Microbe‐mediated adaptation in plants

Interactions with microbial symbionts have yielded great macroevolutionary innovations across the tree of life, like the origins of chloroplasts and the mitochondrial powerhouses of eukaryotic cells. There is also increasing evidence that host‐associated microbiomes influence patterns of microevolutionary adaptation in plants and animals. Here we describe how microbes can facilitate adaptation in plants and how to test for and differentiate between the two main mechanisms by which microbes can produce adaptive responses in higher organisms: microbe‐mediated local adaptation and microbe‐mediated adaptive plasticity. Microbe‐mediated local adaptation is when local plant genotypes have higher fitness than foreign genotypes because of a genotype‐specific affiliation with locally beneficial microbes. Microbe‐mediated adaptive plasticity occurs when local plant phenotypes, elicited by either the microbial community or the non‐microbial environment, have higher fitness than foreign phenotypes as a result of interactions with locally beneficial microbes. These microbial effects on adaptation can be difficult to differentiate from traditional modes of adaptation but may be prevalent. Ignoring microbial effects may lead to erroneous conclusions about the traits and mechanisms underlying adaptation, hindering management decisions in conservation, restoration, and agriculture.