EFSA 4th European Conference on Xylella fastidiosa - Report part 2
In August 2023, the European Food Safety Authority (EFSA) held its 4th European Conference on Xylella fastidiosa in Lyon, France as a satellite event to the ICPP 2023. Experts from the BIOVEXO Project were key participants, alongside experts from the BeXyl project, ERC MultiX project, and the Euphresco network, as well as experts from a range of research institutions, and national and international bodies. This blog is the second in a series intended to provide an in-depth summary of the conference.
In August 2023, the European Food Safety Authority (EFSA) held its 4th European Conference on Xylella fastidiosa in Lyon, France as a satellite event to the ICPP 2023. Experts from the BIOVEXO Project were key participants, alongside experts from the BeXyl project, ERC MultiX project, and the Euphresco network, as well as experts from a range of research institutions, and national and international bodies. This blog is the second in a series intended to provide an in-depth summary of the conference.
After session 1, on the Current status and research updates on Xylella fastidiosa in the EU, session 2 looked at the latest findings on Xylella fastidiosa resistance and control.
The session was co-chaired by BIOVEXO’s Scientific Coordinator, Stéphane Compant (AIT Austrian Institute of Technology, Austria), and Maria Saponari (Institute for Sustainable Plant Protection (IPSP) of the National Research Council of Italy (CNR), Italy).
Session 2 consisted of 6 presentations (click on each below to skip to the summary), followed by an open Q&A discussion:
- How many plant species can X. fastidiosa infect? An update from the EFSA host plants database – Giuseppe Stancanelli, European Food Safety Authority (EFSA)
- Identification and characterization of a X. fastidiosa resistant almond genotype – Ofir Bahar, Agricultural Research Organization (ARO), Volcani Center, Israel
- Enhancing rootstock-mediated systemic immunity against Pierce’s Disease in grafted Chardonnay – Renata Assis (USDA), University of California, USA
- Novel target-oriented peptides with potential for infection control strategies against X. fastidiosa – Luis Alejandro Moll dos Santos, Institute of Food and Agricultural Technology (INTEA), University of Girona (UdG), Spain
- Grapevines primed with lipopolysaccharide demonstrates systemic resistance to Pierce’s disease and reveals an important peroxidase mechanism linked to the immune memory – Nancy Her, University of California, USA
- High prevalence of resistant genotypes to X. fastidiosa in natural olive resources derived from the cultivar Leccino – Pasquale Saldarelli, Institute for Sustainable Plant Protection (IPSP) of the National Research Council of Italy (CNR), Italy
Giuseppe Stancanelli (EFSA) introduced the European Food Safety Authority’s work on establishing a database of Xylella host species. At the time of the confirmation of the detection of Xylella in olive orchards in Apulia, Italy, in 2013, the known number of host plants for the bacterium stood at 132. Two years later, after EFSA completed its first assessment and compiled its first list, this figure more than doubled to 312. A year later, in 2016, this was updated to 359. It was in 2016 that the European Commission mandated that EFSA establish a database of Xylella host plants including information on: subspecies, strains, isolates; natural and artificial infections; and, on tolerance and resistance. Since 2021, the database has been updated twice a year, with the most recent update in June 2023. The database is publicly available: the raw data are published on the Zenodo platform in the EFSA Knowledge Junction community. Giuseppe Stancanelli explained the process for updating the list, which includes a literature search, screening, data extraction, and data cleaning/analysis. The database now lists 690 species of 306 genera and 88 families as Xylella hosts though these numbers drop to 433, 197 and 68 (respectively) where confirmation from at least two modern detection methods are required for verification.
Just over half of the newly identified host plants were infected by the multiplex subspecies, with about a quarter of new hosts infected with a subspecies that was not identified. Whilst most of the new hosts came from the expected families of Fabaceae, Asteraceae, Lamiaceae, and Rosaceae, a significant number of the new hosts identified were trees, such as Salicaceae (e.g. as discussed in presentation 5 in session 1).
Two new sequence types of the subspecies multiplex were discovered in the last year. New host plants are identified each time a new outbreak is investigated in a new area or in a new agroecosystem.
The second presentation was made by Ofir Bahar and discussed the potential identification of a Xylella-resistant almond cultivar. Almonds are threatened by Almond Leaf Scorch (ALS) throughout the Mediterranean basin. To date, only Xylella fastidiosa subspecies fastidiosa has been detected in Israel, so Ofir Bahar hypothesised a single introduction there. He then explained how attempts are being made in Israel to identify Xylella fastidiosa resistance in almond trees. This work involves grafting different genotypes onto infected almond trees (with the cultivar ‘Um ElFahem’ (UEF) used as a susceptible control). One genotype was visually observed to display significantly reduced symptoms 170 days post-grafting (12% of samples showing visible ALS symptoms compared to 65-82% in other genotypes and the control).
In the following year, none of the resistant genotype grafts expressed visible ALS symptoms, while 87% of the UEF grafts appeared symptomatic. Furthermore, only 12% of the resistant genotype grafts tested PCR-positive for Xylella fastidiosa in the second year, compared with 50% in the first year. During the subsequent Q&A session, Ofir Bahar hypothesised that the apparent winter recovery could be due to the failure of the bacteria to become so well established in the resistant cultivar before exposure to winter temperatures/conditions. He also suggested that there was no reason to think that the grafting impaired the xylem flow in a way that might have prevented the spread of Xylella in the resistant cultivar: migration and relative concentration was measured in the grafts using quantitative-PCR, which showed that Xylella fastidiosa migrated similar distances in control samples and in the resistant genotype. However, its population was significantly lower in the resistant genotype.
