Across plants and animals, host-associated microbial communities play fundamental roles in host nutrition, development, and immunity. The factors that shape host-microbiome interactions are poorly understood, yet essential for understanding the evolution and ecology of these symbioses. Plant roots assemble two distinct microbial compartments from surrounding soil: the rhizosphere (microbes surrounding roots) and the endosphere (microbes within roots). Root-associated microbes were key for the evolution of land plants and underlie fundamental ecosystem processes. However, it is largely unknown how plant evolution has shaped root microbial communities, and in turn, how these microbes affect plant ecology, such as the ability to mitigate biotic and abiotic stressors. Here we show that variation among 30 angiosperm species, which have diverged for up to 140 million years, affects root bacterial diversity and composition. Greater similarity in root microbiomes between hosts leads to negative effects on plant performance through soil feedback, with specific microbial taxa in the endosphere and rhizosphere potentially affecting competitive interactions among plant species. Drought also shifts the composition of root microbiomes, most notably by increasing the relative abundance of the Actinobacteria. However, this drought response varies across host plant species, and host-specific changes in the relative abundance of endosphere are associated with host drought tolerance. Our results emphasize the causes of variation in root microbiomes and their ecological importance for plant performance in response to biotic and abiotic stressors.

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Several hypotheses proposed to explain the success of introduced species focus on altered interspecific interactions. One of the most prominent, the Enemy Release Hypothesis, posits that invading species benefit compared to their native counterparts if they lose their herbivores and pathogens during the invasion process. We previously reported on a common garden experiment (from 2002) in which we compared levels of herbivory between 30 taxonomically paired native and introduced old-field plants. In this phylogenetically controlled comparison, herbivore damage tended to be higher on introduced than on native plants. This striking pattern, the opposite of current theory, prompted us to further investigate herbivory and several other interspecific interactions in a series of linked experiments with the same set of species. Here we show that, in these new experiments, introduced plants, on average, received less insect herbivory and were subject to half the negative soil microbial feedback compared to natives; attack by fungal and viral pathogens also tended to be reduced on introduced plants compared to natives. Although plant traits (foliar C:N, toughness, and water content) suggested that introduced species should be less resistant to generalist consumers, they were not consistently more heavily attacked. Finally, we used meta-analysis to combine data from this study with results from our previous work to show that escape generally was inconsistent among guilds of enemies: there were few instances in which escape from multiple guilds occurred for a taxonomic pair, and more cases in which the patterns of escape from different enemies canceled out. Our examination of multiple interspecific interactions demonstrates that escape from one guild of enemies does not necessarily imply escape from other guilds. Because the effects of each guild are likely to vary through space and time, the net effect of all enemies is also likely to be variable. The net effect of these interactions may create ''invasion opportunity windows'': times when introduced species make advances in native communities.

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