The origin, evolution and diversification of MADS-box transcription factors in green plants.

MADS-box genes acted as one of the most important regulatory elements in eukaryotes and particularly diversified throughout the evolution of green plants (Viridiplantae). Although the evolution and function of MADS-box genes have been extensively studied, it remains incomplete regarding the picture of the origin and diversification of MADS-box genes in Viridiplantae. Here, we employed genomic data of 551 representative eukaryotes for systematical analyses on both sequences and protein structures, aiming to clarify the evolutionary footprint of MADS-box genes across green plant lineages. Our analyses suggested the polyphyletic origin of plant Type I genes, and the two subfamilies of plant Type II genes (i.e., MIKC^C and MIKC^∗ genes) have arisen from an ancient gene duplication in the stem group of streptophytes rather than in land plants. The sphere-like structures of MIKC^C proteins were uncovered in chlorophytes and charophytes, likely being an ancestral feature as physical constraint to tetramerization. The architectures of MIKC^C proteins largely evolved in land plants with significantly shortened C-termini where key domains fully exposed, offering a structural necessity for shaping tetramer. Meanwhile, we recovered the diversification of MIKC^C genes in ferns, implying a preacquisition of raw genetic basis for their explosive functional diversity in seed plants.