Three cDNAs with high homology to spermine (Spm) synthases in Arabidopsis were isolated from apple [Malus sylvestris (L.) Mill. var. domestica (Borkh.) Mansf.]. MdACL5-1 and MdACL5-2 have high homology with ACL5 and MdSPMS has high homology with AtSPMS. The similarity of MdSPMS to spermidine synthases (SPDSs) was higher than that of MdACL5s, despite the fact that both are putative Spm synthases. However, MdSPMS could be discriminated from SPDSs by the presence of several characteristic amino acids, i.e., Val-149, Ser-161, Ala-205, and Val-235, in the decarboxylated S-adenosylmethionine (dcSAM)-binding motif of MdSPMS. Both MdACL5-1 and MdSPMS complemented Spm biosynthesis in a yeast mutant deficient in Spm synthase, and ectopic expression of MdACL5-1 in the Arabidopsis dwarf mutant acl5 allowed recovery of the normal phenotype. RNA gel blot analysis showed that MdACL5 and MdSPMS are differentially expressed in tissues and suspension cells. These results suggest that functional MdACL5 and MdSPMS are independently involved in apple fruit development and cell growth.