Project description:The stabilization of host-symbiont mutualism against the emergence of parasitic individuals is pivotal to the evolution of cooperation. One of the most famous symbioses occurs between legumes and their colonizing rhizobia, in which rhizobia extract nutrients (or benefits) from legume plants while supplying them with nitrogen resources produced by nitrogen fixation (or costs). Natural environments, however, are widely populated by ineffective rhizobia that extract benefits without paying costs and thus proliferate more efficiently than nitrogen-fixing cooperators. How and why this mutualism becomes stabilized and evolutionarily persists has been extensively discussed. To better understand the evolutionary dynamics of this symbiosis system, we construct a simple model based on the continuous snowdrift game with multiple interacting players. We investigate the model using adaptive dynamics and numerical simulations. We find that symbiotic evolution depends on the cost-benefit balance, and that cheaters widely emerge when the cost and benefit are similar in strength. In this scenario, the persistence of the symbiotic system is compatible with the presence of cheaters. This result suggests that the symbiotic relationship is robust to the emergence of cheaters, and may explain the prevalence of cheating rhizobia in nature. In addition, various stabilizing mechanisms, such as partner fidelity feedback, partner choice, and host sanction, can reinforce the symbiotic relationship by affecting the fitness of symbionts in various ways. This result suggests that the symbiotic relationship is cooperatively stabilized by various mechanisms. In addition, mixed nodule populations are thought to encourage cheater emergence, but our model predicts that, in certain situations, cheaters can disappear from such populations. These findings provide a theoretical basis of the evolutionary dynamics of legume-rhizobia symbioses, which is extendable to other single-host, multiple-colonizer systems.

Project description:Background: The overproduction of the main auxin indole-3-acetic acid (IAA) in Sinorhizobium meliloti RD64 leads to beneficial effects both in free-living rhizobium and in host plant during symbiosis. These effects could be related to alteration in gene expression induced by IAA. Preliminary studies of these alterations induced in free-living cells have already started. In this work the transcriptional profiles of free-living S. meliloti wild type 1021 and RD64 strains obtained by RNA-seq analysis were used to better investigate this issue. Results: Transcriptome analysis by next-generation sequencing (RNA-seq) allowed the identification of the genes differentially expressed in RD64 cells, grown in a rich medium under aerobic conditions. Most of the up-regulated genes can be classified into two main categories. The first class comprised the main regulator fixJ and the two intermediate regulators fixK and nifA, which control the expression of genes involved in nitrogen-fixation. For these two latter genes the highest expression levels were observed. Several other genes, known to be FixJ targets, such as those involved in denitrification process and proline metabolism, were also identified. The second class of up-regulated genes comprised the gene coding for the sigma factor RpoH1 and other genes regulated in RpoH1-dependent manner in S. meliloti and involved in stress response. The main category of genes negatively affected can be classified in transport of sugars and other small molecules, chemotaxis and cell envelope functions. Under microaerobic condition, quantitative real-time PCR analysis of genes coding for the two-component regulatory system FixJL and for the regulator NifA showed that for all three genes the induction levels measured in RD64 cells were even higher when compared to aerobic conditions. In particular, for the fixJ gene the expression level increased more than tenfold. The up-regulation of nifA gene was also observed under aerobic conditions both when nitrogen level in the medium was limiting and when it was high. Conclusion: This work provided evidence that the overexpression of the auxin IAA in S. meliloti free-living cells triggers the induction of many genes involved in bacterial response to different abiotic stresses and allows the expression of key genes involved in nitrogen fixation.

