Coupling Secretomics with Enzyme Activities To Compare the Temporal Processes of Wood Metabolism among White and Brown Rot Fungi

Abstract

Wood-degrading fungi use a sequence of oxidative and hydrolytic mechanisms to loosen lignocellulose and then release and metabolize embedded sugars. These temporal sequences have recently been mapped at high resolution using directional growth on wood wafers, revealing previously obscured dynamics as fungi progressively colonize wood. Here, we applied secretomics in the same wafer design to track temporal trends on aspen decayed by fungi with distinct nutritional modes: two brown rot (BR) fungi (Postia placenta and Gloeophyllum trabeum) and two white rot (WR) fungi (Stereum hirsutum and Trametes versicolor). We matched secretomic data from three zones of decay (early, middle, and late) with enzyme activities in these zones, and we included measures of total protein and ergosterol as measures of fungal biomass. In line with previous transcriptomics data, the fungi tested showed an initial investment in pectinases and a delayed investment in glycoside hydrolases (GHs). Brown rot fungi also staggered the abundance of some oxidoreductases ahead of GHs to produce a familiar two-step mechanism. White rot fungi, however, showed late-stage investment in pectinases as well, unlike brown rot fungi. Ligninolytic enzyme activities and abundances were also different between the two white rot fungi. Specifically, S. hirsutum ligninolytic activity was delayed, which was explained almost entirely by the activity and abundance of five atypical manganese peroxidases, unlike more varied peroxidases and laccases in T. versicolor These secretomic analyses support brown rot patterns generated via transcriptomics, they reveal distinct patterns among and within rot types, and they link spectral counts with activities to help functionalize these multistrain secretomic data.IMPORTANCE Wood decay, driven primarily by wood-degrading basidiomycetes, is an essential component of global carbon cycles, and decay mechanisms are essential for understanding forest ecosystem function. These fungi efficiently consolidate pretreatment and saccharification of wood under mild conditions, making them promising templates for low-cost lignocellulose conversion. Species are categorized as ligninolytic white rots and polysaccharide-selective brown rots, with considerable undescribed variability in decay mechanism that may manifest in the sequential variation in protein secretion over the progression of decay. Here we resolved spatially a temporal progression of decay on intact wood wafers and compared secretome dynamics in two white and two brown rot fungi. We identified several universal mechanistic components among decay types, including early pectinolytic "pretreatment" and later-stage glycoside hydrolase-mediated saccharification. Interspecific comparisons also identified considerable mechanistic diversity within rot types, indicating that there are multiple avenues to facilitate white and brown rots.

Keywords: laccase; manganese peroxidase; pectinase; proteomics.