Plant plasma membrane water channels conduct the signalling molecule H₂O₂

H₂O₂ is a relatively long-lived reactive oxygen species that signals between cells and organisms. H₂O₂ signalling in plants is essential for response to stress, defence against pathogens and the regulation of programmed cell death. Although H₂O₂ diffusion across membranes is often considered as a passive property of lipid bilayers, native membranes represent significant barriers for H₂O₂. In the present study we addressed the question of whether channels might facilitate H₂O₂ conduction across plasma membranes. The expression of several plant plasma membrane aquaporins in yeast, including PIP2;1 from Arabidopsis (where PIP is plasma membrane intrinsic protein), enhanced the toxicity of H₂O₂ and increased the fluorescence of dye-loaded yeast when exposed to H₂O₂. The sensitivity of aquaporin-expressing yeast to H₂O₂ was altered by mutations that alter gating and the selectivity of the aquaporins. The conduction of water, H₂O₂ and urea was compared, using molecular dynamics simulations based on the crystal structure of SoPIP2;1 from spinach. The calculations identify differences in the conduction between the substrates and reveal channel residues critically involved in H₂O₂ conduction. The results of the calculations on tetramers and monomers are in agreement with the biochemical data. Taken together, the results strongly suggest that plasma membrane aquaporin pores determine the efficiency of H₂O₂ signalling between cells. Aquaporins are present in most species and their capacity to facilitate the diffusion of H₂O₂ may be of physiological significance in many organisms and particularly in communication between different species.