Acts as negative regulator of root hair development redundantly with GIR2 (PubMed:28526410). GIR1 and GIR2 may function as adapter proteins that associate with GL2 and participate in the control of root hair formation (PubMed:28526410). GIR1 and GIR2 may function as adapter proteins that associate with TPL and participate in the repression of root gene expression (PubMed:28526412). Interacts with GL2 (PubMed:28526410). Interacts with TPL (PubMed:28526412). Expressed in root and shoot meristems. No visible phenotype under normal growth conditions, but the double mutant seedlings gir1 and gir2 exhibit excessive root hair formation. Seedlings overexpressing GIR1 exhibit reduced root hair formation.