Symbiosis
Symbiosis has played a key role in the evolution of life on Earth. Symbiotic mergers of once independent species drove the origin of eukaryotes. Moreover, symbiosis has enabled many species to gain novel functions and occupy new ecological niches, thus underpinning the functioning of diverse ecosystems. As endosymbionts, microbes provide their eukaryotic hosts with an array of ecological and physiological innovations, including new metabolic capabilities, such as autotrophy or nitrogen fixation, and protection against infections or environmental stressors. Microbial eukaryotes also commonly host their own endosymbionts, including bacteria and algae. Understanding the stability and resilience of symbioses is key to predicting the response of important ecosystems, such as coral reefs, to global change. Manipulating symbiotic associations also has far-reaching economic, environmental and medical implications, through the potential to improve crop productivity, reduce reliance on fertilisers, and control the insect vectors of infectious diseases.
This collection, guest edited by Professor Michael Brockhurst (University of Manchester) and Dr. Rebecca J Hall (University of Birmingham), will feature microbe-focused studies of symbiosis, ranging from the molecular mechanisms of host-symbiont interactions, their genetic and genomic diversity, to understanding the impacts of symbioses in natural and manmade ecosystems.
Collection Contents
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R-type bacteriocins of Xenorhabdus bovienii determine the outcome of interspecies competition in a natural host environment
Xenorhabdus species are bacterial symbionts of Steinernema nematodes and pathogens of susceptible insects. Different species of Steinernema nematodes carrying specific species of Xenorhabdus can invade the same insect, thereby setting up competition for nutrients within the insect environment. While Xenorhabdus species produce both diverse antibiotic compounds and prophage-derived R-type bacteriocins (xenorhabdicins), the functions of these molecules during competition in a host are not well understood. Xenorhabdus bovienii (Xb-Sj), the symbiont of Steinernema jollieti, possesses a remnant P2-like phage tail cluster, xbp1, that encodes genes for xenorhabdicin production. We show that inactivation of either tail sheath (xbpS1) or tail fibre (xbpH1) genes eliminated xenorhabdicin production. Preparations of Xb-Sj xenorhabdicin displayed a narrow spectrum of activity towards other Xenorhabdus and Photorhabdus species. One species, Xenorhabdus szentirmaii (Xsz-Sr), was highly sensitive to Xb-Sj xenorhabdicin but did not produce xenorhabdicin that was active against Xb-Sj. Instead, Xsz-Sr produced high-level antibiotic activity against Xb-Sj when grown in complex medium and lower levels when grown in defined medium (Grace’s medium). Conversely, Xb-Sj did not produce detectable levels of antibiotic activity against Xsz-Sr. To study the relative contributions of Xb-Sj xenorhabdicin and Xsz-Sr antibiotics in interspecies competition in which the respective Xenorhabdus species produce antagonistic activities against each other, we co-inoculated cultures with both Xenorhabdus species. In both types of media Xsz-Sr outcompeted Xb-Sj, suggesting that antibiotics produced by Xsz-Sr determined the outcome of the competition. In contrast, Xb-Sj outcompeted Xsz-Sr in competitions performed by co-injection in the insect Manduca sexta, while in competition with the xenorhabdicin-deficient strain (Xb-Sj:S1), Xsz-Sr was dominant. Thus, xenorhabdicin was required for Xb-Sj to outcompete Xsz-Sr in a natural host environment. These results highlight the importance of studying the role of antagonistic compounds under natural biological conditions.
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Regulation of hsnT, nodF and nodE genes in Rhizobium tropici CIAT 899 and their roles in the synthesis of Nod factors and in the symbiosis
Rhizobium tropici strain CIAT 899 possesses outstanding agronomic properties as it displays tolerance to environmental stresses, a broad host range and high effectiveness in fixing nitrogen with the common bean (Phaseolus vulgaris L.); in addition, it carries intriguing features such as five copies of the regulatory nodD gene, and the capacity to synthesize a variety of nodulation factors (NFs), even in a flavonoid-independent manner, when submitted to abiotic stresses. However, the roles of several nod genes of the repertoire of CIAT 899 remain to be determined. In this study, we obtained mutants for the hsnT, nodF and nodE genes of CIAT 899 and investigated their expression, NF structures and symbiotic properties. Either in the presence of the flavonoid apigenin, or of salt the expression of hsnT, nodF and nodE in wild-type CIAT 899 was highly up-regulated in comparison to the mutants of all five copies of nodD, indicating the roles that regulatory nodD genes play in the activation of hsnT, nodF and nodE; however, NodD1 was recognized as the main inducer. In total, 29 different NF structures were synthesized by wild-type CIAT 899 induced by apigenin, and 36 when induced by salt, being drastically reduced by mutations in hsnT, nodF and nodE, especially under osmotic stress, with specific changes related to each gene, indicating that the three genes participate in the synthesis of NFs. Mutations in hsnT, nodF and nodE affected differently symbiotic performance (nodule number and shoot dry weight), according to the host plant. Our results indicate that the expression of hsnT, nodF and nodE genes of CIAT 899 is mediated by nodD genes, and although these three genes do not belong to the main set of genes controlling nodulation, they contribute to the synthesis of NFs that will impact symbiotic performance and host specificity.
