Abstract
Flowering is triggered by the transmission of a mobile protein, FLOWERING LOCUS T (FT), from leaves to the shoot apex. FT originates in the phloem of leaf veins. However, the identity of the FT-synthesizing cells in the phloem is not known. As a result, it has not been possible to determine whether the complex regulatory networks that control FT synthesis involve intercellular communication, as is the case in many aspects of plant development. We demonstrate here that FT in Arabidopsis thaliana and FT orthologs in Maryland Mammoth tobacco (Nicotiana tabacum) are produced in two unique files of phloem companion cells. These FT-activating cells, visualized by fluorescent proteins, also activate the GALACTINOL SYNTHASE (CmGAS1) promoter from melon (Cucumis melo). Ablating the cells by expression of the diphtheria toxin gene driven by the CmGAS1 promoter delays flowering in both Arabidopsis and Maryland Mammoth tobacco. In Arabidopsis, toxin expression reduces expression of FT and flowering-associated genes downstream, but not upstream, of FT Our results indicate that specific companion cells mediate the essential flowering function. Since the identified cells are present in the minor veins of two unrelated dicotyledonous species, this may be a widespread phenomenon.