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

人类依赖农作物中的蛋白质。为了生产蛋白质，一些作物如豆类与土壤中的细菌有着独特的关系。这些被称为根瘤菌的细菌帮助植物从空气中获得氮，氮是蛋白质的主要成分之一。然而，氮不仅可以用来制造蛋白质，还可以用来制造化学物质，帮助保护植物免受害虫的攻击。这个项目将研究根瘤菌提供的氮如何被利马豆植物利用，以帮助它们生长并保护自己免受食草昆虫的侵害。利马豆植物有两种方式来保护自己免受昆虫的侵害：它们制造有毒的氰化物，并为蚂蚁提供花蜜，蚂蚁充当保镖。当植物根部有根瘤菌提供氮时，这两种防御都会发生变化。有根瘤菌的植物产生更多的氰化物，但花蜜较少。这些保护措施的变化不仅会影响利马豆植物和以其为食的昆虫，还会影响以植食性昆虫为食的捕食者。该项目的总体目标是了解土壤中的根瘤菌是否会影响整个食物链。 这项研究对于提高不断增长的人口的生活质量具有重要意义，因为豆类家族的许多成员为人类和牲畜提供了营养丰富的作物。这些作物依靠根瘤菌在土壤中重新引入养分，当养分被其他作物耗尽时。本研究将根瘤菌纳入食物网，通过在野生利马豆（Phaseolus lunatus），在那里它已经发展出特定的防御和互惠关系的本地范围内使用田间实验。为了验证根瘤菌影响地上无脊椎动物群落的假设，研究人员将联合收割机结合生理学、生态学和理论方法。具体来说，这项研究将测试根瘤菌如何：i）影响用于吸引蚂蚁的花外花蜜的产生，ii）当细菌通过提供氮合作或通过不提供氮“欺骗”时，影响地上食物网的不同，iii）影响植物防御化合物的产生。更一般地说，原生无脊椎动物群落对根瘤菌处理的反应将为地下氮循环细菌如何影响地上生态系统提供新的功能见解。

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.