Four full-length MADS-box cDNAs from chrysanthemum, designated Chrysanthemum Dendrathema grandiflorum MADS (CDM) 8, CDM41, CDM111, and CDM44, have been isolated and further functionally characterized. Protein sequence alignment and expression patterns of the corresponding genes suggest that CDM8 and CDM41 belong to the FRUITFULL (FUL) clade, CDM111 is a member of the APETALA1 (AP1) subfamily, and CDM44 is a member of the SEPALLATA3 (SEP3) subfamily of MADS-box transcription factors. Overexpression of CDM111 in Arabidopsis plants resulted in an aberrant phenotype that is reminiscent of the phenotype obtained by ectopic expression of the AP1 gene. In addition, CDM111 was able to partially complement the ap1-1 mutant from Arabidopsis, illustrating that CDM111 is the functional equivalent to AP1. Yeast two- and three-hybrid studies were performed to investigate the potential protein interactions and complexes in which these chrysanthemum MADS-box proteins are involved. Based on these studies, we conclude that CDM44 is most likely the SEP3 functional equivalent, because the CDM44 protein interacts with CDM proteins of the AP1/FUL and AG subfamilies, and as a higher order complex with the heterodimer between the presumed B-type CDM proteins.