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The mutual association between the Acacia plants and the symbiotic ants of the genus Pseudomyrmex represents a classic example of mutualism, where the plant offers different nutritional resources to the ants, such as extra-floral nectar, nutritive granules, and refuge in the form of spines for nesting . In exchange for these resources, the ants defend the plant from other insects that eat it (herbivores) either by stinging or by aggressive behavior. It was recently shown that Pseudomyrmex ants would also act as a kind of immune system for the host plant, thus protecting the Acacia plants from potential infections.

The research, which corresponds to Dr. Marcia González-Teuber, researcher DIULS and the Department of Biology of the University of La Serena, together with collaborators, was published in New Phytologist (and in an article by The Economist) and goes back to an ancient observation made in plants of Macaranga, where plants of this species had greater attack of fungi when the ants were absent from the plant. Researchers at the University of La Serena and the Max Planck Institute for Chemical Ecology, Germany, wondered if this observation would also be met in the Mesoamerican Acacia-Pseudomyrmex system, and if so, how this phenomenon would occur.

To answer this question, the authors selected 10 plants of the species Acacia hindsii in Oaxaca, Mexico, and removed the ants manually from a randomly selected branch, cutting the thorns and avoiding re-colonization by a sticky gel trap placed in the base of the branch. As a control, they cut the thorns in a second branch, but did not remove the ants, allowing them to move freely around the branch.

After six weeks, the authors found an 45% infection in the leaves of the experimental branches, compared to only an 14% in those leaves in which the ants were able to move freely. Analysis of microorganism culture, as well as molecular analysis, indicated that the branches without ants were mostly infected by recognized plant pathogens compared to the branches with ants, thus changing the community of microorganisms on the surface of the leaf.

The reason for this phenomenon would apparently be related to the legs of the ants. When the authors cut ants legs and washed them with methanol, they found that this extract was able to inhibit the growth of common plant pathogens, as well as pathogens previously isolated from Acacia leaves. When they analyzed in detail the content of the legs of the ants, they found different types of bacteria living associated with the legs, which apparently would fulfill the function of synthesis of antibiotics. Therefore, the ants that would act as a host of these protective bacteria, would protect their host plants from the attack of other microorganisms.