Calcium-dependent NADPH oxidase that generates superoxide. Required for H(2)O(2) production in response to K(+) deficiency and for the generation of reactive oxygen species (ROS) that regulate cell expansion through the activation of Ca(2+) channels. Monomer and homodimer. In roots, expressed in the epidermis in the proximal regions of the meristem, in the elongation zone, in the differentiation zone and in elongating root hairs. By potassium starvation. Plants have short root hairs and stunted roots. Belongs to the RBOH (TC 5.B.1.3) family.