The adherence and digestion of lignocellulosic biomass by microbes is not such an easy understanding, in fact, it requires very specific molecular binding sites which are made to facilitate this purpose. These kind of specific molecular structures are known as cellulosomes and have a complex of varied enzymatic domains. The cellulosomes work in an efficient way in which they have proper mechanism of actions i.e., they attach to the biomass in the first step and in the next step; they degrade the biomass resulting in components which are further absorbed by the microbes to fulfill their food requirements [13] .

As compared to bacterial cellulase systems, fungal cellulases are structurally less complicated. Fungal cellulases usually consist of 2 separate domains: cellulose binding module (CBM) and a catalytic domain (CD), which has a short polylinker region to its N-terminal to join cellulose binding module (CBM) with it. The CBM has 35 amino acids, and the linker region has a plenty of Serine and Threonine. The major differentiating character between cellulosomes i.e., bound cellulase and free cellulase is that cellulosomes-cohesion has scaffolding and dockerin containing enzyme. Cellulose binding domains (CBMs) replaced by a dockerin in cellulosomal complex in free cellulase, and one scaffolding-born CBM directs the complete cellulosomes complex to cellulosic biomass [14] [19] .

Fermentation technique has been mostly used for the production of cellulases and T. reesai has been widely used in bioprocessing for cellulase production. Solid substrate fermentation could be preferred over liquid media in case of aerobic microorganisms [15] .

The production of cellulase and pectinase using Aspergillus niger on corn cobs as a carbon source is examined. Different parameters are implied to check the optimization including temperature, pH, biomass production and activity of enzyme. The maximum activity of cellulase (1.9 × 10⁻⁴ μg/mL/sec) is produced on 4^th day while pectinase shows maximum activity (1.5 × 10⁻⁴ μg/mL/sec) on 4^th and 5^th day. The temperature range of 50˚C is found to be optimum for cellulase activity (1.3 × 10⁻⁴ μg/mL/sec) while activity of pectinase (1.6 × 10⁻⁴ μg/mL/sec) shows 60˚C as optimum temperature [16] - [19] . The pH 4 is optimum for cellulase activity (2.70 × 10⁻⁴ μg/mL/sec) and pH 6 is for activity of pectinase (1.5 × 10⁻⁴ μg/mL/sec). The study reveals that Aspergillus niger has the capability of producing cellulase and pectinase using corn cobs under SSF [20] .

Bagasse powder is used as a substrate for cellulase production employing novel thermo-stable yeast. Maximum cellulase yield obtains at 50˚C, medium of bagasse powder 4% (w/v) + (NH₄)₂SO₄ = 0.1% (w/v), pH 5.5 and incubation time of 72 hours. Moreover the isolated yield is tolerant to wide ranges of substrate concentration, temperature and pH expressing higher productivity of enzyme [21] . Additionally, C1 exo-gluconase and endo-gluconase using a crude lignocellulosic material are also produced and hence can even be used for ethanol production [21] . Solely, fungi naturally manufacture the require titers of cellulases needed for the entire saccharification (30 - 50 mg enzyme/g of crystalline cellulose) [6] [22] . Many cellulase producing fungi including Trichoderma, penicillium, Botrytis neurospora [7] [23] genra Aspergilli [8] [24] Aspergillus niger and Aspergillus terreus, Rhizopus stolonifer [9] [25] , Fusarium oxysporum [10] [26] are suitable for bioprocessing.

Solid state fermentation is the cheapest way of cellulase production from agro industrial wastes [11] [27] . Interestingly, recent studies report that SSF provides an additional adequate environment for fungi [12] [28] - [30] for various enzymes production. SSF commercialization has been used for production of enzymes (~3.5 billion tons per year). The advantage of exploiting SSF to attain the low cost fermentation system needed and the likelihood of getting it administrated on farms [13] [31] . Moreover, it is environmentally favorable, low energy demanding, inhibiting waste water release and economically feasible [14] [32] . The roles of Cellulases are inevitable in paper, pulp and textile industry of the world. Cellulose has a higher level of crystallinity and thus becomes difficult to be broken down into sub particles. To overcome this problem, Cellulolytic enzymes are used synergistically because the combination of cellulases expresses much more activities than the activity of individual cellulases. The action of these enzymes has been more elaborately explained by a most common and most accepted endo-exo energy model. This model suggests that there is proper mechanism of action employ by such enzymes in which endo-glucanases attack on random sites of the lignocellulosic chains exposing some new sites for cellobiohydrolases to attack. Cellobiohydrolases further performs its function as exoenzymes liberating two main products; one of the products is cellobiose which is the major product of this degradation process. On the other hand, β-glucosidases, which arenot considered legitimate cellulases, play a vital role within the break- down of cellobiose and some other short oligosaccharides to final product is glucose [16] [33] . Cellulase activity is described as the capability of cellulase to digest crystalline cellulose extensively [17] [34] .

