OsMADS1/OsLHS1: Diversified Regulatory Functions in Ensuring Transition and Completion of Sexual Reproduction in Rice

Abstract

Sexual reproduction is a highly adopted mode of propagation in higher plants. Monocot grass species develop fertile florets on the spikelet of their inflorescences (panicle). MADS-box containing SEPALLATA proteins, together with other transcription factors play crucial role during floral meristem specification, organogenesis and meristem determinacy. Unlike four largely
redundant SEP genes in Arabidopsis, five rice SEP genes display both redundant and non-redundant functions in controlling reproductive development. LEAFY HULL STERILE1/OsMADS1, member of a grass-specific LOFSEP clade of rice SEP gene family, is required for specification and development of a fertile floret on the spikelet meristem. OsMADS1 irreversibly promotes spikelet to floret transition by specifying floret meristem identity and repressing spikelet identity. It also suppresses reversion of floret meristem to shoot meristem fate. During later stages, while OsMADS1 maintains floret meristem functions, it also controls floret organ specification and differentiation and determinacy of floret meristem. These diverse functions of OsMADS1 are brought by its genetic and physical interactions with various other genetic regulators and forming higher order complexes at different developmental stages. Thus, all functional data in corroboration with estimated evolutionary divergence time scale of OsMADS1 sub-clade suggest that OsMADS1 is co-evolved with grasses and as a key regulator of rice sexual reproductive habit, it has not only retained its conserved functions but also has acquired some species-specific functions.