EAGER: Nitrogen fixing bacterial endosymbioses in aboveground conifer tissue

作为DNA和蛋白质的重要组成部分，氮（N）通常限制自然生态系统中植物的生长。尽管如此，N到北方针叶林的来源仍然是一个长期存在的生态之谜。在这些生态系统中，土壤和植物中的氮积累远高于已知的氮输入源的预期。一种生活在植物组织内的内生菌可能提供了部分解释。虽然一些森林针叶树可以生长在氮含量极低的土壤中，但通常认为它们不会与可以减少或“固定”大气中N2的细菌合作。本项目旨在确定内生细菌是否在森林松树地上组织中固定N。为此，研究人员将使用一种称为纳米级二次离子质谱仪（NanoSIMS）的方法，这是一种适合测量，可视化和量化细胞内元素及其稳定同位素分布的仪器。 研究人员将松针暴露于稳定的同位素15 N，它只占自然产生的N的一小部分。然后，NanoSIMS将用于可视化已经吸收15 N的内生细菌细胞，以及附近含有15 N的植物细胞。研究人员希望该项目能够提供第一个定量和直接的证明，即细菌内生菌为宿主植物提供固定的大气氮，并确定森林氮输入缺失的潜在来源。森林树木可利用的氮量影响着陆地上碳的储存量和大气中碳的残留量，因此，发现新的森林氮输入途径将提高我们预测未来气候变化的能力。该项目将为加州大学默塞德的不同学生群体提供科学培训，这所大学的大部分学生来自代表性不足的背景，并为加州农村低收入地区的K-8学生提供科学培训。

An important building block of DNA and proteins, nitrogen (N) often limits plant growth in natural ecosystems. Still, the source of N to northern conifer forests is a long-standing ecological mystery. In these ecosystems, N accumulation in the soil and plants is much higher than expected given the known N input sources. Bacteria called endophytes, which live inside plant tissue, may provide part of the explanation. Although some forest conifers can grow in extremely nitrogen limited soil, they are not generally believed to cooperate with bacteria that can reduce, or 'fix' atmospheric N2. This project is designed to determine if endophytic bactera fix N in the aboveground tissues of forest pines. To do this, the investigators will use a method called Nanometer-Scale Secondary Ion Mass Spectrometer (NanoSIMS), an instrument suited to measure, visualize and quantify the distribution of elements and their stable isotopes within cells. The investigators will expose pine needles to the stable isotope 15N, which only makes up a fraction of naturally occurring N. NanoSIMS will then be used to visualize endophytic bacterial cells that have taken up 15N, as well as nearby plant cells containing 15N. The researchers expect this project to provide the first quantitative and direct demonstration that bacterial endophytes provision host plants with fixed atmospheric N, and identify a potential source for missing N inputs to forests. The amount of N available to forest trees affects how much carbon is stored on land and how much remains in the atmosphere, therefore, discovery of a new forest N input pathway will improve our ability to predict future climate-change. The project will provide scientific training to a diverse group of students at UC Merced, a university with a large percentage of students from underrepresented backgrounds, and to K-8 students in a rural, low-income area of California.