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Rhizobium tropici CIAT 899 copA gene plays a fundamental role in copper tolerance in both free life and symbiosis with Phaseolus vulgaris
Rhizobium tropici CIAT 899 is a facultative symbiotic diazotroph able to deal with stressful concentrations of metals. Nevertheless the molecular mechanisms involved in metal tolerance have not been elucidated. Copper (Cu2+) is a metal component essential for the heme-copper respiratory oxidases and enzymes that catalyse redox reactions, however, it is highly toxic when intracellular trace concentrations are surpassed. In this study, we report that R. tropici CIAT 899 is more tolerant to Cu2+ than other Rhizobium and Sinorhizobium species. Through Tn5 random mutagenesis we identify a R. tropici mutant strain with a severe reduction in Cu2+ tolerance. The Tn5 insertion disrupted the gene RTCIAT899_CH17575, encoding a putative heavy metal efflux P1B-1-type ATPase designated as copA. Phaseolus vulgaris plants inoculated with the copA::Tn5 mutant in the presence of toxic Cu2+ concentrations showed a drastic reduction in plant and nodule dry weight, as well as nitrogenase activity. Nodules induced by the copA::Tn5 mutant present an increase in H2O2 concentration, lipoperoxidation and accumulate 40-fold more Cu2+ than nodules formed by the wild-type strain. The copA::Tn5 mutant complemented with the copA gene recovered the wild-type symbiotic phenotypes. Therefore, the copA gene is essential for R. tropici CIAT 899 to survive in copper-rich environments in both free life and symbiosis with P. vulgaris plants.
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Regulation of flagellar, motility and chemotaxis genes in Rhizobium leguminosarum by the VisN/R-Rem cascade
In this paper, we describe the regulatory roles of VisN, VisR and Rem in the expression of flagellar, motility and chemotaxis genes in Rhizobium leguminosarum biovar viciae strains VF39SM and 3841. Individual mutations in the genes encoding these proteins resulted in a loss of motility and an absence of flagella, indicating that these regulatory genes are essential for flagellar synthesis and function. Transcriptional experiments involving gusA–gene fusions in wild-type and mutant backgrounds were performed to identify the genes under VisN/R and Rem regulation. Results showed that the chemotaxis and motility genes of R. leguminosarum could be separated into two groups: one group under VisN/R-Rem regulation and another group that is independent of this regulation. VisN and VisR regulate the expression of rem, while Rem positively regulates the expression of flaA, flaB, flaC, flaD, motA, motB, che1 and mcpD. All of these genes except mcpD are located within the main motility and chemotaxis gene cluster of R. leguminosarum. Other chemotaxis and motility genes, which are found outside of the main motility gene cluster (che2 operon, flaH for VF39SM, and flaG) or are plasmid-borne (flaE and mcpC), are not part of the VisN/R-Rem regulatory cascade. In addition, all genes exhibited the same regulation pattern in 3841 and in VF39SM, except flaE and flaH. flaE is not regulated by VisN/R-Rem in 3841 but it is repressed by Rem in VF39SM. flaH is under VisN/R-Rem regulation in 3841, but not in VF39SM. A kinetics experiment demonstrated that a subset of the flagellar genes is continuously expressed in all growth phases, indicating the importance of continuous motility for R. leguminosarum under free-living conditions. On the other hand, motility is repressed under symbiotic conditions. Nodulation experiments showed that the transcriptional activators VisN and Rem are dramatically downregulated in the nodules, suggesting that the symbiotic downregulation of motility-related genes could be mediated by repressing the expression of VisN/R and Rem.
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Rhizobia and plant-pathogenic bacteria: common infection weapons
More LessPlant-interacting micro-organisms can establish either mutualistic or pathogenic associations. Although the outcome is completely different, common molecular mechanisms that mediate communication between the interacting partners seem to be involved. Specifically, nitrogen-fixing bacterial symbionts of legume plants, collectively termed rhizobia, and phytopathogenic bacteria have adopted similar strategies and genetic traits to colonize, invade and establish a chronic infection in the plant host. Quorum-sensing signals and identical two-component regulatory systems are used by these bacteria to coordinate, in a cell density-dependent manner or in response to changing environmental conditions, the expression of important factors for host colonization and infection. The success of invasion and survival within the host also requires that rhizobia and pathogens suppress and/or overcome plant defence responses triggered after microbial recognition, a process in which surface polysaccharides, antioxidant systems, ethylene biosynthesis inhibitors and virulence genes are involved.
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Reassessment of major products of N2 fixation by bacteroids from soybean root nodules
More LessNH3/ was the principal product from soybean bacteroids, prepared by various procedures, when assayed in solution in a flow chamber under N2 fixation conditions. In addition, small quantities of alanine were produced (reaching 20% of NH3/ under some conditions). Some 15N was assimilated by bacteroids purified from soybean root nodules on Percoll density gradients and shaken with 15N2 and 0·008 atm O2. Under these conditions, accounted for 93% of the 15N fixed into the soluble fraction. This fraction contained no measurable [15N]alanine. Neither these bacteroids nor those prepared by the previously used differential centrifugation method, when incubated with exogenous alanine under non-N2-fixing conditions, gave rise to NH3 from alanine. Therefore, contamination of bacteroid preparations with enzymes of plant cytosolic origin and capable of producing NH3 from alanine cannot explain the failure to detect [15N]alanine [as reported elsewhere: Waters, J. K., Hughes, B. L., II, Purcell, L. C., Gerhardt, K. O., Mawhinney, T. P. & Emerich, D. W. (1998). Proc Natl Acad Sci USA 95, 12038–12042]. Cell-free extracts of the bacteroids as used in the 15N experiments contained alanine dehydrogenase and were able to produce alanine from pyruvate and . Other experiments with alanine dehydrogenase in extracts of cultured rhizobia and bacteroids are reported and discussed in relation to the 15N experiments. Possible reasons for the differences between laboratories regarding the role of alanine are discussed. It is concluded that NH3 is the principal soluble product of N2 fixation by suspensions of soybean bacteroids ex planta and that should continue to be considered the principal product of N2 fixation which is assimilated in vivo in soybean nodules.
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