Influences Of The Polyphenols And Soluble Polysaccharides Of Rice Straw On Cellulase Production By Trichoderma Reesei

Biofuels are considered as a substitute for fossil fuels,and the demand for biofuels is increasing due to shortage of fossil fuels as well as environmental pollution.In the process of ethanol production from lignocellulosic biomass,it is indispensable for lignocellulosic biomass to hydrolyze polysaccharides to monosaccharide with cellulases.Commercial cellulases are usually produced by fungi,such as Trichoderma reesei,which is widely studied because of its high performance in cellulase production.However,an economic bioconversion for lignocellulosic biomass suffers from high cost for cellulase production.Therefore,the efficient production of cellulase with cheap substrates is a key study of the bioconversion of lignocellulosic biomass to biofuels to reduce production cost.In this study,Trichoderma reesei was used as a producer of cellulase,the influence of components of rice straw on cellulase production was investigated.We found that there were certain amounts of polyphenols and soluble polysaccharides in broth.The influences of polyphenols on lactic acid bacteria,yeast and some pathogenic microorganisms have been extensively studied.On the othe hand,the influences of polyphenols on cellulase production by Trichoderma reesei are rarely noticed.Some oligosaccharides and oligosaccharides can be used as inducers to promote production of cellulase,but there is no study about the influences of soluble polysaccharides from rice straw on cellulase production by Trichoderma reesei.Therefore,this study focused on the influences of polyphenols and soluble polysaccharides from rice straw on cellulase production by Trichoderma reesei.In this study,we found that during cellulase production by Trichoderma reesei large amounts of polyphenols were released from rice straw when the latter was used as the carbon source.We identified and quantified the phenolic compounds in rice straw and investigated the effects of the phenolic compounds on cellulase production by T.reesei.The phenolic compounds of rice straw mainly consisted of phenolic acids and tannins.Coumaric acid(CA)and ferulic acid(FA)were the predominant phenolic acids,which inhibited cellulase production by T.reesei.When the concentrations of CA and FA in the broth increased to 0.06 g/L,cellulase activity decreased by 23% compared with that in the control culture.Even though the rice straw had a lower tannin than phenolic acid content,the tannins had a greater inhibitory effect than the phenolic acids on cellulase production by T.reesei.Tannin concentrations greater than 0.3 g/L completely inhibited cellulase production.Thus,phenolic compounds,especially tannins are the major inhibitors of cellulase production by T.reesei.Therefore,we studied the effects of pretreatments on the release of phenolic compounds.Ball milling played an important role in the release of FA and CA,and hot water extraction was highly efficient in removing tannins.By combining ball milling with extraction by water,the 2-fold higher cellulase activity than in the control culture was obtained.Acid-treated,alkali-treated and ball-milled rice straw was used as a substrate by Trichoderma reesei for cellulase production.Of the three pretreated straws,ball-milled straw provided the highest yield of cellulase,which was 1.3-fold higher than commercial cultures containing cellose.The FT-IR of the precipitate from rice straw with 70 % ethanol showed an absorption peak typical of polysaccharides,while high pressure liquid chromatography(HPLC)assays of monosaccharide composition showed that glucose was the major component,while galactose,xylose,arabinose and fructose were present in much smaller quantities.The addition of soluble polysaccharide(SPS)derived from rice straw increased FPase activity by 1.5-fold and β-glucosidase activity by 2.3-fold,respectively,indicating the inductive effect of SPS upon cellulase production,especially upon the yield of β-glucosidase.Ball-milling influenced the molecular weight and monosaccharide composition of SPS and thus affected the performance of SPS in cellulase production.Finally,the crude enzymes produced in culture with SPS were used for saccharification and provided a 1.3-fold higher yield of glucose than that produced in culture without SPS.Our results indicate that SPS could represent a key factor in solving problems caused by the lack of β-glucosidase activity in enzymes produced by Trichoderma reesei.According to aforementioned facts that the soluble polysaccharide in rice straw can promote cellulase production,we should consider the impact of pretreatment on soluble polysaccharides in rice straw.We used alkali-pretreated rice straw to produce cellulase,and found that β-glucosidase activity was lower than that of the culture with untreated rice straw.Polysaccharides were prepared with alkali and water treatments,and their monosaccharide components were analyzed with HPLC.Glucose is the main component of the polysaccharides,while the water-treated polysaccharides contained more glucose than alkali-treated polysaccharides treatment solution.On the other hand,the alkali-treated polysaccharides contained 5-fold higher xylose content than water-treated polysaccharides.The alkali-treated polysaccharides was mainly drived from the destruction of the rice straw structure of hemicellulose.We also studied the effects of these two types of polysaccharides on cellulase production.It was found that cellulase production was not significantly improved by alkali-treated polysaccharides,especially for β-glucosidase activity.These results showed that the major induction of cellulase production by Trichoderma reesei resulted from soluable polysaccharides from undisrupted rice straw.At last,we extracted the soluble polysaccharide with water,followed by the treatment of the resulting residue with alkali.The combination of water-treated polysaccharides and akali-treated rice straw provided 1.2-fold higher FPase activity than alkali-treated rice straw alone,while the combination did 2.5-fold higher β-glucosidase activity.Such the pretreatment method could be more economical to accomplish an efficent cellulase production.