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UBC Theses and Dissertations

Investigating the role of cellulose synthases in the biosynthesis and properties of cellulose in secondary cell walls McDonnell, Lisa Marie


Cellulose synthases are the enzymes responsible for the production of cellulose in plant cell walls. Mutations in any one of the Arabidopsis cellulose synthase (CesA) AtCesA4, AtCesA7, and AtCesA8 genes cause plants to develop collapsed xylem as a result of reduced cellulose content, demonstrating their critical role in secondary cell wall biosynthesis. A thorough characterization of the growth, cell wall properties, and cellulose ultrastructure of the AtCesA4irx⁵-¹, AtCesA7irx³-¹, and AtCesA8irx¹-¹ mutants, presented herein, is the first report of the changes to cellulose microfibril angle, cell wall crystallinity, and cellulose degree of polymerization (DP) in these mutants. This study suggests that the non-redundant functions of individual CesAs may be related to CesA-specific thresholds required for the formation of a cellulose synthesizing complex (CSC), and CesA-specific roles in regulating crystallinity and DP. Additionally, the results illustrate the importance of a fully formed CSC in regulating cellulose microfibril angle. By identifying and characterizing three new CesA genes from spruce (Picea glauca), PgCesA1, PgCesA2, and PgCesA3, which are homologous to the Arabidopsis AtCesA8, A4, and A7 and the Populus trichocarpa PtiCesA8-A, A4, and A7-A genes, respectively, the degree of functional conservation among AtCesA homologs was explored. Expression of PgCesA1 or the PtiCesAs in AtCesAirx plants rescued the collapsed xylem phenotype, thus demonstrating for the first time that orthologs of AtCesA4, A7, and A8 have conserved functions. Lastly, in planta techniques were used to measure interactions between AtCesAs to investigate if specific and consistent interactions exist. The results suggest that CesA8 and A4 can form homodimers in planta, and that there might be weak or transient interactions between AtCesA7-A4 and AtCesA7-A8. Collectively, the results presented suggest, indirectly, an unequal ratio of CesA subunits (AtCesA4:A7:A8) is required for proper cellulose biosynthesis, and that each CesA likely has a unique function which ultimately affects cellulose properties such as cell wall crystallinity and DP. Our conclusions shed new light on the role of CesAs in cellulose biosynthesis in secondary cell walls and elicit questions about the current model of CSC form and function.

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