Fungal endophyte interactions and mechanisms of fungal-mediated plant drought tolerance
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Plants are ubiquitously colonized by diverse communities of horizontally-transmitted fungal endophytes, that can drastically alter plant physiology. Though many endophytes are mutualist, effects are context-dependent and can shift from pathogenic to mutualistic depending on abiotic and biotic factors. However, our understanding of endophyte effects comes almost exclusively from test of individual fungi, which may miss important community level processes that can alter fungal effects. Using Panicum grasses, I examined mechanisms underlying fungal interactions on plant physiology. I studied interactions in the context of plant drought responses, as climate models predict and increase in the intensity and frequency of drought. Scaling up from pairwise endophyte-plant studies will allow us to develop fungal applications that are more generalizable in real-world agricultural settings. Throughout my dissertation, I characterize the effects of altered precipitation and fungal interactions on plant physiology. To examine impacts of altered precipitation, I measured leaf-level and whole-plant carbon and water exchange in C4 grasses grown in extreme dry, extreme wet and mean levels of precipitation. Within this system, both extreme increases and decreases in precipitation inhibited plant gas fluxes, with all plants (Andropogon gerardii, Panicum virgtaum, and Sorghastrum nutans) responding similarly. To understand how fungal interactions effect plant performance, I compared the physiology of P. virgatum grown with six fungal pairs, the corresponding 12 individual fungi, and a no-fungus inoculum in low and high soil moisture. In most cases, plants responses to fungal pairs were non-additive (greater or less than expected) relative to effects of corresponding individual fungi. Furthermore, similarity of fungal stress tolerance and metabolic profiles predicted effects of fungal pairs in high and low soil moisture, respectively. To further understand mechanisms behind fungal interactive effects, I grew P. virgatum with 10 fungal pairs, in which each fungus was paired with one another, the five corresponding fungi, and a no-fungus inoculum in low and high soil moisture. Two of the five species dominated effects on the plants, such that outcomes of interactions could be predicted by the presence of these fungi within a pair. Furthermore, overall fungal effects on plant physiology could be predicted in the plant metabolome.