In non-cyanogenic plants, cyanide is produced during ethylene biosynthesis and is mainly detoxified by the ß-cyanoalanine synthase CAS-C1. Arabidopsis plants lacking CAS-C1 show abnormal root hairs, which stop growing at early stages. Root hair elongates by polarized cell expansion at the tip, and we have observed that CAS-C1-driven GFP fluorescence locates in mitochondria and accumulates in root hair tips during root hair elongation. Genetic crosses have been performed between cas-c1 plants and scn1-1 mutants, defective in the SCN1 protein that regulates the NADPH oxidase RHD2/AtrbohC, and between cas-c1 and rhd2-1, defective in the NADPH oxidase necessary for the generation of ROS and the Ca2+ gradient necessary for root hair elongation. The phenotypic and molecular analysis of these crosses indicates that cas-c1 is hypostatic to scn1-1 and epistatic to rhd2-1. Furthermore, the action of cyanide in root hair development is independent of ROS and of direct NADPH oxidase inhibition by cyanide.