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.

To break the juvenile stage of apple (Malus x domestica Borkh.) we transferred the LFY gene of Arabidopsis into the genome of the apple cv. 'Pinova'. A total of five transgenic clones constitutively overexpressing the LFY gene were obtained. Approximately, 20 shoots of each clone were rooted and transferred to the glasshouse. No flowers were obtained on transgenic plants during the first 2 years of cultivation. Evaluation of the expression of possible LFY targets revealed that no transcripts could be detected for MdAP1-1 and MdAP1-2. MdTFL1 was unaffected. Based on the absence of the LFY core-binding sequence within promoter sequences of MdAP1-1 and MdAP1-2, it was concluded that LFY was not able to induce these genes. The LFY genes of apple were unaffected in transgenic plants and sequence alignments of the C-terminal amino acid sequence showed a high conservation of these proteins. A change in binding ability to DNA can therefore be excluded. Instead of early flowering, the transgenic plants showed an altered phenotype, which is similar to the columnar phenotype of the 'McIntosh Wijcik' mutant of apple. The transgenic plants showed shortened internodes and a significantly reduced length of the regrowing shoot. A negative correlation was observed between the length of the regrowing shoot and the LFY mRNA transcript level. Furthermore, the LFY transgenic apple plants showed an increased shoot diameter at node 20, which was positively correlated with the LFY mRNA transcript level. Based on our results, we assume an alternative role of LFY in apple.

No abstract

No abstract

No abstract

No abstract

No abstract

No abstract