What Is The Main Function Of Cellulose In Plants – Progress and opportunities in the characterization of cellulose – an important regulator of cell wall growth and mechanics
The plant cell wall is a dynamic network of several biopolymers and structural proteins, including cellulose, pectin, hemicellulose, and lignin. Cellulose is one of the main load-bearing components of this complex and heterogeneous structure and, in this way, is an important regulator of cell wall growth and mechanics. The glucan chains of cellulose are assembled through hydrogen bonds and van der Waals forces to form long, thread-like crystalline structures called cellulose microfibrils. The shape, size, and crystallinity of these microfibrils are important structural parameters that affect the mechanical properties of the cell wall, and these parameters are likely important factors in cell wall digestibility for biofuel conversion. Cellulose-cellulose and cellulose-matrix interactions also help regulate cell wall mechanics and growth. As a result, much attention has been paid to extracting valuable structural details about cell wall components from several techniques, individually or in combination, including diffraction/scattering, microscopy, and spectroscopy. In this review, we describe efforts to characterize the organization of cellulose in plant cell walls. X-ray scattering reveals the size and orientation of microfibrils; diffraction reveals unit lattice parameters and crystallinity. The presence of different components of the cell wall, their physical and chemical state, their alignment and orientation were determined by infrared spectroscopy, Raman, nuclear magnetic resonance and frequency generation. Direct visualization of cell wall components, network-like structure, and interactions between different components is also possible through a variety of microscopic imaging techniques, including scanning electron microscopy, transmission electron microscopy, and atomic force microscopy. This review highlights the advantages and limitations of various analytical methods for characterizing the structure of cellulose and its interactions with other wall polymers. We also identify emerging opportunities for future development of tools for structural characterization and multimodal analysis of cellulose and plant cell walls. Ultimately, understanding the structure of plant cell walls at long scales will be essential to establish structural and property relationships to link cell wall structure with growth control and mechanics.
What Is The Main Function Of Cellulose In Plants
The plant cell wall is a complex and heterogeneous network of several structural polymers and proteins. It provides mechanical strength and plays a key role in plant growth, cell differentiation, cell communication, water movement and defense (Cosgrove, 2005). Most higher plants have both primary and secondary cell walls. The primary cell wall is a thin, flexible, highly hydrated structure that surrounds the growing cell, while the secondary cell wall is a stronger, more rigid structure that begins to form when the cell stops growing. These cell wall types differ in function, rheological and mechanical properties, and in the arrangement, mobility and structure of matrix polymers (Cosgrove and Jarvis, 2012). Primary walls are mainly composed of cellulose, pectin and xyloglucans with smaller amounts of arabinoxylans and structural proteins. Hydration of the pectin matrix promotes slippage and separation of cellulose microfibrils during expansion. The strength and stability of the secondary walls results from the more oriented arrangement of cellulose microfibrils and the presence of lignin. Secondary cell walls are mainly composed of cellulose, lignin, xylans and glucomannans and are also less hydrated than primary walls (Cosgrove and Jarvis, 2012).
Peripheral Membrane Proteins Modulate Stress Tolerance By Safeguarding Cellulose Synthases
Cellulose is the main structural component responsible for most of the mechanical strength of the cell wall. The distribution and orientation of cellulose microfibrils within the cell wall helps control cell growth. The alignment of microfibrils provides the cell with mechanical anisotropy that allows preferential expansion in one direction (Jordan and Dumais, 2010). In addition to its biological importance, cellulose is an important raw material for the textile industry, paper, construction materials, and many industrially important chemical products. It is also the most abundant carbohydrate on earth and a promising source of renewable energy.
The chemical structure of cellulose consists of linear chains of glucose units connected by β-1, 4-glycosidic bonds. The glucan chains of cellulose are joined by hydrogen bonds and van der Waals to form an elongated thread-like crystalline structure called cellulose microfibrils (Harris et al., 2010). Important structural properties of cellulose include crystal shape and size, and crystallinity. Many different analytical methods have been used to study the structure and assembly of cellulose microfibrils in cell walls, but a comprehensive understanding at multiple length scales remains elusive.
