Tapping into the maize root microbiome to identify bacteria that promote growth under chilling conditions

Background When maize (Zea mays L.) is grown in the Northern hemisphere, its development is heavily arrested by chilling temperatures, especially at the juvenile phase. As some endophytes are beneficial for plants under stress conditions, we analyzed the impact of chilling temperatures on the root microbiome and examined whether microbiome-based analysis might help to identify bacterial strains that could promote growth under these temperatures. Results We investigated how the maize root microbiome composition changed by means of 16S rRNA gene amplicon sequencing when maize was grown at chilling temperatures in comparison to ambient temperatures by repeatedly cultivating maize in field soil. We identified 12 abundant and enriched bacterial families that colonize maize roots, consisting of bacteria recruited from the soil, whereas seed-derived endophytes were lowly represented. Chilling temperatures modified the root microbiome composition only slightly, but significantly. An enrichment of several chilling-responsive families was detected, of which the Comamonadaceae and the Pseudomonadaceae were the most abundant in the root endosphere of maize grown under chilling conditions, whereas only three were strongly depleted, among which the Streptomycetaceae. Additionally, a collection of bacterial strains isolated from maize roots was established and a selection was screened for growth-promoting effects on juvenile maize grown under chilling temperatures. Two promising strains that promoted maize growth under chilling conditions were identified that belonged to the root endophytic bacterial families, from which the relative abundance remained unchanged by variations in the growth temperature. Conclusions Our analyses indicate that chilling temperatures affect the bacterial community composition within the maize root endosphere. We further identified two bacterial strains that boost maize growth under chilling conditions. Their identity revealed that analyzing the chilling-responsive families did not help for their identification. As both strains belong to root endosphere enriched families, visualizing and comparing the bacterial diversity in these communities might still help to identify new PGPR strains. Additionally, a strain does not necessarely need to belong to a high abundant family in the root endosphere to provoke a growth-promoting effect in chilling conditions

Drought Stress Results in a Compartment-Specific Restructuring of the Rice Root-Associated Microbiomes.

Plant roots support complex microbial communities that can influence plant growth, nutrition, and health. While extensive characterizations of the composition and spatial compartmentalization of these communities have been performed in different plant species, there is relatively little known about the impact of abiotic stresses on the root microbiota. Here, we have used rice as a model to explore the responses of root microbiomes to drought stress. Using four distinct genotypes, grown in soils from three different fields, we tracked the drought-induced changes in microbial composition in the rhizosphere (the soil immediately surrounding the root), the endosphere (the root interior), and unplanted soils. Drought significantly altered the overall bacterial and fungal compositions of all three communities, with the endosphere and rhizosphere compartments showing the greatest divergence from well-watered controls. The overall response of the bacterial microbiota to drought stress was taxonomically consistent across soils and cultivars and was primarily driven by an enrichment of multiple Actinobacteria and Chloroflexi, as well as a depletion of several Acidobacteria and Deltaproteobacteria While there was some overlap in the changes observed in the rhizosphere and endosphere communities, several drought-responsive taxa were compartment specific, a pattern likely arising from preexisting compositional differences, as well as plant-mediated processes affecting individual compartments. These results reveal that drought stress, in addition to its well-characterized effects on plant physiology, also results in restructuring of root microbial communities and suggest the possibility that constituents of the altered plant microbiota might contribute to plant survival under extreme environmental conditions.IMPORTANCE With the likelihood that changes in global climate will adversely affect crop yields, the potential role of microbial communities in enhancing plant performance makes it important to elucidate the responses of plant microbiomes to environmental variation. By detailed characterization of the effect of drought stress on the root-associated microbiota of the crop plant rice, we show that the rhizosphere and endosphere communities undergo major compositional changes that involve shifts in the relative abundances of a taxonomically diverse set of bacteria in response to drought. These drought-responsive microbes, in particular those enriched under water deficit conditions, could potentially benefit the plant as they could contribute to tolerance to drought and other abiotic stresses, as well as provide protection from opportunistic infection by pathogenic microbes. The identification and future isolation of microbes that promote plant tolerance to drought could potentially be used to mitigate crop losses arising from adverse shifts in climate

Soil domestication by rice cultivation results in plant-soil feedback through shifts in soil microbiota.

