Background/Question/Methods  
Most plants harbor a diverse assemblage of non-mycorrhizal fungal endophytes. These fungi can directly influence the host plant, and can instigate trophic cascades that affect surrounding communities of herbivores, plants, and animals. Despite this, biotic mechanisms that influence assembly of fungal endophyte communities are largely unknown. Maize was used to assess the influence of three factors on fungal endophyte communities: production of host defense compounds, genetic diversity of the host neighborhood, and presence of a primary colonizer. Maize has been selectively bred to produce high quantities of benzoxazinoids (BXs) as natural insecticides, compounds also toxic to fungi, bacteria and plants. To determine the influence of BX production on endophyte communities, fields were planted with maize genotypes differing in their ability to produce BXs in two replicated field experiments. To assess the affect of host neighborhood diversity on endophyte community assembly, a non-BX producing genotype (BX-) was planted in plots with either two BX-producing genotypes (BX+), or in plots with only the BX- genotype. Finally, we evaluated the influence of a seed endophyte on communities by planting fields with BX+ and BX- genotypes that were either sterilized or inoculated with Fusarium verticillioides, a common maize endophyte. Endophyte communities were isolated, characterized and tested for tolerance to a toxic BX-byproduct, 2-benzoxazolinone (BOA).
Results/Conclusions
BX production had a significant influence on endophyte community structure. In seedling roots and mature leaves, the community proportion with low tolerance to BOA was significantly greater in BX- plants than in BX+ plants. Mean abundance of BOA-tolerant Fusarium species, including agents of animal toxicosis and corn diseases, was up to 35 times higher in mature leaves of BX+ than in BX- plants. Genetic diversity of the host neighborhood significantly affected endophyte communities. Mean infection density, abundance of Fusarium, and community diversity were greater in communities from plants surrounded by one genotype than in plants grown in a genetically diverse neighborhood. Presence of a primary colonizer also significantly influenced fungal endophyte community assembly. Results from field and related in vitro experiments indicate that F. verticillioides facilitates colonization by species with low tolerance to BOA. Collectively, results suggest that increased colonization by Fusarium may be an unintended outcome of selective breeding for BX production. Production of defense compounds, genetic diversity of the host neighborhood, and presence of a primary colonizer significantly altered endophyte community assembly.