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Apple Malus × domestica Borkh. has a long period of juvenility during which plants are unable to produce flowers and fruits. Flowering is only possible on trees which have passed the transition from juvenility to the adult stage. Several genes have been identified which control this switch and promote or delay flowering in different plant species. FLOWERING LOCUS T (FT) and TERMINAL FLOWER 1 (TFL1) are two flowering genes of Arabidopsis which are similar, but act antagonistically. FT and TFL1 homologous genes are present in the apple genome, but little is known about their function. Therefore we produced transgenic apple lines which over-expressed the MdFT2 gene under the control of the constitutive CaMV 35S promoter. Transgenic shoots expressing the MdFT2 gene driven by the CaMV 35S promoter flowered in vitro. To study the function of MdTFL1 we silenced the gene by expressing an MdTFL1 specific hairpin under the control of the CaMV 35S promoter. Transgenic plants of different apple cultivars flowered in vitro and later in the greenhouse. Transgenic flowers were evaluated for phenotypic differences and tested on their usability for crossbred breeding programs.

Meristems in apple have to pass the vegetative and the reproductive phase change before they can produce flowers. In the previous ten years several genes of apple were identified which are homologous/orthologous to flowering genes of Arabidopsis thaliana. In relation to results obtained in Arabidopsis, the apple genes MdCOL1, MdCOL2, MdFT, MdSOC1, AFL1, AFL2, MdMADS2, MdMADS5, MdTFL1-1 and MdTFL1-2 are potential candidates to be involved in the vegetative and/or reproductive phase change. Gene transcript analyses were performed in leaves of juvenile apple seedlings from a ‘Braeburn’ × ‘Pisaxa’ progeny to evaluate the role of these genes in the vegetative phase change. These analyses revealed an increase of MdFT, AFL1, AFL2 and MdTFL1 mRNA expression in leaves related to the ascending node number suggesting a potential function in the vegetative transition terminated between the 70th and 80th node. Furthermore, we studied seasonal changes of gene transcription in adult apple tree meristems and we showed that activation of AFL1/2, MdMADS2, MdFT, MdSOC1 and suppression of MdTFL1-1/1-2 are involved in the reproductive phase change occurring at the end of May. We also over-expressed MdFT, a homologue of the predicted florigen FT from A. thaliana, in order to determine the function of MdFT. Overexpression of MdFT in Arabidopsis accelerated flowering, whereas flowers were not detected in MdFT transgenic apple lines growing in vitro and for the first year in greenhouse.