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In turn, mycorrhizal fungi and some endophytes may enhance nutrient uptake and growth, alter plant water relations, or deter antagonistic microbes or herbivores ( Arnold et al., 2003 Waller et al., 2005 Arnold and Engelbrecht, 2007 Busby et al., 2015 Estrada et al., 2015 van der Heijden et al., 2015). Pathogens can negatively influence photosynthesis, nutrient- and water uptake and transport, structural integrity, reproduction, and seed germination of their hosts ( Blanchette, 1991 Agrios, 1997 Gallery et al., 2007 Grimmer et al., 2012 Oliva et al., 2014).

Plant-fungus interactions shape plant health and productivity in all terrestrial ecosystems ( Heilmann-Clausen and Boddy, 2008 Kivlin et al., 2011 Tedersoo et al., 2014 Davison et al., 2015). More generally, alteration of the breadth or efficiency of substrate use by dispensable EHB may change fungal niches in short timeframes, potentially shaping fungal ecology and the outcomes of fungal-host interactions. We propose that additive or synergistic substrate use by the fungus-bacterium pair enhances fungal growth in this association. Cases in which the cured strain outperformed the strain harboring the bacterium were observed in only 5% of substrates. These substrates included many that are important for plant- and seed-fungus interactions, such as D-trehalose, myo-inositol, and sucrose, highlighting the potential influence of EHB on the breadth and efficiency of substrate use by an important Fusarium species. Across the majority of substrates (62%), the strain harboring the bacterium significantly outperformed the cured strain as measured by respiration and hyphal density. We examined the effects of an endohyphal bacterium ( Chitinophaga sp., Bacteroidetes) on substrate use by its host, a seed-associated strain of the fungus Fusarium keratoplasticum, by comparing growth between naturally infected and cured fungal strains across 95 carbon sources with a Biolog ® phenotypic microarray. These endosymbiotic bacteria (endohyphal bacteria, EHB) often can be removed from living fungi by antibiotic treatment, providing an opportunity to assess their effects on functional traits of their fungal hosts. 4Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, AZ, USAīacterial endosymbionts occur in diverse fungi, including members of many lineages of Ascomycota that inhabit living plants.