Characterization ofFLOWERING LOCUS CHomologs in Apple as a Model for Fruit Trees

To elucidate the molecular mechanism of juvenility and annual flowering of fruit trees,FLOWERING LOCUS C(FLC), 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 appleFLChomologsMdFLC. 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 theMdFLC1splice variants and MdFLC3 contain the four conserved motives of an MIKC-type MADS protein. The mRNA of variantsMdFLC1aandMdFLC1bcontain intron sequences, and their deduced amino acid sequences lack K- and C-domains. The expression levels ofMdFLC1a,MdFLC1b, andMdFLC1cdecreased during the flowering induction period in a seasonal expression pattern in the adult trees, whereas the expression level ofMdFLC3did not decrease during that period. This suggests thatMdFLC1is 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 ofMdFLC1b,MdFLC1c, andMdFLC3were high during the juvenile phase and low during the transitional and adult phases. Because the expression pattern ofMdFLC3suggests that it plays a specific role in juvenility,MdFLC3was subjected to functional analysis by transformation ofArabidopsis. The results revealed the function ofMdFLC3as a floral repressor. In addition,MdFThad 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.