Symbiotic fungus helps plants to withstand drought

A symbiotic fungus may play an important role in trees' resistance to drought stress, a new genome analysis reveals. Furthermore, the fungus lost several genes that may be harmful for plants, in order to enter a mutually beneficial partnership with trees. The study led by the Swiss Federal Institute for Forest, Snow and Landscape Research WSL and the French research institute Inra was published in the journal Nature Communications.

To take up scarce nutrients and water, trees rely on mycorrhizal symbioses with fungi. Ectomycorrhizal fungi include some of the most conspicuous forest mushrooms, including the iconic Fly Agaric, Golden Chanterelle and King Bolete and also 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.

Common in extreme environments

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 melanized 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 U.S. Department of Energy Joint Genome Institute reported the complete genome and transcript profilings of C. geophilum in a paper published online in the journal Nature Communications.

More water channels produced

Analysis of the genome revealed several surprising details. The research team found that two of the three most highly induced C. geophilum 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 encode an abundance of genes for communication with the host plant via signaling proteins, including small secreted effectors highly expressed upon symbiosis.

In addition to this, C. geophilum has lost hundreds of genes as a result of its intimate alliance with trees. For example, it lacks most of the genes required for breaking down plant cell walls, a critical ability for parasitic as well as free-living saprotrophic fungi that feed off 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 100 millions 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 C. 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.

Dr. Francis Martin
UMR Interactions Arbres/Micro-organismes
Département Ecologie des Forêts, Prairies et milieux Aquatiques
Centre Inra de Nancy, Frankreich
Tel. +33 383 39 40 80
fmartin(at)nancy.inra.fr