Fungal endophytes are a highly diverse group of fungi that colonize plant tissue without visible manifestation of infection. Colonization by endophytes has potential benefits for plants, including increased resistance against herbivores and pathogens. Endophytes in culture have been shown to produce secondary metabolites that can suppress herbivore and pathogen damage. However, it is unknown if these toxins are also synthesized during symbiosis within the host plant, or if endophytes stimulate plant chemical responses, thus affecting how plants interact with their enemies. We tested how endophyte colonization alters the production of defensive compounds in the well-known toxic plant White Snakeroot (Ageratina altissima), a native Indiana wildflower. We inoculated endophyte-free seedlings with one of three treatments: 1) inoculation by a single, dominant endophyte (Colletotrichum sp.), 2) inoculation with rainwater captured underneath wild snakeroot plants as a natural fungal spore source, or 3) application of sterile water as a control. After endophyte communities established in seedlings, we characterized them using a culture-based approach coupled with Sanger sequencing. In order to determine if endophyte community composition and diversity affected plant chemistry, we also extracted phenolics from leaf tissue and performed liquid chromatography-mass spectrometry.
Results/Conclusions
We isolated 17 unique operational taxonomic units (OTUs) from experimentally inoculated snakeroot seedlings. As expected, control plants had minimal fungal colonization, and plants inoculated with Colletotrichum sp. were colonized nearly 100% by Colletotrichum sp. Plants inoculated with rain water as a natural spore source had the most diverse endophyte communities, and an intermediate isolation frequency of 45%. Although Colletotrichum sp. was previously identified as a common endophyte of A. altissima, it was rarely isolated from seedlings inoculated with rain water, suggesting that transmission of Colletotrichum sp. does not depend on rain. Comparing the phenolic profiles across the three treatments revealed that microbial colonizers alter the production of secondary metabolites in plants. Plants treated with Colletotrichum sp. and rain water had a significantly larger breadth of chemical compounds in their tissues than uninoculated seedlings. Additionally, abundance of individual phenolic compounds varied between treatments. Future work can examine if these chemicals were produced by endophtyes, or by the plants themselves. Understanding how fungal endophytes affect the production of plant defensive compounds has wide-ranging implications for both ecology and agriculture. Additionally, due to their unique antimicrobial properties and potentially vital source of plant defensive chemistry, endophytic fungi represent great promise in bioprospecting.