The impact of symbiotic nitrogen fixation on plant defense, herbivores and higher trophic levels

Portland State University, Portland OR

Investigators

Abstract

Belowground nitrogen (N2)-fixing bacteria (rhizobia) are of high ecological and economic importance, yet their role in aboveground food webs is virtually unexplored. To functionally disentangle the complex ecological effects of rhizobia, this project will use multiple rhizobial strains in combination with different host plants, wild lima bean (Fabaceae: Phaseolus lunatus) and wild pea (Fabaceae: Leptospron adenanthum). In the laboratory, these plant-rhizobia systems will be experimentally exposed to varying CO2 and nitrogen availabilities. Both plant species show fundamentally different chemical defenses against herbivores. While P. lunatus has an efficient nitrogen-based defense (via cyanogenesis), L. adenanthum invests in carbon-based defense (via phenolics). In addition to these direct defenses, both plants show an array of indirect defenses (extrafloral nectar and herbivore-induced volatiles) which act through the attraction of carnivorous arthropods. The project will provide an integrated picture of the effects of rhizobia on plant traits, plant-associated organisms of different trophic levels and the impact of variable abiotic conditions on these associations. As plant-microbe-arthropod interactions are relevant for multiple disciplines including chemical ecology, evolutionary biology, global change biology, conservation biology, agriculture and soil sciences, this research will have broad impact. In particular, in times of rapidly changing environments, the value in better understanding the mechanisms controlling natural and agricultural ecosystems cannot be overestimated. As part of this project, one graduate student and several undergraduates will be trained and opportunities for school teachers and K-12 students to participate in the research will be provided in weekly lessons and a three-week summer field course. This project will assess bottom-up effects of rhizobia on generalist and specialist insect herbivores of different feeding guilds, as well as on parasitoid and predatory arthropods. Furthermore, different levels of rhizobial N2-fixation will be considered as rhizobia differ in quality as plant mutualists. While mutualistic rhizobia strains reciprocate, cheating rhizobia reside in root nodules and function as carbon sinks, but provide no nitrogen, and thus reduce the net fitness benefit for the pant host. Beyond lab studies, costs and benefits of the rhizobial symbiosis will be explored in a natural habitat of both plant species (Costa Rica). Specifically, the following questions will be addressed: 1. What are the independent and interacting effects of rhizobial strain, soil nitrogen and ambient carbon dioxide (CO2) on plant traits? 2. How do rhizobia-mediated plant traits affect insect herbivores and carnivores? 3. What are the ecological benefits and defense-associated costs of the rhizobial symbiosis in a mutualist-cheater continuum in nature? Spectrophotometric assays, GC-MS (gas chromatography-mass spectrometry) and LC-MS (liquid chromatography-mass spectrometry), will be used to analyze plant defensive and nutritive compounds. Plant resource allocation to rhizobia and effects of different nitrogen fixation rates will be quantified using isotope labeling techniques (15N) and N2 depletion studies with Argon chambers (setups in which atmospheric N2 is replaced with Argon). Effects of rhizobia-mediated plant traits on insect herbivores and carnivores will be studied with feeding and olfactometer choice trials. Accompanying field studies will serve to evaluate the effects of rhizobial strain and N2-fixation rate on multiple plant defense traits in nature and to test for the predicted bottom-up effects of rhizobia on arthropod herbivores and carnivores in the natural ecosystem.