Plasmid biology of Rhizobium leguminosarum

根瘤菌是一种细菌，它能在豌豆、豆类、小扁豆和紫花苜蓿等豆科植物的根部形成固氮根瘤，这一点值得注意。它们能够让这些植物在不添加合成含氮化肥的情况下生长，而合成含氮化肥的生产和应用在化石燃料消耗方面成本很高，这使得它们在可持续农业中极其重要。生产更高效、更具竞争力的根瘤菌作为豆科植物的接种剂是生物技术的主要目标。然而，引进的根瘤菌之间的相互作用，以及环境中菌株之间遗传转移的潜力，还没有被很好地了解。根瘤菌在细菌中的不同寻常之处在于，它们拥有庞大而复杂的基因组，通常由一条染色体和几个被称为质粒的非常大的染色体外遗传元件组成。这些质粒往往非常稳定，其中一些性质介于质粒和染色体的性质之间。这项建议将集中在豆类根瘤菌生物量viciae菌株VF39和3841的质粒上，这两个菌株都有6个质粒，可以在豌豆、扁豆、蚕豆、豌豆和各种山毛豆品种上形成根瘤。重点将放在决定细菌菌株之间通过接合转移的机制和后果的基因，以及质粒编码基因在碳分解代谢和根际竞争中的作用。这项研究建立在实验室以前研究大质粒的专业知识以及两个菌株的基因组序列的可用性的基础上。两个主要主题将是1)详细分析在pRleVF39b、pRleVF39a和pRL7JI上编码的新型质粒转移和IV型分泌系统，以及2)分析两个菌株基因组中存在的两个不同的、交替的、由质粒编码的核糖醇运输和分解代谢系统。主题1将确定转移质粒所需的所有基因，并确定调节质粒转移的途径。主题2将研究核糖醇分解代谢在根际定植和根瘤侵染中的作用，并将确定两种降解途径中的每一条在核糖醇代谢中的贡献。这项研究将为农业上重要的细菌的多样性、生存和竞争以及水平基因转移提供重要信息。这些信息对设计和应用优良的豆科植物接种剂菌株在可持续作物生产中使用是非常有价值的。

Rhizobia are bacteria that are notable for their ability to form nitrogen-fixing nodules on the roots of legume plants, such as peas, beans, lentils and alfalfa. Their ability to allow these plants to grow without addition of synthetic nitrogen-containing fertilizers, whose production and application has a high cost in fossil fuel consumption, makes them extremely important in sustainable agriculture. Production of more efficient and more competitive rhizobial strains for use as legume inoculants is a major goal in biotechnology. However, the interactions of introduced rhizobial strains, and the potential for genetic transfer between strains in the environment, are not well understood. Rhizobia are unusual among bacteria in that they have large and complex genomes that usually consist of one chromosome and several very large extrachromosomal genetic elements known as plasmids. The plasmids tend to be very stable, and some of them exhibit properties intermediate between those expected of a plasmid and those of a chromosome. This proposal will focus on the plasmids of Rhizobium leguminosarum biovar viciae strains VF39 and 3841, both of which have six plasmids and can form nodules on peas, lentils, faba beans, vetches, and various Lathyrus species. The emphasis will be on the genes that determine the mechanisms and consequences of transfer by conjugation between bacterial strains, and on the role of plasmid-encoded genes in carbon catabolism and rhizosphere competition. The research builds on previous expertise in the lab studying large plasmids, and the availability of genome sequences for both strains. The two main themes will be 1) detailed analysis of the novel plasmid transfer and type IV secretion systems encoded on plasmids pRleVF39b, pRleVF39a and pRL7JI and 2) analysis of the two distinct, alternate, plasmid-encoded systems for ribitol transport and catabolism that are present in the genomes of both strains. Theme 1 will identify all genes required for transfer of the plasmids, and identify the pathways regulating plasmid transfer. Theme 2 will study the role of ribitol catabolism in rhizosphere colonization and nodule infection, and will determine the contributions each of the two degradation pathways makes in ribitol metabolism. The possible reasons for the existence of two parallel pathways will be investigated.This research should provide important information about diversity, survival and competition, and horizontal gene transfer in an agriculturally important bacterium. This information could be very valuable for the design and application of superior legume inoculant strains for use in sustainable crop production.