Physiological heterogeneity within fungal mycelia: an important concept for a functional understanding of the ectomycorrhizal symbiosis

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Abstract
Individual mycelia of filamentous fungi display considerable heterogeneity at the physiological level. Important physiological processes such as nutrient absorption, extracellular enzyme secretion and solute translocation occur differentially within an individual mycelium, and vary according to spatio-temporal changes in patterns of gene expression as the mycelium develops and senesces. In ectomycorrhizal (ECM) fungi, gene expression appears to be strongly influenced by interaction with the soil environment and the host root. The ECM mycelium is thus a complex and dynamic entity wherein discrete regions display particular physiological attributes. Physiological heterogeneity is important in the overall functioning of the symbiosis. In the particular case of movement of phosphorus from soil to host root in the ECM symbiosis, heterogeneity might provide the driving force for the integrated processes of absorption, translocation and transfer. It is suggested that it is only by considering the sum of the seemingly disparate physiological processes within the heterogeneous mycelium that mycorrhizal functioning can be fully understood.