Characterization of Homologs in Apple as a Model for Fruit Trees.

To elucidate the molecular mechanism of juvenility and annual flowering of fruit trees, (), an integrator of flowering signals, was investigated in apple as a model. We performed sequence and expression analyses and transgenic experiments related to juvenility with annual flowering to characterize the apple homologs . The phylogenetic tree analysis, which included other MADS-box genes, showed that both MdFLC1 and MdFLC3 belong to the same FLC group. MdFLC1c from one of the splice variants and MdFLC3 contain the four conserved motives of an MIKC-type MADS protein. The mRNA of variants and contain intron sequences, and their deduced amino acid sequences lack K- and C-domains. The expression levels of , , and decreased during the flowering induction period in a seasonal expression pattern in the adult trees, whereas the expression level of did not decrease during that period. This suggests that is involved in flowering induction in the annual growth cycle of adult trees. In apple seedlings, because phase change can be observed in individuals, seedlings can be used for analysis of expression during phase transition. The expression levels of , , and were high during the juvenile phase and low during the transitional and adult phases. Because the expression pattern of suggests that it plays a specific role in juvenility, was subjected to functional analysis by transformation of . The results revealed the function of as a floral repressor. In addition, had CArG box-like sequences, putative targets for the suppression of flowering by MdFLC binding, in the introns and promoter regions. These results indicate that apple homologs of FLC, which might play a role upstream of the flowering signals, could be involved in juvenility as well as in annual flowering. Apples with sufficient genome-related information are useful as a model for studying phenomena unique to woody plants such as juvenility and annual flowering.