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  1. Stelkens, Rike (Ed.)
    Horizontal gene transfer (HGT) is a major contributor to bacterial genome evolution, generating phenotypic diversity, driving the expansion of protein families, and facilitating the evolution of new phenotypes, new metabolic pathways, and new species. Comparative studies of gene gain in bacteria suggest that the frequency with which individual genes successfully undergo HGT varies considerably and may be associated with the number of protein–protein interactions in which the gene participates, that is, its connectivity. Two nonexclusive hypotheses have emerged to explain why transferability should decrease with connectivity: the complexity hypothesis (Jain R, Rivera MC, Lake JA. 1999. Horizontal gene transfer among genomes: the complexity hypothesis. Proc Natl Acad Sci U S A. 96:3801–3806.) and the balance hypothesis (Papp B, Pál C, Hurst LD. 2003. Dosage sensitivity and the evolution of gene families in yeast. Nature 424:194–197.). These hypotheses predict that the functional costs of HGT arise from a failure of divergent homologs to make normal protein–protein interactions or from gene misexpression, respectively. Here we describe genome-wide assessments of these hypotheses in which we used 74 existing prokaryotic whole genome shotgun libraries to estimate rates of horizontal transfer of genes from taxonomically diverse prokaryotic donors into Escherichia coli. We show that 1) transferability declines as connectivity increases, 2) transferability declines as the divergence between donor and recipient orthologs increases, and that 3) the magnitude of this negative effect of divergence on transferability increases with connectivity. These effects are particularly robust among the translational proteins, which span the widest range of connectivities. Whereas the complexity hypothesis explains all three of these observations, the balance hypothesis explains only the first one. 
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  2. Ramaswami, Mani (Ed.)
    Abstract Social experience and pheromone signaling in olfactory neurons affect neuronal responses and male courtship behaviors in Drosophila. We previously showed that social experience and pheromone signaling modulate chromatin around behavioral switch gene fruitless, which encodes a transcription factor necessary and sufficient for male sexual behaviors. Fruitless drives social experience-dependent modulation of courtship behaviors and physiological sensory neuron responses to pheromone; however, the molecular mechanisms underlying this modulation of neural responses remain less clear. To identify the molecular mechanisms driving social experience-dependent changes in neuronal responses, we performed RNA-seq from antennal samples of mutants in pheromone receptors and fruitless, as well as grouped or isolated wild-type males. Genes affecting neuronal physiology and function, such as neurotransmitter receptors, ion channels, ion and membrane transporters, and odorant binding proteins are differentially regulated by social context and pheromone signaling. While we found that loss of pheromone detection only has small effects on differential promoter and exon usage within fruitless gene, many of the differentially regulated genes have Fruitless binding sites or are bound by Fruitless in the nervous system. Recent studies showed that social experience and juvenile hormone signaling co-regulate fruitless chromatin to modify pheromone responses in olfactory neurons. Interestingly, genes involved in juvenile hormone metabolism are also misregulated in different social contexts and mutant backgrounds. Our results suggest that modulation of neuronal activity and behaviors in response to social experience and pheromone signaling likely arise due to large-scale changes in transcriptional programs for neuronal function downstream of behavioral switch gene function. 
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  3. Abstract

    Chemical signalling in the plant microbiome can have drastic effects on microbial community structure, and on host growth and development. Previously, we demonstrated that the auxin metabolic signal interference performed by the bacterial genusVariovoraxvia an auxin degradation locus was essential for maintaining stereotypic root development in an ecologically relevant bacterial synthetic community. Here, we dissect theVariovoraxauxin degradation locus to define the genesiadDEas necessary and sufficient for indole-3-acetic acid (IAA) degradation and signal interference. We determine the crystal structures and binding properties of the operon’s MarR-family repressor with IAA and other auxins. Auxin degradation operons were identified across the bacterial tree of life and we define two distinct types on the basis of gene content and metabolic products:iac-like andiad-like. The structures of MarRs from representatives of each auxin degradation operon type establish that each has distinct IAA-binding pockets. Comparison of representative IAA-degrading strains from diverse bacterial genera colonizingArabidopsisplants show that while all degrade IAA, only strains containingiad-like auxin-degrading operons interfere with auxin signalling in a complex synthetic community context. This suggests thatiad-like operon-containing bacterial strains, includingVariovoraxspecies, play a key ecological role in modulating auxins in the plant microbiome.

