Biology subjects

Bragard, C.

Publications and source records attributed to Bragard, C..

6 recordsLinked to original sources

bioRxiv2026

An infection assay recapitulating Xanthomonas translucens pv. translucens natural entry into barley hydathodes

Pathogenic bacteria navigate biological host landscapes to enter and establish infection. How Xanthomonas phytopathogenic bacteria enter specific plant tissues is believed to directly determine the systemic vs. local outcome of infection. Non-vascular Xanthomonas infect leaf mesophyll locally from stomatal pores, while vascular ones enter through hydathode water pores on the leaf margin before invading the xylem for systemic movement. The pathovar Xanthomonas translucens pv. translucens (Xtt) is associated with barley infection, causing both bacterial leaf streak and bacterial blight symptoms by colonizing both the mesophyll spaces through stomatal guard cells and the plant vasculature, respectively. How Xtt reaches the vasculature in barley leaves is yet to be established. In this study, we determined the anatomy of barley hydathodes. A protocol allowing efficient hydathode infection by Xtt was established and caused bacterial blight symptoms, closely mimicking the natural infection process. This study establishes Xtt as one of the few demonstrated hydathode pathogens.

plant biology↗

bioRxiv2026

Phytorisque: an Integrated Assessment Tool for Evaluating the Environmental Risk of Pesticides

IntroductionThe environmental impacts of pesticides have raised increasing concern, prompting the development of indicators to assess associated risks across ecosystems. Two main categories are generally distinguished: score-based indicators, which aggregate variables into scores, and risk-based indicators, grounded in the definition of risk as the product of hazard and exposure. Although more data-intensive and more complex to implement, risk-based indicators are recognized to better preserve proportionality with actual risk levels. ObjectivesThis study presents Phytorisque, a model based on the exposure-toxicity ratio to monitor risks associated with pesticide use in Walloon agriculture, from farm to regional scales, and to identify the most contributing active substances in support of risk-reduction policies MethodPhytorisque is a hybrid model that combines mechanistic, empirical, and statistical approaches, integrating quantities of active substances, their ecotoxicological characteristics, and their mobility, persistence, and bioaccumulation properties to generate indices specific to different environmental compartments. ResultsThe indices obtained enable comparison across substances, agricultural sectors, years, and management scenarios. The Phytorisque model provides an integrated assessment of risk across environmental compartments. It can monitor risk evolution over the years for policy impacts evaluation, diagnose the most problematic substances and prospect environmental risks associated with the use of chemical phytoproducts. ConclusionsPhytorisque provides an integrated risk assessment approach adapted to temporal monitoring, diagnosis, and forecasting. It is a relevant operational tool for supporting regional strategies aimed at reducing pesticide-related risks. The model is also transferable to other regions through the adaptation of parameters to local conditions and context.

ecology↗

bioRxiv2022

Novel genetic traits in Xanthomonas unveiled by the complete genome sequencing of three clade-1 xanthomonads

Evolutionary early-branching xanthomonads, also referred to as clade-1 xanthomonads, include major plant pathogens, most of which colonize monocotyledonous plants. Seven species have been validly described, among them the two sugarcane pathogens Xanthomonas albilineans and Xanthomonas sacchari, and Xanthomonas translucens, which infects small grain cereals, diverse grasses, but also asparagus and pistachio trees. Single-gene sequencing and genomic approaches indicated that this clade likely contains more, yet undescribed species. In this study, we sequenced representative strains of three novel species using long-read sequencing technology. Xanthomonas sp. pv. phormiicola strain CFBP 8444 causes bacterial streak on New Zealand flax, another monocotyledonous plant. Xanthomonas sp. strain CFBP 8443 has been isolated from common bean and Xanthomonas sp. strain CFBP 8445 originated from banana. Complete assemblies of the chromosomes confirmed their unique taxonomic position within clade 1 of Xanthomonas. Genome mining revealed novel genetic traits, hitherto undescribed in other members of the Xanthomonas genus. In strain CFBP 8444, we identified genes related to the synthesis of coronatine-like compounds, a phytotoxin produced by several pseudomonads, which raises interesting questions about the evolution and pathogenicity of this pathogen. In addition, strain CFBP 8444 was found to encode a second, atypical flagellar gene cluster in addition to the canonical flagellar gene cluster. Overall, this research represents an important step toward better understanding the evolutionary history and biology of early-branching xanthomonads.

