Tomato flowering depends on overlapping functions of AP1/FUL-like genes in reproductive meristem specification

AP1/FUL-clade transcription factors (TFs) are essential for the initiation and regulation of flowering and have clearly separated functions in Arabidopsis. However, how these functions have diverged across eudicots remains unclear. Here, we performed a detailed analysis to unravel the distinct and overlapping functions of the tomato AP1-ortholog MACROCALYX (MC) and the FUL-like genes FRUITFULL2 (FUL2) and MADS-BOX PROTEIN 20 (MBP20) through integrated molecular, genetic, and genomic approaches. We find that AP1/FUL-like TFs redundantly regulate the floral transition in both the primary shoot and sympodial shoot. In the latter, loss of MC, FUL2, and MBP20 leads to extremely delayed flowering. In the floral and inflorescence meristem, MC is the major player, but FUL2 and MBP20 contribute as well, with a complete loss of reproductive identity in the inflorescence meristem of the triple mutant. The functional differences between the three genes can mainly be attributed to differences in expression level, as the DNA-binding properties of MC and FUL2 are highly similar. Only the TFL1-ortholog SP appears specifically regulated by MC. We reveal that the combined action of AP1/FUL-clade TFs is needed to acquire and retain reproductive activity in tomato, which is probably conserved in many other crops.