Annual Conference 2025 of the Association for General and Applied Microbiology - Jahrestagung 2025 der Vereinigung für Allgemeine und Angewandte Mikrobiologie

Prof. Dr.

Chair

Form and function of arbuscular mycorrhiza symbiosis

FG Symbiotic interactions

Most land plants form symbioses with Glomeromycotina fungi to acquire mineral nutrients from the soil. This so-called arbuscular mycorrhiza (AM) symbiosis is evolutionarily ancient and found in the oldest fossils of land plants, sparking speculations about its importance in the colonization of the land by originally aquatic plants during evolution, at a time when plants had not yet evolved complex root systems for nutrient uptake. The fungi form extended hyphal networks in the soil to scavenge mineral nutrients. These are transported into the root and released via beautifully-shaped, highly branched hyphal structures, the arbuscules inside inner root cells. In return they receive up to 20% of photosynthetically fixed carbon from their host in the form of sugars and lipids. As a consequence, AM contributes significantly to plant nutrition and to global carbon cycles.For symbiosis establishment, AM fungi colonize the root interior and the inside of plant cells. Symbiotic infection of single, already differentiated cells within the tissue context requires a poorly understood cellular remodeling program that is intertwined with mechanisms that control plant development and physiology. In my presentation, I will provide examples of how we investigate the molecular mechanisms underlying development and functioning of this fascinating symbiosis.

Invited lecture

How do zombie fungi adaptively hijack insect host behaviour? A molecular ecology perspective

The evolutionary arms race between parasites and their hosts can culminate into complex extended phenotypes that further disease progression and transmission. The fungus-adaptive changes in behaviour as seen in Ophiocordyceps-infected carpenter ants are a prime example. These "zombie ants" demonstrate a suite of behaviours that are thought to circumvent the colony's social immune responses. Subsequently, the hijacked ant climbs and attaches itself to an elevated position that benefits fungal spore development and dispersal, and does so with a precise daily timing. These fungus-induced behaviours are not unique to Ophiocordyceps infections. Parallel behaviours have also been observed in invertebrates infected by other fungi, viruses and trematodes. The precise mechanisms involved in these behavioural manipulations are unknown. To unravel these mechanisms, we have developed the Ophiocordyceps-ant interaction into an integrative model system that allows us to study parasitic behavioural manipulation in greater detail in the lab. By combining fungal culturing and lab infections with behavioural assays and multiple omics approaches, we propose several comprehensive mechanistic hypotheses about the fungal proteins and ant receptors involved in this phenomenon. These hypotheses include specific fungal "manipulation" effectors of interest and their potential binding to ant proteins involved in light perception, biogenic amine binding and daily rhythms. To test these hypotheses, we are currently, for the first time in this model, integrating functional genetics assays to determine the function of presumed fungal "manipulation" effectors, the host behaviours they elicit, and the host pathways underlying those phenotypes. Our results will provide detailed insights into fungus-insect interactions in general while advancing our understanding of parasitic hijacking of animal behaviour in particular. Moreover, the evident role of biological clocks in these interactions paves the way to study antagonistic organismic interactions at their intersection with the field of chronobiology. We expect our findings to have a translational impact on more sustainable insect pest control practices while our investigations into fungal bioactive proteins and metabolites and their host targets might give rise to the discovery of novel drugs.

Enzyme donation by widespread Rickettsiales in protists and simple aquatic invertebrates

Rickettsiales are intracellular bacteria that depend on their eukaryotic hosts" resources for their metabolism. One clade of aquatic Rickettsiales of the family Midichloriaceae is among the most ubiquitous bacterial phylotypes in aquatic samples, occurring in almost 10% of all aquatic samples. With a large host diversity of protists and early branching animals such as sponges, placozoans, and cnidarians, these Midichloriaceae are regarded mainly as parasites or pathogens and were linked to coral disease outbreaks.Here, I will present evidence for a surprising and highly beneficial role for these intracellular symbionts that inhabit the endoplasmic reticulum in most of their hosts - digestive enzyme donation. Across all host groups, data from genomic, expression, enzyme characterization, and imaging experiments show that these ubiquitous aquatic Midichloriaceae provide enzymes to break down sugar-based macromolecules. Based on the characterization of heterologously expressed enzymes, the symbionts provide digestive enzymes for laminarin, one of the most abundant macromolecules in aquatic systems. In addition, our data show an anti-microbial role, as one broadly-expressed enzyme can attack the cell walls of bacteria. Using in-depth tree-reconciliation analyses, I will discuss the evolutionary chains of events that likely led to the taming of notorious parasites into important intracellular symbionts that can turn dietary fibers into efficient metabolic resources for sugars. These insights into the beneficial roles of a group of symbionts labeled parasitic because of their intracellular lifestyle will provide a better understanding of these common symbioses that can alter physiology, ecology, and gene flow from within host cells. Given their broad host range and geographic distribution, these aquatic Midichloriaceae are at the center of many current frontiers of aquatic sciences on a changing planet, such as biomonitoring and conservation efforts, aquaculture, and the aquarium trade.

FG Symbiotic interactions

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