A single molecule upsets symbiosis

Newswise — Researchers at the Leibniz Institute for Natural Product Research and Infection Biology (Leibniz-HKI) in Jena have conducted a recent study exploring the delicate balance of bacteria and fungi coexistence. The findings highlight that a symbiotic relationship, which proves mutually beneficial, can be remarkably fragile. Specifically, the study focused on the interaction between the bacterial species Mycetohabitans rhizoxinica and the fungus Rhizopus microsporus, revealing that the bacteria can only thrive within the hyphae of the fungus when they produce a specific protein.

In symbiosis, two organisms form a partnership and mutually benefit from each other's presence. However, in endosymbiosis, one organism takes this relationship to a deeper level by residing inside the other. An example of such interdependence can be observed in the case of the fungus Rhizopus microsporus and the bacterium Mycetohabitans rhizoxinica (formerly known as (Para)burkholderia rhizoxinica). The fungus is responsible for causing rice seedling blight, leading to significant crop losses in Asia each year. Interestingly, R. microsporus can only carry out this destructive behavior with the assistance of M. rhizoxinica. The bacterium produces a plant toxin that is utilized and released by the fungus. Without the presence of the bacterium, the fungus loses its ability to form spores and effectively spread. In exchange for this crucial contribution, the fungus provides nutrients to its endosymbiotic partner.

In the natural environment, the fungus Rhizopus microsporus and the bacterium Mycetohabitans rhizoxinica always exist in a state of symbiosis, as explained by Ingrid Richter, a postdoctoral researcher in the Department of Biomolecular Chemistry at Leibniz-HKI. However, through laboratory experiments, the researchers have managed to cultivate these organisms independently. This separation has allowed them to observe that the bacteria retain their ability to infect the fungus. Richter further commented on this finding, stating that even when cultured separately, the bacteria still possess the capability to invade the fungus.

From parasitism to symbiosis?

The recent discovery made by the research team reveals the critical role of a specific bacterial protein, known as an effector protein, in preserving the symbiotic relationship. In particular, the deactivation of the TAL effector 1 (MTAL1) has been observed to lead to uncontrolled bacterial proliferation, causing the fungus to form new cell walls that encapsulate and isolate the trapped bacteria, ultimately resulting in their demise. In highlighting the significance of this finding, Richter noted that similar TAL effectors are already recognized in various bacteria that infect plants, enabling them to invade plant cells.

The relationship between R. microsporus and M. rhizoxinica appears to have undergone a transformation from an initially parasitic association to a symbiotic one. In the absence of the effector protein, the bacteria can still infect the fungus by breaking down the cell wall and penetrating the fungal hyphae. However, the fungus perceives this invasion as parasitic, indicating a shift in the nature of the interaction. The presence of the TAL effector protein, on the other hand, ensures the stability of the symbiotic relationship. In light of these observations, Richter emphasizes the existence of a seamless transition between a mutually beneficial symbiosis and a potentially harmful parasitic relationship, highlighting the complex dynamics between the partners involved.

Close microscopic observation

To gain a more detailed understanding of the infection process, the researchers have devised an advanced system utilizing microfluidic chips with narrow channels to cultivate the fungus. These channels are designed to accommodate only a single fungal hypha. Subsequently, the bacteria are introduced into the system, and the entire process of infection is observed using a microscope over several hours. This unique setup prevents the filamentous hyphae from growing on top of each other, providing clear visibility of the events unfolding. Richter explains that this approach has enabled them to make significant observations. For instance, when the TAL effector is deactivated, the fungus constructs additional transverse walls within the hyphae, leading to distinct regions of separation. Remarkably, these separated areas contain a notably high concentration of bacteria. By employing specific dyes, the researchers can visualize the bacteria that become trapped in these regions, ultimately observing their death after a few hours.

The research team intends to delve deeper into the cellular mechanisms initiated by the effector protein. Richter mentions that their hypothesis is that the TAL effector protein binds to the fungal genome, as is typical for such proteins. However, the exact binding locations are unknown at present. One of the reasons for this knowledge gap is that the complete decoding of R. microsporus' genome has not been accomplished yet. The researchers aim to address this limitation in their future investigations to gain a comprehensive understanding of the interactions between the effector protein and the fungal genome.

The research findings offer valuable insights into the nature of endosymbiotic relationships, which have played a significant role in evolutionary processes. An illustrative example is the mitochondria found in the cells of plants, animals, and fungi, which are believed to have originated as endosymbionts. Despite having their own DNA, mitochondria have become dependent on their host cells and are unable to survive independently, unlike M. rhizoxinica. Richter further highlights that the meticulous microscopic observations have yielded substantial knowledge regarding the various functions performed by different types of hyphae within the fungal mycelium. For instance, the observation of nutrient transport has provided valuable insights into the diverse tasks carried out by distinct hyphal structures. These insights contribute to our understanding of the complex dynamics and functionalities within fungal mycelia.

The research conducted by Ingrid Richter received support from various funding sources. Notably, a Marie Skłodowska-Curie grant provided by the European Union contributed to the project. Additionally, financial support was provided by the German Research Foundation through the Balance of the Microverse Cluster of Excellence and the ChemBioSys Collaborative Research Center. The Jena School for Microbial Communication also played a role in supporting the research. Furthermore, Christian Hertweck, the lead researcher, was awarded the prestigious Leibniz Prize. Lastly, the Swiss National Science Foundation provided additional funding for the project. The combined support from these organizations and grants facilitated the successful execution of the research endeavor.