Fungi: Sex and self defense

The fungus Aspergillus nidulans produces secondary metabolites during sexual development to protect itself from predators.
  1. Milton T Drott  Is a corresponding author
  1. Department of Medical Microbiology and Immunology, University of Wisconsin-Madison, United States

When walking through the woods we often look up, focusing on the birds and the rustling leaves in the canopy above us, but on the ground a drama is unfolding: the fungi are under attack. Looking down you may see a slug grazing on mold that has established itself on an old log, or a mushroom swarmed by fruit flies on the forest floor. So how do fungi protect themselves from these attacks if they cannot physically escape?

Previous research has shown that fungi defend themselves using secondary metabolites, chemical compounds which are not essential for growth but involved in ecological interactions (Janzen, 1977; Rohlfs and Churchill, 2011). These compounds can be toxic to animals and/or drive them away from the fungus. As predators can appear without warning, fungi must be ready with the metabolites at short notice, either by making them ahead of time, or by rapidly creating them in response to a threat (Drott et al., 2017). However, even in the best-studied fungi, it is unclear exactly where and when these defensive chemicals are created, making it difficult to fully understand their ecological purpose.

Now, in eLife, researchers from the University of Göttingen – including Li Liu and corresponding authors Jennifer Gerke and Gerhard H Braus – report that a set of previously identified secondary metabolites called xanthones (Sanchez et al., 2011) are produced during certain life stages of the soil-dwelling fungus Aspergillus nidulans (Liu et al., 2021). Xanthones are synthesized through a series of chemical reactions controlled by a group of genes known as the mdp/xpt cluster. The proteins encoded by the mdp genes make the ‘backbone’ of the metabolite, which is then progressively modified by proteins produced from the xpt genes until the final compound is formed.

To narrow down where xanthones are synthesized in the fungus, Liu et al. added a fluorescent tag to the protein responsible for the final chemical reaction, as this represents the complete synthesis of the secondary metabolites produced by the mdp/xpt pathway. This revealed that xanthones are created in Hülle cells which support the development of cleistothecia, fruiting bodies that allow the fungus to sexually reproduce and last through the winter (Troppens et al., 2020). This suggests that xanthones are not produced throughout the life of the fungus, but are only generated during the stages of the fungus’ sexual lifecycle when cleistothecia form.

Next, Liu et al. set out to determine the role of other genes in the mdp/xpt cluster by creating a set of mutant fungi that are missing one of these genes. They found that each gene plays a specific role in the sequence of chemical reactions that synthesize the xanthones used by the fungi. As a result, none of the mutant strains were able to produce the final xanthones, and instead accumulated intermediate chemical structures that are generated during this pathway. Like xanthones, these intermediates only appeared at times when the fungus was forming cleistothecia.

It is clear from these findings that A. nidulans likely uses xanthones during sexual development; but what role do these secondary metabolites play in ecology? To investigate this, Liu et al. grew fungal colonies and cleistothecia from mutated and non-mutated (or wild-type) strains of A. nidulans and exposed them to arthropods (invertebrates with exoskeletons such as insects and arachnids) that eat fungi (Figure 1). Wild-type colonies – which can produce all of the xanthones – were damaged less heavily by the arthropods than the mutant colonies. Further experiments showed that, in addition to mitigating damage from arthropods, some of the intermediates formed during synthesis can suppress fungal growth when added to other fungi in the laboratory. However, these intermediates did not accumulate to high levels in the wild-type strain and also suppressed the development of A. nidulans, raising doubts about their potential benefit to the fungus when competing with other fungi in nature.

Chemical products of the mdp/xpt gene cluster protect A. nidulans from predators.

A. nidulans produces secondary metabolites called xanthones using a set of genes known as the mdp/xpt cluster. (Top) In the wild-type fungus, xanthones (shown as chemical structures) are produced by Hülle cells (small beige circles) and then accumulate in sexual fruiting bodies called cleistothecia (large black circles). When arthropods attack the wild-type fungus, the xanthones deter these predators and stop them from destroying the cleistothecia. (Bottom) Fungi with lab-induced mutations in the mdp/xpt genes are unable to produce xanthones, making them more susceptible to fungus-eating arthropods.

Image credit: Milton T Drott; Figure created using BioRender.com.

