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Resources, such as nitrogen, are widely hypothesized to underlie the expression and evolution of plant defenses to herbivory. However, resource availability can affect selection on plant defense traits in contrasting ways: resource availability can 1) weaken selection on defense traits by reducing the costs of herbivory, or 2) strengthen selection on defense traits by increasing herbivore pressure. Previous studies have compared herbivore resistance in populations across natural resource gradients to infer how resource availability affects the microevolution of plant defenses. However, because these studies do not manipulate resource availability, they are unable to directly test the effects of resources of plant defense trait evolution. We used a three‐decade‐long nitrogen fertilization field experiment to test how nitrogen availability affects the evolution of an architectural plant defense trait: stem nodding inSolidago altissima. Stem nodding is a genetic dimorphism that helps plants to evade apex‐galling herbivores. By comparing the frequency of defensive nodding versus erect morphs in experimentally fertilized or unfertilized plots 27, 32 and 33 years post‐treatment initiation, we assessed how nitrogen addition affects the evolution of this defense trait. We found that the defensive nodding morph was 3–6 times more common in plots that evolved under nitrogen fertilization compared to those that evolved in unfertilized control plots. This study provides empirical evidence for resource availability driving plant defense evolution and demonstrates that this evolution can occur on time‐scales conducive to study at many long‐term nutrient fertilization experiments. 

Many bacterial traits important to host-microbe symbiosis are determined by genes carried on extrachromosomal replicons such as plasmids, chromids, and integrative and conjugative elements. Multiple such replicons often coexist within a single cell and, due to horizontal mobility, have patterns of variation and evolutionary histories that are distinct from each other and from the bacterial chromosome. In nitrogen-fixing Rhizobium, genes carried on multiple plasmids make up almost 50% of the genome, are necessary for the formation of symbiosis, and underlie bacterial traits including host plant benefits. Thus the genomics and transmission of plasmids in Rhizobium underlie the ecology and evolution of this important model symbiont. Here we leverage a natural population of clover-associated Rhizobium in which partner quality has declined in response to long-term nitrogen fertilization. We use 62 novel, reference-quality genomes to characterize 257 replicons in the plasmidome and study their genomics and transmission patterns. We find that, of the four most frequent plasmid types, two (types II & III) have more stable size, larger core genomes, and track the chromosomal phylogeny (display more vertical transmission), while others (types I & IV – the symbiosis plasmid, or pSym) vary substantially in size, shared gene content, and have phylogenies consistent with frequent horizontal transmission. We also find differentiation in pSym subtypes driven by long-term nitrogen fertilization. Our results highlight the variation in plasmid transmission dynamics within a single symbiont and implicate plasmid horizontal transmission in the evolution of partner quality. 

Summary Microbial communities can rapidly respond to stress, meaning plants may encounter altered soil microbial communities in stressful environments. These altered microbial communities may then affect natural selection on plants. 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 how microbial responses to stress and persistent microbial legacy effects of stress affect natural selection, we grewChamaecrista fasciculataplants in stressful (salt, herbicide, or herbivory) or nonstressful conditions with microbes that had experienced each of these environments in the previous generation.Microbial community responses to stress generally counteracted the effects of stress itself on plant selection, thereby weakening the strength of stress as a selective agent. Microbial legacy effects of stress altered plant selection in nonstressful 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.