Structural characterization approaches currently used to examine plant cell walls are based on four broad categories of methods: diffraction/scattering, spectroscopy, microscopy, and physicochemical analyses. Figure 1 highlights these structural characterization tools and the length scales at which they can reveal information about cell wall structure. Hard state
Nuclear magnetic resonance (NMR) studies led to the discovery of two cellulose allomorphs (VanderHart and Atalla, 1984). The crystal structures of cellulose Iα and Iβ were then determined by X-ray, electron, and neutron diffraction studies (Sugiyama et al., 1991b; Abe et al., 1997; Nishiyama et al., 2002, 2003). Further details of the structural differences between these two forms have been described by infrared (FTIR or IR)-Raman and Fourier transform spectroscopy, which show that the glucan chains have similar conformations but differ in the hydrogen bonding patterns (Atalla and VanderHart , 1999). Selective detection of cellulose allomorphs is also possible by an emerging spectroscopic technique called summation frequency generation (SFG) spectroscopy (Kim et al., 2013). In addition to crystal structure, X-ray diffraction (XRD), NMR, and IR and Raman spectroscopy are widely used to estimate the amount of crystalline cellulose (degree of crystallinity) in plant cell walls. Crystallinity is also determined by some physico-chemical methods, such as the Updegraff method, iodine adsorption, water vapor sorption and wetting enthalpy.
Biological Matrix Composites From Cultured Plant Cells
Figure 1. Tools that allow the characterization of the primary cell wall at different length scales. The crystal structures of cellulose Iα and Iβ are reprinted with permission from Nishiyama et al. (2003). Crystal structure and hydrogen bonding system in cellulose Iα from synchrotron X-ray and neutron fiber diffraction. Journal of the American Chemical Society 125, 14300–14306. Copyright 2003 American Chemical Society. Primary cell wall reprinted with permission from Cosgrove (2014). Reconstructing models of cellulose and primary cell wall assembly. Current Opinion in Plant Biology 22, 122–131. Copyright © 2014 Elsevier Ltd. The schematic was inspired by Martínez-Sanz et al. (2015a). Application of small-angle X-ray and neutron scattering techniques to study the hierarchical structure of plant cell walls: A review. Carbohydrate polymers 125, 120-134.
The supramolecular structure of the primary cell wall has been extensively characterized by microscopic methods. Many structural parameters such as crystallite size as well as fibril dimensions, cross-section and spacing have been directly visualized (Cox and Archa, 1973; Davis and Harris, 2003; Ding et al., 2014). Electron microscopy has been used more frequently to image the fibrillar features of cellulose, but can still introduce artifacts during sample preparation. Therefore, other microscopic techniques, including scanning probe microscopy, fluorescence microscopy, confocal microscopy, and polarized light microscopy (Abe et al., 1997; Thomson et al., 2007; Choong et al., 2016), are now used for visualization. are studied. cell wall in its original state with minimal sample preparation.
In addition to microscopy, the dimensions and packing of cellulose microfibrils are also investigated by scattering and spectroscopic techniques (Fernandez et al., 2011; Newman et al., 2013; Zhang et al., 2016). Because of minimal sample preparation, scattering is ideal for characterizing the cell wall in its native state. Scattering approaches also offer the advantages of allowing the investigation of a large range, thereby enabling the investigation of the arrangement of individual microfibrils as well as aggregates of microfibrils.
Overall, the combination of different methods to characterize the organization of cell wall components opens the door to investigate the interactions between cellulose and other cell wall polysaccharides, potentially revealing different aspects of cell wall assembly (Martinez-Sanz et al., 2015a). For example, a combination of different imaging techniques, such as atomic force microscopy (AFM), transmission electron microscopy (TEM), field scanning electron microscopy (FESEM), and confocal microscopy, have been used to investigate changes in the structure of cellulose microfibrils in the primary. cell walls of the Arabidopsis xxt1 xxt2 double mutant lacking detectable xyloglucan (Xiao et al., 2016). The study showed that the cellulose microfibrils in xyloglucan mutants were more aligned than in wild-type mutants, suggesting that xyloglucan functions as a spacer between cellulose microfibrils in the primary cell wall.
Cell Wall Definition And Examples
This review summarizes methods used to characterize the structure and interactions of cellulose in plant cell walls, particularly cellulose crystallinity, microfibril size and spatial organization, along with cellulose-cellulose and cellulose-matrix interactions. We discuss both established and emerging techniques used for the molecular and microstructural characterization of cellulose structure and highlight the strengths and limitations of each technique. In addition, the review presents several characterization methods that are not currently widely used for the study of plant cell walls, but given their capabilities, can be powerful tools for revealing new information about structure and organization.
) that have been identified as being interchangeable (O’Sullivan, 1997). Natural cellulose exists in the form of cellulose I, which has two allomorphs—cellulose Iα and cellulose Iβ (VanderHart and Atalla, 1984; Sugiyama et al., 1991a).
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