Diversification of a Transcription Factor Family Led to the Evolution of Antagonistically Acting Genetic Regulators of Root Hair Growth

Streptophytes colonized the land some time before 470 million years ago [1–3]. The colonization coincided with an increase in morphological and cellular diversity [4–7]. This increase in diversity is correlated with a proliferation in transcription factors encoded in genomes [8–10]. This suggests that gene duplication and subsequent diversification of function was instrumental in the generation of land plant diversity. Here, we investigate the diversification of the streptophyte-specific Lotus japonicus ROOTHAIRLESS LIKE (LRL) transcription factor (TF) [11, 12] subfamily of basic loop helix (bHLH) proteins by comparing gene function in early divergent and derived land plant species. We report that the single Marchantia polymorpha LRL gene acts as a general growth regulator required for rhizoid development, a function that has been partially conserved throughout multicellular streptophytes. In contrast, the five relatively derived Arabidopsis thaliana LRL genes comprise two antagonistically acting groups of differentially expressed genes. The diversification of LRL genes accompanied the evolution of an antagonistic regulatory element controlling root hair development. •The LRL bHLH transcription factor family diversified during land plant evolution•Expression of individual LRL genes was gradually restricted to specific domains•In derived lineages, there are two sets of antagonistically acting LRL genes•LRL gene function is partially conserved between streptophyte algae and angiosperms Breuninger et al. show that the LRL transcription factor family of angiosperms is derived from a single-copy gene in early diverging plant lineages. The single-copy MpLRL gene acts as a general growth regulator in liverworts. LRL function diversified into two antagonistically acting groups of proteins active in root hair development in angiosperms.