Predominant secretion of cellobiohydrolases and endo-ß-1,4-glucanases in nutrient-limited medium by Aspergillus spp. isolated from subtropical field.

Biological degradation of cellulose from dead plants in nature and plant biomass from agricultural and food-industry waste is important for sustainable carbon recirculation. This study aimed at searching diverse cellulose-degrading systems of wild filamentous fungi and obtaining fungal lines useful for cellooligosaccharide production from agro-industrial wastes. Fungal lines with cellulolytic activity were screened and isolated from stacked rice straw and soil in subtropical fields. Among 13 isolated lines, in liquid culture with a nutrition-limited cellulose-containing medium, four lines of Aspergillus spp. secreted 50-60 kDa proteins as markedly dominant components and gave clear activity bands of possible endo-ß-1,4-glucanase in zymography. Mass spectroscopy (MS) analysis of the dominant components identified three endo-ß-1,4-glucanases (GH5, GH7 and GH12) and two cellobiohydrolases (GH6 and GH7). Cellulose degradation by the secreted proteins was analysed by LC-MS-based measurement of derivatized reducing sugars. The enzymes from the four Aspergillus spp. produced cellobiose from crystalline cellulose and cellotriose at a low level compared with cellobiose. Moreover, though smaller than that from crystalline cellulose, the enzymes of two representative lines degraded powdered rice straw and produced cellobiose. These fungal lines and enzymes would be effective for production of cellooligosaccharides as cellulose degradation-intermediates with added value other than glucose.

Prebiotic Oligosaccharides Prepared by Enzymatic Degradation of Dietary Fibers in Rice Grains.

Rice are consumed mainly as polished rice grains. In the threshing and polishing processes of paddy rice, a considerable amount of husk and bran are separated as by-products. Rice bran is utilized for oil production, whereas rice husk as well as straw is not fully utilized. Defatted rice bran is rich in proteins and non-digestible polysaccharides, while husk and straw consist mainly of plant cell wall components, including cellulose, hemicellulose and pectin. Such non-digestible polysaccharides function in gastrointestinal lumen as dietary fiber, though physiological functions and their application are limited. Non-digestible oligosaccharides have recently been interested as prebiotics from a viewpoint of health benefit via utilization by intestinal microbiota. A diversity of non-digestible polysaccharides in rice bran and husk are good and ecological sources for production of both prebiotic and potentially prebiotic oligosaccharides. In this review, we summarize non-digestible polysaccharides constituting cell wall of rice grains including husk and degradation of the polysaccharides into oligo- and monosaccharides by microbial glycoside hydrolases. Prebiotic potential of such oligosaccharides derived from rice non-digestible polysaccharides are also introduced. Finally, our recent attempt for effective production of cello-oligosaccharides by regulated enzymatic degradation is briefly described.