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Microtubule Function in Meristematic Cells and Chloroplast Light Avoidance Movement in Arabidopsis thaliana



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The microtubule (MT) cytoskeleton plays multiple functions in plant morphogenesis and development including cell division, expansion, hormonal signaling, and response to biotic and abiotic stress. During these diverse processes, the primary role of MTs is to modulate cell wall structure and to direct intracellular movement of organelles that mainly depend on MT organization. In meristematic cells, MT organization system is continuing by the MT organization at cell cortex called cortical MT (CMT) array and in cytoplasm called endoplasmic MT (EMT). The mechanism underlying microtubule organization in plants has been primarily studied using the CMTs array, which lines the plasma membrane and configures the cell wall properties that define plant cell shape. Much less is known about EMTs, which radiate from the nuclear envelope and attach end-on to the plasma membrane, and are abundant in meristematic cells. By studying two mutants having meristematic defects in root tip clasp-1 and knt1-2, I found that EMTs appeared to maintain cells in a meristematic state by providing a structural scaffold that stabilized the cytoplasm to counteract actomyosin-based cytoplasmic streaming forces, thereby preventing premature establishment of a central vacuole and rapid cell elongation. Besides actin filaments, the function of MT orientation in intracellular movement of organelles is necessary for many cellular functions. In plants which generate their own food supply and energy from light, water and carbon dioxide by photosynthesis, the photorelocation movement of chloroplast (CP) to optimize the photosynthesis efficiency is essential. CP movement is believed to be driven mainly by the actin cytoskeleton, but the participation of MTs is currently under debate. By using high-resolution live-cell imaging of MTs during the CP avoidance response, I found evidence that the CMT orientation is important for the CP movement. CP light avoidance movement was significantly affected by MT inhibitors, and changed corresponding to different CMT orientations. The movement of CPs was guided by minus-ends of treadmilling CMTs to travel from the periclinal wall to the anticlinal wall in long distance. It suggests that CMTs might function directly as tracks for stable and long-distance movement of CPs or indirectly by interacting with actin in CP movement.



Microtubule, Endoplasmic microtubule, meristematic cells, chloroplast



Doctor of Philosophy (Ph.D.)






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