Negative regulation of conserved class I bHLH transcription factors evolved independently among land plants.

Basic helix-loop-helix transcription factors encoded by class I genes control a gene regulatory network that positively regulates the development of filamentous rooting cells - root hairs and rhizoids - in land plants. The GLABRA2 transcription factor negatively regulates these genes in the angiosperm . To find negative regulators of class I genes in early diverging land plants we conducted a mutant screen in the liverwort . This identified FEW RHIZOIDS1 (MpFRH1) microRNA (miRNA) that negatively regulates the class I gene Mp. The miRNA and its mRNA target constitute a feedback mechanism that controls epidermal cell differentiation. MpFRH1 miRNA target sites are conserved among liverwort class I mRNAs but are not present in class I mRNAs of other land plants. These findings indicate that while class I genes are ancient and conserved, independent negative regulatory mechanisms evolved in different lineages during land plant evolution.