Abstract
At the molecular level the plant cell walls consist of a few nanometer thick semi-crystalline cellulose fibrils embedded in amorphous matrix polymers such as pectins, hemicelluloses, and lignins. The arrangement of these molecules within the cell wall in different plant tissues, cells and cell wall layers is of crucial importance for a better understanding and thus optimized utilization of plant biomass. During the last years Confocal Raman microscopy evolved as a powerful method in plant science by revealing the different molecules in context with the microstructure. In this study two-dimensional spectral maps have been acquired of micro-cross-sections of spruce (softwood) and beech (hardwood). Raman images have been derived by using univariate (band integration, height ratios) and multivariate methods [vertex component analysis (VCA)]. While univariate analysis only visualizes changes in selected band heights or areas, VCA separates anatomical regions and cell wall layers with the most different molecular structures. Beside visualization of the distinguished regions and features the underlying molecular structure can be derived based on the endmember spectra. VCA revealed that the lumen sided S3 layer has a similar molecular composition as the pit membrane, both revealing a clear change in lignin composition compared to all other cell wall regions. Within the S2 layer a lamellar structure was visualized, which was elucidated to derive from slight changes in lignin composition and content and might be due to successive but not uniform lignification during growth.
Highlights
Plant cell walls comprise very complex and highly variable structures
Lignins are three-dimensional, amorphous heteropolymers that result from the oxidative coupling of three p-hydroxycinnamyl alcohols (p-coumaryl, coniferyl, and sinapyl alcohols) in a reaction mediated by roughly ten enzymes leading to the formation of guaiacyl (G), syringyl (S), and hydroxyphenyl (H) subunits (Boerjan et al, 2003)
SPRUCE EARLYWOOD AND LATEWOOD: UNIVARIATE IMAGING AND VERTEX COMPONENT ANALYSIS Raman mapping gives a fluorescence image by plotting the intensity change of the background (Figure 1A), which in wooden cell walls in many cases looks similar like integration of the main lignin bands between 1711 and 1533 cm−1 (Figure 1B)
Summary
Plant cell walls comprise very complex and highly variable structures. In the thin primary cell wall (0.1–1 μm) built during early growth a network is formed together with pectins, hemicelluloses, and proteins (Keegstra, 2010). These thin walls give already shape and structure to plant cells, tissues, and organs and are sufficiently strong to prevent the cell from rupturing, yet they must be flexible and plastic to accommodate growth (Cosgrove, 2005). The amount and molecular structure of the lignin in the cell walls is one of the limiting factors that determine the digestibility of forage crops (Bhatt et al, 2008; Studer et al, 2011; Yoo et al, 2012). Downregulating the enzymes of the major lignin pathways can affect the course and/or extent of lignification and gives expanded opportunities for engineering the composition and consequent properties of lignin for improved utilization of valuable plant resources (Boerjan et al, 2003; Artz et al, 2008; Gonzalez-Guerrero et al, 2008; Vanholme et al, 2010; Li et al, 2011; Delgado-Cerezo et al, 2012; Thomas et al, 2013; Kim and Ralph, 2014)
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