Abstract
Quorum sensing (QS) plays an important role in the growth, nodulation, and nitrogen fixation of rhizobia. In this study, we show that water-soluble humic materials (WSHM) repress the expression of the QS related genes sinI, sinR, and expR in Sinorhizobium meliloti. This decreased the production of N-acetyl homoserine lactones (AHL) and exopolysaccharides (EPS), and ultimately increased S. meliloti cell density. We also identified a novel regulator, SMc03890 (renamed QsrR), which binds directly to the expR promoter. Deletion of qsrR increased expR expression. WSHM repressed the expression of expR by augmenting the interaction between QsrR and the expR promoter; this was determined by a bacterial-one-hybrid assay. These effects of WSHM on the QS system in S. meliloti may be the underlying mechanism by which WSHM increase the symbiotic nitrogen fixation of Medicago sativa inoculated with S. meliloti. This study provides the first evidence that humic acids regulate the QS of rhizobia and suggests that WSHM could be used as fertilizers to improve the efficiency of symbiotic nitrogen fixation.
Highlights
Quorum sensing (QS) is a bacterial communication mechanism in which cell physiology and behavior are coordinated with population density (Bogino et al, 2015)
We revealed that the water-soluble humic materials (WSHM) that are produced by lignite biodegradation enhanced the growth, cell metabolism, and nutrient transport of Bradyrhizobium liaoningense CCBAU05525, as well as its nodulation with soybean (12∼26% increased yield in soybean grains) (Gao T.G. et al, 2015)
The acetyl homoserine lactones (AHL) detector strain, A. tumefaciens KYC55 did not respond to WSHM but did respond to the AHL produced by A. tumefaciens R10 (Supplementary Figure S1)
Summary
Quorum sensing (QS) is a bacterial communication mechanism in which cell physiology and behavior are coordinated with population density (Bogino et al, 2015). In the QS system of Sinorhizobium meliloti 8530 (Figure 1), SinI is responsible for synthesis of the QS signaling molecules, N-acetyl homoserine lactones (AHL). In addition to regulating the genes involved in nodulation and nitrogen fixation (Hoang et al, 2004), AHL-ExpR complex upregulate the expression of exp operon, which is involved in EPS II synthesis, and down-regulate genes related to bacterial motility, such as visN, visR, and rem (Gurich and Gonzalez, 2009; Mueller and González, 2010). The number of pink nodules induced by the sinI mutant decreased compared
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