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UBC Theses and Dissertations

Understanding functions of a putative galactose oxidase, RUBY, and its homologues in plant cell wall modifications Šola, Krešimir


Cell-to-cell adhesion is essential for establishment of multicellularity. In plants, cell adhesion is mediated through a middle lamella composed primarily of pectic polysaccharides, but the molecular interactions that promote and regulate such adhesion are not fully understood. In Chapter 3, Arabidopsis seed coat mucilage was used as a model system to investigate interactions between cell wall carbohydrates. Using a forward-genetic approach, we have discovered a gene encoding a putative galactose oxidase, RUBY PARTICLES IN MUCILAGE (RUBY), that is required for cell-to-cell adhesion in the seed coat epidermis. Cellular and enzymatic analyses support the hypothesis that RUBY facilitates cross-links in the cell walls via the side-chains of rhamnogalacturonan I (RG-I), a constituent of pectin. These results (Chapter 3) provide genetic evidence for oxidative cross-linking in cell walls and assigns a biological function to the galactose/glyoxal oxidase family of enzymes. To better understand functions of galactose oxidases in plants, Arabidopsis homologues of RUBY, GALACTOSE OXIDASE-LIKE (GOXL) genes, were studied (Chapter 4). The expression patterns of these seven genes suggest that all of the members have functions in specialised tissues. Phylogenetic analyses suggest that the GOXL family likely has two pairs of GOXL paralogues, GOXL1 and GOXL6, and RUBY and GOXL3. Surprisingly, RUBY and GOXL3 are expressed in different tissues, whereas GOXL1 and GOXL6 have similar expression patterns, suggesting genetic redundancy. Functional complementation of ruby mutant and qualitative enzyme assays indicate that GOXL1, GOXL3 and GOXL6 are putative galactose oxidases. Plants with mutations in these genes, apart from RUBY, have no obvious phenotypes. When mutations were introduced in both GOXL1 and GOXL6, a collapsed pollen phenotype appeared, indicating that these genes may be redundant. Pollen collapse occurs at the anthesis, when pollen grains are desiccating, suggesting possible roles in pollen wall folding during controlled pollen dehydration (harmomegathy). Further genetic analysis is required to confirm that these mutations are indeed linked to the pollen phenotype. Taken together, putative Arabidopsis galactose oxidases seem to have specialised roles in tissues that may require mechanical support.

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