Isolation of flowering genes and seasonal changes in their transcript levels related to flower induction and initiation in apple (Malus domestica).

Flower development in apple (Malus domestica Borkh.) extends over two consecutive seasons. During the first season, most shoot apical meristems change to reproductive growth and initiate flowers. After winter dormancy, flower development continues during the second season and ends with anthesis in the spring. To determine the beginning of the transition to reproductive growth at the molecular level and to identify genes involved in this critical phase of flower development, we examined transcript levels of the putative flowering genes MdCOL1, MdCOL2, MdFT, MdSOC1, MdMADS2, MdMADS5, MdTFL1-1 and MdTFL1-2 in vegetative terminal buds of the apple cultivar Pinova during the first season by quantitative real-time PCR. Transcript levels of these genes peaked at the end of April during blooming of coexisting floral buds. Subsequently, there was a large increase in transcription, which started on May 22 for AFL2 and MdMADS2, followed by MdFT and AFL1 one week later. We propose that the increased transcription at the end of May marks the beginning of flower induction. Transcript levels of MdSOC1, MdTFL1-1 and MdTFL1-2 increased at the end of June, suggesting that these genes are involved in flower initiation, which follows flower induction. In contrast, MdMADS5 transcription was too weak to be quantified, and the transcript levels of MdCOL1 and MdCOL2 showed no detectable trends during the study.