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Many plant species form symbioses with ectomycorrhizal fungi, which help them forage for limiting nutrients in the soil such as inorganic phosphate (Pi). The transcriptional responses to symbiosis and nutrient-limiting conditions in ectomycorrhizal fungal hyphae, however, are largely unknown. An artificial system was developed to study ectomycorrhizal basidiomycete Paxillus involutus growth in symbiosis with its host tree Pinus sylvestris at different Pi concentrations. RNA-seq analysis was performed on P. involutus hyphae growing under Pi-limiting conditions, either in symbiosis or alone. We show that Pi starvation and ectomycorrhizal symbiosis have an independent effect on the P. involutus transcriptome. Notably, low Pi availability induces expression of newly identified putative high-affinity Pi transporter genes, while reducing the expression of putative organic acid transporters. Additionally, low Pi availability induces a close transcriptional interplay between P and N metabolism. GTP-related signalling was found to have a positive effect in the maintenance of ectomycorrhizal symbiosis, whereas multiple putative cytochrome P450 genes were found to be downregulated, unlike arbuscular mycorrhizal fungi. We provide the first evidence of global transcriptional changes induced by low Pi availability and ectomycorrhizal symbiosis in the hyphae of P. involutus, revealing both similarities and differences with better-characterized arbuscular mycorrhizal fungi.

We present an analysis of fungal specificity in myco-heterotrophic orchids and monotropes. We argue that specificity represents a continuum and can only be properly assessed using phylogenetic data. Several green orchids associate with wide phylogenetic arrays of Rhizoctonia species, and hence show little specificity, while other green orchids, and all studied achlorophyllous orchids and monotropes, associate with narrow phylogenetic groups of fungi, and hence show significant specificity. In several species, this tight specificity has been shown to apply from seed germination through adulthood under natural conditions, though not necessarily under in vitro conditions. Patterns of specificity have been correlated with patterns of fungal distribution and habitat variation in several myco-heterotrophs. However, studies of other myco-heterotrophs have shown that tight specificity is expressed even when diverse fungi co-exist with the plant. Moreover, in one case, genetic influences of the host plant have been shown to outweigh environmental influences over the patterns of specificity. Major host jumps and intraspecific host-race formation have contributed to the evolution of specialisation in several myco-heterotrophs. Some achlorophyllous orchids associate with wood-decay or parasitic fungi, but many recent studies have revealed associations with ecto-mycorrhizal fungi in orchids, monotropes, and a liverwort. Tracer studies show that autotrophic ecto-mycorrhizal host plants can provide the fixed carbon to nourish myco-heterotrophs linked by a shared fungal partner. Important outstanding questions concern recognition phenomena, the origins and evolution of specificity, the physiology and ecology of carbon exchange, and whether myco-heterotrophs interact with fungi in fundamentally different ways than do autotrophs.

The microbiota of most forest soils is dominated by ecto-mycorrhizal and saprotrophic decomposer fungi which are the main organisms involved respectively in the supply of nutrients to trees and the decomposition of woody plant litter. In recognition of the pivotal role that these fungi play in plant nutrition and nutrient cycles in plantations and natural forests, the activities of their mycelial systems have been independently investigated in recent years. Such studies have revealed remarkable structural and functional similarities between these organisms, particularly in their active foraging for nitrogen and phosphorus in soils. This has highlighted the need to determine the nature and extent of interactions between the two groups of organisms. The use of radioactive tracers, time-sequence photography and digital image analysis of microcosms containing non-sterile soil in which the two groups of fungi are grown with their natural carbon resources have provided novel insights into their interactions. When mycelia of the fungi meet in soil they often interact strongly, resulting in cessation of growth or even die-back of mycelium. Such inhibition can be accompanied by loss of nutrients to the opposing fungus. This chapter reviews current evidence and presents some new data showing that interactions between the mycelial systems of mycorrhizal and saprotrophic fungi have major effects on the functioning of these organisms and upon nutrient and carbon cycling processes in temperate and boreal forest ecosystems.