From: Treseder KK, Lennon JT. 2015. Fungal traits that drive ecosystem dynamics on land. Microbiology and Molecular Biology Reviews 79:243-262.
Chitin
Extracellular chitinase. Chitin is produced within the cell walls of most fungi (Rosenberger 1976) and it is also a primary component of arthropod exoskeletons. It consists of chains of N-acetylglucosamine, and is one of the more abundant N-containing biopolymers in the biosphere (Gooday 1990). Fungi can use extracellular chitinases to break chitin into smaller polymers and, ultimately, glucosamine (Gooday 1990). They can then acquire and metabolize the glucosamine to meet demands for N or C (Allison et al. 2007). The depolymerization of relatively large N-containing polymers into oligomers or monomers, which are more readily taken up by microbes or plants, has been proposed as a rate-limiting step in the N cycle (Schimel and Bennett 2004). Thus, the ability of fungal taxa to produce extracellular chitinases is a trait with particularly important consequences for ecosystem function. Extracellular chitinase production, and the ability to grow on chitin as a sole N or C source in pure culture, has been verified for a number of ectomycorrhizal, ericoid, and saprotrophic fungi (e.g., Bajwa and Read 1986, Leake and Read 1990, Kerley and Read 1995, Read and Perez-Moreno 2003, Tzelepis et al. 2012).
Extracellular protease and peptidase. About 20–40% of soil N is bound in various proteinaceous compounds (Schulten and Schnitzer 1997, Jones et al. 2004, Jones et al. 2005), which fungi can depolymerize via extracellular proteases and peptidases. First, proteases such as serine protease or metalloprotease split long protein chains into shorter chains (Gottesman 1996). Next, amino acids are released from these shorter chains by peptidases such as glycine aminopetidase and leucine aminopeptidase (Sinsabaugh 1994). Collectively, these enzymes produce small peptides and single amino acids, each of which can be taken up by fungi that possess the appropriate membrane transport proteins (Abuzinadah and Read 1986, Chalot et al. 1996, Nehls et al. 1999, Wipf et al. 2002). Mycorrhizal fungi have received particular attention for their capacity to break down proteins as a source of N. In a recent review, Talbot and Treseder (2010) reported that of 53 ericoid and ectomycorrhizal species examined, 46 possessed this trait.
Fungal genes for nitrogen depolymerization
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Bajwa, R. and D. J. Read. 1986. Utilization of mineral and amino N sources by the ericoid mycorrhizal endophyte Hymenoscyphus ericae and by mycorrhizal and non-mycorrhizal seedlings of Vaccinium. Mycol Res 87:269-277.
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Gooday, G. W. 1990. The ecology of chitin degradation. Adv Microb Ecol 11:387-430.
Gottesman, S. 1996. Proteases and their targets in Escherichia coli. Annual Review of Genetics 30:465-506.
Jones, D. L., J. R. Healey, V. B. Willett, J. F. Farrar, and A. Hodge. 2005. Dissolved organic nitrogen uptake by plants – an important N uptake pathway? Soil Biol Biochem 37:413-423.
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Tzelepis, G. D., P. Melin, D. F. Jensen, J. Stenlid, and M. Karlsson. 2012. Functional analysis of glycoside hydrolase family 18 and 20 genes in Neurospora crassa. Fungal Genetics and Biology 49:717-730.
Wipf, D., M. Benjdia, M. Tegeder, and W. B. Frommer. 2002. Characterization of a general amino acid permease from Hebeloma cylindrosporum. FEBS Lett 528:119-124.