Hülle cells are found in other fungi (Dyer and O'Gorman, 2012), and genes resembling the mdp/xpt cluster occur in other species where no sexual cycle has been observed to date (de Vries et al., 2017). It remains to be seen how secondary metabolites that appear at specific life stages – like the ones in this study – translate into these other species. Furthermore, it is unclear how these chemical compounds relate to previous observations that other secondary metabolites with unknown functions are only produced under certain conditions (Georgianna et al., 2010). The findings of Liu et al. emphasize the complicated interplay between fungi and their environment, and spark further questions about about how the fungus' investment in protecting its sexual offspring has impacted its evolution.

References

Article and author information

Author details

  1. Milton T Drott

    Milton T Drott is in the Department of Medical Microbiology and Immunology, University of Wisconsin-Madison, Madison, United States

    For correspondence
    mdrott@wisc.edu
    Competing interests
    No competing interests declared
    ORCID icon "This ORCID iD identifies the author of this article:" 0000-0001-9715-2200

Publication history

  1. Version of Record published: October 14, 2021 (version 1)

Copyright

© 2021, Drott

This article is distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use and redistribution provided that the original author and source are credited.

Metrics

  • 762
    Page views
  • 71
    Downloads
  • 1
    Citations

Article citation count generated by polling the highest count across the following sources: Crossref, PubMed Central, Scopus.

Download links

A two-part list of links to download the article, or parts of the article, in various formats.

Downloads (link to download the article as PDF)

Open citations (links to open the citations from this article in various online reference manager services)

Cite this article (links to download the citations from this article in formats compatible with various reference manager tools)

  1. Milton T Drott
(2021)
Fungi: Sex and self defense
eLife 10:e73723.
https://doi.org/10.7554/eLife.73723

Further reading

    1. Biochemistry and Chemical Biology
    2. Microbiology and Infectious Disease
    Atanas Radkov et al.
    Research Article

    Members of the bacterial T6SS amidase effector (Tae) superfamily of toxins are delivered between competing bacteria to degrade cell wall peptidoglycan. Although Taes share a common substrate, they exhibit distinct antimicrobial potency across different competitor species. To investigate the molecular basis governing these differences, we quantitatively defined the functional determinants of Tae1 from Pseudomonas aeruginosa PAO1 using a combination of nuclear magnetic resonance (NMR) and a high-throughput in vivo genetic approach called deep mutational scanning (DMS). As expected, combined analyses confirmed the role of critical residues near the Tae1 catalytic center. Unexpectedly, DMS revealed substantial contributions to enzymatic activity from a much larger, ring-like functional hot spot extending around the entire circumference of the enzyme. Comparative DMS across distinct growth conditions highlighted how functional contribution of different surfaces is highly context-dependent, varying alongside composition of targeted cell walls. These observations suggest that Tae1 engages with the intact cell wall network through a more distributed three-dimensional interaction interface than previously appreciated, providing an explanation for observed differences in antimicrobial potency across divergent Gram-negative competitors. Further binding studies of several Tae1 variants with their cognate immunity protein demonstrate that requirements to maintain protection from Tae activity may be a significant constraint on the mutational landscape of tae1 toxicity in the wild. In total, our work reveals that Tae diversification has likely been shaped by multiple independent pressures to maintain interactions with binding partners that vary across bacterial species and conditions.

    1. Biochemistry and Chemical Biology
    Kanwal Kayastha et al.
    Research Article

    Lactate oxidation with NAD+ as electron acceptor is a highly endergonic reaction. Some anaerobic bacteria overcome the energetic hurdle by flavin-based electron bifurcation/confurcation (FBEB/FBEC) using a lactate dehydrogenase (Ldh) in concert with the electron-transferring proteins EtfA and EtfB. The electron cryo-microscopically characterized (Ldh-EtfAB)2 complex of Acetobacterium woodii at 2.43 Å resolution consists of a mobile EtfAB shuttle domain located between the rigid central Ldh and the peripheral EtfAB base units. The FADs of Ldh and the EtfAB shuttle domain contact each other thereby forming the D (dehydrogenation-connected) state. The intermediary Glu37 and Glu139 may harmonize the redox potentials between the FADs and the pyruvate/lactate pair crucial for FBEC. By integrating Alphafold2 calculations a plausible novel B (bifurcation-connected) state was obtained allowing electron transfer between the EtfAB base and shuttle FADs. Kinetic analysis of enzyme variants suggests a correlation between NAD+ binding site and D-to-B-state transition implicating a 75° rotation of the EtfAB shuttle domain. The FBEC inactivity when truncating the ferredoxin domain of EtfA substantiates its role as redox relay. Lactate oxidation in Ldh is assisted by the catalytic base His423 and a metal center. On this basis, a comprehensive catalytic mechanism of the FBEC process was proposed.