Fragile symbiosis between bacteria and fungus discovered

Researchers at the Leibniz Institute for Natural Product Research and Infection Biology (Leibniz-HKI) in Jena sheds light on the delicate nature of a mutually beneficial symbiotic relationship between bacteria and fungi. The study focused on the bacterial species Mycetohabitans rhizoxinica and its interaction with the fungus Rhizopus microsporus, revealing that the symbiosis heavily relies on the production of a specific by the bacteria. The research findings have been published in the journal Current Biology.

Symbiosis refers to the close association between two organisms, where both parties derive benefits from each other. In the case of endosymbiosis, one organism takes this relationship to the next level by living within the other. The fungus Rhizopus microsporus and the bacterium Mycetohabitans rhizoxinica (formerly known as (Para)burkholderia rhizoxinica) exemplify a symbiotic partnership wherein they are mutually dependent. The fungus is capable of causing rice seedling blight, a condition that leads to significant crop losses in Asia each year. However, this ability is only possible with the presence of the bacterium, which produces a plant toxin that is subsequently processed and released by the fungus. Without the bacterium, the fungus cannot efficiently form spores and spread. In return for this toxin production, the fungus provides nutrients to its endosymbiont.

“In the wild, these two organisms always exist in symbiosis,” explains Ingrid Richter, a postdoctoral researcher in the Department of Biomolecular Chemistry at Leibniz-HKI. However, the researchers managed to cultivate them separately in the laboratory, leading to the discovery that the bacteria can still infect the fungus even when they are not living in symbiosis. Richter further elaborates on the fragility of this relationship by stating that the symbiosis is reliant on the presence of the specific protein produced by the bacteria.

This study highlights the intricate dynamics of symbiotic relationships and underscores the vulnerability of such interactions. The findings contribute to our understanding of the delicate balance that exists within these mutually beneficial associations.

From parasitism to symbiosis?

The research team has made an intriguing discovery regarding the role of a specific bacterial protein, known as an effector protein, in maintaining the symbiotic relationship between the fungus Rhizopus microsporus and the bacterium Mycetohabitans rhizoxinica. In their experiments, the researchers deactivated a particular effector protein called TAL effector 1 (MTAL1) and observed that the bacteria started to multiply uncontrollably. As a result, the fungus responded by closing off sections of its hyphae with new cell walls, effectively trapping and causing the death of the bacteria. TAL effectors are already known from various bacteria that infect plants, as they enable the bacteria to invade plant .

This finding suggests that the initial interaction between R. microsporus and M. rhizoxinica might have transitioned from a parasitic relationship 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, from the perspective of the fungus, the bacteria are then perceived as parasites. Only when the TAL effector is present does the symbiotic relationship remain stable. Ingrid Richter emphasizes the significance of this discovery, stating that it demonstrates the subtle transition between a symbiosis that benefits both partners and a potentially parasitic relationship that can be detrimental to one partner.

These findings shed new light on the intricacies of symbiotic interactions and illustrate how the presence or absence of specific proteins can determine the nature of the relationship between organisms. The study highlights the delicate balance that exists within symbioses and provides valuable insights into the factors that contribute to their stability or disruption.

Close microscopic observation

In order to closely observe the infection process between Rhizopus microsporus and Mycetohabitans rhizoxinica, the researchers have devised an advanced system using microfluidic chips with narrow channels. These channels accommodate a single fungal hypha each, allowing for precise observation of the events. The bacteria are introduced into the channels, and the infection process is monitored under a microscope for several hours.

The unique design of the microfluidic chips prevents the hyphae from growing on top of each other, granting researchers a clear view of the proceedings. By utilizing this system, Ingrid Richter was able to demonstrate that when the TAL effector is deactivated, the fungus constructs additional transverse walls within the hyphae. These separated regions contain a higher concentration of bacteria, and through the use of specific dyes, it was observed that the trapped bacteria perish after a few hours.

To delve further into the cellular mechanisms triggered by the effector protein, the researchers aim to investigate its binding sites within the fungal genome. Although such binding is typical for these types of proteins, the exact locations remain unknown due to the incomplete decoding of the R. microsporus genome.

The research findings contribute valuable insights into the understanding of endosymbiotic partnerships, which have played a significant role in . Mitochondria, for instance, which serve as energy producers in plant, animal, and fungal cells, were likely endosymbionts in their origins. While mitochondria possess their own DNA, they have become dependent on host cells and cannot survive independently, unlike M. rhizoxinica. Furthermore, the close microscopic examination has provided significant knowledge regarding the distinct functions of different types of hyphae within the fungal mycelium, such as nutrient transport.

This research not only enhances our understanding of the delicate interactions between symbiotic organisms but also contributes to our broader knowledge of evolutionary processes and the functions of various cellular structures.

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