To understand the beneficial effect of ectomycorrhizal symbiosis on the resilience of trees to drought stress, an international consortium, led by INRA and the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL), sequenced the genome of the root symbiont Cenococcum geophilum and characterized its gene expression in symbiotic roots upon high drought stress.
Scientists describe how comparative analysis of 60 fungal genomes allowed them to track the evolution of this mutualistic fungus, the most abundant symbiont found on forest trees. The reported findings provide a better understanding on how trees and fungi developed symbiotic relationships, and how the mutualistic association provides trees with beneficial traits for adaptation to climate change, such as drought stress. The consortium published these results in Nature Communications on 7 September 2016.
Ectomycorrhizal fungi include some of the most conspicuous forest mushrooms, including the iconic Fly Agaric, Golden Chanterelle and King Bolete and also the hidden delicacies, the truffles. The fungal lineages containing mycorrhizal species are separated by tens or hundreds of millions of years, but their symbiosis with trees share remarkable morphological and metabolic similarities. The symbiotic association between the roots of forest trees and ectomycorrhizal fungi is a universal rule; it is essential for the establishment and sustainability of forests, as well as their productivity.
The genome of the most common symbiotic forest fungus is decoded!
The ascomycete Cenococcum geophilum is the most common and globally abundant symbiotic fungus on tree roots in the arctic, temperate and subtropical zones, and particularly in extreme environments. The mycorrhizal root tips are highly resistant to desiccation and are strikingly abundant during soil drought conditions when other mycorrhizal species decline, suggesting an important role in drought resistance and resilience of host trees.
A team led by INRA and WSL and including researchers from the Department of Energy Joint Genome Institute and other academic partners characterized the complete genome of Cenococcum geophilum. They identified the genes and their expression products, molecules at the origin of proteins. Their analysis revealed several surprising details. The research team found that two of the three most highly induced Cenococcum genes in symbiosis are coding for water channels (aquaporins). This dramatic induction of water channel genes is fine-tuned under drought conditions and they likely play a key role in drought adaptation of host plants. The fungus genome also encodes a large set of genes to communicate with the host plant via signaling proteins, including small secreted effectors highly expressed upon symbiosis.
Cenococcum has lost hundreds of genes as a result of its intimate and secular alliance with trees. For example, it lacks most of the genes required for breaking down plant cell walls, a critical ability for free-living saprotrophic fungi that feed on dead organic matter in forest soils. The ectomycorrhizal symbiont has therefore become highly reliant upon the availability of a continuous flux of photoassimilates from its host plant. Interestingly, these genomic adaptations to the mycorrhizal lifestyle are shared with ectomycorrhizal basidiomycetes indicating a striking convergent evolution in fungal lineages separated by several 100 million years of evolution. By combining genome sequences with rigorous physiological and ecological studies, we are entering a time where linking the presence, composition and abundance of soil mycorrhizal communities with important soil processes and forest productivity at an ecosystem scale is possible. This should facilitate the identification of drought-adapted Cenococcum geophilum strains, which can be used to efficiently support their host trees threatened by the forecasted increase in drought periods in many parts of the world.