ELONGATED HYPOCOTYL5 and B-BOX21/22 are required for auxin-induced root hair development

Auxin is a key phytohormone required for root hair development. It acts as a central node that integrates various environmental and hormonal signals. Here, we report that the light signaling components HY5 (ELONGATED HYPOCOTYL 5) and BBX21/22 (B-box 21/22) are not only involved in light-enhanced root hair development but also required for auxin-induced root hair growth in Arabidopsis (Arabidopsis thaliana). Exogenous auxin increases HY5 protein levels in the root differentiation zone through reducing the level of nuclear-localized COP1 (CONSTITUTIVE PHOTOMORPHOGENIC 1), which targets HY5 protein for degradation. Both HY5 and BBX21/22 are required for auxin-induced expression of genes involved in root hair development, including key regulatory genes RSL4/5 (RHD6-LIKE 4/5). Through a transient expression assay, we demonstrated that co-expressing BBX21/22 and HY5 promotes the expression of the reporter gene driven by the RSL4/5 promoter. Genetic analysis revealed that HY5 and BBX21/22 act in the same pathway. Furthermore, this HY5-BBX21/22 module is also required for ethylene and low-phosphate-induced root hair growth. We thus propose that the HY5-BBX21/22 module acts as a key node downstream of auxin that potentially mediates the crosstalk between various environmental and hormonal signals in regulating root hair development.