Oyster mushrooms ( Pleurotus ) are useful for utilizing lignocellulosic biomass

This review shows the biotechnological potential of oyster mushrooms with lignocellulosic biomass. The bioprocessing of plant byproducts using Pleurotus species provides numerous value-added products, such as basidiocarps, animal feed, enzymes, and other useful materials. The biodegradation and bioconversion of agro wastes (lignin, cellulose and hemicellulose) could have vital implication in cleaning our environment. The bioprocessing of lignin depends on the potent lignocellulolytic enzymes such as phenol oxidases (laccase) or heme peroxidases (lignin peroxidase (LiP), manganese peroxidase (MnP) and versatile peroxidase) produced by the organism. The cellulose-hydrolysing enzymes (that is, cellulases) basically divided into endo-β-1,4-glucanase , exo-β-1,4-glucanase I and II, and β-glucosidase, they attack cellulose to release glucose, a monomers units from the cellobiose, while several enzymes acted on hemicellulose to give D-xylose from xylobiose. These enzymes have been produced by species of Pleurotus from lignocellulose and can also be used in several biotechnological applications, including detoxification, bioconversion, and bioremediation of resistant pollutants.


INTRODUCTION
Lignocellulosic materials are the most promising feedstock as natural and renewable resource essential to the functioning of modern industrial societies (Anwer et al., 2014).A large amount of such materials as waste byproducts are being generated through agricultural practices mainly from various agro based industries (Pe´rez et al., 2002).
Large amounts of lignocellulosic waste generated through forestry and agricultural practices, paper-pulp industries, timber industries and many agro-industries, and they are posing serious environmental pollution problems (Howard et al., 2003).Accidentally, much of the lignocellulosic biomass is often disposed of by burning or just lying values without any important used attached to them.
Author(s) agree that this article remains permanently open access under the terms of the Creative Commons Attribution License 4.0 International License lulosic wastes.Pleurotus are characterized by a white spore print, attached to the gills, often with an essentric stip, or no stip at all, and they are commonly known as Oyster mushrooms (Miles and Chang, 1997).Growing oyster mushroom is becoming more popular throughout the world because of their abilities to grow at a wide range of climate conditions and utilizing various lignocelluloses.In nature, Pleurotus species live on parts of plants which are generally poor in nutrients and vitamins.
For spawn running and fruit body development, lignin and cellulose materials such as corn cobs, all cereal straws, paper, wood shavings, sawdust, nutshells and vegetable wastes as well as food industry wastes are sufficient (Baysal et al., 2003).Species of the genus Pleurotus (Fr.) P. Kumm known as oyster mushrooms, rank third place in worldwide production of edible mushrooms after Agaricus bisporus and Lentinula edodes, and comprises various edible species with medical, biotechnological and environmental applications (Cohen et al., 2002).Pleurotus species present high adaptability to produce basidiomata within a wide variety of agroindustrial lignocellulosic wastes due to their production of ligninolytic and hydrolytic enzymes (Mikiashvili et al., 2006).
Agro-residues contain three major structural polymers, cellulose, hemicellulose and lignin, which can be easily utilized or broken down by the lignocellulotic enzymes.Pleurotus species are the most efficient lignin-degrading organisms, with the ability to produce mainly laccases (EC 1.10.3.2),lignin peroxidase (EC 1.11. 10.14) and manganese peroxidase (EC1.11.1.13)(Adebayo et al., 2012a).These enzymes present a non-specific biocatalyst mechanism and have been used for bioremediation process due to their ability to degrade azo, heterocyclic, reactive and polymeric dyes (Baldrian and Snajdr, 2006;Forgacs et al., 2004).White-rot basidiomycetes are among the most potent organisms to biodegrade and detoxify a wide range of wastes and pollutants.These fungi selectively attack lignin and related compounds by producing one or more of phenol-targeting redox enzymes, namely the peroxidases and laccases/phenoloxidases (Ntougias et al., 2012).Prospection for fungi is the ability to secret high levels of lignin-degrading enzymes and novel enzyme variants, with desirable properties for biotechnological applications (Adebayo et al., 2012a).Therefore, the huge amounts of lignocellulosic biomass can be potentially bioconverted into different high value raw materials and products such as bio-ethanol, enriched animal feed, cheap energy sources for microbial cultivation (mushrooms included) and enzyme production, biodegradation and bioremediation of toxic organic compounds (Koutrotsios et al., 2014;Anwar et al., 2014;Asgher et al., 2013;Irshad et al., 2013;Ntougias et al., 2012).The objectives of this review are the compilation of the newer achievements in the technologies developed between oyster mushrooms and Adebayo and Martínez-Carrera 53 lignocellulosic materials.

