The E3 ubiquitin ligase COP1 and transcription factors HY5 and RHD6 integrate light signaling and root hair development.

Light signaling plays a substantial role in regulating plant development, including the differentiation and elongation of single-celled tissue. However, the identity of the regulatory machine that affects light signaling on root hair cell (RHC) development remains unclear. Here, we investigated how darkness inhibits differentiation and elongation of RHC in Arabidopsis (Arabidopsis thaliana). We found that light promotes the growth and development of RHC. RNA-seq analysis showed that light signaling regulates the differentiation of RHC by promoting the expression of specific genes in the root epidermis associated with cell wall remodeling, JA, auxin, and ethylene signaling pathways. Together, these genes integrate light and phytohormone signals with root hair development. Our investigation also revealed that the core light signal factor ELONGATED HYPOCOTYL 5 (HY5) directly interacts with the key root hair development factor ROOT HAIR DEFECTIVE6 (RHD6), which promotes the transcription of RSL4. However, CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1) repressed RHD6 function through the COP1-HY5 complex. Our genetic studies confirm associations between RHD6, HY5, and COP1, indicating that RHD6 largely depends on HY5 for root hair development. Ultimately, our work suggests a central COP1-HY5-RHD6 regulatory module that integrates light signaling and root hair development with several downstream pathways, offering perspectives to decipher single-celled root hair development.