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

共生固氮对植物防御、食草动物和更高营养水平的影响

• 批准号: 1457369
• 负责人: Daniel Ballhorn
• 金额: $ 41.28万
• 依托单位: Portland State University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2015
• 资助国家: 美国
• 起止时间: 2015-03-01 至 2019-02-28
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1457369&HistoricalAwards=false
• 关键词: impact; symbiotic; nitrogen; fixation; plant

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.

地下固氮菌（根瘤菌）具有很高的生态和经济价值，但它们在地上食物网中的作用几乎未被探索。为了从功能上解开根瘤菌复杂的生态效应，该项目将使用多种根瘤菌菌株与不同的宿主植物，野生利马豆（豆科：菜豆lunatus）和野生豌豆（豆科：Leptosporn adenanthum）相结合。在实验室中，这些植物根瘤菌系统将实验暴露于不同的CO2和氮的可用性。这两种植物对食草动物表现出根本不同的化学防御。虽然P. lunatus具有有效的氮基防御（通过氰生成），L。adenanthum投资于碳基防御（通过酚）。除了这些直接防御，这两种植物显示出一系列的间接防御（花外花蜜和草食动物诱导的挥发物），通过吸引食肉节肢动物。该项目将提供根瘤菌对植物性状的影响、不同营养水平的植物相关生物以及可变的非生物条件对这些关联的影响的综合图片。由于植物-微生物-节肢动物相互作用与化学生态学、进化生物学、全球变化生物学、保护生物学、农业和土壤科学等多个学科有关，因此这项研究将产生广泛的影响。特别是在环境迅速变化的时代，更好地了解控制自然和农业生态系统的机制的价值怎么估计也不过分。作为这一项目的一部分，将培训一名研究生和几名本科生，并为学校教师和K-12学生提供参加研究的机会，每周上课和为期三周的夏季实地课程。该项目将评估根瘤菌对不同取食集团的通才和专性食草昆虫以及寄生性和捕食性节肢动物的自下而上的影响。此外，不同水平的根瘤菌固氮将被认为是根瘤菌作为植物互利共生体的质量不同。虽然互惠根瘤菌株互惠，欺骗根瘤菌驻留在根瘤中，作为碳汇，但不提供氮，从而降低了植物宿主的净适合度效益。除了实验室研究之外，还将在两种植物的自然栖息地（哥斯达黎加）探索根瘤菌共生的成本和效益。具体而言，将解决以下问题：1。根瘤菌菌株、土壤氮和环境二氧化碳（CO2）对植物性状的独立和相互作用是什么？2.根瘤菌介导的植物性状如何影响昆虫食草动物和食肉动物？3.在自然界中，根瘤菌共生在互惠-欺骗连续体中的生态效益和防御相关成本是什么？分光光度测定，GC-MS（气相色谱-质谱）和LC-MS（液相色谱-质谱），将用于分析植物防御和营养化合物。将使用同位素标记技术（15 N）和氩气室（其中大气中的N2被氩气取代的设置）的N2消耗研究量化植物资源分配到根瘤菌和不同固氮率的影响。将通过饲养试验和嗅觉仪选择试验研究根瘤菌介导的植物性状对食草性昆虫和食肉性昆虫的影响。伴随的实地研究将有助于评估根瘤菌菌株和固氮率对自然界中多种植物防御性状的影响，并测试根瘤菌对自然生态系统中节肢动物食草动物和食肉动物的自下而上的影响。