A Look at Fungal Laccases

© Jennifer Kerekes

Original publication: Mycena News, April 2012

It is well known that fungi decompose plant matter. But, how do they do this? Fungi are able to decompose plant matter, such as lignin and cellulose, by releasing extracellular enzymes that degrade these polymers.  Some fungi, such as most white-rot basidiomycete fungi, are capable of producing several types of extracellular enzymes, including lignin peroxidases, manganese peroxidases, and laccases.  These enzymes allow the fungus to break down and utilize organic substrates as an energy and nutrient source (Osono, 2007).  Lignin is a recalcitrant compound that provides strength and support to plant cells, bonds cellulose fibers, and makes the limited nitrogen in wood less available.  In addition to lignin degradation, laccases also play an important role in soil organic matter cycling.

Pycnoporus cinnabarinus, a white-rot fungus, is used as a model organism for understanding the role of laccases in lignin degradation. Photography courtesy of Wikipedia.

Laccases have been found in almost all wood decay fungi; however, Phanerochaete chrysosporium, a well studied white-rot basidiomycete, does not produce laccases, and likely breaks down lignin using a variety of peroxidases (Martinez et al. 2004; Larrondo et al. 2003; Hoegger et al. 2006).  Litter-decomposers are also capable of producing extracellular enzymes, such as laccases and manganese peroxidases.  However, litter-decomposers vary in their ability to decompose lignin in leaf litter.  Basidiomycete genera such as Clitocybe, Collybia, Marasmius, and Mycena have been studied for their bleaching activity and enzyme production (Osono, 2007).  Bleaching of leaf surfaces and humus is correlated with ligninolytic activity of fungi in that lignin content has been found to be lower in both bleached leaf surfaces and humus as compared to non-bleached surfaces.

Recently, there has been an interest in linking the relationship between fungal diversity and functional diversity.  Researchers are interested in using molecular markers, such as laccase-encoding genes, as a proxy for functional diversity.  Therefore, looking at laccase-encoding gene diversity would provide information regarding potential for litter degradation and soil organic matter cycling.  Previous studies using molecular approaches have looked at the diversity and distribution of laccase genes from basidiomycetes.  Luis et al. (2004) first described the Cu1F/Cu2R basidiomycete specific laccase primer pair and identified a number of laccase genes from mycelial cultures and fruit-bodies.  They also demonstrated that saprotrophic fungi have a greater diversity of laccase genes as compared to mycorrhizal fungi.  In a follow-up study, they found that soil fungi with laccase genes occupied different niches and showed a vertical distribution in the soil profile with the greatest number of laccase genes found in the upper horizons (Luis et al., 2005).  Understanding soil enzyme functional diversity could significantly increase our understanding of the linkages between resource availability, microbial community structure and function, and ecosystem processes (Caldwell, 2005).

Laccases are a multigene family, and some fungi, such as Coprinopsis cinera, have as many as 17 different laccase genes (Kilaru, Hoegger, & Kües, 2006).  It should also be noted that fungal laccases also appear to be involved in a variety of physiological functions, including fruit-body development, detoxification of phenolic compounds, pigment production, and antimicrobial activity (Levin, Forchiassin, & Ramos, 2002; Thurston, 1994).  In addition, fungal laccases also appear to have roles in stress defense and fungal plant-pathogen/host interaction (Thurston, 1994).

Some ascomycetes, primarily xylariaceous fungi (Pointing et al., 2003), are capable of degrading lignin, though they are less capable than white-rot fungi.  This was recently explored further in a study by Shary and colleagues (2007).  There is also evidence that laccases, or laccase-like genes, are also present in bacteria.  Less is known about the presence, diversity and function of these laccase-like genes in bacteria, compared with fungi (Kellner et al., 2008).  However, fungal basidiomycete laccases are the primary ligninolytic enzymes in the environment and play a critical role in plant matter decomposition.

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About the Author:
Jennifer Kerekes received a Ph.D. in Microbiology in December, 2011 from the University of California at Berkeley, where she worked with Dr.Tom Bruns. She is interested in the ecology and diversity of saprotrophic fungal communities.