Life in earth – the root microbiome to the rescue?

Manipulation of the soil microbiome holds great promise for contributing to more environmentally benign agriculture, with soil microbes such as Pseudomonas promoting plant growth and effectively suppressing pathogenic microorganisms. Next-generation sequencing has enabled a new generation of research into soil microbiomes, presenting the opportunity to better understand and exploit these valuable resources. Soil bacterial communities are both highly complex and variable, and contain vast interspecies and intraspecies diversity, both of which respond to environmental variation. Therefore, we propose that a combination of whole microbiome analyses with in-depth examination of key microbial taxa will likely prove the most effective approach to understanding rhizosphere microbial interactions. This review highlights recent efforts in this direction, based around the important biocontrol bacterium Pseudomonas fluorescens

Plant-associated pseudomonads

This article belongs to the Special Issue Plant-Associated PseudomonadsBacteria belonging to the genus Pseudomonas (the pseudomonads) are a group of Gammaproteobacteria that are characterized by a high metabolic versatility and adaption to different ecological niches [...] Full articl

Bacterial pathogenesis of plants: Future challenges from a microbial perspective

Plant infection is a complicated process. Upon encountering a plant, pathogenic microorganisms must first adapt to life on the epiphytic surface, and survive long enough to initiate an infection. Responsiveness to the environment is critical throughout infection, with intracellular and community-level signal transduction pathways integrating environmental signals and triggering appropriate responses in the bacterial population. Ultimately, phytopathogens must migrate from the epiphytic surface into the plant tissue using motility and chemotaxis pathways. This migration is coupled to overcoming the physical and chemical barriers to entry into the plant apoplast. Once inside the plant, bacteria use an array of secretion systems to release phytotoxins and protein effectors that fulfil diverse pathogenic functions (Fig. 1)(Phan Tran et al., 2011, Melotto & Kunkel, 2013)

Analyzing the Complex Regulatory Landscape of Hfq – an Integrative, Multi-Omics Approach

The ability of bacteria to respond to environmental change is based on the ability to coordinate, redirect and fine-tune their genetic repertoire as and when required. While we can learn a great deal from reductive analysis of individual pathways and global approaches to gene regulation, a deeper understanding of these complex signaling networks requires the simultaneous consideration of several regulatory layers at the genome scale. To highlight the power of this approach we analyzed the Hfq transcriptional/translational regulatory network in the model bacterium Pseudomonas fluorescens. We first used extensive ‘omics’ analyses to assess how hfq deletion affects mRNA abundance, mRNA translation and protein abundance. The subsequent, multi-level integration of these datasets allows us to highlight the discrete contributions by Hfq to gene regulation at different levels. The integrative approach to regulatory analysis we describe here has significant potential, for both dissecting individual signaling pathways and understanding the strategies bacteria use to cope with external challenges

Differential regulation of genes for cyclic-di-GMP metabolism orchestrates adaptive changes during rhizosphere colonization by Pseudomonas fluorescens

Bacteria belonging to the Pseudomonas genus are highly successful colonizers of the plant rhizosphere. The ability of different Pseudomonas species to live either commensal lifestyles or to act as agents of plant-growth promotion or disease is reflected in a large, highly flexible accessory genome. Nevertheless, adaptation to the plant environment involves a commonality of phenotypic outputs such as changes to motility, coupled with synthesis of nutrient uptake systems, stress-response molecules and adherence factors including exopolysaccharides. Cyclic-di-GMP (cdG) is a highly important second messenger involved in the integration of environmental signals with appropriate adaptive responses and is known to play a central role in mediating effective rhizosphere colonization. In this study, we examined the transcription of multiple, reportedly plant-upregulated cdG metabolism genes during colonization of the wheat rhizosphere by the plant-growth-promoting strain P. fluorescens SBW25. While transcription of the tested genes generally increased in the rhizosphere environment, we additionally observed a tightly orchestrated response to environmental cues, with a distinct transcriptional pattern seen for each gene throughout the colonization process. Extensive phenotypical analysis of deletion and overexpression strains was then conducted and used to propose cellular functions for individual cdG signaling genes. Finally, in-depth genetic analysis of an important rhizosphere colonization regulator revealed a link between cdG control of growth, motility and stress response, and the carbon sources available in the rhizosphere

Interaction and Signalling Networks:a report from the fourth 'Young Microbiologists Symposium on Microbe Signalling, Organisation and Pathogenesis'

At the end of June, over 120 microbiologists from 18 countries gathered in Dundee, Scotland for the fourth edition of the Young Microbiologists Symposium on ‘Microbe Signalling, Organisation and Pathogenesis’. The aim of the symposium was to give early career microbiologists the opportunity to present their work in a convivial environment and to interact with senior world-renowned scientists in exciting fields of microbiology research. The meeting was supported by the Microbiology Society, the Society of Applied Microbiology and the American Society for Microbiology with further sponsorship from the European Molecular Biology Organisation and the Royal Society of Edinburgh. In this report, we highlight some themes that emerged from the many interesting talks and poster presentations, as well as some of the other activities that were on offer at this energetic meeting

