Received January 23, 2007
Returned for revision November 28, 2007
Accepted December 10, 2007
A Naturally Occurring Mutation in an Arabidopsis Accession Affects a 
-D-Galactosidase That Increases the Hydrophilic Potential of Rhamnogalacturonan I in Seed Mucilage
Audrey Macquet 1, Marie-Christine Ralet 2, Olivier Loudet 3, Jocelyne Kronenberger 1, Gregory Mouille 4, Annie Marion-Poll 1, and Helen M. North 1*
1 Laboratoire de Biologie des Semences, Unité Mixte de Recherche 204 Institut National de la Recherche Agronomique, AgroParisTech, Institut Jean-Pierre Bourgin, F-78026 Versailles Cedex, France
2 Institut National de la Recherche Agronomique, Unité de Recherche 1268 Biopolymères Interactions Assemblages, Institut National de la Recherche Agronomique, F-44300 Nantes, France
3 Station de Génétique et Amélioration des Plantes, Institut National de la Recherche Agronomique, Institut Jean-Pierre Bourgin, F-78026 Versailles Cedex, France
4 Laboratoire de Biologie Cellulaire, Institut National de la Recherche Agronomique, Institut Jean-Pierre Bourgin, F-78026 Versailles Cedex, France
* To whom correspondence should be addressed. E-mail: helen.north{at}versailles.inra.fr.
The Arabidopsis thaliana accession Shahdara was identified as a rare naturally occurring mutant that does not liberate seed mucilage on imbibition. The defective locus was found to be allelic to the mum2-1 and mum2-2 mutants. Map-based cloning showed that MUCILAGE-MODIFIED2 (MUM2) encodes the putative 
-D-galactosidase BGAL6. Activity assays demonstrated that one of four major -D-galactosidase activities present in developing siliques is absent in mum2 mutants. No difference was observed in seed coat epidermal cell structure between wild-type and mutant seed; however, weakening of the outer tangential cell wall by chemical treatment resulted in the release of mucilage from mum2 seed coat epidermal cells, and the mum2 mucilage only increased slightly in volume, relative to the wild type. Consistent with the absence of -D-galactosidase activity in the mutant, the inner layer of mucilage contained more Gal. The allocation of polysaccharides between the inner and outer mucilage layers was also modified in mum2. Mass spectrometry showed that rhamnogalacturonan I in mutant mucilage had more branching between rhamnose and hexose residues relative to the wild type. We conclude that the MUM2/BGAL6 -D-galactosidase is required for maturation of rhamnogalacturonan I in seed mucilage by the removal of galactose/galactan branches, resulting in increased swelling and extrusion of the mucilage on seed hydration.
