**2. Plant cell wall polysaccharides and lignin**

Plant cell wall [6] polysaccharides are the most abundant organic compounds found in na‐ ture. These compounds consist mainly of polysaccharides such as cellulose, hemicelluloses and pectin, as well as the phenolic polymer lignin. Together, the polysaccharides and lignin provide high complexity and rigidity to the plant cell wall. Cellulose, the major constituent of plant cell wall consists of ß-1,4 linked D-glucose units that form linear polymeric chains of about 8000-12 000 glucose units. In its crystalline form, cellulose consists of chains that are packed together by hydrogen bonds to form highly insoluble structures, called microfibrils. In addition to the crystalline structure, cellulose contains amorphous regions within the mi‐ crofibrils (noncrystalline structure) [3].

Hemicelluloses are heterogeneous polysaccharides consisted by different units of sugars, being the second most abundant polysaccharides in plant cell wall. Hemicelluloses are usually classified according to the main residues of sugars present in the backbone of the structural polymer. Xylan, the most abundant hemicellulose polymer in cereals and hard‐ wood, is composed by ß-1,4-linked D-xylose units in the main backbone, and can be sub‐ stituted by different side groups such as D-galactose, L-arabinose, glucuronic acid, acetyl, feruloyl, and *p*-coumaroyl residues [3]. Another two major hemicelluloses in plant cell wall are galacto(gluco)mannans, which consist of a backbone of ß-1,4-linked D-mannose (mannans) and D-glucose (glucomannans) residues with D-galactose side chains, and xy‐ loglucans that consist of a ß-1,4-linked D-glucose backbone substituted by D-xylose [3]. Moreover, in xyloglucan polymer, L-arabinose and D-galactose residues can be attached to the xylose residues, and L-fucose can be attached to galactose residues. The diversity of side groups that can be attached to the main backbone of xyloglucans confers to this polymers high structural complexity and variability [4]. Figure 1 represents schematic views of the three major hemicellloses.

Pectins are another family of plant cell wall heteropolysaccharides, containing a backbone of α-1,4-linked D-galacturonic acid. The polymers usually contain two different types of re‐ gions. The so-called "smooth" region of pectins contains residues of D-galacturonic acids that can be methylated or acetylated. In the other region, referred as "hairy" region, the back‐ bone of D-galacturonic acids residues is interrupted by α-1,2-linked L-rhamnose residues. In the hairy region, long side chains of L-arabinose and D-galactose residues can be attached to the rhamnose residues [5]. Figure 2 depicts a schematic representation of the hairy region of pectins.

**Figure 1.** Schematic representation of the three major hemicellulose structures. A. Xylan. B. Xyloglucan. C. Galacto‐

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Lignin is a phenolic polymer that confers strength to plant cell wall. Lignin is a highly in‐ soluble complex branched polymer of substituted phenylpropane units, which are joined to‐ gether by ether and carbon-carbon linkages, forming an extensive cross-linked network within the cell wall. The cross-linking between the different polymers described above con‐

mannan (upper left) and Galactoglucomannan (lower right).

improve microorganisms that can be able to efficiently degrade the plant biomass. Also, new strains could be great producers of CWDEs, providing enzymatic cocktails that can be intro‐ duced commercially. Finally, the future perspectives will demonstrate how far we are from cellulosic ethanol and other biomass-derived chemical compounds, regarding to research of

208 Sustainable Degradation of Lignocellulosic Biomass - Techniques, Applications and Commercialization

Plant cell wall [6] polysaccharides are the most abundant organic compounds found in na‐ ture. These compounds consist mainly of polysaccharides such as cellulose, hemicelluloses and pectin, as well as the phenolic polymer lignin. Together, the polysaccharides and lignin provide high complexity and rigidity to the plant cell wall. Cellulose, the major constituent of plant cell wall consists of ß-1,4 linked D-glucose units that form linear polymeric chains of about 8000-12 000 glucose units. In its crystalline form, cellulose consists of chains that are packed together by hydrogen bonds to form highly insoluble structures, called microfibrils. In addition to the crystalline structure, cellulose contains amorphous regions within the mi‐

Hemicelluloses are heterogeneous polysaccharides consisted by different units of sugars, being the second most abundant polysaccharides in plant cell wall. Hemicelluloses are usually classified according to the main residues of sugars present in the backbone of the structural polymer. Xylan, the most abundant hemicellulose polymer in cereals and hard‐ wood, is composed by ß-1,4-linked D-xylose units in the main backbone, and can be sub‐ stituted by different side groups such as D-galactose, L-arabinose, glucuronic acid, acetyl, feruloyl, and *p*-coumaroyl residues [3]. Another two major hemicelluloses in plant cell wall are galacto(gluco)mannans, which consist of a backbone of ß-1,4-linked D-mannose (mannans) and D-glucose (glucomannans) residues with D-galactose side chains, and xy‐ loglucans that consist of a ß-1,4-linked D-glucose backbone substituted by D-xylose [3]. Moreover, in xyloglucan polymer, L-arabinose and D-galactose residues can be attached to the xylose residues, and L-fucose can be attached to galactose residues. The diversity of side groups that can be attached to the main backbone of xyloglucans confers to this polymers high structural complexity and variability [4]. Figure 1 represents schematic

Pectins are another family of plant cell wall heteropolysaccharides, containing a backbone of α-1,4-linked D-galacturonic acid. The polymers usually contain two different types of re‐ gions. The so-called "smooth" region of pectins contains residues of D-galacturonic acids that can be methylated or acetylated. In the other region, referred as "hairy" region, the back‐ bone of D-galacturonic acids residues is interrupted by α-1,2-linked L-rhamnose residues. In the hairy region, long side chains of L-arabinose and D-galactose residues can be attached to the rhamnose residues [5]. Figure 2 depicts a schematic representation of the hairy region of

microorganisms.

