Breaking down the microbiology world one bite at a time
Together We Achieve More
Teamwork is the essence of any successful project. In recent research carried out by Anckaert and his colleagues, the coauthors showed us how the interaction between a soil bacterium (Bacillus velezensis), mycorrhizal fungus (Rhizophagus sp.), and a tomato plant benefits each member of the team. The scientists explored how an antagonistic bacterium cooperates with mycorrhiza to enhance tomato plant resistance to pests in a tripartite cooperation. In the following article, we will dissect these cross-kingdom interactions one by one to understand how the bacterium changes its behavior to fit into the team.
The team
The story begins with the plant, which secretes metabolites via its roots to attract beneficial soil microbes, such as mycorrhiza. Mycorrhiza is one of the keystone examples of beneficial fungi that cooperate with two-thirds of plants on Earth. During the growth of the Mycorrhiza, it forms a tangled, long network of filaments called hyphae. This network works as a nutrient supply system and telecommunication tool. For instance, the mycorrhizal networks extend tens of centimeters away from the roots, thus improving nutrient uptake by the plant. In addition, it connects neighboring plants and transfers warning signals of pathogens between them. As a reward, the plant provides the fungus with nutrients (carbon sources), supporting its growth and development. Similar behavior is followed by the fungus, which secretes nutrients around its hyphae to recruit beneficial microbes. Yet, this part of the relationship remains obscure. We lack knowledge of how this cooperation could emerge and whether it can be established with bacteria that produce antifungal compounds and are used as a biocontrol agent.
1st Clue: Can the antagonistic bacterium (Bacillus velezensis) and mycorrhizal fungus (Rhizophagus sp.) physically interact?
To answer this question, the coauthors used a bi-compartmented Petri-dish (see figure below). In one compartment, they added a 3-month-old root system with a mature mycorrhizal network, while in the other compartment, they inoculated the bacterium. Microscopy revealed that the bacterium spread over the hyphae within 48 hours, covering 5.6 mm/day. After covering the hyphae, the motile bacteria cells switched into sessile ones, establishing a multicellular colony.
To verify whether that observation could happen in more natural conditions the authors customized a setup with two pots connected by a plastic tube covered at both ends with a thin nylon mesh, allowing only mycorrhizal hyphae to connect both plants (figure B below). In that setup, only one pot is inoculated with mycorrhiza and Bacillus velezensis. Interestingly, the bacterium reached the other plant by hitchhiking the mycorrhizal hyphae. Indeed, the antagonistic bacterium physically interacted with the mycorrhiza and benefited from sliding over its hyphae. Yet, that raises the question of whether the fungus also benefited from this interaction or not.
A B
2nd Clue: Did mycorrhizal fungus (Rhizophagus sp.) also benefit from the bacterial interaction?
The first step is to test whether the mycorrhizal hyphae are viable and active. To do so, the coauthors measured the activity of specific respiratory enzymes with and without adding Bacillus velezensis. Surprisingly, they found no difference, indicating that the bacterium did not adversely affect fungal respiration and metabolism. Yet, that was not the only surprise; the scientists discovered that the velocity of the cytoplasmic flow inside the hyphae increases in the presence of the bacterium. This velocity reflects the fungus’s vitality because it allows metabolite translocation from plant to fungus and vice versa. Results showed the fungus remained viable and unaffected by Bacillus velezensis. But what about its antifungal activity?
3rd Clue: How has Bacillus velezensis altered its production of antifungals?
To sort this puzzle, the scientists tested the sensitivity of Rhizophagus sp. to various antifungal compounds (Fengycin and iturin) produced by Bacillus velezensis. Both toxic compounds make pores in the membranes, causing the leakage of the cytoplasm and the death of the fungus. A high concentration of 50 mM of both compounds was found to destabilize Rhizophagus sp. membranes. Near the hyphae, Bacillus velezensis reduced its toxic compound levels to 0.48 mM (iturin) and 0.03 mM (fengycin). Thus, the released antifungals did not adversely affect the Rhizophagus sp. However, could those compounds protect the fungus against competitor soil fungi? Yes! By cultivating Bacillus velezensis in fungus exudates and collecting the produced metabolites the authors found that the growth ofTrichoderma harzianum and Collimonas fungivorans, two harmful fungi, was inhibited. Therefore, this bacterium protects Rhizophagus sp. against competitor soil-dwelling fungi.
4th Clue: How does bacterium-mycorrhizal mutualism affect the plant’s growth?
The last puzzle to be solved is whether the plant also benefits by gaining protection against pathogens. To test this, a major pathogen that causes leaf lesions of a wide range of crops (Botrytis cinerea) was selected to infect tomato plants in the greenhouse. Surprisingly, the data showed that in plants co-inoculated with both Rhizophagus sp. and Bacillus velezensis had a 60% and 75% decrease in disease severity and disease incidence, respectively.
Altogether, studying mutualistic interactions cross-kingdoms offers valuable opportunities to design different consortia with biocontrol potentials and pave the way for a sustainable agriculture solution.
Link to the original post: Anckaert, A. et al. The biology and chemistry of a mutualism between a soil bacterium and a mycorrhizal fungus. Current Biology 34, 4934-4950.e4938, (2024).
Featured image: Made by the author
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