This study provides heretofore unavailable molecular evidence to define the 3D architecture of secondary cell walls
The secondary cell wall is key to plant life and accounts for the majority of plant biomass. It is composed of cellulose, hemicellulose, and lignin. Based on the limited molecular information available relating to secondary cell wall organization, it was proposed that bundles of cellulose microfibrils are coated by a xylan-lignin complex and crosslinked by glucomannans.
Lignin is a complex aromatic biopolymer that serves to strengthen and waterproof plant secondary cell walls. It, therefore, provides mechanical stability and facilitates long-distance water transport in trees. This essential component of plants, however, represents a barrier to the use of plant materials for manufacturing and industrial purposes as the lignin makes the cellulose inherently recalcitrant to chemical and enzymatical treatments. Lignin removal is consequently a key step in paper production and biomass conversion to biofuels.
There has been much research aimed at genetically engineering plants to alter the structure of the cell wall in such a way that increases its digestibility. These efforts have been significantly impeded by the limited understanding of the structure of cell wall polymers and the physical nature of the interactions between lignin and cell wall polysaccharides.
Recent research has made a valuable contribution to the knowledge of the lignocellulose structure using a novel imaging technique. Intact maize stems were analyzed by solid-state NMR spectroscopy using Bruker Avance spectrometers with sensitivity enhancement by dynamic nuclear polarization (DNP). The exceptional resolution and sensitivity of ssNMR with DNP enabled a significantly larger dataset to be obtained than was previously possible. Furthermore, minimal sample preparation was required so the cell wall structure would not be disturbed and give rise to artifacts.
The images obtained revealed abundant electrostatic interactions of lignin with the polar motifs of xylan, rather than with cellulose as was previously thought. The two hydrophobic cores of lignin and bundled cellulose are bridged by xylan in a conformation-dependent manner. Thus, rather than lignin coating the cellulose bundles, lignin self-aggregates to form highly hydrophobic and dynamically unique nanodomains. These lignin domains have extensive surface contacts to xylan, with only limited interpenetration being possible between the separate phases. It was also shown that a distorted xylan structure favors lignin-binding. This contrasts with previous findings that flat xylan conformers bind cellulose. The cell wall structure that emerges from this ssNMR study thus differs substantially from contemporary views.
The novel molecular characteristics discovered in this study advance knowledge of the molecular-level organization of lignocellulosic biomass. This knowledge will enable further study to optimize of post-harvest processing of plant materials for biofuels and biomaterials.
Kang X,et al. Lignin-polysaccharide interactions in plant secondary cell walls revealed by solid-state NMR. Nature Communications 2019;Volume 10:Article number 347. https://www.nature.com/articles/s41467-018-08252-0#Sec9