Revealing changes in molecular composition of plant cell walls on the micron-level by Raman mapping and vertex component analysis (VCA)

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.

From the sap's perspective: The nature of vessel surfaces in angiosperm xylem

“…Our findings suggest that the lumen surface consists of a mixture of cellulose and lignin, consistent with previous studies (Saka and Goring, 1985;Fromm et al, 2003;Kukkola et al, 2004;Gierlinger, 2014;Herbette et al, 2015). This result implies that the lumen surface is neither completely hydrophilic (i.e., zero contact angle) nor completely hydrophobic (contact angle >90°).…”

“…1). Previous studies have found the S3 layer to have the same lignin concentration as the S2 layer in birch (Saka and Goring, 1985), be relatively low in lignin in beech (Fromm et al, 2003), and appearing to have a different lignin composition than the S2 layer (Gierlinger, 2014), such as having a high concentration of the dibenzodioxocin substructure of lignin in birch (Kukkola et al, 2004). Lignin was detected throughout the secondary walls making up pit borders of poplar, including the lumen-facing and pit-chamber-facing surfaces (Herbette et al, 2015).…”

Label‐free live cell imaging by Confocal Raman Microscopy identifies CHO host and producer cell lines

“…CRM and multivariate data analysis approaches have been successfully combined to reveal non‐dominant features and small spectral variations 24, 25, 26.…”

Application of Raman spectroscopy to analyse lignin/cellulose ratio in Norway spruce tree rings

“…It presents spectral information complementary to that from the Fourier-transform infrared spectroscopy (FTIR) and near-infrared spectroscopy (NIR) techniques based on infrared absorption (Rana et al 2008, Tsuchikawa andKobori 2015). Raman spectra of woody tissues reflect in particular the vibrational features of lignin, cellulose, and hemicelluloses (Gierlinger et al 2013). The contribution of hemicelluloses to the Raman spectra overlaps with that of cellulose.…”

“…Raman micro-spectroscopy enables analysis of biological tissues from the leaf level Baranska 2007, Vítek et al 2017) down to the cellular or subcellular level (Kaczor and Pilarczyk 2014, Prats-Mateu et al 2014, Vítek et al 2016. It has been established as a non-invasive analytical tool for chemical and structural analyses of wood (Agarwal and Ralph 1997), including for imaging analysis of cell wall structure (Gierlinger and Schwanninger 2007, Gierlinger et al 2012, 2013.…”