Expression and function analyses of the MIKCC-type MADS-box genes in Akebia trifoliata (Lardizabalaceae) flower development

The evolutionary origin and diversification of flowers in angiosperms remain one of the great unsolved mysteries in plants, while the molecular genetics of floral organ development are the crucial aspect of this question. Here, the expression patterns of all 25 MIKC^C genes in various floral organs of Akebia trifoliata, including the stamens, carpels and sepals of both male and female flowers, were determined by qRT–PCR to preliminary explore the molecular mechanism of flowers development. The phenotypic characteristics of floral organs were also outlined. The results showed that A. trifoliata flowers were monoecious and unisexual. Neither female nor male flowers have petals, but petal-like sepals form the petaloid perianth, especially the curly and enlarged sepals in female flowers that compensate for the loss of petals. Multiple comparison analysis showed that 24 (96%) of the 25 MIKC^C genes exhibited significantly different expression levels between the leaves and floral organs. Further cluster analysis of the expression profiles showed that 11 genes were classified into a clade that mainly regulated the sepal development of male flowers, while the remaining 14 genes were classified into another clade that was associated with the development of other floral organs. These results suggested that many A. trifoliata MIKC^C genes had a broader expression boundary and weaker specificity than the classic ABC(D)E model and that both petaloidy of the perianth and dimorphism of the sepals could be related to rapid functional diversification and subfunctionalization.