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Global climate change and shifting land‐use are increasing plant stress due to abiotic factors such as drought, heat, salinity and cold, as well as via the intensification of biotic stressors such as herbivores and pathogens. The ability of plants to tolerate such stresses is modulated by the bacteria and fungi that live on or inside of plant tissues and comprise the plant microbiome. However, the impacts of diverse classes of beneficial members of the microbiome and the contrasting stresses that impact plants are most commonly studied independently of each other.Our meta‐analysis of 288 experiments across 89 studies moves beyond previous studies in that we simultaneously compare the roles of bacterial versus fungal microbiome members that live within plant tissues and colonize plant surfaces in ameliorating biotic versus abiotic sources of plant stress.The magnitude of microbial stress amelioration can be measured as the greater proportional impact of beneficial microbes on plant performance in more stressful environments. In the plant experiments we examine, the magnitude of microbial stress amelioration is substantial: it is 23% of the effect size of the typical impact of stress and 56% of the effect size of beneficial microbiome members in the absence of stress.The amount of benefit microbiome members confer to plants differs among classes of microbes, depending on whether plants are grown in stressful or non‐stressful environments. In the absence of stress, beneficial bacteria tend to confer greater plant benefits than do fungi. However, symbiotic fungi, especially arbuscular mycorrhizal fungi, more strongly ameliorate plant stress than do bacteria. In particular, beneficial microbes ameliorate salinity, foliar herbivory and fungal pathogen stress.These results highlight the fact that the impacts of beneficial and antagonistic components of the microbiome on plant performance depend on biotic and abiotic environmental contexts. Furthermore, beneficial microbes are especially critical for plant health in stressful environments and thus present opportunities to mitigate negative consequences of global change. A free plain language summary can be found within the Supporting Information of this article. [ABSTRACT FROM AUTHOR]

To establish and spread in a new location, an invasive species must be able to carry out its life cycle in novel environmental conditions. A key trait underlying fitness is the shift from vegetative to reproductive growth through floral development. In this study, we used a common garden experiment and genotyping‐by‐sequencing to test whether the latitudinal flowering cline of the North American invasive plant Medicago polymorpha was translocated from its European native range through multiple introductions, or whether the cline rapidly established due to evolution following a genetic bottleneck. Analysis of flowering time in 736 common garden plants showed a latitudinal flowering time cline in both the native and invaded ranges where genotypes from lower latitudes flowered earlier. Genotyping‐by‐sequencing of 9,658 SNPs in 446 individuals revealed two major subpopulations of M. polymorpha in the native range, only one of which is present in the invaded range. Additionally, native range populations have higher genetic diversity than invaded range populations, suggesting that a genetic bottleneck occurred during invasion. All invaded range individuals are closely related to plants collected from native range populations in Portugal and southern Spain, and population assignment tests assigned invaded range individuals to this same narrow source region. Taken together, our results suggest that latitudinal clinal variation in flowering time has rapidly evolved across the invaded range despite a genetic bottleneck following introduction. [ABSTRACT FROM AUTHOR]