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Paxillus involutus ATCC 200175 v1.0
Paxillus involutus
Photo courtesy of Michael Wood

More than 75% of the carbon in terrestrial ecosystems is stored in forests. More than half of this carbon is found in soil organic matter (SOM). Recent studies have indicated that ectomycorrhizal (EM) fungi provide the dominant pathway through which carbon enters the SOM pool. The same fungi also drive the cycling of other nutrients within the forest community (microbiome) together with other soil microorganisms. Global climate changes as well as increasing anthropogenic nitrogen deposition are compelling reasons to study in detail how EM fungi and associated microflora mediate carbon as well as nitrogen cycling and deposition in terrestrial ecosystems. Likewise, EM fungi are known to protect plants from toxic metals. So, the development of metal-tolerant fungal associations would provide a strategy for active remediation of metal-contaminated soils.

Paxillus involutus is one of the most well studied EM fungi at molecular, physiological, and ecological levels. The fungus forms mycorrhizae (mutually beneficial root systems) with many species of trees, including the genetic model tree species Populus trichocarpa. Comparative analyses of the genome and transcriptome of P. involutus with those of the recently sequenced EM fungus Laccaria bicolor can reveal the evolutionary processes that led to the diversification of EM fungi, and the mechanistic bases of transitions between decomposer and mycorrhizal lifestyles. The complete genome sequence of P. involutus will facilitate studies aiming at identifying the genomic mechanisms that could account for the variations in phenotypic characteristics and adaptations in EM fungi to specific hosts and environments. The P. involutus genome sequence will help to identify the molecular mechanisms controlling the development and function of rhizomorphs (tough root-like structures that facilitate colonization of large areas of soil and serve as a significant repository for carbon storage) by soil fungi. P. involutus has also been one of the most commonly studied fungi for examining the mechanisms of toxic metal tolerance and excess salinity. The Paxillus genome sequence will allow the annotation of genes coding proteins such as transporters involved in metal uptake and proteins involved in metal homeostasis and free radical scavenging. Based on the genome sequences of P. involutus, L. bicolor and P. trichocarpa, it will be possible to construct a gene chip that would allow in-depth exploration of the poplar microbiome, thus adding a needed dimension to climate change research and providing another step in the quest for mechanistic modelling of ecosystem responses.