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This satellite event to the International Congress of Plant Pathology was held in Lyon, France on the 20th August 2023 and organised by Dr Andrea Harper, and Dr Sara Franco Ortega (University of York) and Prof Ville Friman (University of Helsinki), brought together worldwide experts in ecology and evolution, as well as plant pathologist and bacteriologist to show the most recent advances in the bacterial wilt and its causal agent Ralstonia solanacearum species complex research fields.
The main objective of the event was to create a strong global network among researchers working on bacterial wilt disease to better understand and control this notorious disease. This one-day event, opened by the post-doctoral research associate, Dr. Sara Franco Ortega, remarked on the necessity to create strong collaborations between researchers in different parts of the world and strengthen those links already created several months before during the 7th Bacterial Wilt Symposium (Montevideo, Uruguay). The day, divided into four main sessions, featured talks from world-class leaders, including the evolution and ecology of the bacteria, the interaction that it has with the rest of the soil rhizosphere microbiome and the potential exploitation of other bacteria as biocontrol agents, mechanisms of adaptation of the bacteria to the environment and plant resistance and the use of big data. The event also included industry talks, by the CEO of iMEAN, and how the development of in silico predictive models can help study changes in the bacteria metabolism when exposed to different conditions.
The event also included flash talks from early career researchers that showcased their enthusiasm and excellent research trying to tackle the bacterial wilt disease problem. Round tables were held after every session to allow scientific discussions and enhance the interaction between the speakers and the rest of the participants. A total of 33 registered people from 14 different countries represented the necessity of finding global solutions to this disease that threads food security all around the world.
Dr Alice Guidot, researcher at LIPME (Laboratoire des interactions plantes – microbes – environment) in Auzeville (France) and specialised in microbial ecology, focused on ‘What are the molecular bases of host adaptations in R. solanacearum?’ She explained how she used multi-host experimental evolution of the pathogen. In these experiments, the xylem of tomato, eggplant, cabbage or other plants were injected with R. pseudosolanacearum, and after several days, the xylem sap was recovered and reinjected into healthy plants of the same species. After several serial passages corresponding to 300 bacterial generations, evolved clones were isolated and studied to identify the genetic and epigenetic adaptation of the bacterium to each one of these plants. She explained how R. pseudosolanacearum adapts to changing environments by mutations in genes such as efpR, a putative transcription factor regulator of virulence and metabolism and involved in planta fitness gain. She also focused on the role of DNA methylation in the promoter of genes during host adaptation and how fast epigenetic changes can allow rapid adaptation of the bacterium to the plant xylem.
Dr Tiffany Lowe-Power, Assistant Professor at the University of California, Davis (California, USA) talked about the ecology and evolution of one of the molecular weapons of R. solanacearum, the type 6 secretion system (T6SS), used to antagonize other Gram-negative bacteria by depositing effectors into the periplasm or cytosol. She explained the importance of T6SS genes during different stages of the infection and where and how these genes are located in the pathogen’s genome. One of the main research questions was ‘How has evolution shaped T6SS gene content in R. solanacearum?’ and included her experiments with bioinformatic analysis on pangenome analyses on the RSCC and other related species to identify T6SS genes. She concluded that T6SS have different roles that are important in different environments and especially when other bacteria are present.
Dr Clara Torres-Barcelo, researcher at the Plant Pathology Unit at INRAE (Institut national de la recherche agronomique) in Avignon (France). Her talk focused on the importance of bacteriophages as drivers of bacterial ecology and evolution. She focused on two main research questions, if bacteriophages target all the genomic diversity of the R. solanacearum species complex and if specialists’ or generalists’ bacteriophages are more effective as biocontrol tools. She explained the necessity of considering bacterial genetic variability when measuring phage host range by incorporating phylogenetic distances among bacteria. She also gave examples of her research in Reunion and Mauritius islands, and how bacteriophage host range structure depended on each island, concluding that generalist phages may be most efficient as biocontrol agents.
Prof Ville Friman, Professor of Microbiology at the University of Helsinki (Finland) talked about bacteriophage-resistance-virulence trade-offs in R. solanacearum, results obtained in his collaborative work in the UK and in China. He explained how he has curated a collection of bacteriophages isolated from different environmental reservoirs such as rivers in the UK or soil in China, all of them with different phage infectivity to different R. solanacearum strains and genotypes. His main research questions were Can we use phages as biocontrol agents to control bacterial wilt infections and can we optimize the efficacy? Will resistance rise during phage treatment and what consequences will it have on R. solanacearum virulence? Are trade-offs involved? And what are the mechanisms of phage resistance? He explained how multiple bacteriophage applications or the use of bacteriophage cocktails increases the efficacy and despite phage resistance arising, it reduces bacterial growth and virulence due to mutations in quorum sensing signalling system or in structures involved in motility such as pilus.
