Microbial responses to stress do not promote plant tolerance to same or different stressors

Microbes can respond to stress in ways that benefit plants under stress, a phenomenon called microbe-mediated acclimation. However, microbe-mediated acclimation has been tested under a limited number of stressors, and it remains unclear whether plant benefits are stress-specific or whether microbial stress responses provide cross-tolerance to multiple stressors. We inoculated Chamaecrista fasciculata plants with microbial communities from the rhizosphere of field plants that experienced salt, herbicide, or herbivory stress, non-stressful conditions, or a sterile inoculant and grew them under these same four environments in the greenhouse. We did not observe microbe-mediated acclimation to any stressor, nor did microbes respond to stress in ways that provided cross-tolerance to multiple stressors. Instead, inoculation with a live microbial community (absent exposure to stress) exacerbated the fitness effects of herbicide and herbivory stress, and microbial responses to herbicide consistently reduced plant survival across all contemporary environments. Our results suggest that microbial communities do not always respond to stress in ways that promote plant stress tolerance, despite the many cases reported in the literature, and illustrate the need for future research identifying which stressors and environmental contexts are most likely to result in microbe-mediated acclimation versus persistent negative microbial legacy effects of stress.