CHEMICAL COMPOSITIONS OF LIGNOCELLULOSE
Lignocellulosic biomass are the most abundant and less utilized biomass, they includes agricultural wastes, forestry residues, grasses and woody materials, which are good source of substrates for oyster mushroom and in turn resulted into valuable products.Agricultural lignocellulosic biomass basically consists of 40 to 50% cellulose, 25 to 30% hemicellulose and 15 to 20% lignin (Malherbe and Cloete, 2002;Iqbal, et al., 2011;Menon and Rao, 2012).Major structural component of plant cell walls is cellulose, which is responsible for its mechanical strength.Hemicelluloses are macromolecules with repeated polymers of pentoses and hexoses, while lignin contains three aromatic alcohols (coniferyl alcohol, sinapyl alcohol and pcoumaryl alcohol) produced through a biosynthetic process and forms a protective seal around the other two components i.e., cellulose and hemicelluloses (Figure 1) (Sanchez, 2009;Menon and Rao, 2012;Anwar et al., 2014).Composition of lignocellulose is generally depends on its source; whether it is derived from the hardwood, softwood, or grasses.
Table 1 shows the typical chemical compositions in cellulose, hemicellulose and lignin from various lignocellulosic materials.The variation in chemical compositions may be due to the genetic variability among different sources (Iqbal et al., 2011;Menon andRao, 2012: Kumar et al., 2009;Malherbe and Cloete, 2002).The chemical formula of cellulose is (C 6 H 10 O 5 ) n, with structure of single chain polymer shown in Figure 1.
Cellulose is a highly stable polymer consisting of glucose and attached with linear chains up to 12,000 residues.It is majorly com-posed of (1,4)-Dglucopyranose units, which are attached by β-1,4 linkages with an average molecular weight of around 100,000 (Himmel et al., 2007).Cellulose held together by intermolecular hydro-gen bonds in native state, but they have a strong tendency to form intra-molecular and intermolecular hydrogen bonds and this tendency increases the rigidity of cellulose and makes it highly insoluble and highly resistant to most organic solvents.Naturally, cellulose molecules exist as bundles which aggregated together in the form of micro-fibrils order that is, crystalline and amorphous regions (Iqbal et al., 2011;Taherzadeh and Karimi, 2008).
Hemicellulose mainly consists of glucuronoxylan, glucomannan and trace amounts of other polysaccharides.They are majorly found in grasses and straws contain arabinan, galactan and xylan, while mannan is a component of hardwood and softwood (Brigham et al., 1996).Hemicelluloses have sugar backbone which composed xylans, mannans and glucans, with xylans and mannans being the most common (Wyman et al., 2005).Galactans, arabinans and arabinogalactans are also Hemicelluloses were originally believed to be intermediates in the biosynthesis of cellulose (Vercoe et al., 2005).
Lignin has a long-chain, heterogeneous polymer composed largely of phenyl-propane units, most commonly linked by ether bonds.Lignin acts like a glue by filling the gap between and around the cellulose and hemicellulose complexion with the polymers.It is present in all plant biomass and comprised of complex and large polymer of phenyl-propane, methoxy groups and noncarbohydrate poly phenolic substance, which bind cell walls component together (Hamelinck et al., 2005).

