Tandem <scp>MADS</scp> ‐Box Genes <scp>FUL2</scp> and <scp>MADS1</scp> Form a Regulatory Module to Repress Serotonin Biosynthesis via Direct <i>ASMT5</i> Activation in Tomato Fruit

The regulation of serotonin metabolism during fruit development and ripening remains poorly understood, despite its potential roles in plant defence and human nutrition. Here, we demonstrated that the MADS-box transcription factor FUL2 acts as a key repressor of serotonin accumulation in tomato by forming a functional module with MADS1. CRISPR-Cas9-generated ful2-cr mutants exhibited delayed ripening, reduced fruit size and a striking 10-fold increase in serotonin levels, suggesting a previously unrecognised link between FUL2 and secondary metabolism. Immunoprecipitation-mass spectrometry (IP-MS) revealed that FUL2 physically interacts with MADS1, and genetic analyses showed that mads1-cr mutants phenocopied both the developmental and serotonin hyperaccumulation phenotypes of ful2-cr mutants. Furthermore, ChIP-seq and transcriptomic profiling demonstrated that the FUL2-MADS1 complex directly binds CArG-box motifs in the promoter of ASMT5 (a key enzyme in serotonin-to-melatonin conversion), activating its expression while repressing TDC1 (tryptophan decarboxylase). Electrophoretic mobility shift assays (EMSA) and dual-luciferase reporter assays confirmed their cooperative DNA binding and synergistic transcriptional regulation. Our work establishes a MADS-box transcriptional module that gates serotonin flux by coordinately regulating biosynthetic and metabolic genes. These findings provided a framework for engineering serotonin content in crops and deepen understanding of how developmental transcription factors govern specialised metabolism during ripening.