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 the broad host range strain Sinorhizobium fredii NGR234 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 to bridge gaps in signaling cascades.
Highlights
The rhizobium–legume symbiosis is considered to be one of the best-studied model systems of mutualistic interactions between eukaryotic hosts and the Alpha- and Betaproteobacteria that are commonly called “rhizobia.” The symbiosis in the Alphaproteobacteria is initiated by a signal exchange between the legume plant and the microbe that ideally results in the production of nitrogen-fixing plant root nodules (Gage, 2004; Jones et al, 2007; Deakin and Broughton, 2009)
The pNGR234a replicon belongs to the repABC-type plasmids and there is a high degree of synteny with respect to the overall organization of the region encoding the traI and the repABC genes with A. tumefaciens tumor inducing (Ti) plasmid and many other repABCtype plasmids (Table 3, Figure 1A)
In A. tumefaciens the Ti plasmid copy number is increased in the presence of elevated levels of the AI molecule 3-oxo-C8-HSL and in Rhizobium leguminosarum the pRL1J repA transcription is increased in the presence of the same AI (Pappas and Winans, 2003a; McAnulla et al, 2007)
Summary
The rhizobium–legume symbiosis is considered to be one of the best-studied model systems of mutualistic interactions between eukaryotic hosts and the Alpha- and Betaproteobacteria that are commonly called “rhizobia.” The symbiosis in the Alphaproteobacteria is initiated by a signal exchange between the legume plant and the microbe that ideally results in the production of nitrogen-fixing plant root nodules (Gage, 2004; Jones et al, 2007; Deakin and Broughton, 2009). The infection depends in part on the release of plant-produced specific nod gene-inducing flavonoids (Spaink, 2000; Gage, 2004; Mierziak et al, 2014; Nelson and Sadowsky, 2015). While most rhizobia and closely related species are known to establish a symbiosis with a rather small number of plant legume genera, some rhizobia have evolved mechanisms that allow them to nodulate a larger variety of legume plants These strains have been designated “broad host range” strains and are promiscuous with respect to the selection of their host plants (Broughton and Perret, 1999; Krysciak et al, 2015). The traI gene is part of a conserved cluster that shares a high degree of synteny with the well-studied tumor inducing (Ti) plasmid of Agrobacterium tumefaciens (He et al, 2003; Schmeisser et al, 2009; Pinto et al, 2012; Krysciak et al, 2014). pNGR234a is a repABC-type plasmid similar to the A. tumefaciens Ti plasmids and other known rhizobial symbiotic plasmids (Cevallos et al, 2008; Pinto et al, 2012)
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