Evolutionary studies of the bHLH transcription factors belonging to MBW complex: their role in seed development

Background and aims: The MBW complex consist of proteins belonging to three major families MYB, bHLH and WDR, involved in various processes throughout plant development: epidermal cell development, mucilage secretory cells and flavonoid biosynthesis. Recently, it has been reported that TT8, encoding a bHLH transcription factor, is involved in the biosynthesis of flavonoids in the seed coat and it also plays a role in bypassing the postzygotic barrier resulting from an unbalance in genetic loads of the parental lines. Here we focus on the functional evolution, in seed development, of the bHLH proteins that are part of the MBW complex complemented with a literature review. Methods: Phylogenetic analyses performed across seed plants, and expression analyses in the reproductive tissues of four selected angiosperms: Arabidopsis thaliana, Brassica napus, Capsella rubella and Solanum lycopersicum, allowing us to hypothesize on the evolution of its functions. Key results: TT8 expression in the innermost layer of the seed coat is conserved in the selected angiosperms. However, except for Arabidopsis, TT8 is also expressed in ovules, carpels and fruits. The homologs belonging to TT8's sister clade, EGL3/GL3, involved in trichome development, are expressed in the outermost layer of the seed coat, suggesting potential roles in mucilage. Conclusions: The ancestral function of these genes appears to be flavonoid biosynthesis and the conservation of TT8 expression patterns in the innermost layer of the seed coat in angiosperms suggests that their function in postzygotic barriers may also be conserved. Moreover, the literature review and the results of the present study suggest a sophisticated association, linking the mechanisms of action of these genes to the cross-communication activity between the different tissues of the seed. Thus, it provides avenues to study the mechanisms of action of TT8, in the postzygotic triploid block, which is crucial since it impacts seed development in unbalanced crosses.