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Role of a Single Aquaporin Isoform in Root Water Uptake

^a Biochimie et Physiologie Moléculaire des Plantes, Agro-M/Centre National de la Recherche Scientifique/Institut National de la Recherche Agronomique/UM2 Unité Mixte de Recherche 5004, Place Viala, F-34060 Montpellier Cedex 1, France
^b Institut des Sciences du Végétal, Centre National de la Recherche Scientifique, Avenue de la Terrasse, F-91198 Gif-sur-Yvette Cedex, France
^c Laboratoire de Neurobiologie, Centre National de la Recherche Scientifique Unité Mixte de Recherche 7637, Ecole Supérieure de Physique et de Chimie Industrielles, 10 Rue Vauquelin, F-75231 Paris Cedex 05, France
^d Institute of Biochemical Plant Pathology, GSF-National Research Center for Environment and Health, D-85764 Neuherberg/Muenchen, Germany
^e Station de Génétique et Amélioration des Plantes, Institut National de la Recherche Agronomique, Route de Saint-Cyr, F-78026 Versailles Cedex, France

⁴ To whom correspondence should be addressed. E-mail maurel{at}ensam.inra.fr; fax 33-4-67-52-57-37

Aquaporins are ubiquitous channel proteins that facilitate the transport of water across cell membranes. Aquaporins show a typically high isoform multiplicity in plants, with 35 homologs in Arabidopsis. The integrated function of plant aquaporins and the function of each individual isoform remain poorly understood. Matrix-assisted laser desorption/ionization time-of-flight analyses suggested that Plasma Membrane Intrinsic Protein2;2 (PIP2;2) is one of the abundantly expressed aquaporin isoforms in Arabidopsis root plasma membranes. Two independent Arabidopsis knockout mutants of PIP2;2 were isolated using a PCR-based strategy from a library of plant lines mutagenized by the insertion of Agrobacterium tumefaciens T-DNA. Expression in transgenic Arabidopsis of a PIP2;2 promoter–-glucuronidase gene fusion indicated that PIP2;2 is expressed predominantly in roots, with a strong expression in the cortex, endodermis, and stele. The hydraulic conductivity of root cortex cells, as measured with a cell pressure probe, was reduced by 25 to 30% in the two allelic PIP2;2 mutants compared with the wild type. In addition, free exudation measurements revealed a 14% decrease, with respect to wild-type values, in the osmotic hydraulic conductivity of roots excised from the two PIP2;2 mutants. Together, our data provide evidence for the contribution of a single aquaporin gene to root water uptake and identify PIP2;2 as an aquaporin specialized in osmotic fluid transport. PIP2;2 has a close homolog, PIP2;3, showing 96.8% amino acid identity. The phenotype of PIP2;2 mutants demonstrates that, despite their high homology and isoform multiplicity, plant aquaporins have evolved with nonredundant functions.

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