Root hair formation is controlled by environmental signals. We found significantly increased Arabidopsis root hair density and length in response to low-dose vanadate (V). Reactive oxygen species (ROS) production was induced with V treatment. We investigated the possible role of NADPH oxidase in altering root system architecture induced by V by using diphenylene iodonium (DPI), an inhibitor of NADPH oxidase, and an NADPH oxidase mutant (rhd2/AtrbohC). NADPH oxidase was involved in root hair elongation induced by V. As well, ethylene receptor (ETR1) and ROOT HAIR DEFECTIVE (RHD6) participated in inducing root hair formation induced by V. Furthermore, the kinase inhibitors, genistein (tyrosine kinase inhibitor) and K252a (ser/thr kinase inhibitor), and a phosphatase inhibitor, cantharidin (ser/thr phosphatase inhibitor), suppressed root hair formation induced by V. To elucidate the regulation of gene expression in response to V, we investigated transcriptional changes in roots by microarray assay. Exposure to V triggered changes in transcript levels of genes related to cell wall formation, ROS activity and signaling. Several genes involved in root hair formation were also regulated.