Abstract
BackgroundAgricultural residue is more efficient than purified cellulose at inducing lignocellulolytic enzyme production in Penicillium oxalicum GZ-2, but in Trichoderma reesei RUT-C30, cellulose induces a more efficient response. To understand the reasons, we designed an artificially simulated plant biomass (cellulose plus xylan) to study the roles and relationships of each component in the production of lignocellulolytic enzymes by P. oxalicum GZ-2.ResultsThe changes in lignocellulolytic enzyme activity, gene expression involving (hemi)cellulolytic enzymes, and the secretome of cultures grown on Avicel (A), xylan (X), or a mixture of both (AX) were studied. The addition of xylan to the cellulose culture did not affect fungal growth but significantly increased the activity of cellulase and hemicellulase. In the AX treatment, the transcripts of cellulase genes (egl1, egl2, egl3, sow, and cbh2) and hemicellulase genes (xyl3 and xyl4) were significantly upregulated (P <0.05). The proportion of biomass-degrading proteins in the secretome was altered; in particular, the percentage of cellulases and hemicellulases was increased. The percentage of cellulases and hemicellulases in the AX secretome increased from 4.5% and 7.6% to 10.3% and 21.8%, respectively, compared to the secretome of the A treatment. Cellobiohydrolase II (encoded by cbh2) and xylanase II (encoded by xyl2) were the main proteins in the secretome, and their corresponding genes (cbh2 and xyl2) were transcripted at the highest levels among the cellulolytic and xylanolytic genes. Several important proteins such as swollenin, cellobiohydrolase, and endo-beta-1,4-xylanase were only induced by AX. Bray-Curtis similarity indices, a dendrogram analysis, and a diversity index all demonstrated that the secretome produced by P. oxalicum GZ-2 depended on the substrate and that strain GZ-2 directionally adjusted the compositions of lignocellulolytic enzymes in its secretome to preferably degrade a complex substrate.ConclusionThe addition of xylan to the cellulose medium not only induces more hemicellulases but also strongly activates cellulase production. The proportion of the biomass-degrading proteins in the secretome was altered significantly, with the proportion of cellulases and hemicellulases especially increased. Xylan and cellulose have positively synergistic effects, and they play a key role in the induction of highly efficient lignocellulolytic enzymes.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-014-0162-2) contains supplementary material, which is available to authorized users.
Highlights
Agricultural residue is more efficient than purified cellulose at inducing lignocellulolytic enzyme production in Penicillium oxalicum GZ-2, but in Trichoderma reesei RUT-C30, cellulose induces a more efficient response
The highest activities of filter paper activity (FPase), carboxymethyl cellulase or endoglucanase (CMCase), and xylanase were obtained in the a mixture of both (AX) treatment, and the enzymatic activities of beta-glucosidase, beta-xylosidase, and cellobiohydrolase induced by AX were at the intermediate level
A and X were the best substrates for cellulase and xylanase production, respectively; the addition of xylan to the cellulose medium significantly decreased the FPase activity but had little effect on the xylanase activity (Additional file 1: Figure S1A)
Summary
Agricultural residue is more efficient than purified cellulose at inducing lignocellulolytic enzyme production in Penicillium oxalicum GZ-2, but in Trichoderma reesei RUT-C30, cellulose induces a more efficient response. Cellulose itself cannot directly trigger the induction of lignocellulolytic enzymes because it is insoluble [5] Soluble saccharides such as cellobiose, sophorose, lactose, sorbose, and galactose have been demonstrated to induce cellulase synthesis in Trichoderma reesei [6,7,8,9]. Trichoderma longibrachiatum cultured on a mixture of lactose (0.8%) and xylan (0.2%) was found to result in significantly higher levels of both xylanase and cellulase than did cultures on either substrate alone [11]. These reports suggested that complicated interactions exist in the induction of these enzymes
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