BackgroundSoils are a key component of agricultural productivity, and soil microbiota determine the availability of many essential plant nutrients. Agricultural domestication of soils, that is, the conversion of previously uncultivated soils to a cultivated state, is frequently accompanied by intensive monoculture, especially in the developing world. However, there is limited understanding of how continuous cultivation alters the structure of prokaryotic soil microbiota after soil domestication, including to what extent crop plants impact soil microbiota composition, and how changes in microbiota composition arising from cultivation affect crop performance.ResultsWe show here that continuous monoculture (> 8 growing seasons) of the major food crop rice under flooded conditions is associated with a pronounced shift in soil bacterial and archaeal microbiota structure towards a more consistent composition, thereby domesticating microbiota of previously uncultivated sites. Aside from the potential effects of agricultural cultivation practices, we provide evidence that rice plants themselves are important drivers of the domestication process, acting through selective enrichment of specific taxa, including methanogenic archaea, in their rhizosphere that differ from those of native plants growing in the same environment. Furthermore, we find that microbiota from soils domesticated by rice cultivation contribute to plant-soil feedback, by imparting a negative effect on rice seedling vigor.ConclusionsSoil domestication through continuous monoculture cultivation of rice results in compositional changes in the soil microbiota, which are in part driven by the rice plants. The consequences include a negative impact on plant performance and increases in greenhouse gas emitting microbes

Lignin engineering in field-grown poplar trees affects the endosphere bacterial microbiome

Cinnamoyl-CoA reductase (CCR), an enzyme central to the lignin bio-synthetic pathway, represents a promising biotechnological target to reduce lignin levels and to improve the commercial viability of lignocellulosic biomass. However, silencing of the CCR gene results in considerable flux changes of the general and monolignol-specific lignin pathways, ultimately leading to the accumulation of various extractable phenolic compounds in the xylem. Here, we evaluated host genotype-dependent effects of field-grown, CCR-down-regulated poplar trees (Populus tremula x Populus alba) on the bacterial rhizosphere microbiome and the endosphere microbiome, namely the microbiota present in roots, stems, and leaves. Plant-associated bacteria were isolated from all plant compartments by selective isolation and enrichment techniques with specific phenolic carbon sources (such as ferulic acid) that are up-regulated in CCR-deficient poplar trees. The bacterial microbiomes present in the endosphere were highly responsive to the CCR-deficient poplar genotype with remarkably different metabolic capacities and associated community structures compared with the WT trees. In contrast, the rhizosphere microbiome of CCR-deficient and WT poplar trees featured highly overlapping bacterial community structures and metabolic capacities. We demonstrate the host genotype modulation of the plant microbiome by minute genetic variations in the plant genome. Hence, these interactions need to be taken into consideration to understand the full consequences of plant metabolic pathway engineering and its relation with the environment and the intended genetic improvement

Comparative Analysis of Root Microbiomes of Rice Cultivars with High and Low Methane Emissions Reveals Differences in Abundance of Methanogenic Archaea and Putative Upstream Fermenters.

