Degradation of [14C]Lignin-labelled Wheat Lignocellulose by White-rot Fungi

Abstract

[lignin-14C]Lignocellulose was prepared from wheat seedlings, injected with L-[U-' 4C]-phenylalanine, by a procedure that selectively removed radiolabelled low molecular weight compounds and protein. 4C was mainly localized in the acid-insoluble fraction ('Klason lignin') of the lignocellulose and the polymeric nature of the radiolabelled material was confirmed by gel permeation chromatography. Ferulic and p-coumaric acids were found to be esterified to both carbohydrate and lignin fractions of the wheat lignocellulose, but contained only a small proportion of the total 14C. The white-rot fungi Phanerochaete chrysosporium, Sporotrichum pulverulentum and Coriolus versicolor degraded the radiolabelled substrate; the S . pulverulentum culture was the most active, releasing almost 50% of the 14C as 14C02 in 15 d at 37 "C. In all three species I4CO2 evolution commenced after a lag period during which growth occurred, and was greatly enhanced by reducing the nitrogen concentration in the medium. This indicated that grass lignin degradation by these fungi is subject to the same nutrient nitrogen regulation as the degradation of wood and synthetic lignins. In a culture of P . chrysosporium grown on the radiolabelled substrate for 50d at 37"C, I4CO2 was the main product. Low molecular weight degradation products present in the culture medium were acid-soluble and accounted for a much smaller proportion of the original [ 14C]lignin.

Lignocellulose-degrading actinomycetes

“…Thin mesophila and Stm. cvaneus released 7-8% of the 14C in this substrate as 14CO~ in 14 days [17] compared to 20-50% 14CO2 evolved in 15 days by 3 species of white-rot fungi grown on the same substrate [108].…”

“…After extraction to remove low molecular weight material and protease treatment (if [UC]phenylalanine has been used as the precursor), detailed analysis is required to determine the specificity of radiolabelling. A combination of acid, alkali and cellulase treatments together with the application of thin-layer, reversephase and gel permeation chromatographic techniques provide a comprehensive profile of the radiolabelled substrate [108]. Nitrobenzene oxidation to demonstrate that 14C is present in the products derived from lignin has also been applied [36].…”

Biodegradation of lignin-carbohydrate complexes

“…In recent years, studies on the solubilization of lignin from grasses or wood labeled with 14C phenylalanine have proliferated (Crawford 1978;Reid et al 1982;McCarthy et al 1984). The bulk of the radioactive label is incorporated into the lignin rather than into carbohydrate or protein of the plant, but it is clear that lignin purified from the labeled plant tissue contains significant amounts of carbohydrate.…”

“…Phanerochaete chrysosporium will form water-soluble products from lignin isolated from aspen (Reid et al 1982) or wheat seedlings (McCarthy et al 1984), as will a number of actinomycetes. Organisms reported to solubilize grass lignins include Streptomyces viridosporus, S. baddius (Pometto & Crawford 1986), S. cyanus, Thermomonospora mesophila, and Actinomadura sp.…”

Lignocellulolysis by ascomycetes (fungi) of a saltmarsh grass (smooth cordgrass)

“…In all three of the above experiments, it is possible that nitrogen sufficiency (from marsh sediment, green leaves, and other added N) suppressed fungal oxidation of lignin (McCarthy et al, 1984;Reid, 1991), and in the 1984 experiment the physicochemical preexposure of LC may have made it available to bacterial enzymes that would not naturally have been physically capable of contacting the LC (Backa et al, 1993;Flournoy et al, 1993;Robinson, 1990;Srebotnik and Messner, 1990). Background Electron Microscopy of Lignocellulolysis As an alternative to the empirical route for determination of the LC-lytic capabilities of cordgrass ascomycetes, we chose to follow the lead of wood-decay science (Adaskaveg et al, 1991;Blanchette, 1991;Eriksson et al, 1990) and examine cordgrass-ascomycete LC-lysis in as direct a fashion as possible: by electron-microscopically examining the interactions that occur in nature between fungal hyphae and cordgrass LC.…”