Contribution à l'analyse des interactions tripartites entre Brassica napus, Delia radicum et leur microbiote

Microorganisms have a strong influence on plant-insect interactions. We have studied the interaction between oilseed rape (Brassica napus), the cabbage root fly (Delia radicum) and their associated microbial communities. Soil microbial diversity was manipulated with the dilution to extinction approach to assess its effect on plant chemistry and insect life history traits. Diversity variation influenced the fly emergence rate and oviposition, but not plant chemistry. Conversely, herbivory by D. radicum strongly modified root chemistry and both root and rhizosphere microbial communities. We proposed a scenario that in the presence of herbivory, plants would produce defensive compounds but also would recruit, with attractive chemical compounds, soil microorganismsthat may maintain plant defenses. Bacterial communities of adult flies, free of their facultative symbiont Wolbachia, were studied using an antibiotic. We showed that tetracycline decreased fly bacterial diversity, without making them sterile, modified community composition, and that effect lasted over several generations. Lastly, studying bacterial transmission in D. radicum showed two species shared between females and eggs, and two others between larvae and both roots and rhizosphere. This study showed that having a better understanding of plant-insect interactions and how strongly microorganisms can influence their own host or other interacting organisms is a crucial step that could promote microbial applications in a context of insect biological control.Les microorganismes exercent une forte influence sur les interactions plante-insecte. Nous avons étudié l’interaction entre le colza (Brassica napus), la mouche du chou (Delia radicum) et les communautés microbiennes qui leur sont associées. La diversité microbienne du sol a été manipulée par dilution jusqu’à extinction pour évaluer son effet sur la chimie de la plante et les traits d’histoire de vie de l’insecte. La différence de diversité a influencé le taux d’émergence et l’oviposition de la mouche, mais pas la chimie de la plante. A l’inverse, la phytophagie par D. radicum a drastiquement modifié la chimie des racines et les communautés microbiennes des racines et de la rhizosphère. Nous avons proposé le scénario selon lequel les plantes soumises à des attaques de phytophages produiraient des composes défensifs mais recruteraient aussi avec des composés chimiques attractifs, des microorganismes du sol qui permettraient à la plante de maintenir sesdéfenses. Les communautés bactériennes des mouches adultes, sans leur symbiote facultatif Wolbachia, ont été étudiées à l’aide d’un antibiotique. Nous avons montré que la tétracycline diminuait la diversité bactérienne des mouches sans les rendre stériles, modifiait la composition des communautés, et que les effets étaient durables sur plusieurs générations. Enfin, l’étude de la transmission des bactéries chez D. radicum a montré deux espèces partagées entre les femelles et les oeufs, et deux autres entre les larves et les racines et le sol. Cette étude montre qu’avoir une meilleure compréhension des interactions plantesinse

Contribution to the analysis of tripartite interactions between Brassica napus, Delia radicum

