The identity of floral organs in angiosperms is specified by multimeric transcription factor complexes composed of floral homeotic MADS-domain proteins that bind to specific cis-regulatory DNA-elements (‘CArG-boxes’) of their target genes, thus constituting floral quartets. Gymnosperms possess orthologues of floral homeotic genes enconding MIKC-type MADS-domain proteins, but when and how the interactions constituting floral quartets were established during evolution has remained unknown. To better understand the ‘abominable mystery’ of flower origin, in this project a comprehensive study was carried out to detect the dimerization and DNA-binding of several classes of MADS-domain proteins from a gymnosperm, Gnetum gnemon of the Gnetales. Determination of protein-protein interactions by pull-down assays revealed complex patterns of heterodimerization among orthologues of class B, class C and class E floral homeotic proteins and Bsister proteins, while homodimerization was not observed. In contrast, electrophoretic mobility shift assays (EMSAs) revealed that all proteins tested except one bind to CArG-boxes also as homodimers, suggesting that homodimerization is relatively weak, but facilitated by DNA-binding. Proteins able of DNA-based homodimerization include orthologues of class B and C proteins; B and C proteins also form heterodimers in vitro and in yeast, which is in sharp contrast to their orthologues from angiosperms, which require class E floral proteins to ‘glue’ them together in multimeric complexes. Remarkably, the heterodimers of B and C proteins from G. gnemon are not capable of binding to CArG-boxes, suggesting that DNA-binding in vivo is based on homodimers, while heterodimerization of B and C proteins may constitute multimeric, DNA-bound complexes by mediating the interaction between two DNA-bound homodimers. EMSAs and DNase I footprint assays indicated that both B with C proteins and C proteins alone but not B proteins alone can induce DNA-looping to form tetrameric protein-DNA complexes similar to floral quartets. These data suggest that at least some of the gymnosperm orthologues of floral homeotic proteins may have the capability of forming higher-order complexes and that gymnosperm B and C proteins control male organ identity and C proteins controls female organ identity, respectively, by forming quartet-like complexes composed of two homodimers, each bound to a CArG-box.