OYSTER MUSHROOMS
Oyster mushrooms are commercially important in the world mushroom market, and several species are grown commercially on a large and small scale in many countries (Adebayo et al., 2012c).Pleurotus species have been recognized as mushroom with dual functions to humans; both as food and medicine (Chang and Buswell, 2003).They are nutritive with good quantity of proteins, vitamins and minerals.Medicinally, they are been recommended for obese persons and diabetes patients because of low calorie value (Chang and Buswell, 2003) and very low sugar without starch.
Traditionally, extracts from Pleurotus species have been reported to be used in treating some ailments (Osemwegie et al., 2010;Idu et al., 2007).Pleurotus are preeminent wood decomposers; they grow on a wider array of forest and agricultural wastes than species of any other group.They thrive on most all hardwoods, on wood by-products (sawdust, paper, pulp sludge), all the cereal straws, corn and corn cobs, on sugar cane bagasse, coffee residues (coffee grounds, hulls, stalks and leaves), banana fronds, cottonseed hulls, agave waste, soy pulp and on other materials too numerous to mention and difficult to imagine possible.In cultivating oyster mushrooms, several valuable by-products are generated.After the crop cycle is complete, the remaining substrate is rendered into a form animal feeds such as cattle, chickens and pigs.Using the spent straw as a nutritious food source could help replace the wasteful practice of feeding grain in the dairy and cattle industry (Stamets, 2000).

Taxonomy of genus Pleurotus
Oyster mushrooms are cosmopolitan, and belong to the genus Pleurotus (Fungi: Basidiomycetes).Their cap is normally shell-like (about 5 to 20 cm in diameter; 1.9 to 7.8 inches), fleshy, with eccentric or lateral stipe; and their color can be white, cream, yellow, pink, brownish, or dark gray (Martínez-Carrera, 1999).Oyster mushroom was first cultivated by 1917 in Germany by Flank.Evolutionary connection among species in the genus Pleurotus is still not clear and many taxonomic issues remain controversial.The genus Pleurotus is one of the most diverse groups of cultivated mushrooms.Fungal populations are established and developed through sexual and asexual reproduction (Cohen et al., 2002).Conventional methods for classification (fruit body morphology, microscopic observations, mating studies between populations, biochemical analyses) have not provided clear-cut results (Martínez-Carrera, 1999).This taxonomic confusion has always been associated with the genus Pleurotus, especially species belonging to the Pleurotus ostreatus complex due to morphological variations of different specimens and similarity of isolates belonging to different species (Asef, 2012).Molecular studies have shown to be more informative; intra and interspecific heterogeneity was determined using ribosomal and mitochondrial DNA analyses, and phylogenetic studies of ribosomal DNA sequences indicated geographic speciation in several groups.Hilber (1982) reported results of mating reactions as well as the microscopic and macroscopic characteristics of many Pleurotus isolates and concluded that Pleurotus pulmonarius and P. ostreatus were different species, Pleurotus sapidus is more likely to correspond to Pleurotus cornucopiae, and Pleurotus columbinus is simply a variety of P. ostreatus.There are other disagree-ments concerning the exact taxonomic position of the cultivar Pleurotus florida (considered either as a P. pulmonarius or a P. ostreatus strain) and whether Pleurotus cystidiosus and Pleurotus abalonus belong to separate (Han et al., 1974;Bresinsky et al., 1987) or the same species (Jong and Peng, 1975;Moore, 1985).Zervakis and Labarer (1992) classified 23 isolates of genus Pleurotus with dendrogram of taxonomic distances demonstrated the suitability of enzyme isoelectric focusing, as it clearly distinguished the four well-defined species P. cornucopiae, P. cystidiosus, Pleurotus eryngii and Pleurotus flabellatus.The P. ostreatus, whose taxonomy is controversial, was separated from P. pulmonarius, and P. columbinus was classified as a distinct taxon.The delimitation of Pleurotus sajor-caju and P. sapidus strains appeared to be more difficult as they seemed to be closely related, the former with P. pulmonarius and the latter with P. ostreatus.Vilgalys et al. (1993) identified three intersterile groups, P. ostreatus, P. pulmonarius and a new species limited to North America: Pleurotus populinus.Vilgalys et al. (1996) identified 15 intersterility groups of Pleurotus (ostreatus, pulmonarius, populinus, cornucopiae, djamor, eryngii, cystidiosus, levis, dryinus, tuberregium, agaves, abieticola, brazil, australis, purpureo-olivaceus) of which some groups have been added in the later time.Oyster mushroom are generally classified as follow; Scientific Name: Pleurotus spp.; Phylum: Basidiomycotina; Class: Basidiomycetes; Subclass: Holobasidiomycetidae; Family: Polyporaceae; Genus: Pleurotus; Species: sajor caju, sapidus, ostreatus, eous, membranaceous, florida, citrinopileatus, flabellatus, pulmonarius etc.

