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Bacterial poison protects fungi from parasites

IIn the soil, fungi gradually break down the dead organic matter so that the nutrients bound in it become available again for the plants. The mushrooms also break down things that are difficult to digest, such as lignin and chitin. Yoke mushrooms of the genus play in healthy soils Mortierella a central role. For this, these distant relatives of the mold have to defend themselves reasonably successfully against other soil dwellers. Yoke fungi are mainly attacked by tiny nematodes. The species that also occurs in this country Mortierella globalpina but can turn the tables as observed by Michael DiLegge and colleagues at Colorado State University in Fort Collins: Fungal threads attach themselves to the nematode, penetrate the body wall made up of collagen fibers and digest all the cells that they then encounter. In this way, the fungus also protects plant roots from parasitic roundworms.

A mushroom called is also native to this country Mortierella verticillatawho developed a completely different strategy. It can enter into a symbiosis with bacteria that provide toxins against roundworms. Scientists working with Hannah Büttner and Sarah P. Niehs from the Leibniz Institute for Natural Product Research and Infection Biology in Jena recently discovered this. That the variant “NRRL 6337” from Mortierella verticillata so-called necroxime was already known. Since these highly effective poisons from the group of benzolactone enamides fit more into the repertoire of bacteria than of fungi, the suspicion of symbiotic microorganisms arose.

Together with microbiologists from the Doherty Institute in Melbourne, Büttner and her colleagues now actually have bacteria of the genus in the cells of the fungi in question Mycoavidus discovered. Months of antibiotic treatment confirmed that the toxins that are effective against roundworms come from these microbes: after the cure, the necroximes had completely disappeared from the fungal cells, as had the bacterium for which the name was given Mycoavidus necroximicus seems appropriate.


With the help of fluorescence microscopy, the bacteria (green) in the hyphae of the fungus (blue) become visible.
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Photo: Leibniz-HKI

Detected a new genus of mushrooms

Although this inhabitant of fungal cells stubbornly refused to thrive on an artificial nutrient medium, the researchers finally succeeded in isolating its DNA. As a symbiotic partner that is presumably fully supplied, this bacterium has a relatively small genome, a large part of which is used to produce extravagant natural substances. One of the relevant gene clusters was identified as the blueprint for the synthesis apparatus for Necroxime. As the researchers led by Büttner in the Proceedings of the National American Academy of Sciences messages, Overall, the bacterium differs so clearly from previously known representatives of the genus Mycoavidusthat it can be viewed as a new species.

How good is the mushroom Mortierella verticillata can protect against roundworms with bacterial help, was demonstrated using the example of Aphelenchus avenae checked. This nematode stings its mouthparts into fungal cells in order to ingest their nutritious contents. When confronted with fungi whose symbiotic bacteria produce necroxime, these aggressive worms had a much higher death rate than when they came into contact with defenseless specimens that had no bacterial toxins at their disposal. It is possible that soil fungi with symbiotic bacteria can also be useful in biological pest control in the future.

If plant roots are to be protected from roundworms, however, it is important to carefully select the fungi and their symbiotic partners. Some variants of Mortierella verticillata they naturally harbor bacteria that cannot produce necroxime. Such symbioses probably offer the fungus other, as yet unknown, advantages.

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