Legume plant defenses and nutrients mediate indirect interactions between soil rhizobia and chewing herbivores

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Soil bacteria that associate with plant roots promote host vigor. Legume plants form mutualisms with rhizobial bacteria, and legumes grown with rhizobia have more nutrients and defenses than those grown without rhizobia. However, few studies have tested how stressors such as herbivores affect soil rhizobia, and the mechanisms mediating these interactions. Here we tested reciprocal interactions between a chewing herbivore, Sitona lineatus (pea leaf weevil), and Pisum sativum (pea) plants grown with or without rhizobia (Rhizobium leguminosarum biovar. viciae), and the plant-defense and nutritional mechanisms mediating these interactions. We hypothesized that plants grown with rhizobia would have less feeding from S. lineatus due to greater expression of phytohormones or physical defenses. We also predicted that herbivory might impede the mutualism between P. sativum and rhizobia. Our experiments showed that leaf defoliation by S. lineatus was indeed lowest on plants with rhizobia. Plants grown with rhizobia had increased gene transcript expression associated with hormone-related defense (jasmonic acid, ethylene, abscisic acid) as well as physical and antioxidant-related defense, which may explain reduced feeding by S. lineatus. Conversely, S. lineatus feeding reduced the number of root nodules and nodule fresh weight, suggesting a disruption of the symbiosis between plants and rhizobia. Our study shows that aboveground herbivores can engage in mutually antagonistic interactions with soil microbes that are mediated through multiple plant-mediated pathways.