RUBY, a putative galactose oxidase, influences pectin properties and promotes cell-to-cell adhesion in the seed coat epidermis of Arabidopsis thaliana

Cell-to-cell adhesion is essential for establishment of multicellularity. In plants, such adhesion is mediated through a middle lamella composed primarily of pectic polysaccharides. The molecular interactions that influence cell-to-cell adhesion are not fully understood. We have used Arabidopsis thaliana seed coat mucilage as a model system to investigate interactions between cell wall carbohydrates. Using a forward-genetic approach, we have discovered a gene, RUBY PARTICLES IN MUCILAGE (RUBY), encoding a protein that is annotated as a member of the AA5 family of Carbohydrate-Active Enzymes (galactose/glyoxal oxidases) and is secreted to the apoplast late in the differentiation of seed coat epidermal cells. We show that RUBY is required for the galactose oxidase activity of intact seeds, the oxidation of galactose in side-chains of rhamnogalacturonan-I (RG-I) present in mucilage-modified2 (mum2) mucilage, but not in wild-type mucilage, the retention of branched RG-I in the seed following extrusion, and the enhancement of cell-to-cell adhesion in the seed coat epidermis. These data support the hypothesis that RUBY is a galactose oxidase that strengthens pectin cohesion within the middle lamella, and possibly the mucilage of wild-type seed coat epidermal cells, through oxidation of RG-I galactose side-chains.