Endophytic fungi in elm trees help protect them from Dutch elm disease

Endophytes live within a plant without causing diseases. Although many functions of these symbionts have been described, the ecological niche is still unknown in many cases. Some of these fungi inhabit plants hoping some of their bodies to die to become the first decomposers of wood. In other cases, some endophytic fungi inhabit the plant's internal tissues until their host plant weakens for some external factor such as drought, attack or disease. Then, these fungi use this weakness to adopt a state of "opportunistic" pathogens, developing and causing disease.

However, other endophytes have been positively related to health of their host plants, protecting them against herbivore attacks, pathogens and even abiotic damage. It has been shown that these beneficial fungi secrete substances harmful to pathogens or pest organisms. Likewise, these symbionts could boost the defense mechanisms of plants, thereby making them more resistant.

Among the diverse mechanisms that endophytic fungi could utilize to protect plants, we find the nutritional niche overlap in plant's internal tissues that prevent the access of these nutrients by pathogens. In order to study these mechanisms, the team of researchers has assessed in detail the nutritional niche of one of the most aggressive forest pathogenic fungi, the agent responsible for Dutch elm disease (DED), by comparing them with the nutritional niche of three other species of endophytic fungi isolated from healthy trees.The pathogenic fungus was able to use the 54% of the 190 nutritional sources studied, while two of the three endophytes were able to metabolize a greater number of carbon sources (71 and 60%, respectively). However, the third endophyte metabolized only the 22% of the substances. Other two endophytes, which had been previously identified as potential agents of biocontrol of DED, showed a nutritional niche that overlapped with the pathogen, particularly in the case of some essential substances for the metabolism of fungi such as sugars and fatty acids. Besides, all endophytes used phenolic substrates more effectively than the pathogen. This suggests that these compounds are important for the life strategies of endophytes in general. This result is consistent with the suggested role of phenolic compounds as defensive metabolites that are synthesized to counteract pathogen infections.

In conclusion, this study shows that certain endophytes probably compete with pathogens for the same carbon sources inside the plant. Therefore, the action of these fungi in the habitat of the pathogen (in this case, the phloem and xylem of elms) would limit its growth.