土壤微生物对地上生物食物链/网关系的影响是现代生态学研究的热点，其中，根瘤菌与豆科植物形成的独特的生物固氮不仅在维持农业生态系统氮循环中发挥重要作用，还是生态系统中多种植物的直接氮源，满足植物对氮素营养的需求；然而，其对害虫发生、天敌控害的作用机制一直不清楚。本项目以固氮根瘤菌－蒺藜苜蓿－豌豆蚜－蚜茧蜂地上地下互作系统为主线，以测序种苜蓿A17品种及其生物固氮缺失型突变体nip为材料，从植物的结瘤固氮作用、氮营养代谢、诱导防御机制，蚜虫的种群参数、营养识别通路、行为和生理免疫途径，寄生蜂的识别与定位、寄生效率、种群参数等方面，系统研究生物固氮在介导蚜虫种群发生、取食为害、天敌控害中的功能和作用，揭示地下根瘤菌对地上苜蓿－豌豆蚜－寄生蜂三者关系生态调控的分子机制。研究成果为害虫生态调控提供新思路。

Exploring the mechanisms that underlie the effects of soil organisms belowground on multiple species interactions aboveground has becoming a central theme in ecology, especially the effect of beneficial microbe on the interactions among plant-insect and natural enemies. By using Rhizobia, legume plant Medicago truncatula, pea aphids as well as the parasitoid wasp Aphidius ervi, this project aim to determine the effects of underground biological nitrogen fixation on the aboveground interaction among Medicago truncatula - pea aphid - parasitoid wasps. The nitrogen fixation mutant nip and wild-type alfalfa A17 are comparatively examined to study the nitrogen-fixing, carbon and nitrogen transport and allocation, and systemic defense pathways of plant; the population abundance, individual growth, nitrogen nutrition perception as well as immune response of pea aphid; as well as the plant foraging efficiency, prey localization as well as parasitic parameter of Aphidius ervi. These studies can not only determine the role of plant nitrogen fixation signaling in aphid infestation, as well as natural enemy performance, but clarify the mechanisms involved in the responses of Medicago truncatula-pea aphid-Aphidius ervi to belowground nitrogen fixation. Furthermore, we also reveal the bottom-up effect of nitrogen fixation on the two physiological aspect of host plant including plant nitrogen metabolism and resistant phytohormone mediated defensive signaling pathway, which in turn affect pea aphid nutritional perception and immune effects against parasitoids as well as foraging behavior of parasitoid wasps. The outcomes of this project are of great significance to explore how mutualistic root microbes impose significant indirect effects on the performance of parasitoid of insect herbivores and develop theory of ecological regulation of tri-trophic interactions among plants, herbivores, and natural enemies.