Overexpression of BpMADS4 from silver birch (Betula pendula Roth.) induces early-flowering in apple (Malus x domestica Borkh.)

To shorten the juvenile stage of apple (Malus x domestica Borkh.) the BpMADS4 gene from silver birch (Betula pendula Roth.) was constitutively overexpressed in 25 transgenic apple clones. All clones were characterized by PCR, RT-PCR and Real Time PCR. Solitary flowers were produced on in vitro shoots of eight transgenic clones and most of them appeared to be morphologically normal. Twenty shoots of each clone were rooted and transferred to a glasshouse. Glasshouse plants of clones T1165, T1187 and T1190 developed flowers. Several plants of T1165 and T1187 started floral initiation within 3-4 months following transfer to the glasshouse. Primary flowers were solitary and in a terminal position on the main shoot. Lateral flower clusters, consisting of three to five individual flowers, were also found. Pollen vitality and tube germination of glasshouse-grown flowers were investigated, and there were no significant differences compared to pollen of non-transgenic control plants. Preliminary crosses using flowers of glasshouse plants resulted in small apple fruits. It would seem that this is the first report on in vitro flower induction in transgenic apple.