Jena. The symbiosis with a bacterium protects a fungus of the genus Mortierella against its predator - a nematode. Since fungi of this genus are mainly found in healthy soils, the results offer new possibilities for nature-friendly agriculture. An international research team reports on this hidden triangular relationship in the scientific journal PNAS.

Even the smallest living organisms on our planet form highly complex networks of interaction. Microorganisms regulate their conflicts in communities via chemical signals, the natural products. But some of the agents are on secret missions.

Unusual similarities

Researchers at the Leibniz Institute for Natural Product Research and Infection Biology – Hans Knöll Institute – (Leibniz-HKI) came across a striking parallel in the literature: The fungus Mortierella verticillata is said to produce a toxic substance that is also generated in a similar form by a bacterium.

"It is highly unlikely that organisms as different as fungi and bacteria produce such similar substances," explains Christian Hertweck. He is a Professor of Natural Product Chemistry at Friedrich Schiller University Jena and heads a research department at Leibniz-HKI. His team became suspicious and set out to find the hidden player.

And indeed there was another microorganism which had remained undercover until now. But Hertweck's team was able to track it down: Using fluorescence microscopy, the researchers discovered a previously undescribed bacterium that lives in the hyphae of the fungus. They named it Candidatus Mycoavidus necroximicus. "The discovery fuelled our suspicion that it is not the fungus that produces the toxin, but the bacterium inside the fungus," states Hannah Büttner, a doctoral researcher in Hertweck's group and one of the first authors of the study.

Detective work in the laboratory

Detective work was needed to investigate whether the bacterium was actually responsible for the toxin - called necroxime. The research team conducted a series of comparative studies in the laboratory. "We treated the fungus with antibiotics to kill the endosymbiont, which is the bacterium inside the fungus. We then compared the metabolic profile with that of the untreated fungus. In the bacteria-free fungus, the necroxime could no longer be detected," explains Büttner.

The bacterium is dependent on the fungus it lives inside, so it cannot be cultivated without its host. However, the scientists succeeded in isolating bacterial DNA from the hyphae of the fungus. From these DNA fragments, the Australian partners around molecular biologist Sacha Pidot were able to decode the entire genome of the bacterium. "The genome of the bacterium shows a high potential for the synthesis of natural products. One gene cluster also encodes biosynthetic enzymes that are a perfect match for the formation of necroxime," Hannah Büttner notes.

Ecological significance

The researchers assume that the toxin produced by the bacteria protects the fungus from predators such as nematodes, which are also found in the soil. Experiments with these nematodes showed the toxic effect: In the presence of the toxin-producing bacteria, the worms are killed. Image analyses carried out by the research group led by systems biologist Thilo Figge at Leibniz-HKI further confirmed the effect.

Application possibilities

The discovery of the hidden bacterium and the decoding of its ecological function for the fungus are of great importance. Fungi of the genus Mortierella indicate a healthy soil structure. They are also used in agriculture as plant growth-promoting fungi. "Our results therefore open up new approaches for the development of natural biocontrol agents in agriculture," concludes study leader Hertweck.

The language of microorganisms

The study, published in the scientific journal PNAS, was carried out within the framework of the Collaborative Research Centre (SFB) "ChemBioSys" at the Friedrich Schiller University Jena, which is funded by the German Research Foundation. Experts from microbiology, chemistry and bioinformatics have been studying complex microbial communities and the chemical signalling compounds that regulate these relationships for about eight years. The CRC scientists are currently applying for a third funding period. Besides Hannah Büttner, Sarah Niehs is the second lead author of the study. She recently started working at Stanford University in the USA.

Original publication

Büttner H, Niehs SP, Vandelannoote K, Cseresnyés Z, Dose B, Richter I, Gerst R, Figge MT, Stinear TP, Pidot SJ, Hertweck C (2021) Bacterial endosymbionts protect beneficial soil fungus from nematode attack. Proc Natl Acad Sci U S A, 118 (37) e2110669118, doi.org/10.1073/pnas.2110669118