Several indications implied that the Arabidopsis FT ( FLOW-ERING LOCUS T) gene provides a possible functional link between the systemic pathways and the cell-autonomous pathways to flowering. FT is a major integrator of the genetic pathways to flowering in short and long days (4, 5); it encodes a signaling factor (6, 7) and is not expressed in the SAM proper (8) but can be detected, upon induction, in shoot apices (SAPs) containing young leaves (9). Flowering is delayed in mutant ft plants (10, 11), and when FT is overexpressed, flowering occurs earlier with a determinate inflorescence (12, 13). FT is regulated by the flowering-time gene CONSTANS in both long-and short-day plants (14,15), and grafting experiments in Arabidopsis have shown that systemic induction of flowering by CONSTANS is most likely mediated by FT (16,17). It was recently shown that a small fraction of heat-shock-induced FT RNA, originating in a single leaf, is found in the SAPs, suggesting that the FT mRNA itself may represent a major component of florigen (18).We chose tomato, a photoperiod-insensitive plant, to test the premise that orthologs of the Arabidopsis FT gene can initiate a conserved, long-distance, flower-promoting pathway in diverse flowering systems. The generality of the florigen hypothesis was supported by interspecies grafting experiments (2). Grafting results are independent of the validity of promoters, the resolution of in situ hybridization patterns, inferences derived from the activation of upstream genes, or interpretations of clonal analysis. The perennial habit; the compound shoots, which permit the analysis of multiple vegetative͞floral transition events in one plant (19); and the ease of grafting render tomato as a useful experimental platform for investigating the nature of florigen. We expanded the analysis in tomato with parallel experiments in long-day Arabidopsis and short-day tobacco.The primary shoot of tomato is terminated by an inflorescence, after which the apparent main axis consists of an upright array of reiterated axillary branches called sympodial units (SUs). Each SU arises from the most proximal axillary bud of the preceding unit and consists of three vegetative nodes and a terminal inflorescence (Fig. 1A). The distinction between the primary and compound sections of sympodial plants provides two basic criteria for flowering time: the number of leaves to the first inflorescence in the primary shoot and the number of leaves between inflorescences in the compound part. Here, we identify the tomato FT ortholog as SINGLE FLOWER TRUSS (SFT), a gene regulating primary shoot flowering time, sympodial habit, and flower morphology. All aspects of the sft phenotype were complemented by graft-transmissible SFT signals, suggesting that all are the consequence of a common flowering-time defect. Significantly, graft-transmissible SFT signals substituted for light dose and two inductive photoperiodic stimuli in different species as well. ResultsThe Tomato Ortholog of FT Is Disrupted in Late-Flowering sft Mutants....

The florigen paradigm implies a universal flowering-inducing hormone that is common to all flowering plants. Recent

SummaryCONSTANS-Like (COL) proteins are plant-specific nuclear regulators of gene expression but do not contain a known DNA-binding motif. We tested whether a common DNA-binding protein can deliver these proteins to specific cis-acting elements. We screened for proteins that interact with two members of a subgroup of COL proteins. These COL proteins were Tomato COL1 (TCOL1), which does not seem to be involved in the control of flowering time, and the Arabidopsis thaliana CONSTANS (AtCO) protein which mediates photoperiodic induction of flowering. We show that the C-terminal plant-specific CCT (CO, CO-like, TIMING OF CAB EXPRESSION 1) domain of both proteins binds the trimeric CCAAT binding factor (CBF) via its HAP5/NF-YC component. Chromatin immunoprecipitation demonstrated that TCOL is recruited to the CCAAT motifs of the yeast CYC1 and HEM1 promoters by HAP5. In Arabidopsis, each of the three CBF components is encoded by several different genes that are highly transcribed. Under warm long days, high levels of expression of a tomato HAP5 (THAP5a) gene can reduce the flowering time of Arabidopsis. A mutation in the CCT domain of TCOL1 disrupts the interaction with THAP5 and the analogous mutation in AtCO impairs its function and delays flowering. CBFs are therefore likely to recruit COL proteins to their DNA target motifs in planta.