Life cycle of Pleurotus (Oyster mushroom)
Pleurotus mushrooms show the typical life cycle of Basidiomycetes, a major fungal group (Figure 2).It begins with the germination of a basidiospore in a suitable substrate, which gives rise to a monokaryotic mycelium containing genetically identical nuclei (n) and capable of indefinite independent growth.When two compatible monokaryotic mycelia are in close contact, they are able to establish a fertile dikaryon by hyphal fusion or plasmogamy.This dikaryon (n+n), having clamp connexions and binucleate in each hyphal compartment, contains two genetically different nuclei (one from each monokaryon) throughout the mycelium.When environmental conditions are appropriate (temperature, light, relative humidity), the dikaryotic mycelium will differentiate into fruit bodies having specialized structures called basidia.In these club-shaped, binucleate cells, which are formed in the lamellae (hymenium) of each fruit body, karyogamy (fusion of the paired nuclei; 2n) and meiosis (recombination and segregation) take place.The four resulting haploid nuclei move to the sterigmata on the basidium, to form four new basidiospores.When the

Lignocellulose biomass as substrate for Pleurotus cultivation
In commercial cultivation of mushroom, the availability of good substrates is major limiting factor, which determine mushrooms growth rate and quality of yield produced.The cost of mushroom is directly dependent on the substrate availability and utilizable potential of mushroom species.Pleurotus species have been known to grow on a wider array of forest and agricultural wastes than any other mushroom species.Cultivation of edible mushroom with agricultural residues is a value-added process to convert these materials, which otherwise considered to be a wastes, into human and animal food (Zhang et al., 2002).It represents one of most efficient biological ways by which these residues can be recycled (Madan et al., 1987).Oyster mushrooms (Pleurotus spp.) are in the third place after the white button and shiitake among the world mushroom production (Gyorfi and Hajdu, 2007).Due to a large variety of non-specific lignocellulosic enzymes produced by them, they can be cultivated on a number of agricultural wastes (Zhang et al., 2002).Varieties of agrowastes have been used in cultivating oyster mushroom, with most of them proved promising and enhanced the yield performance (Table 2).Amongst various cereal straws, paddy straw was reported to be the best substrate for the cultivation of oyster mushroom (Bano and Srivastava, 1962;Jandaik and Kapoor, 1974;Khanna and Garcha, 1982), whereas, next to the paddy straw, wheat straw proved to be the best substrate for the cultivation of Pleurotus species (Bano and Rajarathnam, 1982;Bhatti et al., 1987;Thampi et al., 1996;Bonatti et al., 2004).Sorghum straw was also effectively used to cultivate P. sajor-caju (Bahukhandi and Munjal, 1989;Patil et al., 1989).Similarly, Garcha et al. (1984) and Diwakar et al. (1989) reported the utility of pearl millet stalks in the cultivation of P. sajor-caju.Rye straw waste (Pal and Thapa, 1979), lawn grass (Yamashita et al., 1983), maize cobs (Bhatti et al., 1987), banana waste (Bonatti et al., 2004) and maize straw (Bahukhandi and Munjal, 1989) were reported as suitable substrates for cultivations of different Pleurotus species.Bhandari et al. (1991) successfully cultivated P. sajor-caju on straws of millets.Many other types of substrates were also reported to be useful for the cultivation of various species of Pleurotus species as shown in Table 2.

