CELLULOLYTIC FUNGI

            Cellulose is the primary structural component of plant cell wall. Cellulose is a linear polymer of D-glucose residue linked by β (1-4) glycosidic bonds. In plant cell wall, the cellulose fibers are embedded in and cross linked by a matrix of several polysaccharide that are composed of glucose as well as other monosaccharides. Cellulase enzymes are capable of hydrolyzing the β (1-4) linkages of cellulose, to convert it into simpler forms.

            A large number of microorganisms are capable of degrading cellulose, but only a few of these microorganisms produce significant quantities of cell-free enzymes capable of completely hydrolysing crystalline cellulose in vitro. Fungi are the main cellulase-producing microorganisms, though a few bacteria and actinomycetes have also been recently reported to yield cellulase activity. Such fungi that produce cellulase enzymes are called cellulolytic fungi.

            A cellulase enzyme system consists of three major components: endo-ß-glucanase, exo-ß-glucanase and ß-glucosidase. The mode of action of each of these being –

            1. Endo-p-glucanase, 1,4-ß-D-glucan glucanohydrolase, CMCase, Cx: "random" scission of cellulose chains yielding glucose and cello-oligo saccharides.

            2. Exo-P-glucanase, 1,4-ß - D-glucan cellobiohydrolase, Avicelase, C1: exo-attack on the non-reducing end of cellulase with cellobiose as the primary structure.

            3. ß-glucosidase, cellobiase: hydrolysis of cellobiose to glucose.

            Fungi of the genera Trichoderma and Aspergillus are thought to be cellulase producers, and crude enzymes produced by these fungi are commercially available for agricultural and industrial use. Fungal species of the genus Trichoderma produce relatively large quantities of endo-ß-glucanase and exo-ß-glucanase, but only low levels of ß-glucosidase, while those of the genus Aspergillus produce relatively large quantities of endo-ß-glucanase and ß-glucosidase with low levels of exo-ß- glucanase production.

            Trichoderma reesei, is a mesophilic and filamentous fungi. T. reesei has the capacity to secrete large amounts of cellulolytic enzymes (cellulases and hemicellulases). These cellulases have industrial application in the conversion of cellulose, a major component of plant biomass, into glucose.

            T. reesei isolate QM6a was originally isolated from the Solomon Islands during World War II because of its degradation of canvas and garments of the US Army. All strains currently used in biotechnology and basic research were derived from this one isolate. Recent advances in the biochemistry of cellulose enzymology, the mechanism of cellulose hydrolysis (cellulolysis), strain improvement, molecular cloning and process engineering are bringing T. reesei cellulases closer to being a commercially viable route to cellulose hydrolysis. Several industrially useful strains have been developed and characterised, e.g. Rut-C30, RL-P37 and MCG-80. The genome of this organism was released in 2008. This organism also has a mating type dependent characterised sexual cycle.

            USE IN INDUSTRY

            T. reesei is an important commercial and industrial micro-organism due to its cellulase production ability. Many strains of T. reesei have been developed since its discovery, with heavy emphasis on increasing cellulase production. These "improvement programs" originally consisted of classical (ionising-radiation-based and chemical-based) mutagenesis, which led to strains capable of producing 20 times as much cellulase as the QM6a isolate. The ultimate aim in the creation of hypercellulolytic strains was to obtain a carbon catabolite derepressed strain. This derepression would allow the T. reesei strain to produce cellulases under any set of growth conditions, even in the presence of glucose.

            However, with the advent of modern genetic engineering tools such as targeted deletion, targeted knockout, and more, a new generation of strains dubbed "hyperproducers" has emerged. Some of the highest performing industrial strains produce up to 100 grams of cellulases per litre, more than 3 times as much as the RUT-C30 strain (which itself produces twice as much as the parent strain NG14 from which it was derived).

            USE IN AGRICULTURE

            Agricultural and industrial wastes are among the causes of environmental pollution. Their conversion into useful products may ameliorate the problems they cause. These wastes which include cereals, straw, leaves, corncobs, etc,. A large quantity is left on farmlands to be decomposed by microorganism such as bacteria and fungi.

            Proper biotechnological utilization of these wastes in the environment will eliminate pollution and convert them into useful by-products.

            Cellulase (a complex multienzyme system) acts collectively to hydrolyze cellulose from agricultural wastes to produce simple glucose units. Celluloses are synthesized by cellulolytic fungi such as the Chaetomium, Funsarium Myrothecium and Trichoderma species. Other species include the Penicillium and Aspergillus species. Such cellulases are in turn used to convert the cellulose present in agricultural waste to turn it into useful products.

Prema Iswary,               

Assistant Professor,     

Department of Botany.

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