Cycling in degradation of organic polymers and uptake of nutrients by a litter-degrading fungus

Abstract

Wood and litter degrading fungi are the main decomposers of lignocellulose and thus play a key role in carbon cycling in nature. Here, we provide evidence for a novel lignocellulose degradation strategy employed by the litter degrading fungus Agaricus bisporus (known as the white button mushroom). Fusion of hyphae allows this fungus to synchronize the activity of its mycelium over large distances (50 cm). The synchronized activity has a 13-h interval that increases to 20 h before becoming irregular and it is associated with a 3.5-fold increase in respiration, while compost temperature increases up to 2°C. Transcriptomic analysis of this burst-like phenomenon supports a cyclic degradation of lignin, deconstruction of (hemi-) cellulose and microbial cell wall polymers, and uptake of degradation products during vegetative growth of A. bisporus. Cycling in expression of the ligninolytic system, of enzymes involved in saccharification, and of proteins involved in nutrient uptake is proposed to provide an efficient way for degradation of substrates such as litter.

Fig. 1

Cumulative CO₂ production (A), CO₂ production rate (B), and respiratory quotient (C) by sterile compost (dark grey), compost without A. bisporus (light grey), and compost inoculated with A. bisporus (black). O₂ levels were below 17.97% (the limit at which O₂ could be measured) at time points indicated with a black dot (C), resulting in an underestimation of the actual O₂ consumption and therefore overestimation of the RQ.

Fig. 2

Compost temperature measured by six sensors spaced 15–25 cm apart in a 40 × 60 cm box (A) and the maximum peak delay between the sensors during a burst (B). LUC = left upper corner, RUC = right upper corner, LM = left of the middle, RM = right of the middle, LDC = left bottom corner, RDC = right bottom corner.

Fig. 3

Typical temperature profiles of two parts of compost (dark and light grey) pressed together (A), placed 3 cm apart (B), or separated by aluminium foil (C). Arrows indicate where heat transfer through the aluminium foil was detected by the temperature sensor. Each experiment was performed five times. The average difference between two temperature sensors picking up a burst (i.e. the time delay of a burst) for each condition was tested using Mann–Whitney U, P < 0.05.

Fig. 4

Experimental setup (A) and typical temperature profile of compost divided in four areas, two of which were inoculated with A15 (B and C, black lines) and two with Bisp015 (B and C, grey lines) in diagonal positions in a box. Numbers in panel B and C relate to the location of temperature sensors in panel A. The temperature profiles of A15 and Bisp015 in panel B and C are representative of two independent experiments.

Fig. 5

Compost temperature (black line) and mycelial growth expressed as grey value (grey line, based on the area in the white dashed boxes) during growth of A. bisporus from PIII compost into fresh PII compost (border indicated by the dashed line). Scans of the compost after 150, 163, 187, 239, and 360 h are shown with the position of the temperature sensor indicated (white dot). These data are representative of two independent experiments.

Fig. 6

Schematic representation of experimental setup for the transcriptome analysis of respiratory bursts (A) and analysis of differentially expressed genes. Samples were harvested in between bursts (end of blue line), during bursts (end of red line), or after a burst (end of green line). For each point, RNA was isolated from three biological replicates. Up‐ (B, top panel) and down‐ (B, lower panel) regulated genes before a burst, during a burst, and after a burst (i.e. greater than or equal to twofold up or downregulated, with an fpkm > 10 in at least one of the three conditions and a q‐value < 0.05). Relative abundance of FPKM of differentially expressed genes annotated as CAZYs, MEROPS proteases, P450 enzymes, heme‐thiolate peroxidases (HTP), gluthatione‐S‐transferase (GST), sugar transporters, amino acid transporters, and other transporters (C).

Fig. 7

Expression values (FPKM) of differentially expressed classes of CAZYs related to lignin degradation (A), cellulose degradation (B), hemicellulose degradation (C), pectin degradation (D), microbial cell wall degradation and remodelling (E), carbohydrate esterases (F), other GHs and expansins (G). Bars represent expression values before (inter‐burst) during (burst) and after bursts (after burst). The number of differentially expressed genes that are considered for each CAZY class are listed in the legenda of each subfigure.