Members of the tomato FRUITFULL MADS-box family regulate style abscission and fruit ripening

The tomato (Solanum lycopersicum) protein MADS-RIN plays important roles in fruit ripening. In this study, the functions of two homologous tomato proteins, FUL1 and FUL2, which contain conserved MIKC domains that typify plant MADS-box proteins, and which interact with MADS-RIN, were analysed. Transgenic functional analysis showed that FUL1 and FUL2 function redundantly in fruit ripening regulation, but exhibit distinct roles in the regulation of cellular differentiation and expansion. Over-expression of FUL2 in tomato resulted in a pointed tip at the blossom end of the fruit, together with a thinner pericarp, reduced stem diameter, and smaller leaves, but no obvious phenotypes resulted from FUL1 over-expression. Dual suppression of FUL1 and FUL2 substantially inhibited fruit ripening by blocking ethylene biosynthesis and decreasing carotenoid accumulation. In addition, the levels of transcript corresponding to ACC SYNTHASE2 (ACS2), which plays a key role in ethylene biosynthesis, were significantly decreased in the FUL1/FUL2 knock-down tomato fruits. Overall, our results suggest that FUL proteins can regulate tomato fruit ripening through fine-tuning ethylene biosynthesis and the expression of ripening-related genes.