Plant Mechanics, Optimization and Remodeling

Abstract

The thesis presented here is focused on plant mechanics and structural optimization; the major finding of the work is that the micro-structure of the Arabidopsis root does sense and show reactions to external mechanical stresses; such reactions involve re-orientation of the microtubule (MT) cytoskeleton closer to the maximum principal tensile stress direction after a significant bending moment is applied. If the root is free of external mechanical stresses (only having internal turgor pressure) then there are two scenarios: in the first scenario, within the cell division zone, the microtubules direction is perpendicular to the main axis of the cell (along maximum principal tensile stress direction and aligned with hoop stress).In the second scenario, within the cell elongation zone, microtubules make a 45 degree angle with the main axis of the cell (possibly due to maximum shear stress). Another focus of the work presented here is to draw inspiration from nature and apply the “self-optimizing” rules found in natural tissues to engineering frame structural design. This was achieved by simulating frame structures based on two different theories: Wolff’s theory (for natural tissues) and Michell’s theory (for engineering comparative analysis). The performance of the two frame structures studied was evaluated against each other, and it was shown that, for an example of a cantilever beam, structures created based on Wolff’s theory are easier to generate under dimensional restrictions and have greater strength than analogous frame structures modeled based on Michell’s theory. In order to observe microtubule re-orientation, Arabidopsis root cells were observed by means of a confocal microscope, and the data were analyzed using image processing to find the dominant pattern of microtubules. The influence of gravity on microtubules direction was also studied by rotating control samples in different directions; gravity was found to have negligible impact on microtubule orientation. The root cell was then simulated numerically to study the direction of principal stresses, and confirm the re-orientation of the microtubules closer to the maximum principal tensile stress direction. For the strength comparison of the frame structures based on the two theories (Wolff and Michell), a cantilever domain was defined, and the curves were then generated for a computer programming environment, and results were later exported for finite element analysis.