BIOCONVERSION OF AGRO-WASTE INTO USEFUL PRODUCTS
Conversion of lignocellulose into food and feed rich in protein by fungi offers an alternative for developing unconventional source of proteins as food/feed (Mane et al., 2007).Large quantity of lignocellulose materials are produced as by-products and its accumulation in the environment could result to environmental problem.The utilization of waste (reuse or recycle) is very important, in order to keep our environment free of any obnoxious condition.Alternative methods of the utilizing these agricultural wastes are needed to mitigate the environment pollution problems associated with current disposal methods, such as open-field burning and soil incorporation.Pleurotus species as primary wood rot fungi are able to colonize different types of agricultural wastes as substrates.Thus, they are cultivated on different lignocellulose wastes.Exploitation of the substrate varies with the species, strain and cultivation technology (Zadrazil and Dube, 1992).In fact, the prospect of utilization of such largely unexploited materials is significant, especially with the cultivation of mushroom to generate new value-added products.
Rumen microorganisms convert cellulose and other plant carbohydrates in large amounts to acetic, propionic and butyric acids, which ruminant animals can use as energy and carbon sources (Ezeji et al., 2006;Martin et al., 2006;Albores et al., 2006); these microbes also have potential for commercial bioprocessing of lignocellulosic wastes anaerobically in liquid digesters.
Studies on combination of an integrated system of composting, with bio-inoculants (strains of P. sajor-caju, Trichoderma harzianum, Aspergillus niger and Azotobacter chroococcum) and subsequent vermicomposting showed an accelerated composting process of wheat straw, besides producing a nutrientenriched compost (Singh and Sharma, 2002).Koutrotsios et al. (2014) reported a high reduction of both fibre and the content of hemicelluloses and cellulose of spent cultivation substrate of P. ostreatus, an increase in crude protein of spent substrate is also reported compared to initial substrate (Table 3).

ENZYMES PRODUCED BY PLEUROTUS SPECIES IN SEVERAL LIGNOCELLULOSE BIOMASS
Basidiomycetes fungi especially Pleurotus species are the most efficient lignin-degrading organisms that produce mainly laccases (EC 1.10.3.2),lignin peroxidase (EC 1.11.10.14) and manganese peroxidase (EC1.11.1.13)(Adebayo et al., 2012a).These enzymes present a non-specific biocatalyst mechanism and have been used for bioremediation process due to their ability to degrade azo, heterocyclic, reactive and polymeric dyes ( Baldrian and Snajdr, 2006;Forgacs et al., 2004).Fungi prospection is the ability to secret high levels of lignindegrading enzymes and novel enzyme variants, with desirable properties for biotechnological applications.On the other hand, alternative low cost substrates like agricultural residues for enzyme production using solid state fermentation (SSF) offer economic and environmental advantages.Different types of agro-industrial wastes have been reported from literature as raw materials for diverse value-added products.The cost of raw materials in enzymes production, which translates into 40 to 60% has make enzymes production generally expensive (Hacking, 1987).In this perspective, the utilization of wastes as growth substrates for the production of enzymes is more economical and profitable.Agro-industrial wastes were omnipresence, high biodegradability, and rich in carbon with optimum potential to serve as substrate for enzymes production.Several species of Pleurotus have been isolated and identified as producer of lignocellulotic enzymes with better performance on agro waste as substrate (Table 4).

Biodegradation of lignocellulosic biomass
Lignocellulose degrading mushroom species are used in various SSF applications such as biodegradation of hazardous compounds and biological detoxification of toxic agro-industrial wastes (Soccol and Vandenberghe, 2003;Philippoussis and Diamantopoulou, 2011), biotransformation of agro-industrial residues to mushroom food and animal feed (Philippoussis, 2009), compost and product developments such as biologically active metabolites, enzymes, food, flavour compounds and other added value compounds (Philippoussis and Diamantopoulou, 2011).It has been reported that some species of Pleurotus are able to colonize different types of vegetable wastes, increasing their digestibility (Villas-Boas et al., 2002;Zhang et al., 2002;Mukherjee and Nandi, 2004).Salmones et al. (2005) reported that three species of Pleurotus consumed lignocellulose with varied degradation percentage from each strain.In particular P. ostreatus efficiently degraded hemicellulose on coffee pulp and wheat straw and lignin on wheat straw, whereas Pleurotus djamor showed a preference for the cellulose of coffee pulp.P. pulmonarius was distinguished by a high consumption of lignin when grown on coffee pulp.Generally, greater decrease in lignocellulosic compounds was observed for wheat straw samples than for coffee pulp (Salmones et al., 2005).The cellulose has been reported as the most biodegraded component of the lignocellulose by fungi due to its content of the substrate in the production of fruiting bodies (Geetha and Sivaprakasam, 1998;Thomas et al., 1998), followed by the hemicellulose which is less abundant than the cellulose and least is the lignin (Salmones et al., 2005).

