Expression analysis of apple DORMANCY-ASSOCIATED MADS-box genes in buds during dormancy release

Most temperate fruit tree species enter dormancy in autumn and are released from dormancy after being exposed to a specific period of chilling. In peach (Prunus persica) and Japanese apricot (Prunus mume) trees, DORMANCY-ASSOCIATED MADS-box (DAM) genes may be key factors controlling chilling requirements. However, the factors regulating dormancy and chilling requirements in apple (Malus x domestica) have not been fully characterized. Among 11 SVP-like genes in the apple genome, four (i.e., MDP0000322567, MDP0000259294, MDP0000527190, and MDP0000255146) are closely related to Prunus species DAM genes. MDP0000322567 (MdDAM1) and MDP0000259294 (MdDAM2) were highly expressed in dormant buds of 'Fuji', and partial cDNA fragments for these genes were cloned and sequenced. The MdDAM1 and MdDAM2 expression levels in dormant 'Fuji' apple buds collected monthly in Kyoto, Japan, were analyzed by quantitative reverse transcription polymerase chain reaction. Our data indicated that MdDAM1 was stably expressed at similar levels from December to February, but was up-regulated in terminal buds during dormancy release. In contrast, MdDAM2 expression was down-regulated during the same period. These results suggest that apple MdDAM2 expression is inversely correlated with dormancy release, similar to the expression of some Prunus species DAM genes.