Les microorganismes exercent une forte influence sur les interactions plante-insecte. Nous avons étudié l’interaction entre le colza (Brassica napus), la mouche du chou (Delia radicum) et les communautés microbiennes qui leur sont associées. La diversité microbienne du sol a été manipulée par dilution jusqu’à extinction pour évaluer son effet sur la chimie de la plante et les traits d’histoire de vie de l’insecte. La différence de diversité a influencé le taux d’émergence et l’oviposition de la mouche, mais pas la chimie de la plante. A l’inverse, la phytophagie par D. radicum a drastiquement modifié la chimie des racines et les communautés microbiennes des racines et de la rhizosphère. Nous avons proposé le scénario selon lequel les plantes soumises à des attaques de phytophages produiraient des composes défensifs mais recruteraient aussi avec des composés chimiques attractifs, des microorganismes du sol qui permettraient à la plante de maintenir sesdéfenses. Les communautés bactériennes des mouches adultes, sans leur symbiote facultatif Wolbachia, ont été étudiées à l’aide d’un antibiotique. Nous avons montré que la tétracycline diminuait la diversité bactérienne des mouches sans les rendre stériles, modifiait la composition des communautés, et que les effets étaient durables sur plusieurs générations. Enfin, l’étude de la transmission des bactéries chez D. radicum a montré deux espèces partagées entre les femelles et les oeufs, et deux autres entre les larves et les racines et le sol. Cette étude montre qu’avoir une meilleure compréhension des interactions plantesinsecMicroorganisms have a strong influence on plant-insect interactions. We have studied the interaction between oilseed rape (Brassica napus), the cabbage root fly (Delia radicum) and their associated microbial communities. Soil microbial diversity was manipulated with the dilution to extinction approach to assess its effect on plant chemistry and insect life history traits. Diversity variation influenced the fly emergence rate and oviposition, but not plant chemistry. Conversely, herbivory by D. radicum strongly modified root chemistry and both root and rhizosphere microbial communities. We proposed a scenario that in the presence of herbivory, plants would produce defensive compounds but also would recruit, with attractive chemical compounds, soil microorganismsthat may maintain plant defenses. Bacterial communities of adult flies, free of their facultative symbiont Wolbachia, were studied using an antibiotic. We showed that tetracycline decreased fly bacterial diversity, without making them sterile, modified community composition, and that effect lasted over several generations. Lastly, studying bacterial transmission in D. radicum showed two species shared between females and eggs, and two others between larvae and both roots and rhizosphere. This study showed that having a better understanding of plant-insect interactions and how strongly microorganisms can influence their own host or other interacting organisms is a crucial step that could promote microbial applications in a context of insect biological control

A field indicator for rhizosphere effect monitoring in arable soils

International audienceFor the agroecological transition, the rhizosphere is a critical interface for plants to acquire resources and to enhance plant health with limited inputs. In the present study, we developed a new indicator to estimate and monitor the intensity of plant-soil-microbiota interactions under field conditions.A Rhizosphere Effect Indicator (REI) was calculated by comparing individual and aggregated variables of bulk soil to those of the rhizosphere (i.e. soil enzyme activities and nutrient fluxes) every 2 weeks in the fields of three farms along a crop-diversification gradient. The diversity and structure of microbial communities in bulk soil and the rhizosphere in each field were assessed at flowering.The REI revealed statistically distinct dynamics and intensities of rhizosphere functioning among the three farms. Soil enzyme activities contributed more to the rhizosphere effect than nitrate and phosphate fluxes. Molecular analysis distinguished the two soil compartments and all crop-diversification levels. An integrated REI that provided a single value monitored the rhizosphere effect reliably.Methodological limits due to sampling of the rhizosphere under field conditions must be overcome to develop the REI, but the REI can serve as a complementary tool to traditional soil analysis for agroecological cropping system design and evaluation

Influence of Belowground Herbivory on the Dynamics of Root and Rhizosphere Microbial Communities

International audienceRecent studies are unraveling the impact of microorganisms from the roots and rhizosphere on interactions between plants and herbivorous insects and are gradually changing our perception of the microorganisms' capacity to affect plant defenses, but the reverse effect has seldom been investigated. Our study aimed at determining how plant herbivory influences the dynamics of root and rhizosphere microbial community assemblages and whether potential changes in root metabolites and chemical elements produced during herbivory can be related to microbial community diversity. We conducted our study on oilseed rape (Brassica napus) and its major belowground herbivore, the cabbage root fly (Delia radicum). We further assessed the influence of initial soil microbial diversity on these interactions. Different microbial diversities based on a common soil matrix were obtained through a removal-recolonization method. Root and rhizosphere sampling targeted different stages of the herbivore development corresponding to different perturbation intensities. Root bacterial communities were more affected by herbivory than some rhizosphere bacterial phyla and fungal communities, which seemed more resistant to this perturbation. Root herbivory enhanced the phylum of gamma-Proteobacteria in the roots and rhizosphere, as well as the phylum of Firmicutes in the rhizosphere. Herbivory tended to decrease most root amino acids and sugars, and it increased trehalose, indolyl glucosinolates, and sulfur. Higher abundances of four bacterial genera (Bacillus, Paenibacillus, Pseudomonas, and Stenotrophomonas) were associated following herbivory to the increase of trehalose and some sulfur-containing compounds. Further research would help to identify the biological functions of the microbial genera impacted by plant infestation and their potential implications in plant defense

