Lignin

After cellulose, lignin is the second most abundant organic substance produced during NPP. Lignin is a complex and dense amorphous secondary cell wall polymer found in the trachea elements and sclerenchyma of terrestrial plants. The basic structure of lignin is based on the phenyl propanoid unit, consisting of an aromatic ring and a 3-C side chain (Fig. 12.9). The conversion of phenylalanine and tyrosine in the shikimic and phenylpropanoid pathways to p-coumaric acid by ammonia lyase is the starting point of the phenylpropanoid metabolic pathway that forms the monolignol precursors synapyl, coniferyl, and coumaryl alcohols. Lignin synthesis begins with phenoxy radical coupling, a random self-replicating

FIGURE 12.9 Proposed structure of a softwood lignin showing the large hydrocarbon content that among other characteristics leads to the hydrophobic character of the plant secondary cell wall.

FIGURE 12.9 Proposed structure of a softwood lignin showing the large hydrocarbon content that among other characteristics leads to the hydrophobic character of the plant secondary cell wall.

polymerization, of the monolignols. After polymerization, the lignin monolignol subunits are referred to as p-hydroxyphenol, guaiacyl, and syringyl residues, respectively. The synthesis of lignin involves the deposition of monolignols onto a protein template to create an amorphous polymer structure. The random assembly of hydrocarbons is hydrophobic in nature and provides structural rigidity and a barrier against pests and pathogens. In most dicots, the lignin structure contains guaiacyl and syringyl residues. Grass lignins also contain p-hydroxyphenol in small amounts. Lignin is thought to be cross-linked to hemicellulose via a cell wall protein called extensin (Fig. 12.10). The actual details of lignification are poorly understood. This has contributed to the incomplete understanding of lignin decomposition.

The dense nature, hydrophobicity, and nonspecific structure of lignin make it difficult for enzymes to attack. It is thought that lignin must be broken into smaller fragments before extensive anaerobic decomposition will proceed. Lignin depoly-merization produces a water-soluble, acid-precipitable product not unlike soil humic acids. The gram-negative aerobic bacteria Pseudomonadaceae, Azotobacter, and Neisseriaceae and common actinomycetes Nocardia and Streptomyces can degrade lignin but not to the same extent as fungi. Whether bacteria can cause complete decomposition and use any lignin C for growth has not been well established. Bacteria may attack parts of the lignin structure to remove the barrier shielding energy-rich cellulose and hemicellulose. Bacteria found in the guts of ruminants and some arthropods also have limited ability to degrade lignin. Lignin is solubilized in termites by gut-inhabiting streptomycetes to liberate the energy-rich cellulose and hemicellulose. The effect of the high pH (9-11) in termite foreguts and the proportion of true lignase activity relative to depolymerization and by-product formation are not known.

Fungi are the most efficient lignin degraders in nature, playing a key role in biotic C cycling. Fungal species that degrade lignin are often grouped into soft rot, brown rot, and white rot fungi based on the color of the decaying substrate. Various fungi represented by Imperfecti and Ascomycetes cause the soft rot of wood. Soft

FIGURE 12.10 A three-dimensional model of the secondary cell wall of plants. The model allows visualization of interactions among petic substances, hemicellulose, cellulose, lignin, and extensin.

FIGURE 12.10 A three-dimensional model of the secondary cell wall of plants. The model allows visualization of interactions among petic substances, hemicellulose, cellulose, lignin, and extensin.

Phenolic/Lignin structure rots prefer polysaccharides but have the ability to remove methylated side chains (R-O-CH3) and cleave aromatic rings, but cannot completely degrade the lignin structure. The soft rot fungi are important in mesic environments and appear to degrade hardwood lignin more effectively than that of softwoods, with Chaetomium and Preussia being representative organisms. Box 12.3 shows the vulnerability to degradation of C found in specific positions in the monolignol coniferyl alcohol over a 2-year period.

The majority of wood decay is performed by brown and white rot fungi from the basidiomycetes. Brown rots lack ring-cleaving enzymes but readily degrade hemicellulose and cellulose of intact wood. They modify and degrade lignin through demethylation and removal of methylated side chains producing hydroxylated phenols. The oxidation of the lignin aromatic structures causes a characteristic brown color. The separation of polysaccharides from lignin is thought to occur through nonenzymatic oxidation via the production of low-molecular-weight

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