Rice cultivation worldwide accounts for ∼7 to 17% of global methane emissions. Methane cycling in rice paddies is a microbial process not only involving methane producers (methanogens) and methane metabolizers (methanotrophs) but also other microbial taxa that affect upstream processes related to methane metabolism. Rice cultivars vary in their rates of methane emissions, but the influence of rice genotypes on methane cycling microbiota has been poorly characterized. Here, we profiled the rhizosphere, rhizoplane, and endosphere microbiomes of a high-methane-emitting cultivar (Sabine) and a low-methane-emitting cultivar (CLXL745) throughout the growing season to identify variations in the archaeal and bacterial communities relating to methane emissions. The rhizosphere of the high-emitting cultivar was enriched in methanogens compared to that in the low emitter, whereas the relative abundances of methanotrophs between the cultivars were not significantly different. Further analysis of cultivar-sensitive taxa identified families enriched in the high emitter that are associated with methanogenesis-related processes. The high emitter had greater relative abundances of sulfate-reducing and iron-reducing taxa which peak earlier in the season than methanogens and are necessary to lower soil oxidation reduction potential before methanogenesis can occur. The high emitter also had a greater abundance of fermentative taxa which produce methanogenesis precursors (acetate, CO2, and H2). Furthermore, the high emitter was enriched in taxa related to acetogenesis which compete with methanogens for CO2 and H2 These taxa were enriched in a spatio-specific manner and reveal a complex network of microbial interactions on which plant genotype-dependent factors can act to affect methanogenesis and methane emissions.IMPORTANCE Rice cultivation is a major source of anthropogenic emissions of methane, a greenhouse gas with a potentially severe impact on climate change. Emission variation between rice cultivars suggests the feasibility of breeding low-emission rice, but there is a limited understanding of how genotypes affect the microbiota involved in methane cycling. Here, we show that the root microbiome of the high-emitting cultivar is enriched both in methanogens and in taxa associated with fermentation, iron, and sulfate reduction and acetogenesis, processes that support methanogenesis. Understanding how cultivars affect microbes with methanogenesis-related functions is vital for understanding the genetic basis for methane emission in rice and can aid in the development of breeding programs that reduce the environmental impact of rice cultivation

Development of a multigenic metabarcoding of prokaryotic microbiota: application to Bacteria and Archaea of agronomical interest encountered in the rice rhizosphere of Camargue

It is our hypothesis that soil and plant microbiota are playing an eminent role in shaping both natural ecosystems and agricultural production systems. It is now widely admitted that bacterial communities show a great diversity in the rhizosphere and in the endosphere as endophytes. These microbial communities affect the fitness of both the hosts and the microbes and further play important roles in C and N cycles. Much less is known about the role played (if any) by archaea within both the plant rhizosphere and endosphere. Interestingly, a recent study has revealed a relatively high archaeal concentration in internal plant tissues (Müller et al., 2015). The objective of our study is to develop a multigenic metabarcoding approach to accurately describe the biodiversity of relevant microbial communities interacting with rice paddies in the Rhône delta region. This pilot agro-ecosystem was selected because it is annually subjected to several abiotic stresses (water stress associated to rice paddies flooding, soil salinity, etc.) that are likely to enhance archaeal growth. We will retain universal prokaryotic primers and archaeal specific primers within the 16S rRNA gene, two housekeeping genes, gyrB(encoding the ß subunit of the DNA gyrase) and rpoB (encoding the ß subunit of the RNA polymerase) for bacteria, and the thermosome TF55 gene (encoding a type II chaperonin) for archaea. Primers evaluation will be conducted on mock cultures and by sampling rice plants during both rice paddies flooding and drying periods. This study is likely to be a first step towards better understanding the dynamics of Camargue rice paddies microbial communities in relation with abiotic constraints

Exploring within-host diversity of plant viruses in rice agrosystems using two high-throughput sequencing strategies