Influential Insider: Wolbachia, an Intracellular Symbiont, Manipulates Bacterial Diversity in Its Insect Host

International audienceFacultative intracellular symbionts like the α-proteobacteria influence their insect host phenotype but little is known about how much they affect their host microbiota. Here, we quantified the impact of infection on the bacterial community of the cabbage root fly by comparing the microbiota of -free and infected adult flies of both sexes. We used high-throughput DNA sequencing (Illumina MiSeq, 16S rRNA, V5-V7 region) and performed a community and a network analysis. In both sexes, infection significantly decreased the diversity of bacterial communities and modified their structure and composition by reducing abundance in some taxa but increasing it in others. Infection by was negatively correlated to 8 bacteria genera ( was the most impacted), and positively correlated to and . We suggest that might antagonize for being entomopathogenic (and potentially intracellular), but would favor and because they might protect the host against chemical plant defenses. Although they might seem prisoners in a cell, endocellular symbionts can impact the whole microbiota of their host, hence its extended phenotype, which provides them with a way to interact with the outside world

Temporal dynamics of bacterial and fungal communities during the infection of Brassica rapa roots by the protist Plasmodiophora brassicae.

International audienceThe temporal dynamics of rhizosphere and root microbiota composition was compared between healthy and infected Chinese cabbage plants by the pathogen Plasmodiophora brassicae. When inoculated with P. brassicae, disease was measured at five sampling dates from early root hair infection to late gall development. The first symptoms of clubroot disease appeared 14 days after inoculation (DAI) and increased drastically between 14 and 35 DAI. The structure of microbial communities associated to rhizosphere soil and root from healthy and inoculated plants was characterized through high-throughput DNA sequencing of bacterial (16S) and fungal (18S) molecular markers and compared at each sampling date. In healthy plants, Proteobacteria and Bacteroidetes bacterial phyla dominated the rhizosphere and root microbiota of Chinese cabbage. Rhizosphere bacterial communities contained higher abundances of Actinobacteria and Firmicutes compared to the roots. Moreover, a drastic shift of fungal communities of healthy plants occurred between the two last sampling dates, especially in plant roots, where most of Ascomycota fungi dominated until they were replaced by a fungus assigned to the Chytridiomycota phylum. Parasitic invasion by P. brassicae disrupted the rhizosphere and root-associated community assembly at a late step during the root secondary cortical infection stage of clubroot disease. At this stage, Flavisolibacter and Streptomyces in the rhizosphere, and Bacillus in the roots, were drastically less abundant upon parasite invasion. Rhizosphere of plants colonized by P. brassicae was significantly more invaded by the Chytridiomycota fungus, which could reflect a mutualistic relationship in this compartment between these two microorganisms

Can soil microbial diversity influence plant metabolites and life history traits of a rhizophagous insect?: A demonstration in oilseed rape.

International audienceInteractions between plants and phytophagous insects play an important part in shaping the biochemical composition of plants. Reciprocally plant metabolites can influence major life history traits in these insects and largely contribute to their fitness. Plant rhizospheric microorganisms are an important biotic factor modulating plant metabolites and adaptation to stress. While plant-insects or plant-microorganisms interactions and their consequences on the plant metabolite signature are well-documented, the impact of soil microbial communities on plant defenses against phytophagous insects remains poorly known. In this study, we used oilseed rape (Brassica napus) and the cabbage root fly (Delia radicum) as biological models to tackle this question. Even though D. radicum is a belowground herbivore as a larva, its adult life history traits depend on aboveground signals. We therefore tested whether soil microbial diversity influenced emergence rate and fitness but also fly oviposition behavior, and tried to link possible effects to modifications in leaf and root metabolites. Through a removal-recolonization experiment, 3 soil microbial modalities ("high," "medium," "low") were established and assessed through amplicon sequencing of 16S and 18S ribosomal RNA genes. The "medium" modality in the rhizosphere significantly improved insect development traits. Plant-microorganism interactions were marginally associated to modulations of root metabolites profiles, which could partly explain these results. We highlighted the potential role of plant-microbial interaction in plant defenses against Delia radicum. Rhizospheric microbial communities must be taken into account when analyzing plant defenses against herbivores, being either below or aboveground