Aspergillus terreus ion for cellulase production using rice straw as a substrate under solid state fermentation is reported and Response surface methodology implicating Box-Behnken-design apply to optimize temperature and pH. The filter paper activity which is predicted under optimized conditions is 9.73 U/g and the approved activity is 10.96 U/g. The study shows the use of pretreated rice straw with mild alkali to produce fermentable sugars with 74.19% adequacy [35] .

Rice grass (Spartina spp.) use lignocellulosic material by implicating Aspergillus species under solid state fermentation process. The study reveals the efficacy of using rice grass (Spartina sp.) as the major substrate for yielding cellulase through a novel isolated strain of Aspergillus sp. (SEMCC-3.248) in solid-state fermentation. The parameters which are optimized for cellulase production, are rice grass 2.5 g, 1.5 g of wheat bran, 4 mL of nutrient medium ((NH₄)₂SO₄ 14 g/L, CaCl₂ 4 g/L, KH₂PO₄ 2 g/L, soluble starch 2.62 g/L, MgSO4.7H2O 0.2 g/L and peptone 1.51 g/L), 1 mL of inoculum, moisture level 70%, pH 5.0, temperature 32˚C and 5 days of incubation period. Following these optimum conditions, the total cellulase activity is 1.14 FPIU/gds [36] .

Aspergillus niger HQ-1 is studied for cultivation and optimization using solid state fermentation process. Plackett-Burman design (PBD) is used to identify optimum incubation temperature, moisture content and culture pH for cellulase activity. The optimal regions containing three significant factors, is determined. Furthermore, response surface analysis and Box-Behnken design (BBD) are employed to find interactive effect between the three variables on the activity of cellulases. The optimum conditions are expressed to be temperature 33.5˚C - 33.7˚C, moisture level 70.3% - 70.6% and pH 4.626 - 4.662. Moreover, the activity of cellulase or hydrolysis of chitosan is high at 50˚C and pH 5.6. This cellulase hydrolyzing activity is further improved by some metal ions Mg²⁺, Mn²⁺, K⁺ and Ca²⁺ while is inhibited by Ba²⁺, Zn²⁺, Co²⁺, Cu²⁺, Fe³⁺ and Ag⁺ [37] .

Corn cobs are used for the production of cellulase enzyme using Alternaria alternata through solid state fermentation. Different optimizing parameters are implied to check the maximum yield of cellulase including incubation period (1 - 7 days), pH (3.0 - 9.0) and temperature (25˚C - 40˚C). The optimal cultural conditions are like incubation period of 96 hours, pH 6.0 and incubation temperature as 35˚C expressing cellulase activity as 15.06 μg/mL, 26.4106 μg/mL and 31.2406 μg/mL respectively [38] .

Eichhornia crassipes (water hyacinth) is used for cellulase production and the growth medium is enriched with water hyacinth mixture in ratio 1:05 (V/V) as energy source. Maximum cellulase is produced after incubation time of 6 days, temperature 30˚C, 150 rpm shaking speed at pH 5.0. Cellulases show maximum activity at optimum conditions including 40˚C temperature and pH 5.0. Vmax and Km are observed to be 58.3 μmol/ mL/min and 4.7 mg/mL respectively [39] .

Solid state fermentation methodology is employed for endo-cellulases production of by Aspergillus japonicus C03. The temperature for maximum production is observed to be 50˚C - 55˚C for cellulase production with the optimum pH of 4.0. Moreover, this enzyme is capable to bear the pH change of 4.0 - 7.0. It is also observed that Manganese and Copper enhanced the cellulase activity up to 64% [40] .

Soybean hulls are used to yield cellulolytic enzymes under solid state fermentation by Aspergillus oryzae and Trichoderma reesei cultures. It is observed that crystallinity is extremely increased by mild acid, alkali and steam pretreatments. The steam pretreated hulls is first inoculated with T. reesei and shows more cellulase activities (4 filter paper units (FPU)/g-ds, 45 IU/g-ds endo-cellulase and 0.6 IU/g-ds β-glucosidase) than untreated soybean hulls (0.75 FPU/g-ds, 7.29 IU/g-ds endocellulase and 0.06 IU/g-ds β-glucosidase). In case of A. oryza, the pretreated hulls produce more endo-cellulases (47.10 IU/g-ds) than the untreated hulls (30.82 IU/g-ds). The work shows an interrelationship between enzymatic production and physiochemical characteristics [11] [41] .

Castor bean is used as a substrate for cellulase production using Aspergillus japonicus URM5620 under solid state fermentation. A full factorial design (2⁴) is used to study the effects of different parameters like substrate concentration, pH, moisture level, incubation period and temperature on enzyme yield. The optimum conditions are observed to be substrate concentration 5.0 g, pH 6.0, moisture level 15%, 120 h of incubation period and 25˚C temperature. The optimization processes describes clearly the impact on enzyme production [41] .

