CoachThe signal the leaf exports was named florigen in 1937 and identified seventy years later as a small protein, the product of the gene FLOWERING LOCUS T (FT). In the leaf, the clock and phytochrome together switch on FT only when the day length is right (in Arabidopsis, a long-day plant, when light coincides with the afternoon peak of the clock-controlled activator CONSTANS); the FT protein enters the phloem, travels with the sugar stream to the shoot apex, and there, with a partner protein, switches on the genes that convert the meristem. The same protein is the florigen of rice, a short-day plant, in which the clock wires it the other way round. Grafting experiments, the transport of the protein in the phloem, and the flowering of plants in which FT is expressed from a leaf-specific promoter all agree: the flower is decided in the leaf and executed in the bud.
