Significance Tip-growing root hairs are an excellent model system for deciphering the molecular mechanism underlying reactive oxygen species (ROS)-mediated cell elongation. Root hairs are able to expand in response to external signals, increasing several hundred-fold their original size, which is important for survival of the plant. Although their final cell size is of fundamental importance, the molecular mechanisms that control it remain largely unknown. In this study, we propose a molecular mechanism that links the auxin–auxin response factors module to activation of RSL4, which directly targets genes encoding ROS-producing enzymes such as NADPH oxidases (or RESPIRATORY BURST OXIDASE HOMOLOG proteins) and secreted type III peroxidases. Activation of these genes impacts on apoplastic ROS homeostasis, thereby stimulating root hair cell elongation.