Financial support: David Liénard was supported by a grant from the “Conseil Régional de Haute Normandie”. Corresponding author

Although aquaporins have been shown to increase membrane water permeability in many cell types, the physiological role of this increase was not always obvious. In this report, we provide evidence that in the leafy stage of development (gametophore) of the moss Physcomitrella patens, aquaporins help to replenish more rapidly the cell water that is lost by transpiration, at least if some water is in the direct vicinity of the moss plant. Three aquaporin genes were cloned in P. patens: PIP2;1, PIP2;2 and PIP2;3. The water permeability of the membrane was measured in protoplasts from leaves and protonema. A significant decrease was measured in protoplasts from leaves and protonema of PIP2;1 or PIP2;2 knockouts but not the PIP2;3 knockout. No phenotype was observed when knockout plants were grown in closed Petri dishes with ample water supply. Gametophores isolated from WT and pip2;3 mutant were not sensitive to moderate water stress, but pip2;1 or pip2;2 gametophores expressed a “water stress phenotype". The knockout mutant leaves were more bent and twisted, apparently suffering from an important loss of cellular water. We propose a model to explain how the aquaporins PIP2;1 and PIP2;2 delay leaf dessication in a drying atmosphere. We suggest that in ancestral land plants, some 400 million years ago, aquaporins were already used to facilitate the absorption of water. www.plantphysiol.org on January 16, 2018 Published by Downloaded from Copyright © 2008 American Society of Plant Biologists. All rights reserved.