Laccase
Laccases are glycosylated blue multi-copper oxidoreductases (BMCO) that use molecular oxygen to oxidize various aromatic and nonaromatic compounds through a radical catalyzed reaction mechanism (Claus, 2004;Baldrian, 2006).Laccases couple the electron reduction of dioxygen into two molecules of water with the oxidation of a vast variety of substrates, such as phenols, arylamines, anilines, thiols and lignins (Figure 3a) (Thurston, 1994).Four copper ions in their catalytic center mediate the redox process.These are classified as being type-1 (T1), type-2 (T2) and two type-3 (T3 and T3'), based on the copper's coordination and spectroscopic properties (Messerschmidt and Huber, 1990).The oxidation reactions catalyzed by laccases lead to the formation of free radicals which act as intermediate substrates for the enzymes (Figure 3b) (Ferraroni et al., 2007).These mediators can leave the enzyme site and react with a broad range of high-redox potential substrates and thus create non-enzymatic routes of oxidative polymerizing or depolymerizing reactions (Figure 1c) (Dashtban et al., 2010).Ultimately, laccase-mediator system (LMS) becomes involved in a range of physiological functions such as lignolysis (Figure 3d), lignin synthesis, morphogenesis, pathogenesis and detoxification (Mayer and Staples, 2002).

Heme peroxidase
Lignin peroxidases (LiP): Lignin peroxidase is one of the heme-containing glycoproteins and plays a major role in the biodegradation of lignin cell wall constituent (Piontek, 2001).LiPs catalyze the H 2 O 2 -dependent oxidative depolymerization of a variety of non-phenolic lignin compounds (diarylpropane), β-O-4 non-phenolic lignin model compounds and a wide range of phenolic compounds (e.g.guaiacol, vanillyl alcohol, catechol, syringic acid, acteosyringone) (Wong, 2009).LiPs oxidize the substrates in multi-step electron transfers and form intermediate radicals, such as phenoxy radicals and veratryl alcohol radical cations (Figure 3e).These intermediate radicals undergo non-enzymatic reactions such as radical coupling and polymerization, side-chain cleavage, demethylation and intramolecular addition and rearrangement (Figure 3f) (Wong, 2009).Unlike the other peroxidises, LiP is able to oxidize non-phenolic aromatic substrates and does not require the participation of mediators due to its unusually high redox potential (Figure 3g) (Wong, 2009;Wang et al., 2008).The crystal structure of the first LiP has shown that the heme group is buried in the interior of the protein and has access to the outer medium through a channel (Dashtban et al., 2010).Although, the size of the channel is not sufficient to allow the large polymer lignin to access the heme group, small molecule substrates can find a suitable binding site (Piontek, 2001).

Manganese peroxidases (MnP):
Manganese Peroxidases are extracellular glycoproteins and are secreted in multiple isoforms which contain one molecule of heme as iron protoporphyrin IX (Asgher et al., 2008).MnP catalyzes the peroxide dependent oxidation of Mn (II) to Mn (III) (Figure 3h), which is then released from the enzyme surface in complex with oxalate or with other chelators (Figure 3i).Chelated Mn (III) complex acts as a reactive low molecular weight, diffusible redox-mediator (Figure 3, j) of phenolic substrates including simple phenols, amines, dyes, phenolic lignin substructures and dimers (Wong, 2009;Wesenberg et al., 2003;Asgher et al., 2008).The oxidation potential of Mn (III) chelator is only limited to phenolic lignin structures.However, for the oxidation of non-phenolic substrates by Mn (III), reactive radicals must be formed in the presence of a second mediator (Figure 3k).Organic acids, such as oxalate and malonate are the primary compounds that act as second mediators in the production of reactive radicals like carbon centered radicals (acetic acid radicals, COOHC•H 2 ), peroxyl radicals (COOH-CH 2 OO•), superoxide (O 2 •-) and formate radicals (CO 2 •-) (Wong, 2009;Wesenberg et al., 2003;Asgher et al., 2008).In the absence of H 2 O 2 (e.g. in fungi lacking H 2 O 2 -generating oxidases), these radicals can be used by MnP as a source of peroxides and increase the lignin-degrading efficiency of the fungi (Wong, 2009;Wesenberg et al., 2003;Asgher et al., 2008).

