Xylan Interaction Dominates in Sorghum Secondary Cell Walls ✉ Yu Gao 1,2, Andrew S
Total Page：16
File Type：pdf, Size：1020Kb
ARTICLE https://doi.org/10.1038/s41467-020-19837-z OPEN A grass-specific cellulose–xylan interaction dominates in sorghum secondary cell walls ✉ Yu Gao 1,2, Andrew S. Lipton 3, Yuuki Wittmer4, Dylan T. Murray 4 & Jenny C. Mortimer 1,2,5 Sorghum (Sorghum bicolor L. Moench) is a promising source of lignocellulosic biomass for the production of renewable fuels and chemicals, as well as for forage. Understanding secondary cell wall architecture is key to understanding recalcitrance i.e. identifying features which fi 1234567890():,; prevent the ef cient conversion of complex biomass to simple carbon units. Here, we use multi-dimensional magic angle spinning solid-state NMR to characterize the sorghum sec- ondary cell wall. We show that xylan is mainly in a three-fold screw conformation due to dense arabinosyl substitutions, with close proximity to cellulose. We also show that sorghum secondary cell walls present a high ratio of amorphous to crystalline cellulose as compared to dicots. We propose a model of sorghum cell wall architecture which is dominated by interactions between three-fold screw xylan and amorphous cellulose. This work will aid the design of low-recalcitrance biomass crops, a requirement for a sustainable bioeconomy. 1 Joint BioEnergy Institute, Emeryville, CA 94608, USA. 2 Environmental Genomics and Systems Biology Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, USA. 3 Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, WA 99354, USA. 4 Department of Chemistry, University of California, Davis, CA 95616, USA. 5 School of Agriculture, Food and Wine, Waite Research Institute, Waite ✉ Research Precinct, University of Adelaide, Glen Osmond, SA 5064, Australia. email: [email protected] NATURE COMMUNICATIONS | (2020) 11:6081 | https://doi.org/10.1038/s41467-020-19837-z | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-020-19837-z ignocellulosic biomass, a renewable source of organic car- lignin and polysaccharides, and stiffen the cell wall and maintain bon, will be a major feedstock for sustainable biorefineries structural integrity11,18–23. L 1,2 producing bio-based fuels, chemicals, and materials . These Although the chemical structures of individual cell wall com- biorefineries are required to address the twin global demands of ponents are relatively well defined, the interaction between the cell increased energy availability3,4 and reduced emissions5. Grasses, wall components that form the three-dimensional network such as sorghum (Sorghum bicolor L. Moench), Miscanthus × remains poorly understood for grasses. Methods to access the giganteus, and switchgrass (Panicum virgatum L.), are promising molecular information of intact plant cell walls were limited by the as dedicated bioenergy crops, due to their high photosynthetic need for additional chemical extraction and pre-treatments, until a efficiency and ability to grow in a range of environmental groundbreaking effort by Hong and colleagues which employed conditions4,6. However, plant cell walls, which comprise most of multi-dimensional solid-state Nuclear Magnetic Resonance lignocellulosic biomass, have evolved to provide many crucial (ssNMR) to reveal the architecture of native primary cell walls in functions to the plant, including structural support, and protec- Arabidopsis24–29. More recently, the architecture of secondary tion from herbivory and pathogens7. The resulting highly recal- plant cell walls was also investigated via ssNMR. Simmons et al. citrant cell wall architecture is an impediment to the efficient found direct evidence for the presence of xylan in both two- and deconstruction of plant cell walls into simple monomers8,9. three-fold screw conformations in the dicot secondary cell wall. In Improved understanding of the architecture of intact grass plant a cross-polarizing experiment, which emphasizes the immobile cell walls at a molecular level will provide key insights into this components, the xylan-cellulose interaction was dominated by recalcitrance, and support the predictable design of both the xylan in a two-fold screw conformation, with only a minor pro- biomass deconstruction process, as well as the development of portion of three-fold xylan detected30. Terrett et al. reported engineered bioenergy crops. similar xylan-cellulose interactions in the secondary plant cell walls All plant cells are surrounded by a thin, extensible, primary cell of softwoods due to the even pattern of substitution of softwood wall. Thickened secondary cell walls are only deposited in certain GAX. Additionally, galactoglucomannan, the other major hemi- cell types at the cessation of cell expansion, but form the majority