Functional Studies of Japanese Apricot DAM Genes

Bud dormancy allows plants to adapt to adverse environmental condition and synchronize their developmental phase. Bud dormancy of woody perennials consists of three different stages; paradormancy, endodormancy, and ecodormancy. Endodormant buds differ from the other types of dormant buds in that they cannot resume growth under favorable conditions, which suggests the possible existence of internal factors inhibiting bud outgrowth. Because endodormant buds require a certain amount of chilling accumulation for dormancy release, the genes showing chilling-mediated differential expression would be the candidates for the internal factors. To search for the candidates, we performed RNA subtraction and 454-pyrosequencing analyses to examine gene expression patterns. Among the genes up-regulated preferentially in endodormant buds, we identified a Japanese apricot (Prunus mume) MADS-box gene, DORMANCY-ASSOCIATED MADS6 (PmDAM6). Seasonal expression analysis and cold treatment indicated that PmDAM6 was down-regulated when the buds are exposed to certain amounts chilling hours that varied with the cultivars. Negative correlation was found between the PmDAM6 expression and endodormancy release. To clarify the biological function of PmDAM6, we performed transgenic studies with poplar plants and found that growth of 35S:PmDAM6 poplar plants was retarded. Moreover, growth cessation and terminal bud set were observed in 35S:PmDAM6 poplars under environmental conditions in which wild-type poplars continued shoot tip growth. These results collectively suggest that PmDAM6 may participate in signaling pathways leading to growth cessation and bud set. We discuss the possible functions of PmDAM6 in maintaining bud dormancy and preventing the bud outgrowth.