Root hairs are unicellular extensions of root epidermal cells that help plants increase water and nutrient uptake and improve soil anchorage, both of which are crucial for the globally recognized goal of yield improvement with reduced fertilizer use. Previous studies have implicated numerous genes and phytohormones in the control of root hair development. This work uncovers the molecular mechanism of ethylene (ET)-promoted root hair growth and identifies a transcriptional complex consisting of EIN3/EIL1 and RHD6/RSL1 as the key regulator of root hair initiation and elongation. As ET mediates the effects of various root hair stimuli, this work also elucidates a convergent signaling network that integrates diverse environmental cues and intrinsic signals to modulate plant organ development.
Root hairs are an extensive structure of root epidermal cells and are critical for nutrient acquisition, soil anchorage, and environmental interactions in sessile plants. The phytohormone ethylene (ET) promotes root hair growth and also mediates the effects of different signals that stimulate hair cell development. However, the molecular basis of ET-induced root hair growth remains poorly understood. Here, we show that ET-activated transcription factor ETHYLENE-INSENSITIVE 3 (EIN3) physically interacts with ROOT HAIR DEFECTIVE 6 (RHD6), a well-documented positive regulator of hair cells, and that the two factors directly coactivate the hair length-determining gene RHD6-LIKE 4 ( RSL4) to promote root hair elongation. Transcriptome analysis further revealed the parallel roles of the regulator pairs EIN3/EIL1 (EIN3-LIKE 1) and RHD6/RSL1 (RHD6-LIKE 1). EIN3/EIL1 and RHD6/RSL1 coordinately enhance root hair initiation by selectively regulating a subset of core root hair genes. Thus, our work reveals a key transcriptional complex consisting of EIN3/EIL1 and RHD6/RSL1 in the control of root hair initiation and elongation, and provides a molecular framework for the integration of environmental signals and intrinsic regulators in modulating plant organ development.