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Abstract

Correct timing of flowering is a crucial factor for reproductive success in plants, and the time at which the shoot apical meristem (SAM) switches fate from vegetative to reproductive growth is tightly regulated by intrinsic and extrinsic factors. An important environmental input is day length. More than 70 years ago grafting experiments provided evidence that plants produce in their leaves a signal that translocates to the SAM to induce flowering. The nature of this signal, termed “florigen”, has long eluded discovery. Recently, studies in Arabidopsis have shown that two genes, CONSTANS (CO) and FLOWERING LOCUS T (FT), that are predominately expressed in leaves, are central for the plant to respond to inductive photoperiod. It was further shown that FT is the primary transcriptional target of CO in the leaf and that FT mRNA can be detected at the SAM after heat shock induced expression in the leaves. However, whether movement of the FT mRNA is necessary and / or sufficient to induce flowering is unclear. Here we show that expression of a functional artificial miRNA targeting FT-mRNA (amiR-FT) under control of the FD- (SAM-specific) promotor does not suppress flowering. In contrast, plants expressing amiR-FT from either the 35S- or the SUC2-promotor mimic the late-flowering of the ft mutant phenotype. Together, these results suggest that FT mRNA is required in the vasculature but might not be necessary at the apex to induce flowering. To assess the role of FT protein movement during floral transition, we developed a novel assay in which a large FT fusion protein is retained in the tissue where it is expressed. Upon exposure to a protease the mature FT protein is released and free to act. Using this assay we demonstrate that release of FT protein in the vasculature is sufficient to induce flowering. These results suggest an important role for the movement of FT protein in the induction of flowering by photoperiod.