cellulose in softwoods, binds the same cellulose microfibrils. Both of biomass. Secondary cell walls are composed of polysaccharides cellulose-bound xylan and galactoglucomannan are also associated (e.g., cellulose, xylan) and a complex polyphenolic network of with lignin31. Kang et al. employed ssNMR to investigate the lignin. Cellulose makes up 35–45% of the dry weight of grass lignin-polysaccharide interactions in the secondary plant cell walls secondary cell walls10. It consists of flat-ribbons of α-1,4-linked of maize (Zea mays). Results showed a conformation-dependent glucan chains which self-assemble into a crystalline fibrillar bridge behavior of xylan in the secondary plant cell walls, linking structure via hydrogen bonding. The resulting fibril has surfaces the cellulose fibrils and lignin, in which two-fold screw xylan coats with distinct hydrophilicity that allow potential interactions with the cellulose microfibril hydrophilic surface and the three-fold other cell wall components. However, cellulose crystallites are not screw xylan connects the lignin nanodomain via electrostatic perfect, and there is a significant fraction of cellulose that is less interactions32. ordered, referred to as amorphous cellulose. Hence, two major Using Arabidopsis with genetically engineered xylan struc- domains of cellulose can be distinguished: a crystalline cellulose tures, Grantham et al. determined that an even pattern of xylan domain and an amorphous cellulose domain. Xylan is the major substitution is critical for the formation of the two-fold screw hemicellulose, which makes up 20–35% of the dry weight of grass conformation and its interaction with cellulose16. Evenly pat- secondary cell walls11–13. Xylan has a α-1,4-linked xylosyl unit terned decorations on the xylan backbone allow the substitutions backbone. However, unlike in dicot plants, grass xylan is highly to orient along one side of the molecule, leaving the undecorated substituted by α-L-arabinofuranosyl units at the O-2 and O-3 side of the resultant flat-ribbon shape free to hydrogen bond with positions on the xylosyl units of the backbone and comparatively the hydrophilic surface of the cellulose microfibril16,33,34. low density of substitution with α-1,2-glucuronosyl or α-1,2-(4- Although the substitution pattern of the xylan backbone in O-methyl)-glucuronosyl units14. This xylan structure is known as grasses is not known, the large amounts of arabinosyl units on glucuronoarabinoxylan (GAX), though the exact substitution grass xylan suggests that it may not follow the dicot and soft- pattern is dependent on tissue and species. In addition, grass wood pattern, which may in turn alter its interaction with xylan is also substituted with acetyl groups on the backbone cellulose11,12,17. xylosyl units at the O-2 and O-3 positions, and some arabinosyl Here, to explore xylan-cellulose interactions in grasses, we units can also carry acetyls at the O-2 position13,15. Grass xylan is perform multi-dimensional ssNMR analysis on sorghum to reveal acetylated to a lesser extent compared to dicot plants15. Xylans the native architecture of its secondary cell wall. The results are believed to be present in two major conformations, two-/ suggest a lack of two-fold screw xylan, with the majority of the three-fold screw conformations, and the conformation is highly xylan showing a three-fold screw conformation. However, this dependent on the pattern of substitution. For instance, the even three-fold screw xylan shows relatively high rigidity as compared pattern of acetyls and glucuronosyl/4-O-methyl-glucuronosyl to Arabidopsis and softwood reported earlier, and close proximity units on Arabidopsis thaliana (Arabidopsis) allows these sub- with less ordered amorphous cellulose. We propose that sorghum stitutions to lie on one side of the xylan backbone, where every xylan-cellulose interactions are dominated by xylan in the three- two xylosyl units have a 360° turn (two-fold screw) forming a flat- fold screw conformation and amorphous cellulose, in contrast to ribbon shape. Without this even pattern, the xylan will have a the interactions between xylan in the two-fold screw conforma- 360° turn every three xylosyl units (three-fold screw) forming a tion and crystalline cellulose in softwoods and dicot plants. We helical shape due to steric hindrance16. Additionally, another also show that the fraction of amorphous cellulose in the sor- characteristic feature of grass secondary cell wall is the presence ghum secondary cell wall is approximately three-fold higher than of hydroxycinnamic acids (i.e., ferulic acid and p-coumaric acid), that in Arabidopsis, a model dicot plant. These findings provide which are esterified to the O-5 of the α-1,3-arabinosyl substitu- molecular level understanding