Versatile peroxidases (VP):
Versatile Peroxidases are glycoproteins with combine properties capable of oxidizing typical substrates of other basidiomycetes peroxidases including Mn (II) and also veratryl alcohol (VA), MnP and the typical LiP substrate, respectively (Figure 3) (Wesenberg et al., 2003;Asgher et al., 2008;Ruiz-Duenas et al., 1999).VPs form an attractive ligninolytic enzyme group due to their dual oxidative ability to oxidize Mn (II) and also phenolic and nonphenolic aromatic compounds (Wesenberg et al., 2003).It has been found that VPs can also efficiently oxidize high redox-potential compounds such as dye Reactive Black 5 (RB5) as well as a wide variety of phenols, including hydroquinones (Gomez-Toribio et al., 2001).It has been suggested that VPs can oxidize substrates spanning a wide range of potentials, including low-and high-redox potentials.This is a result of their hybrid molecular structures which provide multiple binding sites for the substrates (Camarero et al., 1999).This makes VPs superior to both LiPs and MnPs, which are not able to efficiently oxidize phenolic compounds in the absence of veratryl alcohol or oxidize phenols in the absence of Mn (II), respectively (Ruiz-Duenas et al., 1999).Similar to the MnP mechanism, Mn (III) is released from VPs and acts as a diffusible oxidizer of phenolic lignin and free phenol substrates (Figure 3h, i and j).Like other members of heme peroxidases, heme is buried in the interior of the protein and has access to the outer medium through two channels (Camarero et al., 1999).The function of the first channel is similar to that described for LiP and is conserved among all heme peroxidases.Conversely, the second channel is found to be specific to VP and MnP and is where the oxidation of Mn (II) to Mn (III) takes place (Ruiz-Duenas et al.,1999).

Other enzymes and mechanisms involved in lignin degradation
In addition to ligninases, other fungal extracellular enzymes which act as accessory enzymes have been found to be involved in lignin degradation.These include oxidases generating H 2 O 2 , which provide the hydrogen peroxide required by peroxidases, and myceliumassociated dehydrogenases, which reduce lignin-derived compounds (Figure 3l) (Martinez et al., 2005).Oxidases generating H 2 O 2 include aryl-alcohol oxidase (AAO) (EC 1.1.3.7)found in P. eryngii (Guillen et al., 1992).In addition, aryl-alcohol dehydrogenases (AAD) (a flavoprotein) and quinone reductases (QR) are also involved in lignin degradation by P. eryngii (Figure 3, m) (Guillen et al., 1992).Moreover, it has been shown that cellobiose dehydrogenase (CDH), which is produced by many different fungi under cellulolytic conditions, is also involved in lignin degradation in the presence of H 2 O 2 and chelated Fe ions (Henriksson et al., 2000).It is proposed that the effect of CDH on lignin degradation is through the reduction of quinones, which can be used by ligninolytic enzymes or the support of a Mn-peroxidase reaction (Figure 3n) (Henriksson et al., 2000).Previous studies have shown the involvement of non-enzymatic mechanisms in plant cell-wall polysaccharide degradation.Mechanisms are mostly assisted by oxidation through the production of free hydroxyl radicals (•OH).Many white and brown-rot fungi have been shown to produce hydrogen peroxide (H 2 O 2 ) which enters the Fenton reaction and results in release of •OH (Figure 3o) (Guillen et al., 1992;Suzuki et al., 2006).By attacking polysaccharides and lignin in plant cell walls in a nonspecific manner, these radicals create a number of cleavages which facilitate the penetration of the cell wall by lignocellulolytic enzymes (Call and Mücke, 1997;Wang et al., 2006).Pathways by which fungi generate free •OH radicals are: cellobiose dehydrogenase (CDH) catalyzed reactions, low molecular weight peptides/quinone redox cycling and glycopeptide-catalyzed Fenton reactions (Renganathan et al., 1990).
βglucosidases are only active on cellobiose, and release glucose monomers units from the cellobiose (Figure 4A).

