Michiel Op De Beeck, Carl Troein, Carsten Peterson, Per Persson, and Anders Tunlid
Fenton reaction facilitates organic nitrogen acquisition by an ectomycorrhizal fungus
New Phytologist 218, 335-43 (2018)
Abstract:
• Boreal trees rely on their ectomycorrhizal fungal symbionts to acquire growth-limiting nutrients, such as nitrogen (N), which mainly occurs as proteins complexed in soil organic matter (SOM). The mechanisms for liberating this N are unclear as ectomycorrhizal fungi have lost many genes encoding lignocellulose-degrading enzymes present in their saprotrophic ancestors. We hypothesized that hydroxyl radicals (˙OH), produced by the ectomycorrhizal fungus Paxillus involutus during growth on SOM, are involved in liberating organic N.
• Paxillus involutus was grown for 7 d on N-containing or N-free substrates that represent major organic compounds of SOM.˙OH production, ammonium assimilation, and proteolytic activity were measured daily.
• ˙OH production was strongly induced when P. involutus switched from ammonium to protein as the main N source. Extracellular proteolytic activity was initiated shortly after the oxidation. Oxidized protein substrates induced higher proteolytic activity than unmodified proteins. Dynamic modeling predicted that ˙OH production occurs in a burst, regulated mainly by ammonium and ferric iron concentrations.
• We propose that the production of˙OH and extracellular proteolytic enzymes are regulated by similar nutritional signals. Oxidation works in concert with proteolysis, improving N liberation from proteins in SOM. Organic N mining by ectomycorrhizal fungi has, until now, only been attributed to proteolysis.
LU TP 17-42
• Boreal trees rely on their ectomycorrhizal fungal symbionts to acquire growth-limiting nutrients, such as nitrogen (N), which mainly occurs as proteins complexed in soil organic matter (SOM). The mechanisms for liberating this N are unclear as ectomycorrhizal fungi have lost many genes encoding lignocellulose-degrading enzymes present in their saprotrophic ancestors. We hypothesized that hydroxyl radicals (˙OH), produced by the ectomycorrhizal fungus Paxillus involutus during growth on SOM, are involved in liberating organic N.
• Paxillus involutus was grown for 7 d on N-containing or N-free substrates that represent major organic compounds of SOM.˙OH production, ammonium assimilation, and proteolytic activity were measured daily.
• ˙OH production was strongly induced when P. involutus switched from ammonium to protein as the main N source. Extracellular proteolytic activity was initiated shortly after the oxidation. Oxidized protein substrates induced higher proteolytic activity than unmodified proteins. Dynamic modeling predicted that ˙OH production occurs in a burst, regulated mainly by ammonium and ferric iron concentrations.
• We propose that the production of˙OH and extracellular proteolytic enzymes are regulated by similar nutritional signals. Oxidation works in concert with proteolysis, improving N liberation from proteins in SOM. Organic N mining by ectomycorrhizal fungi has, until now, only been attributed to proteolysis.
LU TP 17-42