Prof Friman also covered the talk ‘Microbial “helpers” in the plant’s rhizosphere’ by Professor Zhong Wei from Nanjing Agricultural University (China) who couldn’t participate in the meeting due to visa problems. This talk explained the role of root exudates on rhizosphere microbiome composition and pathogen suppression and the role of RIN genes (Ripening inhibitor) in tomato root exudates patterns.
Prof Marc Valls, a Professor at the Department of Genetics, Microbiology and Statistics at the University of Barcelona (Spain) and a Research leader at CRAG (Centre for Research in Agricultural Genomics), Barcelona, focused on explaining the gene reprogramming that occurs when R. solanacearum resides in environmental reservoirs, such as in soil or in watercourses, and in planta, using tomato as a model during the different stages of the disease (apoplast, early stages during the colonisation when it reaches the xylem or late stages when it multiplies in the xylem). He also showcased similarities that occur in the gene expression of R. solanacearum when in the xylem and when resides in rivers or other watercourses, especially in terms of type 3 secretion system gene expression.
Dr Andrea Harper, a Senior Lecturer in Plant Biology at the University of York (UK), focused on reservoir hosts of R. solanacearum, including the nightshades Solanum dulcamara and Solanum nigrum. She presented the newly generated long-read genomes of these plant species, which show a resistant phenotype to the pathogen. She showed how comparative genomics and transcriptomics analysis have revealed genes only present in resistant plants compared with other susceptible hosts such as tomato, potato or aubergine. It also included studies of the differences between the soil microbiome composition in reservoir hosts compared with susceptible tomato species when exposed to the pathogen, concluding that the study of reservoir hosts is “essential to identifying metabolites/genes/genetic markers associated with resistance or to isolate beneficial microbes”.
Dr Remi Peyraud, CEO and co-founder of the life modelling company iMEAN in Toulouse (France) linked with synthetic biology and the big-data era to decipher life complexity. He explained how his company used mathematical models and a knowledge-based approach using experimental data and combining it with multi-omic data to create models of species, including a mathematical model of R. solanacearum which can link environment, phenotype and genotype. His model started with R. pseudosolanacearum GMI1000 strain but has been extended to the species complex as it was based on input from different research groups about substrate usage capacity, as well as PacBio sequencing of the genomes, to construct a genome-scale metabolic model which includes in planta infections considering multi-organ modelling of tomato.
Prof Stephan Genin, head of the Ralstonia Adaptation and Pathogenesis team in LIPME (Laboratoire des interactions plantes – microbes – environnement) in Auzeville (France), talked on nutritional virulence of R. solanacearum and how resource uptake is integral to the virulence programme of the pathogen, which questions such as Which are the preferred nutritional sources of R. pseudosolanacearum GMI1000 and are the metabolic pathway specific to strains of the species complex? Or does R. solanacearum need to modify the plant metabolism to support its growth? He showed untargeted metabolomics of xylem sap between infected and healthy plants to identify metabolites that can be used as nutrients, concluding that some metabolites that did not appear in healthy plants but only in infected ones, due to R. pseudosolanacearum altering plant metabolism. He also showed similarities and differences between the use of metabolites between strains and concluded that tomato xylem sap is not nutrient-limiting for R. pseudosolanacearum growth.
There were specific sessions where 6 early career researchers (ECRs) had time to explain their research. These sessions included ECRs from Germany, the Netherlands, Uruguay, Spain and the UK including talks by PhD students such as Daria Evseeva (Universität Tübingen, Germany) who explained the evolution of a novel lineage from Martinique island or Weiqi Zhang (CRAG, Barcelona, Spain) who explained the role of pathogenesis-related proteins (PRs) in tomato plant defence and won the best ECR talk price.
There were also postdoctoral researchers such as Dr Rekha Gopalan-Nair (Max Planck Institute, Germany) who explained the role of two-parted secretion (TPS) regions on virulence and pathogenicity or interbacterial interactions or Dr Florien Gorter (Wageningen University and Research) who talked about host range of a new R. pseudosolanacearum (phylotype I) strain detected in Dutch surface waters and bittersweet. She also announced the first call for the 8th International Wilt Symposium in the Netherland in 2026. Dr Virginia Ferreira (Universidad de la Republica Uruguay) explained differences in the rhizospheric bacterial communities of potato genotypes with diverse defense responses and Dr Sara Franco Ortega (University of York, UK) explained how plants detect phage-resistant R. solanacearum strains.
A total of 83.3% of the participants responded the scientific content of the event was very good and 88.9% that the organisation of the event was also really good. Instead, 66.7% said the networking opportunities were really good and 27.8% that were good, in particular highlighting the success of round tables after each session. All the participants responded that they would join any other bacterial wilt event in the near future.
The satellite event was followed by the International Congress of Plant Pathology. Among the satellite event participants, 7 presented a poster related to R. solanaceaerum or bacterial wilt research and another 6 participants presented a talk in other concurrent sessions. In general, this satellite event provided a massive opportunity for researchers all around the world to know each other expertise and new results and identify where future collaborations would be appropriate.
Andrea Harper, Ville Friman and Sara Franco Ortega