One of the best-known effects of the plant hormone auxin on plant development is the inhibition of root elongation, yet the underlying mechanism remains poorly understood. Using tomato (Solanum lycopersicum) as a model, Ivanchenko et al. demonstrate that auxin acts by increasing the level of hydrogen peroxide (H2O2) in the root tip. Increasing the auxin level triggers accumulation of H2O2, leading to inhibition of root cell elongation and root growth. The diageotropica (dgt) mutation in tomato, which confers auxin resistance, affects this pathway by reducing the auxin responsiveness of tissues and by disrupting the H2O2 homeostasis in the root tip.