The Cotyledons Produce Sufficient FT Protein to Induce Flowering: Evidence from Cotyledon Micrografting in Arabidopsis

In Arabidopsis, long-distance movement of FLOWERING LOCUS T (FT) protein from the leaf to the shoot apex triggers flower development. In wild-type Arabidopsis plants under long-day conditions, FT is mainly expressed in the cotyledon but is weakly expressed in the first true leaf prior to floral induction. To test the importance of the cotyledon in floral induction, we developed a cotyledon micrografting (Cot-grafting) method that, unlike other grafting methods, allows the FT protein from the graft to be transported via its native route from leaves to the shoot apex. By using Cot-grafting, we found that grafting a single wild-type cotyledon onto an ft-10 mutant strongly suppressed the ft-10 late flowering phenotype. Neither Y-grafting wild-type shoots nor butt-grafting wild-type roots to ft-10 plants resulted in comparably accelerated flowering in the ft-10 recipient plants. ft-10 mutants grafted with a 35S::FT cotyledon flowered as early as wild-type plants. When phloem-specific tracers were applied to a donor cotyledon, the tracers were detected in the vein of the true leaf of recipient plants 6 d after Cot-grafting. Also, macromolecule trafficking of an FT: yellow fluorescent protein: hemagglutinin fusion occurred across the graft junction 6 d after Cot-grafting. These results suggest that Cot-grafting, which allows protein movement in a manner consistent with the natural flow of FT protein from the leaf to the shoot apex, can efficiently suppress the late flowering of ft-10 mutants. Our results further suggest that in Arabidopsis, the cotyledon is an important organ for producing FT protein to induce flowering.