Abstract
Bacterial endosymbionts occur in diverse fungi, including members of many lineages of Ascomycota that inhabit living plants. 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. 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. Across the majority of substrates (62%), the strain harboring the bacterium significantly outperformed the cured strain as measured by respiration and hyphal density. 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. Cases in which the cured strain outperformed the strain harboring the bacterium were observed in only 5% of substrates. We propose that additive or synergistic substrate use by the fungus-bacterium pair enhances fungal growth in this association. 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.
Highlights
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)
In this study we focus on a lineage of bacteria that has not yet been evaluated for phenotypic modulation of fungi: the Bacteroidetes, a diverse clade of Gram-negative, nonendospore forming bacteria that are known from soils and from diverse symbiotic associations (Krieg et al, 2010; Thomas et al, 2011)
Comparative genomics and phenotypic assays have recently highlighted the importance of certain proteobacterial Endohyphal bacteria (EHB) among foliar endophytic Ascomycota (Arendt, 2015; Baltrus et al, 2016)
Summary
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). Endohyphal Bacterium Alters Substrate Use 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) Outcomes of such interactions are influenced by genetic and environmental factors (Schafer and Kotanen, 2003; Gallery et al, 2010; see Agrios, 1997; Jones and Dangl, 2006), and can be shaped by additional microorganisms that alter fungal phenotypes (Frey-Klett et al, 2007; Márquez et al, 2007). EHB are known among diverse fungi (Hoffman and Arnold, 2010; Desirò et al, 2015; Shaffer et al, 2016), but only a few have been developed as model systems in which their effects have been observed
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