The virome is an essential component of the phytobiome as viruses correspond to the most abundant and diverse biological entity in Nature and interact with all living organisms. The recent advances in high-throughput sequencing (HTS) has proven efficient for characterizing within-host diversity of several viruses, which has shed light on the influence of this diversity on the global fitness and virulence of viruses. We here aimed at evaluating the impact of rice management on the within-host diversity of Rice yellow mottle virus (RYMV) in order to optimize the cultural practices and to limit the burden of Rice yellow mottle disease in Africa. We worked on rice plants collected from traditional and intensive rice fields in East Africa, which is the center of origin of Rice yellow mottle virus. Interestingly, several strains with contrasted pathogenic and epidemiological parameters co -exist in this region and recombination between strains were identified recently. We selected twelve RYMV sympto-matic samples from fields managed under different types of practices that were analyzed using two HTS strategies: a metagenomics-based approach and a RYMV specific sequencing approach. Combining both strategies, we tested if the RYMV was associated with other viral species in rice, and if some cases of co-infections between RYMV strains occurred. Both approaches indicate a significant within-host RYMV diversity, which allows us to perceive the diversification processes of RYMV strains at the field scale. The detection of recombinants and the comparison of the within-host genetic diversity depending on the cultivation practices are underway

Cr-resistant rhizo- and endophytic bacteria associated with Prosopis juliflora and their potential as phytoremediation enhancing agents in metal-degraded soils

Prosopis juliflora is characterized by distinct and profuse growth even in nutritionally poor soil and environmentally stressed conditions and is believed to harbor some novel heavy metal-resistant bacteria in the rhizosphere and endosphere. This study was performed to isolate and characterize Cr-resistant bacteria from the rhizosphere and endosphere of P. juliflora growing on the tannery effluent contaminated soil. A total of 5 and 21 bacterial strains were isolated from the rhizosphere and endosphere, respectively, could tolerate Cr up to 3000 mg l-1. These isolates also exhibited tolerance to other toxic heavy metals such as, Cd, Cu, Pb and Zn, and high concentration (174 g l-1) of NaCl. Moreover, most of the isolated bacterial strains showed one or more plant growth-promoting activities. The phylogenetic analysis of the 16S rRNA gene indicated a higher and wider range of population of Cr-resistant bacteria in the endosphere than rhizosphere and the predominant species included Bacillus, Staphylococcus and Aerococcus. As far as we know, this is the first report detecting rhizo- and endophytic bacterial population associated with P. juliflora growing on the tannery effluent contaminated soil. The inoculation of three isolates to ryegrass (Lolium multiflorum L.) improved plant growth and heavy metal removal from the tannery effluent contaminated soil suggesting that these bacteria could enhance the establishment of the plant in contaminated soil and also improve the efficiency of phytoremediation of heavy metal-degraded soils

Going back to the Roots: Impact of the microbiome on plant health and growth

Conferencia científicaPlant roots are colonized by an astounding number of microorganisms that can reach cell densities much greater than the number of plant cells. Various studies have shown that members of the plant microbiome contribute to plant tolerance to abiotic (e.g. drought) and biotic (e.g. diseases) stress factors, but also to plant nutrition, growth and development. For the vast majority of plant-associated microorganisms, however, there is limited knowledge on their support functions and the mechanisms involved. Novel ‘omics technologies have provided more in-depth knowledge of the diversity and functioning of the plant microbiome and significant advances are being made to uncover mechanisms, genes and metabolites involved in the multi-trophic interactions in the plant microbiome. To better understand this intriguing complexity, both reductionists’ and systems approaches are needed to identify the biotic and abiotic factors involved in microbiome assembly and activity. Here, new results are presented on the role of rhizosphere and endosphere bacteria in protection of plants against soil-borne pathogens. For the rhizosphere bacteria, we showed that representatives of the Proteobacteria protect plants from pathogen infection by the production of chlorinated peptides and alter root architecture and plant growth via modulation of sulfur assimilation. In-depth metagenomic sequencing of the endosphere allowed de novo assembly of high quality bacterial genomes and revealed various yet unknown biosynthetic genes and pathways with new potential for plant protection and antibiotic discovery. An overview will be given on the wealth of genes and functions of the plant microbiome.Máster y Programa de Doctroado "Biología Celular y Molecular", y el Departamento de Microbiología, Univesidad de Málaga . Campus de Excelencia Internacional Andalucía Tech