Cell wall dynamics: novel tools and research questions

Abstract

Years ago, a classic textbook would define plant cell walls based on passive features. For instance, a sort of plant exoskeleton of invariable polysaccharide composition, and probably painted in green. However, currently, this view has been expanded to consider plant cell walls as active, heterogeneous, and dynamic structures with a high degree of complexity. However, what do we mean when we refer to a cell wall as a dynamic structure? How can we investigate the different implications of this dynamism? While the first question has been the subject of several recent publications, defining the ideal strategies and tools needed to address the second question has proven to be challenging due to the myriad of techniques available. In this review, we will describe the capacities of several methodologies to study cell wall composition, structure, and other aspects developed or optimized in recent years. Keeping in mind cell wall dynamism and plasticity, the advantages of performing long-term non-invasive live-imaging methods will be emphasized. We specifically focus on techniques developed for Arabidopsis thaliana primary cell walls, but the techniques could be applied to both secondary cell walls and other plant species. We believe this toolset will help researchers in expanding knowledge of these dynamic/evolving structures.

Keywords: Biophysics; cell wall composition; cell wall structure; live-imaging; mechanics; plant cell wall.

Fig. 1.

The plant cell wall is a chemically diverse structure composed mainly of polysaccharides (cellulose, hemicelluloses, and pectin), with secondary cell walls also including lignin. These molecules are arranged in a complex 3D structure that gives the cell wall its strength and rigidity. New techniques for studying plant cell walls provide insights into cell wall composition (A–E) and mechanical characteristics (F–I). (A) Chemical characterization provides accurate absolute quantification of the different cell wall components, although with no possibilities of in situ studies. (B) Histochemical methods such as using monoclonal antibodies against cell wall epitopes or click chemistry carbohydrate probes allow the location of specific components on the cell walls (e.g. hemicelluloses are marked in green on the figure). (C) Fourier-transform infrared (FTIR) spectroscopy enables relative quantifications of specific structural components (e.g. hemicelluloses) but the resolution is limited. (D) Raman spectroscopy also enables relative quantifications, in this case with higher resolution. (E) Solid-state NMR (ssNMR) allows a high level of detail in the study of chemical composition, but the data interpretation is complicated, and it is not compatible with in situ analysis. (F) An automated confocal-microextensometer (ACME) enables the measurement of extensibility and other mechanical properties in vivo in individual cells. (G) Atomic force microscopy (AFM) uses a probe (cantilever) to determine cell wall stiffness and topography, and generates an image with nanometric resolution, although it is restricted to the first cell layer. (H) Brillouin microscopy can be used to study mechanical properties in different cell layers with a micrometric resolution. The frequency shift caused by the Brillouin effect is a measurement of stiffness, whereas the amplitude of the signal can be related to viscosity. (I) Ultrastructural microscopy techniques can be used to image macromolecules at submolecular resolution, allowing the observation of cell wall microstructure.

Fig. 2.

Microfluidics devices allow the study of dynamic responses. The schema represents a basic implementation for a microfluidics device with two inlets, which permits changing of the growing conditions and/or the delivery of specific treatments. The growth of the root tip can be observed under a microscope.