Soil nutrient inputs shift endophytic phenotypes in a manner consistent with a significant role for species interactions in community assembly

Michael R. Fulcher, Foreign Disease-Weed Science Research Unit, USDA-ARS, Seth A. Spawn-Lee, University of Wisconsin-Madison, Zoe A. Hansen, Michigan State University, Mitch Johnson, University of Minnesota, Zewei Song, Beijing Genomics Institute, Georgiana May, Ecology, Evolution, and Behavior, University of Minnesota, Eric W. Seabloom and Elizabeth T. Borer, Ecology, Evolution, and Behavior, University of Minnesota, Saint Paul, MN, Linda L. Kinkel, Plant Pathology, University of Minnesota, Saint Paul, MN

Soil nutrient inputs shift endophytic phenotypes in a manner consistent with a significant role for species interactions in community assembly

M.R. Fulcher, S.A. Spawn-Lee, Z.A. Hansen, M. Johnson, Z. Song, G. May, E. Seabloom, E. Borer, L.L. Kinkel

1) Background/Question/Methods
Endophytic microbes influence plant health and productivity in both natural and agricultural environments, but their community assembly processes remain poorly understood. We studied the response of foliar endophyte communities in a natural grassland to eight years of annual nitrogen, phosphorus and potassium (NPK) amendment. Bacterial and fungal endophytes were isolated from healthy leaves (n=6 per treatment, NPK-amended or control) of Andropogon gerardii. A random collection of isolates (n=120 bacteria and fungi each, 10 of each kingdom from each leaf) were assigned to genus based on 16S or ITS2 sequence similarity and characterized for nutrient utilization with Biolog SF-P2 plates. Network analysis was used to quantify resource competition between sympatric isolates. Inhibition of a collection of indicator microbes was also recorded for all fungal isolates in vitro.

2) Results/Conclusions
Fungal, but not bacterial, community composition, nutrient utilization and inhibitory phenotypes varied dramatically between NPK and control treatments. Phylogenetic analysis showed both intra- and intergenic differentiation between treatments. The growth efficiency of fungi was greater in control leaves for five of seven nutrient classes, but fungi from NPK-amended leaves exhibited greater growth efficiency on aromatic and P-rich compounds. Fungal niche width was significantly greater than bacterial niche width in control leaves, but this advantage disappeared in NPK-amended leaves. The loss of this fungal competitive advantage was associated with increased inhibition strength against bacteria. Network analyses revealed fungal isolates that were the weakest competitors for nutrients (high node in-degree) produced significantly larger zones of inhibition against bacteria than strong nutrient competitors. Our results show widely applied soil nutrients alter the structure and function of endophyte communities. Moreover, the data suggest roles for resource competition and antagonistic microbial interactions in the endophytic community assembly process.