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ScienceWise - Jan/Feb 2008

Mixing cell biology with mechanical engineering

Article Illustration
Two engineers and a biologist: (from left) Shankar Kalyanasundaram, Hung Kha and Richard Williamson gaze at a tray of Arabidopsis, a plant ‘guinea pig’ being studied in order to understand the connection between composition, structure and mechanics of cell walls
Article Illustration
The deformed shape of the plant cell wall (modelled on the left) which was obtained by undertaking a finite element analysis. The colours represent different levels of displacement from lowest (dark blue) to highest (red), as shown in the legend at the top left corner (units in nanometer). The applied forces result in relatively large displacements (yellow areas) at the right edge of the cell wall and small displacements at the constrained left edge ( blue areas).
Article Illustration
A computer model of a simplified plant cell primary wall. The red ‘beam’ elements represent the cellulose microfibrils which are ‘tethered’ by the xyloglucan chains (blue ‘beam’ elements). The green arrows (on the right hand side) are forces applied at one edge of the cell wall, while the other edge is constrained (represented by the green triangles on the left hand side). Research will be undertaken to incorporate pectic polysaccharides and structural proteins, which occupy the space between the xyloglucan chains, into the model.
Article Illustration
A colourised scanning electron micrograph showing how cell shapes go astray when the mechanical properties are wrong. The main image is a mutant that makes less cellulose. The inset is the wild type with normal cellulose levels. Reprinted with permission from Arioli et al 1998, Science 279, 717-720.

The amazing structural properties of plants

Researchers at the Research School of Biological Sciences (RSBS) are working with engineers from the Department of Engineering to better understand the mechanical properties of cell walls.

A large part of a plant’s growth is generated through the expansion of its cells. As the contents of a cell expands, there’s increasing pressure on the cell walls. This forces the cell walls to stretch. Depending on the mechanical properties of the cell’s walls, the cell can grow in any number of shapes and forms. Cells that make up plant stems, for example, might experience significant elongation with little circumferential growth. On the other hand, spherical cells in fruit can expand dramatically in all directions.

Professor Richard Williamson heads up the Cell Wall Laboratory at RSBS. He’s been researching the biosynthesis of cell walls for many years and has pioneered the use of cell wall mutants in investigating the composition and properties of cell walls. “Plants combine crystalline cellulose microfibrils with other polysaccharides to make fibre-reinforced composites that form the wall surrounding every plant cell,” says Professor Williamson. “Small isodiametric cells begin with primary walls that grow to form the shapes of the much more diverse cells that build different plant organs. Only when growth has finished can cells form the thick, inextensible secondary walls familiar from wood and textile fibres.

“An understanding of the cell wall’s mechanical properties is central to understanding plant growth since it’s the mechanics of the primary wall that largely shape the cell that develops during growth. The mechanical properties of this primary wall determine whether it expands isotropically (equally in all directions) or anisotropically. Isotropic expansion will produce large, near isodiametric cells familiar in storage organs such as the potato. On the other hand, anisotropic expansion, depending on the ratio of growth rates in different directions, can produce any shape up to the extremely long thin cells of textile fibres. In all cases, the primary wall’s mechanical properties decisively influence the final cell shape.

“The structure of cell walls has been extensively studied by biologists but how this structure determines wall mechanics has not received the same attention.”

Biologists may not have the appropriate suite of skills for this type of analysis but materials engineers do. Consequently, Professor Williamson has working collaboratively with engineers to model the mechanical properties of cell walls. His collaborator in the research is Dr Shankar Kalyanasundaram from the Department of Engineering. Dr Kalyanasundaram is a materials engineer with expertise in mechanics, computer modelling and finite element analysis.

“The biologists might be able to test the individual components that make up the structure of the cell wall, but they don’t have the expertise to model the various components as a system,” says Dr Kalyanasundaram. “How the structure of a cell wall gives rise to its mechanical properties is an important research area, and we need this understanding if we are to better understand cell expansion and the role it plays in plant growth.

“The mechanical properties of any material always reflect its underlying structure. We’re developing a computational model for simulating primary wall structure that will allow us to explore in silico the relationship between wall structure, mechanical properties and growth.”

The approach being explored by the researchers has elicited a great deal of interest. The project began with a post-doctoral fellow Dr Sigrid Tuble and has recently been supported with an ARC Discovery grant. With this support, the researchers have recruited Dr Hung Kha as a postdoctoral research fellow to carry out the computer modelling and analysis.” The relationship between the microstructure and properties of materials is a familiar one in engineering where powerful methods have been developed to predict effective properties from microstructure. “Now we’re refining and testing the model to ensure it correctly predicts the effects on wall properties when you remove or modify individual wall polymers or polymer classes. Being structure based, the finite element model can predict how removing or modifying one component affects the mechanical properties of the cell wall. These predictions can then be tested because some of the changes can be made in real walls by methods such as selective polymer extraction, or degradation or change following mutation.”

“The control of cell expansion and cell shape during plant growth are central problems in plant developmental biology,” says Professor Williamson. “By adopting this materials engineering approach we hope for big breakthroughs but it’s not just biology that stands to learn.

“Down the road it’s expected that there could well be a wide range of important industrial applications flowing from this research. Cotton and wood industries, for example, depend heavily on the mechanical properties of mature fibres. The cotton industry, as just one example, has expressed interest in modifying growth anisotropy to produce longer, finer fibres.” So, having seen what the engineers can do, are the biologists impressed? “It’s been a very exciting collaboration because it really is two very different disciplines,” says Professor Williamson. “It’s a question of seeing how the biology can be simulated using engineering techniques that haven’t really been applied to it before.”

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