Salt Stress Enhances Early Symbiotic Gene Expression in Medicago truncatula and Induces a Stress-Specific Set of Rhizobium-Responsive Genes

Salt stress is a major agricultural concern inhibiting not only plant growth but also the symbiotic association between legume roots and the soil bacteria rhizobia. This symbiotic association is initiated by a molecular dialogue between the two partners, leading to the activation of a signaling cascade in the legume host and, ultimately, the formation of nitrogen-fixing root nodules. Here, we show that a moderate salt stress increases the responsiveness of early symbiotic genes in Medicago truncatula to its symbiotic partner, Sinorhizobium meliloti while, conversely, inoculation with S. meliloti counteracts salt-regulated gene expression, restoring one-third to control levels. Our analysis of early nodulin 11 (ENOD11) shows that salt-induced expression is dynamic, Nod-factor dependent, and requires the ionic but not the osmotic component of salt. We demonstrate that salt stimulation of rhizobium-induced gene expression requires NSP2, which functions as a node to integrate the abiotic and biotic signals. In addition, our work reveals that inoculation with S. meliloti succinoglycan mutants also hyperinduces ENOD11 expression in the presence or absence of salt, suggesting a possible link between rhizobial exopolysaccharide and the plant response to salt stress. Finally, we identify an accessory set of genes that are induced by rhizobium only under conditions of salt stress and have not been previously identified as being nodulation-related genes. Our data suggest that interplay of core nodulation genes with different accessory sets, specific for different abiotic conditions, functions to establish the symbiosis. Together, our findings reveal a complex and dynamic interaction between plant, microbe, and environment.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

Preparation and Gut Microbiota Modulatory Property of the Oligosaccharide Riclinoctaose

In relation to available polysaccharides, oligosaccharides have a low molecular weight, less viscosity, and complete water solubility. These properties endow oligosaccharides with significant biological properties including the microbiota regulation ability. In this study, a homogeneous oligooctasaccharide, riclinoctaose, was biosynthesized from succinylglycan riclin by enzymatic degradation. Monosaccharide composition, Fourier-transform infrared, electrospray ionization mass spectrometry, and nuclear magnetic resonance spectrometry analysis indicated that riclinoctaose is an oligooctasaccharide consisting of one galactose and seven glucose residues, with a pyruvate group linked to the terminal glucose residue. The effects of dietary riclinoctaose on the gut microbiota of mice were evaluated. We found that the dietary riclinoctaose significantly altered intestinal microbiota with the increased growth of beneficial intestinal bacteria including Bifidobacteria and Lactobacillus and decreased the abundance of pernicious bacteria such as Gammaproteobacteria. The level of short-chain fatty acids (SCFAs) was significantly elevated in the riclinoctaose cecum. Our results suggested that riclinoctaose as a prebiotic may have a great potential application in functional foods.

Characterization and rheological properties analysis of the succinoglycan produced by a high-yield mutant of Rhizobium radiobacter ATCC 19358

Succinoglycan is an industrially important exopolysaccharide biosynthesized by bacteria. In this study, mutant strain 18052 N-11 was obtained from the wild type strain Rhizobium radiobacter ATCC 19358 by NTG mutagenesis. It has a high yield succinoglycan of 32.5 g/L cultured in a 15 L-fementer for 72 h. Succinoglycan SG-A from the wild type strain has two components, and the molecular weights were 1.55 × 10⁷ Da and 1.26 × 10⁶ Da, respectively. While, succinoglycan SG-N from the mutant strain was a homogeneous polysaccharide, and the molecular weight was 1.01 × 10⁷ Da. The molecular weight of both succinoglycan was higher than those reported in literatures. DSC thermogram of SG-A showed a higher endothermic peak than that of SG-N due to the higher crystallinity of SG-A. The dynamic frequency sweep test of SG-A and SG-N showed that the elastic modulus G' and viscosity modulus G" curves intersected at 65 °C, indicating the thermally induced order-disorder conformation. The results of effect of concentrations (2.5-15%) and temperatures (25-75 °C) on apparent viscosity of SG-A and SG-N showed that the succinoglycan solutions exhibited non-Newtonian, shear-thinning behavior. Both SG-A and SG-N showed an excellent emulsification activity. The characterizations and rheological properties make SG-A and SG-N prominent candidates in food, cosmetics, pharmaceutical and petroleum industries.

Characterization of Mutations That Affect the Nonoxidative Pentose Phosphate Pathway in Sinorhizobium meliloti

Sinorhizobium meliloti is a Gram-negative alphaproteobacterium that can enter into a symbiotic relationship with Medicago sativa and Medicago truncatula Previous work determined that a mutation in the tkt2 gene, which encodes a putative transketolase, could prevent medium acidification associated with a mutant strain unable to metabolize galactose. Since the pentose phosphate pathway in S. meliloti is not well studied, strains carrying mutations in either tkt2 and tal, which encodes a putative transaldolase, were characterized. Carbon metabolism phenotypes revealed that both mutants were impaired in growth on erythritol and ribose. This phenotype was more pronounced for the tkt2 mutant strain, which also displayed auxotrophy for aromatic amino acids. Changes in pentose phosphate pathway metabolite concentrations were also consistent with a mutation in either tkt2 or tal The concentrations of metabolites in central carbon metabolism were also found to shift dramatically in strains carrying a tkt2 mutation. While the concentrations of proteins involved in central carbon metabolism did not change significantly under any conditions, the levels of those associated with iron acquisition increased in the wild-type strain with erythritol induction. These proteins were not detected in either mutant, resulting in less observable rhizobactin production in the tkt2 mutant. While both mutants were impaired in succinoglycan synthesis, only the tkt2 mutant strain was unable to establish symbiosis with alfalfa. These results suggest that tkt2 and tal play central roles in regulating the carbon flow necessary for carbon metabolism and the establishment of symbiosis.IMPORTANCESinorhizobium meliloti is a model organism for the study of plant-microbe interactions and metabolism, especially because it effects nitrogen fixation. The ability to derive the energy necessary for nitrogen fixation is dependent on an organism's ability to metabolize carbon efficiently. The pentose phosphate pathway is central in the interconversion of hexoses and pentoses. This study characterizes the key enzymes of the nonoxidative branch of the pentose phosphate pathway by using defined genetic mutations and shows the effects the mutations have on the metabolite profile and on physiological processes such as the biosynthesis of exopolysaccharide, as well as the ability to regulate iron acquisition.

