Plant Cell Walls; the Fabric of Life

Our laboratory focuses on plant cell walls for two main reasons. The diversity of plant forms we see around us is determined by differential growth that is determined by the size and shape it cell. In turn the size and shape of these cells is determined by the physical properties of the cell wall. As a consequence, cell walls are fundamental to all aspects of plant development. Secondly, the effects of man-made climate change are becoming increasingly obvious. Decreasing our reliance on fossil fuels is dependent upon us replacing many of our existing fuels chemical and materials with alternatives with neutral or even negative CO₂ emissions. Plants make up more than 80% of all biomass on the planet and most of this material is inf the form of plant cell walls. The latest estimates suggest there are more than 3 trillion trees on earth and the photosynthesis required for each kilogram of dry wood removes 1.65-1.8kg of CO₂ from the atmosphere. By altering the composition and properties of the plant cell wall, we believe it is possible to generate cell walls that are much better suited to use either as generating novel by materials, or as a rich source of sugars to generate biofuels or other chemicals.

Cellulose is the abundance most abundant component of plant cell walls and its remarkable structural property play a central role in determining the physical properties of the plant cell wall. It also offers a rich source of glucose that have the potential to be made into a wide variety of different chemicals.

Our system: from weeds to trees

Much of our fundamental work is performed in the model plant Arabidopsis, however nearly everything we learn in Arabidopsis is broadly applicable to other plant species. This was clearly demonstrated in our work on the control of radial growth in trees. Using genes identified in Arabidopsis we were able to generate Poplars that grew faster and generated nearly twice as much biomass as their unmodified controls (Etchells et al. 2015). We have also identified a novel plant virus that down regulates lignin deposition in trees (Allen et al. 2022).

Projects

Exploiting proteomics to explore the fundamentals of cellulose synthesis.

Arabidopsis molecular genetics continues to be essential an essential part of our work, however, while we have previously relied heavily on transcriptomics and expression data to uncover co-expression networks, we are now increasingly incorporate proteomics in our study. In particular, proximity labelling is a powerful tool for identifying proteins that co locate in situ. We currently use this method to identify components required to synthesis cellulose, traffic the cellulose synthase complex around the cell and identify important regulators of this process.

Synthesising novel celluloses microfibrils in plants

While higher plant synthesise probably all initially synthesise a similar cellulose microfibril based on 18 individual glucose chains that is determine in large part by the organisation of the cellulose synthase rosettes that forms, hexameric structure, composed of six different lobes (Wilson et al. 2021). At the centre of this complex are the catalytic subunits know as CESA proteins that we extensively study to understand the importance in the variation of CESA protein structure (Kumar 2019).  We have successfully used CESA proteins from lower plants to synthesise cellulose in higher plants and have had good success in altering the properties of the cellulose that is synthesised. In particular we can make dramatic changes to the crystallinity of the cellulose microfibril that allow the sugars to be more easily released from the wall facilitating their use as a feedstock for biofuel or other chemical production.  This is the first towards making rational changes to the properties of cellulose microfibrils by altering the structure of individual CESA proteins. 

Huge natural variation in cellulose microfibrils structure exists in the plant world. In particular different species of algae synthesis cellulose  microfibrils of entirely different shapes and sizes and in some cases the same algae is able to make both small and large cellulose microfibrils. Our improved understanding of the fundamentals of cellulose synthase complex trafficking and assembly will allow us to understand how to successfully express these algal celluloses in higher plants.  The ultimate aim would be to generate higher plants that are able to make cellulose microfibrils with structures. comparable to those found in algae and enable us to generate a new generation of plant-based materials.

Modifications controlling protein localisation.

