Divergent expression of six expansin genes during apple fruit ontogeny

Six cDNAs encoding expansins were isolated from cultured shoots (MdExp1), the flesh of ripening fruit (MdExp2, MdExp3, MdExp4), and fruit pedicel tissue (MdExp5, MdExp6) of apple (Malus x domestica Borkh.). We report here the expression characteristics of these MdExp genes during fruit ontogeny. MdExp1 was expressed in an up-down-up cycle from full-bloom stage to just before ripening. The expression of MdExp2 and MdExp5 was initiated when cell enlargement was started after the cell proliferation stage; the expression of MdExp5 was limited to the early rapid growth period. The up-down-up cycles were also detected in the transcripts of MdExp3 and MdExp4 throughout both growth stages, but the expression of MdExp3 was increased in the ripening stage. Transcripts of MdExp6 were not detected in any growth stage, but were induced by wounding. These results suggest that apple fruit requires at least five expansin genes during ontogeny, and that MdExp3 is a ripening-related expansin gene.