Elucidating genetic and molecular mechanisms underlying symbiotic specificity in the Medicago-Sinorhizobium mutualism

土壤中氮素的有效性是农业生产的一个主要限制因素。尽管工业化肥的使用为养活世界做出了巨大贡献，但氮肥的生产需要化石燃料，过量的化肥会造成严重的环境污染。相比之下，豆科植物可以通过与被称为根瘤菌的固氮土壤细菌共生来制造自己的氮肥。这种共生最终形成了专门的根器官，称为根瘤，在根瘤中，细菌将大气中的氮转化为氨，供植物使用。豆科植物-根瘤菌共生的一个重要特性是其高度的特异性。这种专一性可以发生在相互作用的不同阶段，从最初的细菌感染和结瘤(结瘤专一性)到与固氮效率相关的后期根瘤发育(固氮专一性)。本项目旨在阐明在豆科植物-根瘤菌相互作用中调节共生专一性的遗传和分子机制。这些知识将有助于制定新的战略，以提高共生固氮对可持续农业的好处。该项目将有助于培养研究生、本科生和高中生。该研究小组将特别鼓励代表性不足的本科生加入该项目。尽管我们最近在理解导致根瘤发育的信号通路方面取得了进展，但结瘤能力和固氮效率自然变化背后的分子机制仍然不太清楚。在紫花苜蓿-紫花苜蓿互作中，朱实验室最近克隆了两个宿主基因NFS1和NFS2，它们调控着紫花苜蓿Rm41的固氮专一性，以及一个包含两个受体样蛋白的NS1基因，该基因限制了Rm41的侵染和结瘤。多态的NFS基因编码富含半胱氨酸的结节特异性多肽(NCR)。与NCR多肽作为内共生菌向固氮菌分化的效应因子的主要概念相反，这一发现表明特定的NCR对共生持久性具有负面调节作用。NFS1和NFS2以等位基因特异性和根瘤菌特异性的方式诱导细菌细胞死亡和根瘤早期衰老，其功能依赖于宿主的遗传背景。本项目一方面研究NCR多肽如何诱导固氮所需的终末类菌分化，另一方面一些NCR变异体导致类杆菌的裂解。具体地说，该项目将确定亲共生肽和反共生肽是否与相同的细菌靶标相互作用，这些多肽的氨基酸如何影响它们的功能，以及这些多肽的功能如何受到遗传背景的影响。这项研究的具体目标包括：(1)NFS1和NFS2基因的表征及其在与Rm41共生发育的负面调节中的作用；(2)NFS1抑制因子的克隆和表征；以及(3)限制Rm41结瘤的NS1基因座的表征。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

The availability of nitrogen in the soil is a major limiting factor for agricultural production. Although the use of industrial fertilizers has contributed substantially to feeding the world, the production of nitrogen fertilizers requires fossil fuels and excessive fertilizers cause serious environmental pollution. In contrast, plants of the legume family can make their own nitrogen fertilizer by forming symbiosis with nitrogen-fixing soil bacteria, called rhizobia. This symbiosis culminates in the formation of specialized root organs, called nodules, within which the bacteria convert atmospheric nitrogen into ammonia for use by the plant. One important property of the legume-rhizobial symbiosis is its high level of specificity. Such specificity can take place at various stages of the interaction, ranging from initial bacterial infection and nodulation (nodulation specificity) to late nodule development associated with nitrogen fixation efficiency (nitrogen fixation specificity). This project aims to elucidate genetic and molecular mechanisms that regulate symbiotic specificity in the legume-rhizobia interaction. Such knowledge will facilitate development of novel strategies to enhance the benefits of symbiotic nitrogen fixation for sustainable agriculture. This project will contribute to the training of students at graduate, undergraduate and high school levels. The research group will particularly encourage the participation of members of under-represented undergraduates to join the project.Despite recent advances in our understanding of the signaling pathways leading to root nodule development, the molecular mechanisms underlying natural variation in nodulation capacity and nitrogen fixation efficiency are still not well understood. In the Medicago truncatula- Sinorhizobium meliloti interaction, the Zhu laboratory recently cloned two host genes, NFS1 and NFS2 that regulate nitrogen fixation specificity concerning S. meliloti Rm41 and an NS1 locus containing two receptor-like kinases that restricts infection and nodulation by Rm41. The polymorphic NFS genes encode nodule-specific cysteine-rich (NCR) peptides. In contrast to the predominant notion of NCR peptides as effectors of endosymbionts' differentiation to nitrogen-fixing bacteroids, this finding demonstrated that specific NCRs negatively regulate symbiotic persistence. NFS1 and NFS2 provoke bacterial cell death and early nodule senescence in an allele-specific and rhizobial strain-specific manner, and their function is dependent on the host's genetic background. This project addresses how NCR peptides, on one hand, can induce terminal bacteroid differentiation needed for nitrogen fixation, and on the other hand, some NCR variants result in lysis of bacteroids. Specifically, the project will determine whether the pro- and anti-symbiotic peptides interact with the same bacterial targets, how amino-acids of the peptides affect their function, and how the functions of these peptides are affected by the genetic background. The specific objectives of the research include: (1) characterization of the NFS1 and NFS2 genes and their roles in negative regulation of symbiotic development with Rm41; (2) cloning and characterization of a suppressor of NFS1; and (3) characterization of the NS1 locus that restricts nodulation by Rm41.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.