Table_5_Influence of Belowground Herbivory on the Dynamics of Root and Rhizosphere Microbial Communities.xlsx

<p>Recent studies are unraveling the impact of microorganisms from the roots and rhizosphere on interactions between plants and herbivorous insects and are gradually changing our perception of the microorganisms' capacity to affect plant defenses, but the reverse effect has seldom been investigated. Our study aimed at determining how plant herbivory influences the dynamics of root and rhizosphere microbial community assemblages and whether potential changes in root metabolites and chemical elements produced during herbivory can be related to microbial community diversity. We conducted our study on oilseed rape (Brassica napus) and its major belowground herbivore, the cabbage root fly (Delia radicum). We further assessed the influence of initial soil microbial diversity on these interactions. Different microbial diversities based on a common soil matrix were obtained through a removal-recolonization method. Root and rhizosphere sampling targeted different stages of the herbivore development corresponding to different perturbation intensities. Root bacterial communities were more affected by herbivory than some rhizosphere bacterial phyla and fungal communities, which seemed more resistant to this perturbation. Root herbivory enhanced the phylum of γ-Proteobacteria in the roots and rhizosphere, as well as the phylum of Firmicutes in the rhizosphere. Herbivory tended to decrease most root amino acids and sugars, and it increased trehalose, indolyl glucosinolates, and sulfur. Higher abundances of four bacterial genera (Bacillus, Paenibacillus, Pseudomonas, and Stenotrophomonas) were associated following herbivory to the increase of trehalose and some sulfur-containing compounds. Further research would help to identify the biological functions of the microbial genera impacted by plant infestation and their potential implications in plant defense.</p

Table_3_Influence of Belowground Herbivory on the Dynamics of Root and Rhizosphere Microbial Communities.xlsx

<p>Recent studies are unraveling the impact of microorganisms from the roots and rhizosphere on interactions between plants and herbivorous insects and are gradually changing our perception of the microorganisms' capacity to affect plant defenses, but the reverse effect has seldom been investigated. Our study aimed at determining how plant herbivory influences the dynamics of root and rhizosphere microbial community assemblages and whether potential changes in root metabolites and chemical elements produced during herbivory can be related to microbial community diversity. We conducted our study on oilseed rape (Brassica napus) and its major belowground herbivore, the cabbage root fly (Delia radicum). We further assessed the influence of initial soil microbial diversity on these interactions. Different microbial diversities based on a common soil matrix were obtained through a removal-recolonization method. Root and rhizosphere sampling targeted different stages of the herbivore development corresponding to different perturbation intensities. Root bacterial communities were more affected by herbivory than some rhizosphere bacterial phyla and fungal communities, which seemed more resistant to this perturbation. Root herbivory enhanced the phylum of γ-Proteobacteria in the roots and rhizosphere, as well as the phylum of Firmicutes in the rhizosphere. Herbivory tended to decrease most root amino acids and sugars, and it increased trehalose, indolyl glucosinolates, and sulfur. Higher abundances of four bacterial genera (Bacillus, Paenibacillus, Pseudomonas, and Stenotrophomonas) were associated following herbivory to the increase of trehalose and some sulfur-containing compounds. Further research would help to identify the biological functions of the microbial genera impacted by plant infestation and their potential implications in plant defense.</p