Nitrogen acquisition from mineral‐associated proteins by an ectomycorrhizal fungus

Summary In nitrogen (N)‐limited boreal forests, trees depend on the decomposing activity of their ectomycorrhizal (ECM) fungal symbionts to access soil N. A large fraction of this N exists as proteinaceous compounds associated with mineral particles. However, it is not known if ECM fungi can access these mineral‐associated proteins; accordingly, possible acquisition mechanisms have not been investigated. With tightly controlled isotopic, spectroscopic, and chromatographic experiments, we quantified and analyzed the mechanisms of N acquisition from iron oxide mineral‐associated proteins by Paxillus involutus , a widespread ECM fungus in boreal forests. The fungus acquired N from the mineral‐associated proteins. The collective results indicated a proteolytic mechanism involving formation of the crucial enzyme–substrate complexes at the mineral surfaces. Hence, the enzymes hydrolyzed the mineral‐associated proteins without initial desorption of the proteins. The proteolytic activity was suppressed by adsorption of proteases to the mineral particles. This process was counteracted by fungal secretion of mineral‐surface‐reactive compounds that decreased the protease–mineral interactions and thereby promoted the formation of enzyme–substrate complexes. The ability of ECM fungi to simultaneously generate extracellular proteases and surface‐reactive metabolites suggests that they can play an important role in unlocking the large N pool of mineral‐associated proteins to trees in boreal forests.