DISSERTATION RESEARCH: One plant, many mutualists: the tritrophic response to plant defenses mediated by nitrogen-fixing rhizobia.

Portland State University, Portland OR

Investigators

Abstract

Humans depend on protein from crop plants. To produce protein, some crop plants such as beans have a unique relationship with bacteria in the soil. These bacteria, called rhizobia, help plants obtain nitrogen, one of the major components of protein, from the air. Nitrogen can be used to make more than proteins, however; it can also be used to create chemicals that help protect plants from attack by pest insects. This project will examine how nitrogen provided by rhizobia is used by lima bean plants to help them grow and protect themselves against plant-eating insects. Lima bean plants have two ways to protect themselves from insects: they make poisonous cyanide and they provide nectar to ants, which act as bodyguards. Both of these defenses change when plants have rhizobia in their roots providing nitrogen. Plants with rhizobia make more cyanide but less nectar. These changes in protection can affect not only lima bean plants and the insects that feed on them but also the predators that feed on the plant-eating insects. The overall goal of this project is to understand whether rhizobia in the soil can impact an entire food chain. This research has relevance for improving the quality of life among a growing human population because many members of the bean family provide nutrient-rich crops for humans and livestock. These crops depend on rhizobia to re-introduce nutrients in the soil when nutrients have been depleted by other crops. This research incorporates rhizobia into food webs by using field experiments in the native range of wild lima bean (Phaseolus lunatus), where it has evolved specific defensive and mutualistic relationships. To test the hypothesis that rhizobia influence aboveground invertebrate communities, researchers will combine physiological, ecological and theoretical approaches. Specifically, this research will test how rhizobia: i) affect production of extrafloral nectar used to attract ants, ii) impact aboveground food webs differently when bacteria cooperate by providing nitrogen or "cheat" by not providing nitrogen, and iii) influence production of plant defensive compounds. More generally, the native invertebrate community's response to rhizobia treatments will provide new functional insights into how belowground nitrogen-cycling bacteria affect aboveground ecosystems.