Epigenetic aspects of floral homeotic genes in relation to sexual dimorphism in the dioecious plant Mercurialis annua.

In plants, dioecy characterizes species carrying male and female flowers on separate plants and occurs in about 6% of angiosperms; yet, the molecular mechanism(s) underlying dioecy is essentially unknown. The ability for sex-reversal by hormone application raised the hypothesis that the genes required for the expression of both sexes are potentially functional but regulated by epigenetic means. The proteomic analysis of nuclear proteins isolated from flower buds of females, males and feminized males of the dioecious plant Mercurialis annua showed differentially expressed nuclear proteins implicated in chromatin structure and function including floral homeotic proteins. Focusing on floral genes we showed that class B genes are mainly expressed in male flowers, while class D genes, as well as SUPERMAN-like genes, were mainly expressed in female flowers. Cytokinin-induced feminization of male plants was associated with down-regulation of male-specific genes concomitantly with up-regulation of female-specific genes. No correlation was found between the expression of class B and D genes and the changes in DNA methylation or chromatin conformation of these genes. Thus, we could not confirm DNA methylation or chromatin conformation of floral genes as the major determinant regulating sexual dimorphisms. Rather, determination of sex in M. annua, might be controlled through upstream of floral genes by sex-specific factor(s) that affect hormonal homeostasis. A comprehensive model is proposed for sex-determination in M. annua.