Exploring the transcriptome of the ectomycorrhizal symbiosis

The tree rhizosphere hosts a large community of microbes that compete and interact with each other and with plant roots. Within this cortege of microorganisms, ectomycorrhizal fungi are almost ubiquitous. Mycelium of symbiotic fungi and root tips form a novel composite organ, so-called ectomycorrhiza, which is the site of nutrient transfer between the symbionts. To examine gene-activity changes associated with the development of the symbiosis, we have performed expression profiling using cDNA arrays of poplar, eucalypt and ectomycorrhizal fungi. A marked change in the gene expression in the mycobiont and the host-plants was observed at multiple levels: (a) a general activation of the protein synthesis machinery probably supporting an intense cell division/proliferation, (b) an increased accumulation of transcripts coding for cell surface proteins probably involved in the symbiotic interface formation, and (c) the upregulation of energy metabolism in roots. This suggests a highly dynamic environment in which symbionts are sending and receiving signals, are exposed to high levels of stress conditions and are remodeling their tissues. With multiple gene profiling programmes dealing with ectomycorrhizal associations, we will have in the future an unparalleled opportunity to ask which genetic features are responsible for common/divergent traits involved in this symbiosis. Possible breakthroughts will be in characterisation of common transcriptional and transduction networks, identification of novel surface proteins and new insights into unique metabolic routes critical for mycorrhiza functioning.