Foliar herbivory is known to directly affect phyllosphere microbiomes through altering plant phenotypes. However, how plant evolutionary responses to herbivory shape phyllosphere microbiomes is unclear. Here we use different invasive populations of the plant Ambrosia artemisiifolia that vary in reassociation timespan with a native specialist herbivore, to test whether renewed selection imposed by the herbivore is accompanied by evolutionary shifts in leaf chemistry and correlated changes in phyllosphere microbial communities. In common garden experiments we found directional changes in phyllosphere fungal communities with increasing duration of reassociation, accompanied by increased phyllosphere fungal alpha diversity and community complexity. These changes were associated with shifts in concentrations of plant metabolites, expression levels of their underlying biosynthetic genes, and increased plant herbivore resistance. Invasive plant reassociation with specialist insects can thus reshape phyllosphere fungal communities via changes in plant chemistry, demonstrating the role of plant evolutionary responses to herbivores in modulating microbial communities.

Chemical signals are crucial in mediating ecological and evolutionary adaptation of plants to their environments. Invasive plants can release distinct secondary metabolite mixtures in their new ranges, enhancing their mutualistic interactions and improving their performance, but genetic mechanisms of such adaptations are unexplored. We used Triadica sebifera plants to investigate evolutionary changes in chemical signals (flavonoids and strigolactones) that enhance arbuscular mycorrhizal (AM) fungal associations. We found plants from invasive populations produced higher concentrations of the flavonoid quercetin and the strigolactone 5-deoxystrigol, and had higher AM fungal colonization rates and biomass, relative to those from native populations. Also, applications of either chemical increased AM fungal colonization. Higher expression of genes in flavonoid (FLS) and strigolactone (CCD8) biosynthesis pathways that increased levels of quercetin and 5-deoxystrigol, respectively, were confirmed. These results provide insights into genetic mechanisms that contribute to higher AM fungal colonization and plant invasion success.

Non-native plants are typically released from specialist enemies in new ranges, but continue to be attacked by generalists, but whether they shift relative allocation to constitutive or induced defenses is unknown. We compared herbivory on co-occurring native and non-native species and also constitutive and induced defenses. Non-natives suffered less damage than natives and constitutive defenses of non-natives was lower than that of native congeners, whereas induced defense was the opposite. The strength of constitutive defenses for a species was correlated with the intensity of herbivory experienced, for non-natives, whereas induced defenses showed the reverse. The defenses of natives were not related to herbivory pressure. Finally, the strength of induced defenses correlated positively with growth, suggesting a novel mechanism for the evolution of increased competitive ability. These results expand our understanding of fundamental tradeoffs in constitutive and induced defenses and provide novel insight into how herbivory pressure affects defense allocation.