Accumulation of recalcitrant xylan in mushroom-compost is due to a lack of xylan substituent removing enzyme activities of Agaricus bisporus

The ability of Agaricus bisporus to degrade xylan in wheat straw based compost during mushroom formation is unclear. In this paper, xylan was extracted from the compost with water, 1 M and 4 M alkali. Over the phases analyzed, the remaining xylan was increasingly substituted with (4-O-methyl-)glucuronic acid and arabinosyl residues, both one and two arabinosyl residues per xylosyl residue remained. In the 1 M and 4M KOH soluble solids of spent compost, 33 and 49 out of 100 xylosyl residues, respectively, were substituted. The accumulation of glucuronic acid substituents matched with the analysis that the two A. bisporus genes encoding for alpha-glucuronidase activity (both GH115) were not expressed in the A. bisporus mycelium in the compost during fruiting. Also, in a maximum likelihood tree it was shown that it is not likely that A. bisporus possesses genes encoding for the activity to remove arabinose from xylosyl residues having two arabinosyl residues.