Further information on the work described by Ofir Bahar was also presented in a poster during the poster presentation session.
The third presentation was made by Renate Assis (USDA). She explained the impact of the introduction of Glassy Winged Sharpshooter (GWSS) in California, a highly efficient Xylella vector, which has required increasingly aggressive use of pesticides to combat the spread of Pierce’s disease (PD) in grapes as the GWSS develop pesticide resistance. They are investigating bioengineered rootstocks that could provide systemic immunity to PD to grafted vines. In the Q&A session, she explained that progress to field testing has been slow as regulatory approval is required and thorough lab testing, then greenhouse testing, had to be completed before permits for field work would be granted. Regular testing and reporting to the FDA is also required to maintain permits.
The fourth presentation was given by Luis Alejandro Moll dos Santos (INTEA), whose work has focussed on the use of peptides to fight Xylella. Peptides typically exhibit a broad spectrum of activity against specific target pathogens and can boost host plant survival by either stimulating the host plants defence mechanisms to fight the pathogen, or inhibiting the pathogens deleterious processes, such as biofilm formation. One of the key elements of a successful Xylella infection is the evasion of the host defence response, so priming represents an interesting avenue for exploration. Additionally, peptides have a low environmental impact. Luis Alejandro Moll dos Santos explained that their work has identified two natural peptides which possess potential dual activity (high bactericidal and antibiofilm activities) in almonds. They explained how they set up a screening platform in order to evaluate the different activities of several peptides. This allowed the identification of peptides with interesting properties which are currently being studied in in planta experiments under greenhouse conditions. So far, the preliminary results seem to be promising. The peptide with the most publicly available information is BP178. Luis Alejandro Moll dos Santos explained that they had previously published an article on this in Phytopathology and, furthermore, noted that they plan to publish information on the new peptides they have identified soon. However, they continue to work on improving their understanding of how these peptides work using in silico and in vitro approaches in order to increase their efficacy.
The use of peptides was an area of interest for a number of the posters presented in Lyon, including posters from the ANCoSIX project and the USDA’s European Biological Control Laboratory (EBCL).
In a follow-up discussion with Robert Shatters and Mélanie Tannières from the EBCL in Montpellier, France, they explained that their work, rather than generating transgenic plants by Agrobacterium-mediated plant transformation, focuses on the use of modified Agrobacterium tumefaciens to produce a modified gall on the plant (as this ‘gall’ provides a benefit to the plant, they call it a symbiont) in which selected peptides will be produced and diffused through the host plant’s vascular system. Importantly, this technology can potentially cure diseased plants, and confer resistance to mature fruit trees and perennial woody plants. In addition, the specially modified Agrobacterium tumefaciens does not persist in either the locally modified host plant cells or the soil/environment.
Robert Shatters and Mélanie Tannières explained that the strategy of using modified Agrobacterium tumefaciens to produce symbionts was evaluated to combat Citrus Greening Disease (CGD) and showed promising results in reducing symptoms and bacterial load in plant tissues. In Citrus, their proprietary Symbiont technology has advanced to the field trial stage in Florida. They note that this is of significance to Xylella as, like CGD, it is caused by a vascular plant pathogenic bacterium, transmitted by an insect vector, which produces a biofilm in the host plant’s vascular tissue. Thus, modified Agrobacterium tumefaciens could potentially offer a means of stimulating the production of plant defence elicitors and Xylella-inhibiting compounds in olives, grapevine, or other Xylella host plants.
The fifth presentation, by Nancy Her, looked, in particular at the lipopolysaccharide (LPS) isolated from Xylella fastidiosa and its ability to produce a priming effect of the immune response in grape vines. Their research demonstrated that LPS pre-treatment of grapevines, successively inoculated with Xylella, resulted in a reduction of PD symptoms and of the bacterium titer in comparison with control naïve (un-primed) plants. In addition, LPS pre-treatment also decreased the formation of tyloses, a typical response to Xylella fastidiosa infections in grapevine. Additionally, a pattern was identified in the transcriptional reprogramming of LPS-primed specimens. Unlike naïve vines during the post-pathogen challenge phase, primed vines revealed an increased number of differentially expressed genes temporally and spatially. This allowed them to observe links between expression of a cationic peroxidase, VviCP1, and disease resistance and speculate that a mechanism relying on a peroxidase mediator is important to the grape vines’ immune memory during defence priming.
In the final presentation, Pasquale Saldarelli (CNR), who leads BIOVEXO work package 4, the large scale validation of control strategies for integrated pest management, talked about the knowledge accumulated from various projects on the resistance traits of the cultivar Leccino to Xylella fastidiosa through the analysis of spontaneous olive seedlings surviving in the epidemic area of Salento, Italy. As the first region in which Xylella was discovered, Apulia allows the greatest time for observations of trends in Xylella symptoms.
Based on the analysis of these spontaneous seedlings, they were able to map the parentage of resistant specimens and deduce that cross-breeding with Leccino cultivar frequently confers a resistant phenotype to progeny, even when Leccino is crossed with the susceptible cultivars Cellina di Nardò and Ogliarola salentina. Resistance is characterized by mild symptoms, low level of bacterial population, and limited distribution in the canopy. Pasquale Saldarelli concluded that this provided a confirmation of findings previously published.
The session closed with a Q&A discussion and then a video presentation of the BIOVEXO project.
The video of session 2 and slides from the presentations and Q&A discussion are available on the #xylella23 Conference website.
We hope you found this summary of the Latest findings on Xylella fastidiosa resistance and control interesting and informative. Our summary of the third session of the 4th European Conference on Xylella fastidiosa, Xylella fastidiosa vectors biology, will be published soon.
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