UBC Theses and Dissertations
Genetic analysis of the role of cellulose in Arabidopsis seed coat development and mucilage adherence Griffiths, Jonathan Stewart
Primary plant cell walls are comprised largely of the polysaccharides cellulose, hemicellulose, and pectins, and can also contain up to 10% protein. These cell wall components interact non-covalently and covalently to form a functional cell wall. Interactions between cellulose and pectins are poorly understood, and are the focus of this research. Arabidopsis seed coat epidermal cells produce three distinct types of cell walls: an outer primary wall; mucilage, a specialized wall composed primarily of pectins; and a rigid columella. When seeds are hydrated, mucilage expands rapidly, breaking the outer wall, to form a mucilage halo that surrounds and remains adherent to the seed. The columella appears to be composed primarily of cellulose, and is therefore an excellent model for investigating cellulose biosynthesis. Cell wall biosynthesis and polysaccharide interactions were examined during seed coat development and in mucilage adherence to better understand cell wall assembly and function. Cellulose is synthesized by the CELLULOSE SYNTHASE A (CESA) family of glucosyltransferases. It has been proposed that at least three different CESAs are required to form a functional Cellulose Synthase Complex (CSC). I investigated the contribution of CESA2, CESA5 and CESA9 in cellulose biosynthesis during seed coat development. Based on seed coat epidermal cell morphology and cellulose quantification, all three CESAs have non-redundant roles in secondary wall biosynthesis, while CESA5 specifically functions in mucilage biosynthesis. CESA3 is expressed in the seed coat during mucilage biosynthesis and missense mutations in CESA3, isoxaben resistant 1 (ixr1-1 and ixr1-2), result in altered mucilage structure and pectin distribution, and reduced cellulose amounts in seeds. The mechanism of mucilage adherence was examined by comparing two loss of function mutants that disrupt adherence, cesa5-1 and sos5-2. SOS5 encodes an arabinogalactan protein hypothesized to influence adherence through CESA5. However, the phenotype of each single mutant differs and a cesa5 sos5 double mutant has an enhanced phenotype. Therefore, it is unlikely that SOS5 promotes mucilage adherence through CESA5. SOS5 may influence mucilage structure through galactans, as it is required for the proper function of the !- galactosidase, MUM2. This demonstrates a role for AGPs in galactan metabolism and cell wall polysaccharide interactions.
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