Universal florigenic signals triggered by FT homologues regulate growth and flowering cycles in perennial day-neutral tomato

The transition from vegetative to floral meristems in higher plants is programmed by the coincidence of internal and environmental signals. Classic grafting experiments have shown that leaves, in response to changing photoperiods, emit systemic signals, dubbed 'florigen', which induce flowering at the shoot apex. The florigen paradigm was conceived in photoperiod-sensitive plants: nevertheless it implies that although activated by different stimuli in different flowering systems, the signal is common to all plants. Tomato is a day-neutral, perennial plant, with sympodial and modular organization of its shoots and thus with reiterative regular vegetative/reproductive transitions. SINGLE FLOWER TRUSS a regulator of flowering-time and shoot architecture encodes the tomato orthologue of FT, a major flowering integrator gene in Arabidopsis. SFT generates graft-transmissible signals which complement the morphogenetic defects in sft plants, substitute for light dose stimulus in tomato and for contrasting day-length requirements in Arabidopsis and MARYLAND MAMMOTH tobacco. It is discussed how systemic signals initiated by SFT interact with the SELF PRUNING gene to regulate vegetative to reproductive (VIR) transitions in the context of two flowering systems, one for primary apices and the other for sympodial shoots.