Microbial responses to stress cryptically alter natural selection on plants

Microbial communities can rapidly respond to stress, meaning plants may encounter altered soil microbial communities in stressful environments. Stress may therefore affect plant natural selection not only directly, but also indirectly via changes to the microbial community. Because stress can cause lasting changes to microbial communities, microbes may also cause legacy effects on plant selection that persist even after the stress ceases.

To explore microbial indirect and legacy effects, we grew plants in stressful (salt, herbicide, or herbivory) or non-stressful conditions with microbes that had experienced each of these environments in the previous generation.

Microbial indirect effects generally counteracted the direct effects of stress on plant selection, thereby weakening the strength of stress as a selective agent. Microbial legacy effects altered plant selection in non-stressful environments, suggesting that stress-induced changes to microbes may continue to affect selection after stress is lifted.

These results suggest that soil microbes may play a cryptic role in plant adaptation to stress, potentially reducing the strength of stress as a selective agent and altering the evolutionary trajectory of plant populations.