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UBC Theses and Dissertations

The cell biology of cellulose deposition in secondary cell walls of protoxylem tracheary elements in Arabidopsis thaliana Watanabe, Yoichiro


Cellulose is the most abundant polymer in nature and is a major component of both primary and secondary cell walls in plants. The cellulose produced in these different walls are synthesized by completely independent sets of non-redundant CELLULOSE SYNTHASE (CESA) enzymes. In the last decade, live cell imaging techniques have answered a number of fundamental questions regarding CESA dynamics and organization in the primary cell wall. However, attempts to repeat these experiments in cells producing secondary cell walls has been met with limited success due to the fact that cells forming secondary walls are deep inside plant organs. The development of an inducible system driving the ectopic expression of the master regulator for protoxylem tracheary element development, VASCULAR RELATED NAC-DOMAIN7 (VND7), has generated a valuable biological tool to track secondary cell wall synthesis via live-cell imaging. With these tools, I was able to directly visualize secondary cell wall-specific CESA complexes moving around the plasma membrane, and to quantify that they move at a significantly faster rate than primary cell wall-specific complexes. Additionally, bundling of the underlying cortical microtubules causes the densities of the CESA complexes to be much higher during secondary wall synthesis than during primary wall synthesis, giving a possible explanation for the rapid and abundant development of these walls. Analysis of the transition from primary to secondary cell wall production revealed that primary wall-specific CESAs are selectively targeted into distinct pre-vacuolar compartments for degradation to the lytic vacuole, while secondary cell wall-specific CESAs accumulate. Finally, cesa mutants were investigated to explore the effects of the loss of each of the three CESAs involved in secondary cell wall cellulose synthesis on both the wall patterning and localization of their interacting partners. While the loss of a CESA causes significant defects in secondary cell wall cellulose patterning, the loss of CESA7 specifically resulted in the complete loss in patterning, indicating a possible role for CESA7 in anchoring the CESA complexes to the underlying cortical microtubules. Taken together, these results refine our model of how plant cells coordinate their cellulose synthesis machinery during secondary cell wall production.

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