The absence of quorum sensing signals triggers the expression of the symbiotic genes in broad host range rhizobia. Sinorhizobium fredii NGR234

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. Overall design: To study the influence of flavonids as well as the complete absence of quorum sensing signals on gene expression in Sinorhizobium fredii NGR234, we analyzed in total 12 samples: Thereby we compared the NGR234 wild type to NGR234 wild type treated with 1 µM apigenin as a flavoniod. Furthermore, we compared the gene expression in NGR234 quorum sensing double mutant strain to the NGR234 double mutant supplemented with 1 µM apigenin.