The water uptake capacity of plants roots (i.e. their hydraulic conductivity, Lp(r)) is determined in large part by aquaporins of the Plasma membrane Intrinsic Protein (PIP) sub-family. In the present work, we investigated two stimuli, salicylic acid (SA) and salt, because of their ability to induce in Arabidopsis roots a concomitant accumulation of reactive oxygen species (ROS) and inhibition of Lp(r). Inhibition of Lp(r) by SA was partially counteracted by preventing the accumulation of hydrogen peroxide (H(2)O(2)) with exogenous catalase. In addition, exogenous H(2)O(2) was able to reduce Lp(r) by up to 90% in <15 min. Based on the lack of effects of H(2)O(2) on the activity of individual aquaporins in Xenopus oocytes and on a pharmacological dissection of H(2)O(2) action on Lp(r), we propose that ROS do not gate Arabidopsis root aquaporins through a direct oxidative mechanism but act through cell signalling mechanisms. Expression in transgenic roots of PIP-GFP fusions and immunogold labelling indicated that external H(2)O(2) enhanced in <15 min the accumulation of PIPs in intracellular structures tentatively identified as vesicles and small vacuoles. Exposure of roots to SA or salt also induced an intracellular accumulation of the PIP-GFP fusion proteins and these effects were fully counteracted by co-treatment with exogenous catalase. In conclusion, the present work identifies SA as a novel regulator of aquaporins and delineates in the Arabidopsis root a ROS-dependent signalling path. Several abiotic and biotic stress-related stimuli potentially share this path, which involves a H(2)O(2)-induced internalization of PIPs, to down-regulate root water transport.