Diversity, detection and exploitation: linking soil fungi and plant disease

Plant-associated fungi are incredibly diverse, comprising over a million species of mycorrhiza, endophytes, saprophytes and pathogens worldwide. This diverse fungal community is highly important for plant health. Many fungi are effective biocontrol agents that can kill or suppress fungal pathogens, with pathogen biocontrol found for both individual microorganisms and plant-associated fungal consortia. Meanwhile, increased plant community diversity aboveground corresponds to an increase in below-ground fungal community diversity, which contributes in turn to improved rhizosphere soil health and pathogen suppression. In this review, we discuss the role of fungal diversity in soil health and plant disease suppression and the various mechanisms by which mycorrhizal and endophytic fungi combat plant pathogenic fungi. We also discuss the array of diagnostic tools, both well-established and newly developed, which are revolutionising fungal pathogen detection and rhizosphere community analysis

Communication, co-operation and social interactions:a report from the third 'Young Microbiologists Symposium on Microbe Signaling, Organization and Pathogenesis'

The third Young Microbiologists Symposium took place on the vibrant campus of the University of Dundee, Scotland, from the 2nd to 3rd of June 2014. The symposium attracted over 150 microbiologists from 17 different countries. The significant characteristic of this meeting was that it was specifically aimed at providing a forum for junior scientists to present their work. The meeting was supported by the Society for General Microbiology and the American Society for Microbiology, with further sponsorship from the European Molecular Biology Organization, the Federation of European Microbiological Societies, and The Royal Society of Edinburgh. In this report, we highlight some themes that emerged from the many exciting talks and poster presentations given by the young and talented microbiologists in the area of microbial gene expression, regulation, biogenesis, pathogenicity, and host interaction.</p

Cryopreservation of a soil microbiome using a Stirling 1 cycle approach - a genomic assessment

Soil microbiomes are dynamic systems that respond to biotic and abiotic environmental factors such as those presented at seasonal scales or due to long-term anthropogenic regime shifts. These can affect the composition and function of microbiomes. Investigation of microbiomes can uncover hidden microbial roles in health and disease and discover microbiome-based interventions. Collections of soil samples are kept by various institutions in either a refrigerated or occasionally frozen state, but conditions are not optimised to ensure the integrity of soil microbiome. In this manuscript, we describe cryopreservation with a controlled rate cooler and estimate the genomic content of an exemplar soil sample before and after cryopreservation. The first hypothesis was to test the genomic integrity of the microbiome. We also enriched the soil sample with a liquid medium to estimate the growth of bacteria and compared their growth before and after cryopreservation. Sequence-based rRNA metabarcoding was used to demonstrate that the controlled rate cooler maintains intact the DNA content of the microbiome. Two methods of cryopreservation were applied and compared with control aliquots of soil. An optimised cryopreservation of soil samples is essential for the development of microbiome research in order to retain stable, functionally intact microbiomes. Our results showed that metabarcoding of 16S and ITS rRNA were useful methods to estimate successful cryopreservation. The soil microbiome after enrichment with liquid medium exhibited a similar response of cryopreserved soil and this was estimated with the comparison of the ten most abundant bacterial taxa. These findings support a successful process of cryopreservation and are promising for future use of this technology. To the best of our knowledge, this study is the first report of cryopreservation of soil using a Stirling cycle cooling approach

The diguanylate cyclase AdrA regulates flagellar biosynthesis in Pseudomonas fluorescens F113 through SadB

Flagellum mediated motility is an essential trait for rhizosphere colonization by pseudomonads. Flagella synthesis is a complex and energetically expensive process that is tightly regulated. In Pseudomonas fluorescens, the regulatory cascade starts with the master regulatory protein FleQ that is in turn regulated by environmental signals through the Gac/Rsm and SadB pathways, which converge in the sigma factor AlgU. AlgU is required for the expression of amrZ, encoding a FleQ repressor. AmrZ itself has been shown to modulate c-di-GMP levels through the control of many genes encoding enzymes implicated in c-di-GMP turnover. This cyclic nucleotide regulates flagellar function and besides, the master regulator of the flagellar synthesis signaling pathway, FleQ, has been shown to bind c-di-GMP. Here we show that AdrA, a diguanylate cyclase regulated by AmrZ participates in this signaling pathway. Epistasis analysis has shown that AdrA acts upstream of SadB, linking SadB with environmental signaling. We also show that SadB binds c-di-GMP with higher affinity than FleQ and propose that c-di-GMP produced by AdrA modulates flagella synthesis through SadBThis work was supported by funding from MINECO/FEDER EU Grant RTI2018 093991-BI00 to R.R. and M.M. C.M. was funded by a FPI fellowship from MINECO. EB-R was the recipient of fellowships from Fundación Tatiana Pérez de Guzmán el Bueno (Medioambiente 2016) and the FPU program from MECD (FPU16/05513). Short stays of R.R. and C.M. at John Innes Centre were funded by MECD (Salvador de Madariaga and FPU, respectively