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Sequencing Study Assesses Global Diversity of Symbiotic Plant Root Fungi

NEW YORK (GenomeWeb) – Fungi involved in symbiotic relationships with plant roots appear capable of surprisingly long-range dispersals, according to a study appearing online today in Science.

An international team led by investigators in Estonia did pyrosequencing on fungal DNA from more than 1,000 plant root samples, looking at the symbiotic fungi from plants at diverse environmental sites around the world. From the DNA-based fungal taxa they identified, the researchers found that communities of these so-called "arbuscular mycorrhizal" fungi tend to coincide with local environmental conditions and along spatial gradients.

Still, their results suggest almost all of the fungal taxa have made their way to far-flung locations despite relatively limited means of dispersing themselves.

"Along with a better understanding of the functional attributes of cosmopolitan taxa throughout their ranges, such information will not only enhance our understanding of microbial biogeography but will also facilitate ecosystem restoration and sustainable agriculture," senior author Martin Zobel, an ecology and earth sciences researcher at the University of Tartu, and his colleagues wrote.

Roughly 80 percent of land plants have some form of arbuscular mycorrhizal fungus nestled in their roots, helping them obtain nutrients, withstand limited water conditions, and/or ward off pathogens in exchange for carbon the plant produces, the team noted.

Consequently, the fungi are believed to impact not only individual plants, but also between-plant relationships and plant community structures. Even so, not all that much is known about the nature and diversity of fungi found in association with plant roots, the researchers explained, beyond morphological descriptions of a few hundred arbuscular mycorrhizal fungi and DNA-based descriptions of hundreds more.

"Because many [arbuscular mycorrhizal] fungi are unculturable," they wrote, "identification of arbuscular mycorrhizal] fungal taxa in the environment is principally dependent on DNA-based methods; these asexual organisms are classified into approximately species-level taxonomic units using clustering or sequence-matching algorithms."

With an eye to untangling arbuscular mycorrhizal fungal diversity more systematically in different locations and environments, the team collected 1,104 root samples from vegetation plots in forest, grassland, shrubland, semi-desert, and successional sites from locations around the world ranging from tropical to subtropical to temperate.

Based on more than 912,000 quality-filtered reads generated by Roche 454 sequencing for 836 samples, the researchers defined almost 250 DNA-based "virtual taxa" associated with 161 plant species — a symbiotic fungi set that included 68 percent of the previously described arbuscular mycorrhizal taxa and 10 new taxa.

Almost all of the taxa turned up in more than one climactic zone and more than one continent, with a third of the arbuscular mycorrhizal fungal tax appearing on all six of the continents tested.

Likewise, when researchers folded in data from an existing arbuscular mycorrhizal fungal virtual taxa database, they found that far more fungi than plant species are shared between continents, though arbuscular mycorrhizal fungal community diversity did seem to diminish with increasing distance from the equator.

Their phylogenetic analysis, based on small subunit ribosomal RNA gene sequences, suggested most current fungal symbionts come from taxa that appeared after continents moved apart, fueling speculation about how slow-moving organisms from the taxa may have reached such diverse and widespread locations.

At the local level, meanwhile, the team noted that both environmental conditions and spatial distance appeared to affect the types of symbiotic fungi found in plant roots.

"We suggest that addressing both global and local dispersal of [arbuscular mycorrhizal] fungi and the role of dispersal agents such as birds, large mammals (including humans), wind, surface water, and seawater can illuminate the processes governing [arbuscular mycorrhizal] fungal diversity patterns," the study's authors concluded.