Brown rot (BR) decay mechanisms employ carbohydrate-active enzymes (CAZymes) as well as a unique non-enzymatic chelator-mediated Fenton (CMF) chemistry to deconstruct lignocellulosic materials. Unlike white rot fungi, BR fungi lack peroxidases for lignin deconstruction, and also lack some endoglucanase/cellobiohydrolase activities. The role that the CMF mechanism plays in “opening up” the wood cell wall structure in advance of enzymatic action, and any interaction between CMF constituents and the selective CAZyme suite that BRs possess, is still unclear. Expression patterns for CMF redox metabolites and lytic polysaccharide monooxygenase (LPMO–AA9 family) genes showed that some LPMO isozymes were upregulated with genes associated with CMF at early stages of brown rot by Gloeophyllum trabeum. In the structural studies, wood decayed by the G. trabeum was compared to CMF-treated wood, or CMF-treated wood followed by treatment with either the early-upregulated LPMO or a commercial CAZyme cocktail. Structural modification of decayed/treated wood was characterized using small angle neutron scattering. CMF treatment produced neutron scattering patterns similar to that of the BR decay indicating that both systems enlarged the nanopore structure of wood cell walls to permit enzyme access. Enzymatic deconstruction of cellulose or lignin in raw wood samples was not achieved via CAZyme cocktail or LPMO enzyme action alone. CMF treatment resulted in depolymerization of crystalline cellulose as attack progressed from the outer regions of individual crystallites. Multiple pulses of CMF treatment on raw wood showed a progressive increase in the spacing between the cellulose elementary fibrils (EFs), indicating the CMF eroded the matrix outside the EF bundles, leading to less tightly packed EFs. Peracetic acid delignification treatment enhanced subsequent CMF treatment effects, and allowed both enzyme systems to further increase spacing of the EFs. Moreover, even after a single pulse of CMF treatment, both enzymes were apparently able to penetrate the cell wall to further increase EF spacing. The data suggest the potential for the early-upregulated LPMO enzyme to work in association with CMF chemistry, suggesting that G. trabeum may have adopted mechanisms to integrate non-enzymatic and enzymatic chemistries together during early stages of brown rot decay.
Bibliographical noteFunding Information:
We thank Novozymes Bioenergy for the gift of the Cellic® CTec2 and HTec2 – Enzymes. Funding. YZ appreciates support from the National Natural Science Foundation of China (No. 31890772). We also acknowledge support from the National Institute of Food and Agriculture, U.S. Department of Agriculture, the UMass Center for Agriculture, Food and the Environment under project number S1075 – MAS00503. The contents are solely the responsibility of the authors and do not necessarily represent the official views of the USDA or NIFA. MY and YK were supported by grant funding from JSPS KAKENHI 18H02252. DOE co-authors were funded by the DOE Office of Science, Office of Biological and Environmental Research under the Genomic Science Program (FWP ERKP752).
© Copyright © 2020 Zhu, Plaza, Kojima, Yoshida, Zhang, Jellison, Pingali, O’Neill and Goodell.
- LPMO upregulated enzyme expression
- SANS (small-angle neutron scattering)
- brown rot of lignocellulose
- chelator-mediated Fenton (CMF) degradation
- non-enzymatic activity
- wood decay fungi