Molecular cloning and functional characterization of two apple S-adenosylmethionine decarboxylase genes and their different expression in fruit development, cell growth and stress responses.

Two full-length S-adenosylmethionine decarboxylase (SAMDC) cDNAs, MdSAMDC1 and MdSAMDC2, were isolated from apple [Malus sylvestris (L.) Mill. var. domestica (Borkh.) Mansf.]. Both cDNAs encoded tiny and small ORFs in addition to the SAMDC ORFs, and genomic sequences of MdSAMDC1 and MdSAMDC2 contained two or three introns in the 5' upstream regions, respectively. Yeast complementation experiment indicated that two MdSAMDCs encoded functional proteins, and that the tiny and small ORFs possibly repressed their translation efficiency. RNA gel blot analysis showed that MdSAMDC1 were differentially regulated in fruits depending on the developmental stage and in cell suspension during the culture period, but MdSAMDC2 did not. In contrast, MdSAMDC2 was positively induced by cold and salt stresses, but MdSAMDC1 was not. These results suggest that MdSAMDC1 is mainly involved in fruit development and cell growth while MdSAMDC2 in stress responses, compared with their respective counterpart.