Stimulus-induced downregulation of root water transport involves reactive oxygen species-activated cell signalling and plasma membrane intrinsic protein internalization

Abstract

The water uptake capacity of plant roots (i.e. their hydraulic conductivity, Lpr) is determined in large part by aquaporins of the plasma membrane intrinsic protein (PIP) subfamily. In the present work, we investigated two stimuli, salicylic acid (SA) and salt, because of their ability to induce an accumulation of reactive oxygen species (ROS) and an inhibition of Lpr concomitantly in the roots of Arabidopsis plants. The inhibition of Lpr by SA was partially counteracted by preventing the accumulation of hydrogen peroxide (H2O2) with exogenous catalase. In addition, exogenous H2O2 was able to reduce Lpr by up to 90% in <15 min. Based on the lack of effects of H2O2 on the activity of individual aquaporins in Xenopus oocytes, and on a pharmacological dissection of the action of H 2O2 on Lpr, we propose that ROS do not gate Arabidopsis root aquaporins through a direct oxidative mechanism, but rather act through cell signalling mechanisms. Expression in transgenic roots of PIP-GFP fusions and immunogold labelling indicated that external H2O 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 an ROS-dependent signalling pathway in the roots of Arabidopsis. Several abiotic and biotic stress-related stimuli potentially share this path, which involves an H2O 2-induced internalization of PIPs, to downregulate root water transport. © 2008 The Authors.