Overexpression of Arabidopsis FT gene in apple leads to perpetual flowering

Transgenic apple plants that overexpressed Arabidopsis FLOWERING LOCUS T (FT) under the control of the rolC promoter showed early flowering, while the introduction of FT driven by the 35S promoter induced flower development directly from transgenic apple callus in vitro, but vegetative growth was not maintained and the explant died. GFP-FT fusion proteins were detectable in transgenic apple tissues but never caused early flowering in the transformants. Under the control of the rolC promoter, the fused protein was localized in vascular tissues and fluorescence was detectable in companion cells in the stem and petiole. The transgenic apple lines that expressed AtFT driven by the rolC promoter showed differences in inflorescence architecture and floral organ number from those typical of nontransformed apple. Flowers of transgenic apple lines often contained more numerous petals, fewer stamens, and no pistils, and the pollen grains were incapable of germinating. The transgenic apple lines showed perpetual flowering without the requirement for low temperature and obvious photoperiodism. The expression patterns of six floral meristem identity genes and floral organ genes in flowers of transgenic apple lines were investigated. Among the floral meristem identity genes, expression of MdFT2 was suppressed and that of AFL2 was dramatically enhanced in the transformants. Of the floral organ genes, expression of MdMADS13, which functions as a class B gene, was unchanged from that of the flower of wild type one, whereas expression of MdMADS-NB, a possible class C gene, was suppressed.