Rice straw (RS) is the byproducts of paddy crop as well as an abundant resource of lignocellulosic biomass. Its worldwide production is 800 to 1000 million tonnes. More than 75% of RS is produced by Asia countries like China and India (20). Crop straw returning to field is a widely used method for facilitating sustainable agricultural production in many countries (29). However, straw is hard to degrade due to its recalcitrant structure and composition of lignocellulosic material including lignin, hemicellulose and cellulose (20). Furthermore, the presence of silica (7.5–13.9%) in RS make it more recalcitrant than other straw residues including wheat straw and corn straw (20).
Straw-degrading microorganisms playing an important role in straw decomposition process, which could greatly speed up the straw ripening process, increase farmland fertility, and play important role in keeping sustainable development of agriculture (8). Microbial degradation of lignocellulose depends on the secretion of cellulase including exoglucanase (EC3.2.1.91), endoglucanase is also called CMCase (EC3.2.1.4), and β-glucosidase (EC3.2.1.21)(22). Cellulase activity is an important indicator of lignocellulose-degradation ability of the microbial strains. Digestion of straw lignocellulose to saccharides requires the synergy of these three enzymes (9). The high cost of cellulase is one of the main bottlenecks for the lignocellulose decomposition (10). Thus, it is quite important to screening microorganisms with effective cellulase activity and applying on-site plants for enzyme production.
Many straw-degrading microorganisms are well explored for straw degradation, including Trichoderma, Chaetomium, Penicillium, Aspergillus, Acremonium, Rhizopus, etc (19, 24, 25, 28, 4). The most widely used cellulase producing strain is Trichoderma reesei. However, T. reesei often has insufficient β-glucosidase activity, resulting in cellobiose accumulation and reduced enzymatic hydrolysis efficiency (27). By increasing β-glucosidase activity, the competitive product inhibition of cellobiose can be overcome to a certain extent (9). P. purpureus and P. ropeum have been shown to exert higher activity on lignocellulose compared with T. reesei, because they produce more β-glucosidase and hemicellulas (3, 11). P. griseofulvum can produce more β-glucosidase to supplement traditional cellulase products. However, their straw decomposition efficiencies still need increasing. It is necessary to screen different cellulolytic strains. And, natural habitat is a key factor influencing cellulase characteristics. Hence it is important to explore new habitats to search for novel strains with high cellulase producing potential.
As culture condition is important for converting lignocellulosic biomass to saccharides. For filamentous fungi, cellulase gene expression was mainly regulated by transcription activation factors, carbon source metabolism inhibitors, pH and nitrogen source (7). Carbon sources are key to enzyme production because they induce the expression of cellulase and allow microorganisms to secrete various types of enzymes (7). Studies have shown that pH of media strongly influence many enzymatic reactions by affecting the transport of various chemical products and enzymes on cell membranes (12). Naturally, nitrogen stimulates fungal cell growth, which in turn enhances biomass formation and cellulase enzyme expression(2). Thus, culture condition optimization is quite important for RS decomposition.
In this study, RS-degrading strains was isolated from alpine regions to screen microorganisms with better characters. Culture conditions were optimized for cellulase production by the newly isolated fungus P. griseofulvum A2, including nitrogen, pH and incubation time. The main aim is screen new strain with increased cellulase, in order to overcome the limitations that prevent a RS decomposition.