Some mutant strains of Aspergillus sp. SU14 for the production of cellulase are employed. Aspergillus sp. SU14 spores are frequently treated with ultraviolet irradiation, (Co60) γ-rays and N-methyl-N’-nitro-N-nitros- oguanidine. Aspergillus sp. SU14-M15 is a mutant strain with cellulase production 2.2-fold more than that of wild type. The optimum requirements for growth are examined to be medium containing wheat-bran enriched with urea 1% (w/w), rice starch 1% (w/w), Tween 80 0.05% (v/w) and MgCl₂ 2.5 mM, moisture 50% (v/w), pH 3.5 with aeration area of 3/100. When, 25% of 48 hours seeding culture is inoculated for 3 days at 35˚C, the resultant cellulase production is 8.5 times more than the conventional type of cellulase production [42] .

Cellulase production on carboxymethyl cellulose by Aspergillus niger which is isolated from various sources of soil, is also studied with shaking flasks incubation with ambient temperature. All isolated strains show cellulase activity with the maximum yield at day 4 (0.07162 IU/mL/min) produce by Aspergillus niger isolated by rice growing field whereas minimum activity (0.02911 IU/mL/min) is by Aspergillus niger, isolated from street soil. The experiment reveals the cellulolytic capability of Aspergillus niger in almost all soil environments. Moreover, the results display that this robust strain can be isolated from rice growing fields for the production of commercial cellulase [43] .

Cellulases are amongst the most important group of enzyme in industry [84] and they have been employed to reduce the faded look and protruding fibers in fabrics and garments and to give them softness. Before that, pumice stone was used traditionally for this purpose [24] [85] - [87] .

Cellulases from H. insolens are frequently used in bio-stoning along with proteases and trichoderma [88] . Cellulases give better finish and digest small fibers that cause roughness of the fabrics [70] [89] . They have been employed for defibrillation and softening of fabrics [89] . Cellulases are good localizing agents and are used to eliminate color variation of fibers [89] [90] .

Cellulase CBH I and EG III have excellent cleansing properties and are used in textile cleaning. It has been reported that T. reesai producing EG III variants are suitable for the modification detergents. Similarly, T. harzianum and T. viride are also used as naturally producing sources of cellulases like A. niger [91] [111] .

Cellulase production from Humicola species (H. grisea and H. insolens) is effective under mild alkaline conditions and at elevated temperatures. So, they are mostly used as additives in detergents and washing powders [92] [93] . Cellulases are added to detergents for the breakdown of hydrogen bonding under harsh environmental conditions such as alkaline or thermophile conditions [94] [111] .

Cellulases are employed in food industry to extract and clarify olive oil, fruit and vegetable juices, in the production purposes and fruit nectars [70] [94] . In brewing industry, glucanases are used as additives for the improvement of malting of barley. Decent color extraction and maceration could be achieved using glucanase and hemicellulose [70] [94] . Carotenoids, which have been used as food coloring agents can be extracted through cellulases [94] . Cellulases along with pectinases and hemicellulases have been used to modify nutritive quality of forages [95] . Digestibility and performances of animal feed has been reported to improve using cellulases [96] . Bedford et al. [97] reported that better digestibility and feed conversion ratio of cereal feed could be achieved through the addition of Trichoderma cellulases [97] [111] .

In paper and pulp industry, hemicellulases and cellulases have been used to modify the biochemical pulping of coarse pulp and to improve strengthening [97] [98] . These are equally useful for the depolarization of recycled pulp and for the efficiency improvement and drainage of paper mills [99] [100] . Cellulases have been employed to remove toners and coatings from paper [101] [102] . Microbial cellulases have also been used for the characterization of fiber pulp. Manufacturing of biodegradable cardboard can be easily done using cellulases [103] and also used to improve the soft texture of paper, manufacturing of sanitary paper and paper towels [104] - [106] .

Production of biofuel is one of the most recently investigated applications of cellulases in the bioconversion of lignocellulosic wastes. Although, abundant cellulosic residues are available but major disadvantage of this biodegradation is cost effectiveness of the process. Cellulases have ability to convert lignocellulosic material into fermentable sugars like glucose, maltose, used as substrates to form bioethanol and other products. Certain microorganisms have been reported which have ability of direct conversion of biomass to various alcohols [107] - [109] , but they are not used as efficient source commercially. This technique involves multistep process to convert lignocellulosic material into bioethanol. In the pretreatment process, fraction of hemicellulose and lignin is improved for further processing. Then the residues are hydrolyzed at 50˚C to produce fermentable sugars and in the final step, microorganisms have been employed to convert cellulosic wastes into alcohol [110] [111] .