Moderate Water Deficit Improves Tomato Fruit Nutritional Quality Through SlNAP2‐Mediated Transcriptional Activation of Lycopene and Ascorbic Acid Biosynthesis Genes

Moderate water deficit is an effective strategy to improve fruit nutritional quality, yet the underlying molecular mechanism remains largely elusive, limiting its application in high-quality crop cultivation. Here, we report that moderate water deficit simultaneously elevates the contents of α-carotene, β-carotene, lutein, lycopene, soluble solids, soluble protein, and ascorbic acid in tomato (Solanum lycopersicum L.) fruits, with no significant reduction in fruit yield under our experimental conditions. Through transcriptome analysis and RT-qPCR, we identified SlNAP2 as a central transcription factor regulating this process. Moderate water deficit upregulates SlNAP2 expression. Knockout of SlNAP2 significantly reduces fruit nutritional quality, whereas its overexpression enhances it under moderate water deficit. Importantly, moderate water deficit promotes the improvement of tomato fruit nutritional quality in a SlNAP2-dependent manner. We further demonstrate that SlNAP2 directly binds to and activates the promoters of key lycopene biosynthesis genes (SlPSY2, SlPDS, SlZDS) and ascorbic acid biosynthesis genes (SlGMP, SlGGP), and that moderate water deficit enhances this transcriptional activation. Functional analyses further support the role of these five genes as downstream effectors that mediate SlNAP2-dependent lycopene and ascorbic acid accumulation. Collectively, our findings reveal that moderate water deficit enhances tomato fruit nutritional quality by boosting SlNAP2-mediated transcriptional activation of lycopene and ascorbate biosynthesis, establishing SlNAP2 as a central regulator linking water deficit to fruit quality and providing mechanistic insights into stress-promoted metabolite accumulation.