprehensive framework for understanding the functional roles of chloride in the interaction between these metabolic pathways.