Protein S-acylation is a comparatively poorly understood protein modification that involved the transfer of a fatty acid group to a cysteine residue and greatly increases the hydrophobicity of proteins. As part of a study of CESA proteins, it became clear they were extensively modified by S-acylation. In fact, we estimate that the single cellulose synthase complex may have more than 100 acyl chains added to it, that is likely to dramatically alter its physical properties and also determine how it partitions within the membrane (Kumar et al. 2016). To identify other proteins that were modified by S-acylation, we undertook a global study of in Arabidopsis and identified more than 1000 modified proteins. The work also confirmed that many other proteins involved  in cellulose synthesis were also modified in this way. The study was also  a graphic illustration of the increase power of approaches based on proteomic. 

Acyl groups are added to cysteines by a group of proteins, known as protein, acyltransferases (PATs). Mutants in PATs exhibit interesting but complex pleiotropic phenotypes. To understand the basis of these phenotypes we really need to identify the substrate for individual PATs. We are currently exploring combining all skills in  BioID with our knowledge of S-acylation sites to identify PAT protein substrates.

References

Kumar, M., Carr, P., and Turner, S.R. (2022). An Atlas of Arabidopsis Protein S-Acylation Reveals Its Widespread Role in Plant Cell Organization and Function. Nature Plants 8, 670-681. doi:10.1038/s41477-022-01164-4

Allen, H., Zeef, L., Morreel, K., Goeminne, G., Kumar, M., Gomez, L.D., Dean, A.P., Eckmann, A., Casiraghi, C., McQueen-Mason, S.J., Boerjan, W., and Turner, S.R. (2022). Flexible and Digestible Wood Caused by Viral-Induced Alteration of Cell Wall Composition. Curr. Biol. 32, 3398-3406.e3396. doi:https://doi.org/10.1016/j.cub.2022.06.005

Wilson, T.H., Kumar, M., and Turner, S.R. (2021). The Molecular Basis of Plant Cellulose Synthase Complex Organisation and Assembly. Biochem. Soc. Trans. 49, 379-391. doi:10.1042/bst20200697

Kumar, M., Mishra, L., Carr, P., Pilling, M., Gardner, P., Mansfield, S.D., and Turner, S.R. (2018). Exploiting Cellulose Synthase (Cesa) Class-Specificity to Probe Cellulose Microfibril Biosynthesis. Plant Physiol. 177, 151-167. doi:10.1104/pp.18.00263

Kumar, M., Atanassov, I., and Turner, S. (2017). Functional Analysis of Cellulose Synthase (Cesa) Protein Class Specificity. Plant Physiol. 173, 970-983. doi:10.1104/pp.16.01642

Kumar, M., Wightman, R., Atanassov, I., Gupta, A., Hurst, C.H., Hemsley, P.A., and Turner, S. (2016). S-Acylation of the Cellulose Synthase Complex Is Essential for Its Plasma Membrane Localization. Science 353, 166-169. doi:10.1126/science.aaf4009

Turner, S.R., and Somerville, C.R. (1997). Collapsed Xylem Phenotype of Arabidopsis Identifies Mutants Deficient in Cellulose Deposition in the Secondary Cell Wall. Plant Cell 9, 689-701. doi:10.2307/3870425

Funded PhD studentship

The start date is the end of September 2024. Further information and informal enquires may be made to Professor Simon Turner ([email protected]), https://www.research.manchester.ac.uk/portal/simon.turner.html. Application should be made online, further information can be found at https://www.bmh.manchester.ac.uk/study/research/funded-programmes/bbsrc-dtp/projects/

Simon Turner currently holds the George Harrison Professor of Botany at the University of Manchester. He has held this position since 2004 having joined the University as a lecturer in 1995. This followed his work as a postdoctoral worker in the lab of Professor Chris Somerville at the Carnegie Institute at Stanford University. During this period he identified the first irx mutants that allowed him to identify the genes required for cellulose synthesis in the plant secondary cell wall that started his research into cellulose synthesis. Cellulose synthesis continues to be one of his main interests particularly the relationship between the enzyme complex that synthesises cellulose and relationship between this complex and the microfibrils it produces. Other research interests include the regulation of plant vascular development and how it can be manipulated to increase plant growth. He chairs the University’s biological safety committee.