Project description:Background: The overproduction of the main auxin indole-3-acetic acid (IAA) in Sinorhizobium meliloti RD64 leads to beneficial effects both in free-living rhizobium and in host plant during symbiosis. These effects could be related to alteration in gene expression induced by IAA. Preliminary studies of these alterations induced in free-living cells have already started. In this work the transcriptional profiles of free-living S. meliloti wild type 1021 and RD64 strains obtained by RNA-seq analysis were used to better investigate this issue. Results: Transcriptome analysis by next-generation sequencing (RNA-seq) allowed the identification of the genes differentially expressed in RD64 cells, grown in a rich medium under aerobic conditions. Most of the up-regulated genes can be classified into two main categories. The first class comprised the main regulator fixJ and the two intermediate regulators fixK and nifA, which control the expression of genes involved in nitrogen-fixation. For these two latter genes the highest expression levels were observed. Several other genes, known to be FixJ targets, such as those involved in denitrification process and proline metabolism, were also identified. The second class of up-regulated genes comprised the gene coding for the sigma factor RpoH1 and other genes regulated in RpoH1-dependent manner in S. meliloti and involved in stress response. The main category of genes negatively affected can be classified in transport of sugars and other small molecules, chemotaxis and cell envelope functions. Under microaerobic condition, quantitative real-time PCR analysis of genes coding for the two-component regulatory system FixJL and for the regulator NifA showed that for all three genes the induction levels measured in RD64 cells were even higher when compared to aerobic conditions. In particular, for the fixJ gene the expression level increased more than tenfold. The up-regulation of nifA gene was also observed under aerobic conditions both when nitrogen level in the medium was limiting and when it was high. Conclusion: This work provided evidence that the overexpression of the auxin IAA in S. meliloti free-living cells triggers the induction of many genes involved in bacterial response to different abiotic stresses and allows the expression of key genes involved in nitrogen fixation. mRNA profiles of S. meliloti wild type 1021 and its derivative IAA-overproducing RD64 strain were generated in triplicate by using HiSeq2000 sequencer

Project description:The rhizosphere and rhizoplane are nutrient-rich but selective environments for the root microbiome. Here, we deciphered a posttranscriptional network regulated by the homologous trans-small RNAs (sRNAs) AbcR1 and AbcR2, which rewire the metabolism of the nitrogen-fixing α-rhizobium Sinorhizobium meliloti during preinfection stages of symbiosis with its legume host alfalfa. The LysR-type regulator LsrB, which transduces the cell redox state, is indispensable for AbcR1 expression in actively dividing bacteria, whereas the stress-induced transcription of AbcR2 depends on the alternative σ factor RpoH1. MS2 affinity purification coupled with RNA sequencing unveiled exceptionally large and overlapping AbcR1/2 mRNA interactomes, jointly representing ⁓6% of the S. meliloti protein-coding genes. Most mRNAs encode transport/metabolic proteins whose translation is silenced by base pairing to two distinct anti-Shine Dalgarno motifs that function independently in both sRNAs. A metabolic model-aided analysis of the targetomes predicted changes in AbcR1/2 expression driven by shifts in carbon/nitrogen sources, which were confirmed experimentally. Low AbcR1/2 levels in some defined media anticipated overexpression growth phenotypes linked to the silencing of specific mRNAs. As a proof of principle, we confirmed AbcR1/2-mediated downregulation of the l-amino acid AapQ permease. AbcR1/2 interactomes are well represented in rhizosphere-related S. meliloti transcriptomic signatures. Remarkably, a lack of AbcR1 specifically compromised the ability of S. meliloti to colonize the root rhizoplane. The AbcR1 regulon likely ranks the utilization of available substrates to optimize metabolism, thus conferring on S. meliloti an advantage for efficient rhizosphere/rhizoplane colonization. AbcR1 regulation is predicted to be conserved in related α-rhizobia, which opens unprecedented possibilities for engineering highly competitive biofertilizers. IMPORTANCE Nitrogen-fixing root nodule symbioses between rhizobia and legume plants provide more than half of the combined nitrogen incorporated annually into terrestrial ecosystems, rendering plant growth independent of environmentally unfriendly chemical fertilizers. The success of symbiosis depends primarily on the capacity of rhizobia to establish competitive populations in soil and rhizosphere environments. Here, we provide insights into the regulation and architecture of an extensive RNA posttranscriptional network that fine-tunes the metabolism of the alfalfa symbiont S. meliloti, thereby enhancing the ability of this beneficial bacterium to colonize nutrient-rich but extremely selective niches, such as the rhizosphere of its host plant. This pervasive RNA regulation of metabolism is a major adaptive mechanism, predicted to operate in diverse rhizobial species. Because RNA regulation relies on modifiable base-pairing interactions, our findings open unexplored avenues for engineering the legumes rhizobiome within sustainable agricultural practices.

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Genetic regulation of nitrogen fixation in rhizobia.

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Genetic regulation of nitrogen fixation in rhizobia.

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