Evolution and diversity of basic helix-loop-helix transcription factors regulating the tissue formation in land plants

Abstract

Basic helix-loop-helix (bHLH) transcription factors are widely conserved across eukaryotes and play a central role in development, metabolism, and other life processes in plants and animals. In land plants, bHLH transcription factors are diversified and may contribute to complex tissue formation and environmental adaptation by regulating downstream gene expression network. Molecular genetic and evolutionary developmental studies using model plants reveal the function and evolution of bHLH transcription factors that regulate tissue formation in land plants. The formation of stomata, gas-exchange structures in plant epidermis, is regulated by heterodimers of bHLH transcription factors belonging to subfamily Ia (e.g., SPEECHLESS, MUTE, FAMA) and subfamily IIIb (e.g., ICE1/SCREAM), and this regulatory mechanism is conserved in land plants. We found that Ia and IIIb bHLHs are still retained in the stomata-less bryophyte Marchantia polymorpha, and that these heterodimers regulate the formation of a unique diploid tissue known as the seta. In this article, we discuss the functional differentiation and co-option of bHLH transcription factors in plant tissue formation and the evolution of gene expression regulation by bHLH homodimers and heterodimers, using the Ia-IIIb bHLH transcription factor module as an example.