Novel Genes and Regulators That Influence Production of Cell Surface Exopolysaccharides in Sinorhizobium meliloti

Sinorhizobium meliloti is a soil-dwelling alphaproteobacterium that engages in a nitrogen-fixing root nodule symbiosis with leguminous plants. Cell surface polysaccharides are important both for adapting to stresses in the soil and for the development of an effective symbiotic interaction. Among the polysaccharides characterized to date, the acidic exopolysaccharides I (EPS-I; succinoglycan) and II (EPS-II; galactoglucan) are particularly important for protection from abiotic stresses, biofilm formation, root colonization, and infection of plant roots. Previous genetic screens discovered mutants with impaired EPS production, allowing the delineation of EPS biosynthetic pathways. Here we report on a genetic screen to isolate mutants with mucoid colonial morphologies that suggest EPS overproduction. Screening with Tn5-110, which allows the recovery of both null and upregulation mutants, yielded 47 mucoid mutants, most of which overproduce EPS-I; among the 30 unique genes and intergenic regions identified, 14 have not been associated with EPS production previously. We identified a new protein-coding gene, emmD, which may be involved in the regulation of EPS-I production as part of the EmmABC three-component regulatory circuit. We also identified a mutant defective in EPS-I production, motility, and symbiosis, where Tn5-110 was not responsible for the mutant phenotypes; these phenotypes result from a missense mutation in rpoA corresponding to the domain of the RNA polymerase alpha subunit known to interact with transcription regulators.IMPORTANCE The alphaproteobacterium Sinorhizobium meliloti converts dinitrogen to ammonium while inhabiting specialized plant organs termed root nodules. The transformation of S. meliloti from a free-living soil bacterium to a nitrogen-fixing plant symbiont is a complex developmental process requiring close interaction between the two partners. As the interface between the bacterium and its environment, the S. meliloti cell surface plays a critical role in adaptation to varied soil environments and in interaction with plant hosts. We isolated and characterized S. meliloti mutants with increased production of exopolysaccharides, key cell surface components. Our diverse set of mutants suggests roles for exopolysaccharide production in growth, metabolism, cell division, envelope homeostasis, biofilm formation, stress response, motility, and symbiosis.

Succinoglycan Production Contributes to Acidic pH Tolerance in Sinorhizobium meliloti Rm1021

In this work, the hypothesis that exopolysaccharide plays a role in the survival of Sinorhizobium meliloti at low pH levels is addressed. When S. meliloti was grown at pH 5.75, synthesis of succinoglycan increased, whereas synthesis of galactoglucan decreased. Succinoglycan that was isolated from cultures grown at low pH had a lower degree of polymerization relative to that which was isolated from cultures grown at neutral pH, suggesting that low-molecular weight (LMW) succinoglycan might play a role in adaptation to low pH. Mutants unable to produce succinoglycan or only able to produce high-molecular weight polysaccharide were found to be sensitive to low pH. However, strains unable to produce LMW polysaccharide were 10-fold more sensitive. In response to low pH, transcription of genes encoding proteins for succinoglycan, glycogen, and cyclic β(1-2) glucans biosynthesis increased, while those encoding proteins necessary for the biosynthesis of galactoglucan decreased. While changes in pH did not affect the production of glycogen or cyclic β(1-2) glucan, it was found that the inability to produce cyclic β(1-2) glucan did contribute to pH tolerance in the absence of succinoglycan. Finally, in addition to being sensitive to low pH, a strain carrying mutations in exoK and exsH, which encode the glycanases responsible for the cleavage of succinoglycan to LMW succinoglycan, exhibited a delay in nodulation and was uncompetitive for nodule occupancy. Taken together, the data suggest that the role for LMW succinoglycan in nodule development may be to enhance survival in the colonized curled root hair.

Structural and Functional Properties, Biosynthesis, and Patenting Trends of Bacterial Succinoglycan: A Review

The exopolysaccharide succinoglycan is produced mainly by a large number of soil microbes of Agrobacterium, Rhizobium or Pseudomonas genera etc. Structural properties of succinoglycan are unique in terms of its thermal stability and superior viscosifying property. Unlike the other highly commercialized bacterial exopolysaccharides like dextran or xanthan, mass scale application of succinoglycan has not been that much broadly explored yet. Bacterial succinoglycan is found suitable as a viscosifying and emulsifying agent in food industry, in gravel packing or fluid-loss control agent etc. In this present review, the key aspects of succinoglycan study, in particular, developments in structural characterizations, exo/exs operon system involved in biosynthesis pathway, commercial applications in food and other industries and patenting trends have been discussed.