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Title: The absence of the N-acyl-homoserine-lactone autoinducer synthase genes tral and ngrl increases the copy number of the symbiotic plasmid in Sinorhizobium fredii NGR234

Author
item GROTE, JESSICA - Universitat Hamburg
item KRYSCIAK, DAGMAR - Universitat Hamburg
item PETERSEN, KATRIN - Universitat Hamburg
item GULLERT, SIMON - Universitat Hamburg
item SCHMEISSER, CHRISTEL - Universitat Hamburg
item FORSTNER, KONRAD - University Of Wurzburg
item Krishnan, Hari
item SCHWALBE, HARALD - Goethe University
item KUBATOVA, NINA - Goethe University
item STREIT, WOLFGANG - Universitat Hamburg

Submitted to: Frontiers in Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/4/2016
Publication Date: 11/18/2016
Publication URL: https://handle.nal.usda.gov/10113/5695344
Citation: Grote, J., Krysciak, D., Petersen, K., Gullert, S., Schmeisser, C., Forstner, K., Krishnan, H.B., Schwalbe, H., Kubatova, N., Streit, W. 2016. The absence of the N-acyl-homoserine-lactone autoinducer synthase genes tral and ngrl increases the copy number of the symbiotic plasmid in Sinorhizobium fredii NGR234. Frontiers in Microbiology. 7:1858. Available: http://dx.doi.org/10.3389/fmicb.2016.01858.

Interpretive Summary: Rhizobia are soil-dwelling bacteria that form nodules on the roots of legume plants. The nodules are specialized structures where atmospheric nitrogen is fixed by the bacterium, which in turn, is utilized by legumes for growth and development. This process is termed biological nitrogen fixation and it enables legumes to grow in nitrogen-poor soils. As part of the symbiosis that leads to nodule formation, a series of signal exchange occur between bacteria and legume roots. Previously it has been thought flavonoids, phenolic compounds released from roots, are indispensible for nodulation. In this study we describe a novel signaling method that occurs even in the absence of plant-released flavonoids. The results of our study reveal a previously unknown mechanism for regulation of nodulation. Information obtained from this study will help scientists to better understand the factors that limit the formation of nitrogen-fixing nodules on legumes. Such an understanding should enable scientists to manipulate biological nitrogen fixation so that farmers can increase yields with minimal use of nitrogen fertilizers.

Technical Abstract: Plant-released flavonoids induce the transcription of symbiotic genes in rhizobia and one of the first bacterial responses is the synthesis of so called Nod factors. They are responsible for the initial root hair curling during onset of root nodule development. This signal exchange is believed to be essential for initiating the plant symbiosis with rhizobia affiliated with the alphaproteobacteria. Here, we provide evidence that in broad host range rhizobia the complete lack of quorum sensing molecules results in an elevated copy number of its symbiotic plasmid (pNGR234a). This in turn triggers the expression of symbiotic genes and the production of Nod factors in the absence of plant signals. Therefore, increasing the copy number of specific plasmids could be a widespread mechanism of specialized bacterial populations bridging gaps in signalling cascades and providing a competitive advantage.