Master of recycling: bacterial poison protects fungi from parasites

In 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.

In healthy soils, yoke fungi of the genus

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

However, it can turn the tables, as Michael DiLegge and colleagues from Colorado State University in Fort Collins have observed: 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 fungus called

, which has developed a completely different strategy, is also

native to

this country. 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. It

was already known

that the "NRRL 6337" variant of

contains so-called necroxime. 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 have actually discovered bacteria of the genus

in the cells of the fungi in question

.

Months of antibiotic treatment confirmed that the toxins that are effective against roundworms come from these microbes: after the treatment, the necroximes had completely disappeared from the fungal cells, as had the bacterium, for which the name

seems appropriate.

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 around Büttner report in the

, the bacterium differs so significantly overall from previously known representatives of the genus

that it can be viewed as a new species.

How well the fungus

can arm itself against

with bacterial help was

tested

using the example of

. 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

naturally harbor bacteria that cannot produce necroxime.

Such symbioses probably offer the fungus other, as yet unknown, advantages.