Acetylation of Pinus radiata delays hydrolytic depolymerisation by the brown-rot fungus Rhondonia placenta

Abstract

Acetylation of wood can provide protection against wood deteriorating fungi, but the exact degradation mechanism remains unclear. The aim of this study was to determine the effect of acetylation of Pinus radiata wood (weight percent gain 13, 17 and 21%) on the expression of genes involved in decay by brown-rot fungus Rhodonia placenta. Gene expression analysis using qRT-PCR captured incipient to advanced decay stages. As expected the initiation of decay was delayed as a result the degree of acetylation. However, once decay was established, the rate of degradation in acetylated samples was similar to that of unmodified wood. This suggests a delay in decay rather than an absolute protection threshold at higher acetylation levels. In accordance with previous studies, the oxidative system of R. placenta was more active in wood with higher degrees of acetylation and expression of cellulose active enzymes was delayed for acetylated samples compared to untreated samples. The reason for the delay in the latter might be because of the slower diffusion rate in acetylated wood or that partially acetylated cellobiose may be less effective in triggering production of saccharification enzymes. Enzymes involved in hemicellulose and pectin degradation have previously not been focused on in studies of degradation of acetylated wood. Surprisingly, CE16 carbohydrate esterase, assumed to be involved in deacetylation of carbohydrates, was expressed

Initial Rhodonia placenta Gene Expression in Acetylated Wood: Group-Wise Upregulation of Non-Enzymatic Oxidative Wood Degradation Genes Depending on the Treatment Level

“…Hunt et al [26] reported a decreased diffusion of K + ions with an increasing level of acetylation, which supports the theory to be involved in their wood decaying system [64]. Oxaloacetate dehydrogenase (CyOx Ppl112832) is presumed to be involved in oxalic acid synthesis, which is important for sequestering and reducing Fe 3+ to Fe 2+ [13]. Glyoxylate dehydroxygenase (GlyD Ppl121561) is likely involved in oxalic acid synthesis, by catalyzing the production of oxalate through oxidation of glyoxalate [65].…”

“…Each sample was run with three technical replicates. β-tubulin was used as an endogenous control [13]. A list with all used primers including the number of the Joint Genome Institute (JGI) can be found in Table 2.…”

“…Wood can be either physically (heat-treated wood) or chemically (for example wood acetylation and furfurylation) modified. Acetylation is a widely studied wood modification method [8,[10][11][12][13]. During acetylation, hydroxyl groups in the wood are replaced by acetyl groups [1,8], leading to reduced hygroscopicity and volumetric swelling which bulks the cell walls and reduces water absorption [8,[14][15][16].…”

“…Previous studies (mostly on R. placenta) have shown that brown-rot fungi are not killed when growing on acetylated wood and can express genes needed for both the non-enzymatic oxidative degradation process and the enzymatic wood degradation process [11,13,[68][69][70]. Even though the fungi express the genes needed for degradation, the wood remains intact for a prolonged period of time as compared with untreated wood materials.…”

“…Even though the fungi express the genes needed for degradation, the wood remains intact for a prolonged period of time as compared with untreated wood materials. Since the fungi express genes involved in wood degradation in modified wood, before mass loss occurs, at similar levels as in untreated wood where degradation has begun, it is clearly shown that the fungi are attempting to degrade the wood but fail [11,13,[69][70][71]. Despite previous research, it is still not known which regulatory mechanisms in the brown-rot degradation machinery are prevalent during degradation of acetylated wood.…”