The roles microbes play in shaping plant communities have historically been underestimated. Recent improvements in our abilities to detect, identify, and monitor microbial inhabitants of plant tissues are driving appreciation of amazingly complex dynamics. Microbial endophytes can modify plants at genetic, physiologic, and ecologic levels, inducing profound changes in the way plants respond to their environment. Microscale examination of Bouteloua eriopoda (black grama) and Atriplex canescens (four-wing saltbush) has revealed diverse fungal communities associated with individual plants at the cellular and subcellular levels. To explore thresholds of plant fitness defined by microbial communities, we transferred fungi inhabiting two perennial grasses, B. eriopoda and Sporobolus cryptandrus (sand dropseed), and one shrub, A. canescens, to a variety of nonhost plant species. Dramatic, whole-plant differences in morphology and biomass between treated and untreated plants were observed. In most cases, endophyte transfer at the cellular level produced larger plants with greater reproductive potential than the untransformed counterparts. We hypothesize that these transformed plants will continue to grow, reproduce, and disperse more rapidly than their native counterparts, propagating changes from the plant-microbe interface to ecologically significant scales.