Spatial specificity of MADS-box transcription factors in floral organs

HighlightMADS-box transcription factors specify the identity of floral organs at the primordia stage, but they also have late and spatially-restricted roles in developing or mature floral organs, which we explore in this review. Floral homeotic genes, most of them encoding MADS-box transcription factors, are classically pictured as early specifiers of floral organ identity, as implied by the famous ABC model. Yet, floral homeotic genes remain expressed throughout floral organ development, sometimes with a cell type-specific expression pattern, and late functions have been firmly established for some of these genes. Here, we review how floral homeotic MADS-box genes are expressed in floral organs during their development, highlighting general trends for their spatial or temporal-specific expression, in particular for B-class genes in petals that are systematically distally-enriched. We focus on chosen examples from the literature to discuss the different roles associated with this specific expression, and we then explore the possible molecular mechanisms by which MADS-box transcription factors can adopt spatially or temporally restricted functions in floral organs, either by a simple restriction of their presence, or by a different mode of action in given cell types. Altogether, this review highlights that late roles of floral MADS-box transcription factors have been largely unexplored, and that these regulators might have multiple different functions according to the cell types in which they are present.