Genetic and spatial interactions between FT, TSF and SVP during the early stages of floral induction in Arabidopsis.

Flowering is controlled by a network of pathways that converge to regulate a small number of floral integrator genes. We studied the interactions in Arabidopsis between three of these integrators, flowering locus T (FT), twin sister of FT (TSF) and suppressor of overexpression of constans 1 (SOC1), as well as their repression by the MADS box transcription factor short vegetative phase (SVP). FT is a mobile signal transmitted from the leaf to the meristem to initiate flowering. Using mRNA null alleles, we show that FT and the closely related TSF are not essential for flowering, but that the double mutant is photoperiod-insensitive. Inactivation of both genes also fully suppresses the early-flowering phenotype caused by over-expression of constans (CO), a transcriptional regulator in the photoperiod pathway. In addition, we demonstrate that TSF and FT have similar biochemical functions by showing that they interact in yeast with the same bZIP transcription factors. Expression of FT or TSF from promoters specific for phloem companion cells drives early flowering of the double mutant, so no expression of either gene is required in the meristem. Furthermore, TSF, like FT, is repressed by SVP, but the triple mutant svp-41 ft-10 tsf-1 expresses SOC1 in the meristem sooner and flowers earlier than ft-10 tsf-1. Thus we distinguish the functions of SVP in repressing FT and TSF in the leaf and SOC1 in the meristem. In addition, a time course of in situ hybridizations suggested that repression of SVP and activation of SOC1 proceed simultaneously in the meristem. These observations clarify the relationships between these early regulators of the floral transition, and further emphasize the relatedness of mechanisms acting in the leaf and meristem to control flowering time.