The Brown Rot Two-Step
Understanding the biomechanisms brown rot fungi use to degrade wood could lead to new tools for more efficient biofuel production.
Wood’s complex structure of cellulose, long chains of linked sugar molecules, embedded in a scaffolding of polyaromatic lignin makes it highly resistant to biological or chemical decomposition. Brown rot fungi, however, possess a unique ability to attack the cellulose fraction of wood while avoiding the surrounding lignin. This study provides evidence that brown rot fungi accomplish this using a two-step process: (1) by secreting a set of chemicals and enzymes that open up the lignin framework, and then (2) releasing a second set of enzymes that break down the cellulose chains into sugars that are absorbed by the fungi.
Understanding the newly discovered two-step mechanism of this degradation process could lead to the development of new biotechnology approaches for efficient and cost-effective conversion of wood cellulose into biofuels or bioproducts while leaving the lignin intact as a potential useful byproduct.
Wood-decomposing fungi are essential players in breaking down plant biomass in forest ecosystems and could provide important clues on how to more efficiently convert lignocellulose—the primary building block of wood cell walls—to biofuels and other products. Among these organisms, brown rot fungi are unique in their ability to selectively degrade the cellulose in wood while leaving the lignin portion mainly intact. To accomplish this task, these fungi generate highly reactive oxygen species that alter the chemical structure of wood and work in tandem with enzymes that break down cellulose chains. However, reactive oxygen species could just as easily damage the fungal enzymes as the wood structure, so researchers have long hypothesized that the fungi spatially segregate the oxidant generation process from the secreted enzymes using sets of chemical barriers. However, in this study, scientists found evidence that brown rot fungi separate the oxidants and enzymes in time rather than in space. This two-step wood decomposition mechanism was discovered by designing a simple, yet elegant experiment: brown rot fungi were grown in one direction along thin wood specimens separating the stages of wood decay linearly across the substrate. The wood was then cut into sections and analyzed for patterns of gene expression using whole-transcriptome shotgun sequencing (RNA-seq), assayed for relevant enzyme activity, and imaged using confocal and fluorescence microscopy. The researchers found that at early time points in the brown rot colonization, there was evidence of lignocellulose oxidation by reactive oxygen species and an increase in expression of genes important for plant cell wall-swelling. Both of these activities would weaken the structural integrity of wood and make it easier for enzymes to access cellulose chains. Only at later time points of colonization did brown rot fungi begin to produce glycoside hydrolase enzymes that break down cellulose chains into their component sugars. This unique fungal “pretreatment” strategy predates chemical pretreatment approaches used in industrial biomass processing by millions of years and could provide important new clues for improved conversion of woody plant materials into renewable cellulosic biofuels.
Jonathan S. Schilling
University of Minnesota
This work was supported by the U.S. Department of Energy (DOE), Office of Science, Office of Biological and Environmental Research under award numbers DE-SC0004012 (DOE Early Career Research) and DE-SC0012742. This research also used resources at the Environmental Molecular Sciences Laboratory, which is a DOE Office of Science user facility.
Zhang, J., G. N. Presley, K. E. Hammel, J.-S. Ryu, J. R. Menke, M. Figueroa, D. Hu, G. Orr, and J. S. Schilling. 2016. “Localizing Gene Regulation Reveals a Staggered Wood Decay Mechanism for the Brown Rot Fungus Postia placenta,” Proceedings of the National Academy of Sciences (USA) 113(39), 10968-973. DOI: 10.1073/pnas.1608454113.