     
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  6. Fischer, Reinhard (Ed.)
    ABSTRACT Glyphosate is a commonly used herbicide with a broad action spectrum. However, at sublethal doses, glyphosate can induce plant growth, a phenomenon known as hormesis. Most glyphosate hormesis studies have been performed under microbe-free or reduced-microbial-diversity conditions; only a few were performed in open systems or agricultural fields, which include a higher diversity of soil microorganisms. Here, we investigated how microbes affect the hormesis induced by low doses of glyphosate. To this end, we used Arabidopsis thaliana and a well-characterized synthetic bacterial community of 185 strains (SynCom) that mimics the root-associated microbiome of Arabidopsis . We found that a dose of 3.6 × 10 −6 g acid equivalent/liter (low dose of glyphosate, or LDG) produced an ∼14% increase in the shoot dry weight (i.e., hormesis) of uninoculated plants. Unexpectedly, in plants inoculated with the SynCom, LDG reduced shoot dry weight by ∼17%. We found that LDG enriched two Firmicutes and two Burkholderia strains in the roots. These specific strains are known to act as root growth inhibitors (RGI) in monoassociation assays. We tested the link between RGI and shoot dry weight reduction in LDG by assembling a new synthetic community lacking RGI strains. Dropping RGI strains out of the community restored growth induction by LDG. Finally, we showed that individual RGI strains from a few specific phyla were sufficient to switch the response to LDG from growth promotion to growth inhibition. Our results indicate that glyphosate hormesis was completely dependent on the root microbiome composition, specifically on the presence of root growth inhibitor strains. IMPORTANCE Since the introduction of glyphosate-resistant crops, glyphosate has become the most common and widely used herbicide around the world. Due to its intensive use and ability to bind to soil particles, it can be found at low concentrations in the environment. The effect of these remnants of glyphosate in plants has not been broadly studied; however, glyphosate 1,000 to 100,000 times less concentrated than the recommended field dose promoted growth in several species in laboratory and greenhouse experiments. However, this effect is rarely observed in agricultural fields, where complex communities of microbes have a central role in the way plants respond to external cues. Our study reveals how root-associated bacteria modulate the responses of Arabidopsis to low doses of glyphosate, shifting between growth promotion and growth inhibition. 
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  8. Abstract

    Although plastid genome (plastome) structure is highly conserved across most seed plants, investigations during the past two decades have revealed several disparately related lineages that experienced substantial rearrangements. Most plastomes contain a large inverted repeat and two single‐copy regions, and a few dispersed repeats; however, the plastomes of some taxa harbour long repeat sequences (>300 bp). These long repeats make it challenging to assemble complete plastomes using short‐read data, leading to misassemblies and consensus sequences with spurious rearrangements. Single‐molecule, long‐read sequencing has the potential to overcome these challenges, yet there is no consensus on the most effective method for accurately assembling plastomes using long‐read data. We generated a pipeline,plastidGenomeAssemblyUsingLong‐read data (ptGAUL), to address the problem of plastome assembly using long‐read data from Oxford Nanopore Technologies (ONT) or Pacific Biosciences platforms. We demonstrated the efficacy of the ptGAUL pipeline using 16 published long‐read data sets. We showed that ptGAUL quickly produces accurate and unbiased assemblies using only ~50× coverage of plastome data. Additionally, we deployed ptGAUL to assemble four newJuncus(Juncaceae) plastomes using ONT long reads. Our results revealed many long repeats and rearrangements inJuncusplastomes compared with basal lineages of Poales. The ptGAUL pipeline is available on GitHub:https://github.com/Bean061/ptgaul.

     
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  9. null (Ed.)
  10. Plants have an innate immune system to fight off potential invaders that is based on the perception of nonself or modified-self molecules. Microbe-associated molecular patterns (MAMPs) are evolutionarily conserved microbial molecules whose extracellular detection by specific cell surface receptors initiates an array of biochemical responses collectively known as MAMP-triggered immunity (MTI). Well-characterized MAMPs include chitin, peptidoglycan, and flg22, a 22-amino acid epitope found in the major building block of the bacterial flagellum, FliC. The importance of MAMP detection by the plant immune system is underscored by the large diversity of strategies used by pathogens to interfere with MTI and that failure to do so is often associated with loss of virulence. Yet, whether or how MTI functions beyond pathogenic interactions is not well understood. Here we demonstrate that a community of root commensal bacteria modulates a specific and evolutionarily conserved sector of theArabidopsisimmune system. We identify a set of robust, taxonomically diverse MTI suppressor strains that are efficient root colonizers and, notably, can enhance the colonization capacity of other tested commensal bacteria. We highlight the importance of extracellular strategies for MTI suppression by showing that the type 2, not the type 3, secretion system is required for the immunomodulatory activity of one robust MTI suppressor. Our findings reveal that root colonization by commensals is controlled by MTI, which, in turn, can be selectively modulated by specific members of a representative bacterial root microbiota.

     
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