The smallest creatures colonize not only us humans, but also all kinds of plants. They may even benefit from it: biologists have found that the coexistence of benign and pathogenic bacteria on the leaves can improve the health of the plant. This could help develop natural alternatives to synthetic pesticides.
Microbes live on the leaves of plants, similar to those on the skin or in the intestines of humans. Some of them can cause diseases in the plant, but commensal bacteria also thrive on the leaves. These have neither positive nor negative effects on the host they colonize. But how can plants harbor so many microbes and still remain healthy?
Natural defense against infection studied
Or Shalev from the Max Planck Institute for Biology in Tübingen and his colleagues also asked themselves this question. “In agriculture, many methods such as the use of pesticides or pruning of plants are used to get rid of pathogens, so it is surprising to see that many wild plants can live in harmony with their pathogens,” explains Shalev. The scientists suspect the commensal bacteria to be responsible. They suspect that these may have an effect on their harmful relatives and thus protect the plants from diseases.
To test this hypothesis, the researchers infected plants of the species Arabidopsis thaliana only with commensal bacteria, only with harmful pathogens, or with a mixture of benign and pathogenic Pseudomonas strains. All the bacteria examined came from the same genus. In order to simulate the situation in nature as realistically as possible, the research team planted the test plants in soil that corresponded to their natural locations. In addition, they sprayed them with a suspension that simulated the transmission of bacteria through wind and rain. The researchers also exposed the plants to bacteria other than the Pseudomonas strains that are specifically examined in this series of experiments.
In order to understand the development and change in microbial colonization over time, the researchers used “genome barcoding”, which allows the respective strain to be identified based on a specific DNA sequence of the bacteria.
Bacterial mix promotes plant health
The results showed that some bacteria actually promote plant health: When pathogenic and commensal bacteria of the genus Pseudomonas were co-infected, the plant grew significantly better and there were significantly fewer pathogenic bacteria. In the test plants, however, which were only infected with the harmful Pseudomonas strains, the researchers observed a significantly higher number of pathogenic bacteria and slower plant growth. according to them, this shows that the benign microbes actually had a positive effect on the health of the test plants.
According to the researchers, the commensal bacteria use specific mechanisms that ensure that their pathogenic relatives are less able to spread. They observed that colonization of commensal Pseudomonas stimulated a plant immune response that suppressed the growth of its cognate pests. “This is an unusual result because all of the infected bacteria were from the same genus and this immune response only specifically targeted one lineage of the pathogen. This is evidence of how specific the immune response mediated by the plant was,” Shalev emphasizes.
Commensals in the team against pathogens
However, this defense against the pathogens could not be demonstrated in all plants. Whether the protection against pathogenic Pseudomonas works depends heavily on the genetic properties of the plant and the bacteria that colonize it. The research team also emphasizes the work of the commensal bacteria as a team. “Another important aspect of our work was the identification and description of an existing collective protective effect. We didn’t identify single individuals as actors, but a synergy of the collective,” explains Shalev.
The newly gained knowledge could be of great importance for agriculture. The mechanisms by which plants can coexist in harmony with pathogens have yet to be understood. However, the results from follow-up studies could hold the key to developing novel solutions in agriculture to reduce the use of synthetic pesticides.
Source: Max Planck Institute for Biology Tübingen; Specialist article: Nature Ecology & Evolution, doi: 10.1038/s41559-022-01673-7