Abstract

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.