Water-Soluble Humic Materials Regulate Quorum Sensing in Sinorhizobium meliloti Through a Novel Repressor of expR

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.

Fulvic acid increases forage legume growth inducing preferential up-regulation of nodulation and signalling-related genes

“…1 demonstrate how application of FA can increase vegetative yield at very low concentration. This information supports existing indications of a potential yield effect of HS in M. sativa and similar forage legumes ( Tan and Tantiwiramanond, 1983 ; Little et al, 2013 , 2014 ; Xu et al , 2018 ). Figure 1 also shows how the grass species L. perenne did not have increased vegetative biomass from FA application.…”

“…Conversely, VFA had a significantly higher cell density of 1.88×10 10 . These results indicate that adding FAs to liquid cultures can increase growth of S. meliloti , with a similar result demonstrated in a study comparing the presence of a HS substance with no addition ( Xu et al , 2018 ). Furthermore, the effect of FAs on increasing microbial cell growth without the presence of a plant interaction agrees with other published studies; HS addition has been shown to increase growth of Bradyrhizobium liaoningense in liquid culture ( Guo Gao et al , 2015 ) and increase general microbial population growth in soil microbial cells ( Visser, 1985 ).…”

“…It is possible that FAs may also directly influence N-fixing bacteria such as Sinorhizobium . This has been reported in other studies ( Little et al , 2014 ; Xu et al , 2018 ); however, these studies did not include nutritional controls. Improved symbiotic association of leguminous crops with Rhizobium is important with the current emphasis on growing more leguminous crops globally, due to their N-fixing activity ( Lüscher et al , 2014 ; Preissel et al , 2015 ; Foyer et al , 2016 ; Iannetta et al , 2016 ; Reckling et al , 2016 ).…”

“…The important forage crop M. sativa has been found to increase in vegetative biomass after FA application but with variable responses ( Little et al, 2013 , 2014 ). Another study linked bulk soluble HS fractions to increased biomass of M. sativa and, moreover, nodulation with stimulated Sinorhizobium growth, but this study did not include nutritional controls and compared HS application with no addition ( Xu et al , 2018 ). Studies using various HSs including FAs have been carried out in other important legumes and forage grasses ( Verlinden et al , 2010 ; Aydin et al , 2012 ; Traversa et al , 2014 ; Daneshvar Hakimi Maibodi et al , 2015 ; Chang et al , 2016 ); however, again, no nutritional controls were used.…”

Transcription attenuation-derived small RNA rnTrpL regulates tryptophan biosynthesis gene expression in trans

“…Under such conditions, destabilization of sinI mRNA by rnTrpL may impede the autoinducer accumulation and, as a consequence, quorum sensing may begin at higher population density than under conditions of Trp insufficiency. In line with this, it was recently shown that quorum sensing begins at higher population density, when S. meliloti cultures are supplied with water-soluble humic materials as additional nutrient sources (73). Indeed, quorum sensing is known to be influenced by environmental factors in addition to the population density (74).…”