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Plant J. 2010 Aug;63(3):469-83. doi: 10.1111/j.1365-313X.2010.04256.x. Epub 2010 May 20.

Walls are thin 1 (WAT1), an Arabidopsis homolog of Medicago truncatula NODULIN21, is a tonoplast-localized protein required for secondary wall formation in fibers.

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CNRS, Université de Toulouse, UPS, UMR 5546, Surfaces Cellulaires et Signalisation chez les Végétaux; BP 42617, F-31326, Castanet-Tolosan, FranceVIB Department of Plant Systems Biology, UGent Department of Plant Biotechnology and Genetics, 9052 Gent, BelgiumInstitut fuer Biologie III, Albert Ludwigs Universitaet Freiburg, 79104 Freiburg, GermanyUnité de Recherche en Génomique Végétale INRA-CNRS, 91057 Evry, FranceUmeå Plant Science Center, Department of Forest Genetics and Plant Physiology, Swedish University of Agricultural Sciences, 90183 Umeå, Sweden.


By combining Zinnia elegans in vitro tracheary element genomics with reverse genetics in Arabidopsis, we have identified a new upstream component of secondary wall formation in xylary and interfascicular fibers. Walls are thin 1 (WAT1), an Arabidopsis thaliana homolog of Medicago truncatula NODULIN 21 (MtN21), encodes a plant-specific, predicted integral membrane protein, and is a member of the plant drug/metabolite exporter (P-DME) family (transporter classification number: TC 2.A.7.3). Although WAT1 is ubiquitously expressed throughout the plant, its expression is preferentially associated with vascular tissues, including developing xylem vessels and fibers. WAT1:GFP fusion protein analysis demonstrated that WAT1 is localized to the tonoplast. Analysis of wat1 mutants revealed two cell wall-related phenotypes in stems: a defect in cell elongation, resulting in a dwarfed habit and little to no secondary cell walls in fibers. Secondary walls of vessel elements were unaffected by the mutation. The secondary wall phenotype was supported by comparative transcriptomic and metabolomic analyses of wat1 and wild-type stems, as many transcripts and metabolites involved in secondary wall formation were reduced in abundance. Unexpectedly, these experiments also revealed a modification in tryptophan (Trp) and auxin metabolism that might contribute to the wat1 phenotype. Together, our data demonstrate an essential role for the WAT1 tonoplast protein in the control of secondary cell wall formation in fibers.


Arabidopsis; NODULIN21-like; fiber development; plant drug/metabolite exporter; secondary cell wall; tonoplast

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