Hemicellulose biodegradation
The second most abundant renewable biomass which accounts for 25 to 35% of lignocellulosic biomass is hemicellulose (Saha, 2000).They are heterogeneous polymers built up by pentoses (D-xylose, L-arabinose), hexoses (D-glucose, D-galactose), sugar acids (Ferulic acid and 4-O-methyl-D-glucuronic acid and acetyl group.Many enzymes are responsible for the degradation of hemicellulose.The α-D-glucuronidase attack 4-O-methyl-D-glucuronic acid, while endo-β-1,4-xylanase break the xylan chains and α-Larabinofuranosidase attack end chain of L-arabinose.The further reactions are cutting off ferulic acid and removing of the acetyl groups by feruloyl esterase and acetylxylan esterase, respectively.Reduction of xylan to xylobiose is done by α-Dgalactosidase.The β-D-xylosidase release D-xylose, a monomers unit from xylobiose (Figure 4B).Like cellulose, hemicellulose is also an important source of fermentable sugars for biorefining applications (Figure 5).Xylanases are being produced and used as additives in feed for poultry and as additives to wheat flour for improving the quality of baked products at the industrial scale.

CONCLUSION
The challenges facing the whole world about the increased  production of lignocelluloloses materials from agricultural and forestry practices without efficient utilisation, has informed the alternative uses for this natural and renewable resources through a biotechnological manipulation of oyster mushroom.Majority of developing countries are still grappling with socio economic issues including food security, developing technological solutions in the agriculture, agro-processing and other related issues.Lignocellulolytic fungi, especially Pleurotus species, have attracted a great deal of interest as potential biomass degraders for large-scale applications due to their ability to produce vast amounts of valuable products and extracellular lignocellulolytic enzymes.Agro-waste materials which are majorly composed of lignin, cellulose and hemicellulose serves as major source of carbon and energy for Pleurotus species cultivation.Oyster mushroom and their enzymes serve as an efficient alternative for the biodegradation and bioconversion of lignocelluloses and other resistant pollutants.Lignocellulose biotechnology by oyster mushroom could produce numerous value-added products such as fruit body, extracellular enzymes, animal feed and other products.The bioprocessing of lignin depends on the potent lignocellulolytic enzymes such as phenol oxidases (laccase) or heme peroxidases (lignin peroxidase (LiP), manganese peroxidase (MnP) and versatile peroxidase) produced by the organism.The cellulose-hydrolysing enzymes (that is, cellulases) basically divided into endo-β-1,4-glucanase, exo-β-1,4glucanase I and II, and β-glucosidase, they attack cellulose to release glucose, a monomers units from the cellobiose, while several enzymes (α-D-glucuronidase, , endo-β-1,4-xylanase, α-Larabinofuranosidase, feruloyl esterase, acetylxylan esterase, α-D-galactosidase and β-D-xylosidase) acted on hemicellulose to give D-xylose from xylobiose.The bioconversion of lignocelluloses majorly involves transformation of lignin, hydrolysis of cellulose and hemicellulose to simple sugar, which can then play vital role in fermentation process.This bioprocess sometime refers to as fractionation.
In fact, the prospect of utilisation of such largely unexploited materials is significant, especially when accomplished by combining environmentally sound management with the generation of new value-added products.An indicative case complying with such prerequisites is the controlled solid state fermentation of various plant residues leading to the production of edible mushrooms.The productions of the protein rich foods at low cost could solve the problem of insufficient supply of quantity and quality of the bodybuilding material (protein) in feeding man.
Beside, that the enzymes produced serves in biodegradation or bioconversion of agro-waste, they can also be used in several biotechnological applications, including detoxification, bioconversion and bioremedia-tion of resistant pollutants.Several of these enzymes have previously reported of having the ability to degrade and  mineralize toxic chemicals, such as polycyclic aromatic hydrocarbons (PAHs), atrazine, organophosphorus and wastewaters.

Figure 4 .
Figure 4. Model of enzymatic degradation with site of action in cellulose and hemicellulose.

Figure 5 .
Figure 5. Different chemical reaction in bioconversion of lignocellolosic biomass.

Table 2 .
Yield performance of Pleurotus spp. on various agro-residues ND = Not determined.

Table 4 .
Enzymes produced by Pleurotus species in several agricultural wastes.