Root hairs are an extensive structure of root epidermal cells and are critical for nutrient acquisition, soil anchorage, and environ- mental 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 en- hance root hair initiation by selectively regulating a subset of core root hair (H) 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 in- trinsic regulators in modulating plant organ development. Overall design: Col-0, ein3 eil1, rhd6 rsl1 and ein3 eil1 rhd6 rsl1 were grown vertically on MS medium for 6 d and then treated with or without ET at 10 ppm for 4 h. Two biological replicates were prepared for each condition. Library preparation and high-throughput sequencing were conducted in accordance with the manufacturers' instructions. Root total RNA was extracted using the RNeasy Plant Mini Kit (Qiagen). Sequencing libraries were prepared using the TruSeq mRNA preparation kit (Illumina). RNA quality and library quality were examined by a Bioanalyzer 2100 instrument (Agilent), and paired-end, 100-bp deep sequencing was per-formed on an Illumina HiSeq 2000 platform.