**2. Plant cell wall polysaccharides and lignin**

crofibrils (noncrystalline structure) [3].

views of the three major hemicellloses.

pectins.

**Figure 1.** Schematic representation of the three major hemicellulose structures. A. Xylan. B. Xyloglucan. C. Galacto‐ mannan (upper left) and Galactoglucomannan (lower right).

Lignin is a phenolic polymer that confers strength to plant cell wall. Lignin is a highly in‐ soluble complex branched polymer of substituted phenylpropane units, which are joined to‐ gether by ether and carbon-carbon linkages, forming an extensive cross-linked network within the cell wall. The cross-linking between the different polymers described above con‐ fers the complexity and rigidity of the plant cell wall, which is responsible for the protection of plant cell as a whole. In addition to offer protection against mechanical stress and osmotic lysis, the plant cell wall is an effective barrier against pathogens, including many microor‐ ganisms. However, during the course of evolution some microorganisms, in order to sur‐ vive, developed efficient strategies to degrade plant cell wall components, mainly the polysaccharides [6].

trients represents a challenge for all living organisms, especially for microorganisms. Saprophytism, one of the most common lifestyle of microorganisms, involves living in dead or decaying organic matter, mainly composed by plant biomass. In this context, microorgan‐ isms developed cellular mechanisms in order to take energy from plant biomass, and one of this mechanisms involves the production and secretion of carbohydrate-active enzymes. These enzymes degrade the plant cell wall, releasing sugars monomers that can be used as substrates for the metabolism of the microorganism. The microbial use of plant biomass is pivotal for life on Earth, because it is responsible for large portions of carbon flux in the bio‐ sphere. In addition, plant cell wall-degrading enzymes (CWDEs) have a broad range of in‐ dustrial applications, such as within the food and feed industry and for sustainable

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The capacity to degrade lignocellulose is mainly distributed among fungi and bacteria. Cel‐ lulolytic bacteria can be found in different genus such as *Clostridium*, *Ruminococcus*, *Caldicel‐ lulosiruptor*, *Butyrivibrio, Acetivibrio*, *Cellulomonas, Erwinia, Thermobifida, Fibrobacter, Cytophaga,* and *Sporocytophaga*. Bacterial degradation of cellulolytic material is more restrict to biomass containing low amounts of lignin, once bacteria are poor producers of lignanas‐ es. Plant biomass produced in aquatic environment, containing little amounts of lignin, is typically degraded by bacteria, which are better adapted for an aquatic environment than fungi [7]. Cellulolytic bacteria can also be found in digestive tracts of herbivore animals [8]. Fungal cellulose utilization is distributed within the entire kingdom, from the protist-like

Concerning to lignin degradation, many white-rot basidiomycetes and some actinomycetes are able to produce lignin-degrading enzymes, especially peroxidases. For instance, *Phanero‐ chaete chrysosporium* and *Phlebia radiata* are well known producers of extracellular peroxidas‐ es [9], as well as *Coriolus tersicolor*, which was shown to produce the intracellular haem peroxidase upon the induction by phenolic compounds [10]. A white-rot basidiomycete, *Rigidoporous lignosus*, is known to secrete two oxidative enzymes, laccase and Mn peroxi‐

The fungi *Hypocrea jecorina* (*Trichoderma reesei*) is the most important organism used in cellu‐ lase production [12, 13] and it has been the focus of cellulases research for over 50 years. Degradation of cellulose is performed by cellulases, a high specific class of enzymes able to degrade the cellulose glycosidic bonds. The filamentous fungi *Aspergillus niger* is known to produce a wide range of hemicellulose-degrading enzymes and it has been used for many industrial applications. As discussed above, hemicellulose is a complex class of polysacchar‐ ides composed by different units of sugars. In order to degrade hemicellulose, the organism should be able to produce a large set of enzymes (hemicellulases), acting in a synergistic way to hydrolyze such complex substrate. Therefore, the ascomycetes *T. reesei* and *A. niger* are considered the most important microorganisms for cellulase/hemicellulase production, and constitute the source of these enzymes for industrial applications, including the produc‐

The plant cell wall-degrading machinery of aerobic and anaerobic microorganisms differs significantly, regarding to its macromolecular organization. The cellulase/hemicellulase

production of many chemicals and fuels.

*Chytridomycetes* to the advanced *Basidiomycetes*.

tion of biofuels from plant biomass.

dase, responsible for solubilizing the lignin in a synergistic way [11].

Plant cell wall degradation mechanisms are pivotal for the lifestyle of many microorgan‐ isms, once they should be able to degrade the plant polymers to acquire nutrients from plants. For instance, saprophytic fungi inhabit dead organic materials like decaying wood and leaves. In order to take energy from these materials, these fungi need to produce en‐ zymes capable of degrading the majority of plant cell wall polysaccharides present in the bi‐ omass. The main mechanism through which fungi and other microorganisms degrade plant biomass consists of production and secretion of enzymes acting synergistically in the plant cell wall, releasing monomers that can be used by the microorganism as chemical energy. The next section will discuss mechanisms of cell wall degrading enzymes (CWDE) produc‐ tion by fungi, the most important producers of carbohydrate-active enzymes.

**Figure 2.** Schematic view of the hairy region of pectin.
