Lignin is present in abundance in nature and is the most resistant component entering into the soil. It is present in plant cell wall giving structural support and resistance against microbial attack. The bioavailability of nutrients in soil is reduced due to slow rate of lignin degradation. Lignin is an amorphous heteropolymer insoluble in water containing phenyl propane unit joined together with different types of linkages. The units may be linked by strong ether group (C-O-C) or by carbon (C-C). These links may be between two phenol ring, between two propane side chain’s or between a ring and a side chain. Lignin has three basic monomer:
- p-phenyl monomer derived from coumarly alcohol
- guaiacyl monomer derived from coniferyl alcohol
- syringyl monomer derived from sinapyl alcohol
On the basis of monomers lignin is of three type and linked together in complex network:
- Syringyl lignin polymerized by syringyl propane
- Guaiacyl lignin polymerized by guaiacyl propane
- Hydroxy-phenyl lignin polymerized by hydroxyl-phenyl propane
The delignification process includes breakdown of the covalent bond of the lignin. Microorganisms that can degrade lignin specifically are basidiomycetes white rot fungi, brown rot fungi and some actinomycetes. Lignin degradation is a multi-enzymatic process involving an array of enzyme in addition to four lignolytic oxidases (Pollegioni et al., 2015). Enzymes involved in lignin breakdown are oxidases, peroxidases and laccases (Martinez et al., 2005). Laccases catalyzes phenolic compound. Three different stages of lignin decomposition is observed:
- Stage one involves esterification of the exposed methoxy group
- Second stage is depolymerisation. This stage involves extracellular enzymatic cleavage (Killham 1994)
- Stage three involves splitting of the phenolic ring after initial side chain is removed
Lignin degrading fungal enzyme laccase, manganese-peroxidase (Mn-peroxidase) play a role in the detoxification of xenobiotics and disease suppression due to their ability to degrade melanins (Butler and Day 1998). Melanins play a role in pathogenicity or virulence by protecting plant pathogen from being recognized by the plant defence system. Lignin degrading enzyme ligninase produced by fungi can completely degrade melanin (Butler and Day 1998).
White rot fungi employ different type of heme-containing peroxidases which includes lignin peroxidases (LiP), manganese peroxidases (MnP), versatile peroxidases (VP) and dy-decolorizing peroxidases (DyP) (Lambertz et al., 2016). Fungi also secrete various copper containing oxidative laccases that assist in lignin degradation. Gonzalo de Gonzalo (2016) provides an overview on bacteria secreting oxidative enzyme that assist in lignin degradation.
The extracellular enzyme produced by fungi involve in lignin degradation include general oxidase, generating hydrogen per oxide (H2O2) which is required by peroxidases and the mycelium associated dehydrogenases (Martinez et al., 2005). The H2O2 generating oxidases include aryl-alcohol oxidase (EC 22.214.171.124) and glyoxal oxidase found in some fungi. Quinone reductases (EC 126.96.36.199) too are involved in lignin degradation by fungi (Guillen et al., 1997). More over cellobiose hydrogenase (EC 188.8.131.52) an enzyme produced by many different fungi under cellulolytic condition also degrades lignin in the presence of water and chelated iron ions (Henriksson et al., 1995).
Lignin reduces emmissions of green house gases by acting as a physical barrier to the decomposition of organic matter. The fungus producing lignin degrading enzymes detoxify xenobiotics ( Gul et al., 2014) and moreover play a role in disease suppression due to their ability to degrade fungal melanin in soil. Lignin does influence soil microbial population, bioremediates contaminated soil and improves soil physico-chemical structure.
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