Highly diverse communities of microbial symbionts occupy eukaryotic organisms, including plants. While many well-studied symbionts may be characterized as either parasites or as mutualists, the prevalent but cryptic endophytic fungi are less easily qualified because they do not cause observable symptoms of their presence within their host. Here, we investigate the interactions of an endophytic fungus, Fusarium verticillioides with a pathogen, Ustilago maydis, as they occur within maize (Zea mays). We used experimental inoculations to evaluate metabolic mechanisms by which these three organisms might interact. We assessed the impacts of fungal-fungal interactions on endophyte and pathogen growth within the plant, and on plant growth. We find that F. verticillioides modulates the growth of U. maydis and thus decreases the pathogen's aggressiveness toward the plant. With co-inoculation of the endophyte with the pathogen, plant growth is similar to that which would be gained without the pathogen present. However, the endophyte may also break down plant compounds that limit U. maydis growth, and obtains a growth benefit from the presence of the pathogen. Thus, an endophyte such as F. verticillioides may function as both a defensive mutualist and a parasite, and express nutritional modes that depend on ecological context.
Bibliographical noteFunding Information:
Research described here was supported by an NSF grant, En-Gen 0723451 to G. May and H.C. Kistler. The F. verticillioides strains used in these experiments are publically available at the UM Culture Collection and the U. maydis are available by request from G. May. The Minnesota Super-Computing Institute (MSI) provided computational resources; metabolite analyses were conducted at the Center for Mass Spectrometry and Proteomics, and sequencing and real-time PCR at BioMedical Genomics Center (BMGC), all at the University of Minnesota, St. Paul. The authors gratefully acknowledge the assistance of Alyssa Bernardo, Peter Lenz, and other members of the May and Kistler labs.
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