microbiology↗

bioRxiv2022

Measuring the threat from a distance: insight into the complexity and perspectives for implementing sentinel plantation to test host range of Xylella fastidiosa

The sentinel plantation concept consists of assessing the impact of exotic factors, such as pests and pathogens, on plants of interest by planting them out of their native range. This tool is a way to enhance knowledge for pest risk analysis (PRA) by guiding decisions on how quarantine organisms should be regulated and where to focus prevention and surveillance efforts for an early detection. In this study, the sentinel method was used in the case of research on Xylella fastidiosa, a plant pathogenic bacterium that has recently been found established in southern Europe, but whose potential impact and possible host range are still poorly documented in northern areas where the bacterium is not known to occur. To improve knowledge on the susceptibility of potential hosts of X. fastidiosa in northern Europe, a sentinel plantation of Prunus domestica cv. Opal, Quercus petraea and Salix alba was established in the X. fastidiosa-infected area of Majorca. In order to assess the circulation of the bacterium in the sentinel plot and around it, surveys of the local flora and insect vectors were carried out, as well as the planting of a network of rosemary "spy plants". Symptomatic monitoring and molecular analyses were performed on the sentinel plants for four years. During these years, X. fastidiosa was never detected in our sentinel plants most likely because of the low infectivity pressure recorded in the surroundings. This study underlines the complexity of conducting sentinel plantation assays combined with X. fastidiosa research, highlighting the need for long-term investigation and questioning the efficiency of the sentinel tool. However, this study is placed in perspective with other valuable sentinel plantations. It also highlights the complementarity of the tool and proposes elements to improve or reorient the implementation of future sentinel projects.

ecology↗

bioRxiv2022

Salicaceae as potential host plants of Xylella fastidiosa in European temperate regions

The discovery of three subspecies of Xylella fastidiosa in Europe has triggered major attention on the potential spread up North of the bacteria. Assessing the susceptibility of a previously unexposed European flora is a key element that remains widely unknown. Under biosafety facility, we evaluated the susceptibility of Salicaceae such as Populus tremula, Populus canescens, Salix alba and Salix caprea by mechanically inoculating the KLN59.3 GFP-labelled X. fastidiosa at 22 {degrees}C and at 28 {degrees}C. Bacterial movement and multiplication in plants were investigated by PCR, real-time PCR, confocal or scanning electron microscopy. Nine months post-inoculation, 100 % of the plants tested positive for X. fastidiosa, with the exception of 57% for P. canescens under the 22 {degrees}C-growing conditions. Bacteria were detected up to 120 cm from the inoculation point for S. alba. They were detected in the roots of all species and were successfully isolated for S. alba and P. tremula. Estimates of average CFU/g of plant tissue per species ranged from 1.5E + 03 to 3.5E + 06, with the lowest figures for P. canescens and the highest for P. tremula together with high number of totally obstructed vessels observed by confocal microscopy. The possibility of insect transmission was also evaluated using an experimental set up based on Mallorca Island. There, transmission by P. spumarius of both X. fastidiosa ST1 and ST81 was proven on S. alba. We thus demonstrated that indigenous European Salicaceae such as S. alba or P. tremula are new potential hosts for X. fastidiosa.

plant biology↗

bioRxiv2020

Repeated gain and loss of a single gene modulates the evolution of vascular pathogen lifestyles

Vascular pathogens travel long distances through host veins leading to life-threatening, systemic infections. In contrast, non-vascular pathogens remain restricted to infection sites, triggering localized symptom development. The contrasting features of vascular and non-vascular diseases suggest distinct etiologies, but the basis for each remains unclear. Here, we show that the hydrolase CbsA acts as a phenotypic switch between vascular and non-vascular plant pathogenesis. cbsA was enriched in genomes of vascular phytopathogenic bacteria in the Xanthomonadaceae family and absent in most non-vascular species. CbsA expression allowed non-vascular Xanthomonas to cause vascular blight while cbsA mutagenesis resulted in reduction of vascular or enhanced non-vascular symptom development. Phylogenetic hypothesis testing further revealed that cbsA was lost in multiple non-vascular lineages and more recently gained by some vascular subgroups, suggesting that vascular pathogenesis is ancestral. Our results overall demonstrate how the gain and loss of single loci can facilitate the evolution of complex ecological traits.

microbiology↗