Tracheophytes Contain Conserved Orthologs of a Basic Helix-Loop-Helix Transcription Factor That Modulate Genes.

() genes, which contain the root hair-specific -element (RHE) in their regulatory regions, function in root hair morphogenesis. Here, we demonstrate that an basic helix-loop-helix transcription factor, ROOT HAIR DEFECTVE SIX-LIKE4 (RSL4), directly binds to the RHE in vitro and in vivo, upregulates genes, and stimulates root hair formation in Arabidopsis. Orthologs of RSL4 from a eudicot (poplar []), a monocot (rice []), and a lycophyte () each restored root hair growth in the Arabidopsis mutant. In addition, the rice and RSL4 orthologs bound to the RHE in in vitro and in vivo assays. The orthologous genes contain RHEs in their promoter regions, and RSL4 was able to bind to its own RHEs in vivo and amplify its own expression. This process likely provides a positive feedback loop for sustainable root hair growth. When and its orthologs were expressed in cells in non-root-hair positions, they induced ectopic root hair growth, indicating that these genes are sufficient to specify root hair formation. Our results suggest that RSL4 mediates root hair formation by regulating genes and that this mechanism is conserved throughout the tracheophyte (vascular plant) lineage.