Tackling Multi-Host Pathogenicity of Xylella by Assisted Immunity

Project Information

MultiX

Grant agreement ID: 884235

DOI

10.3030/884235

EC signature date 4 June 2020

Start date 1 January 2021

End date 30 June 2026

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 2 499 260,00

EU contribution

€ 2 499 260,00

Coordinated by

LUDWIG-MAXIMILIANS-UNIVERSITAET MUENCHEN
Germany

Periodic Reporting for period 3 - MultiX (Tackling Multi-Host Pathogenicity of Xylella by Assisted Immunity)

Reporting period: 2024-01-01 to 2025-06-30

Summary of the context and overall objectives of the project

Plant diseases represent a significant threat to global food security. One of the pathogens ranked in the “list of priority pests” for Europe is Xylella fastidiosa. This Gram-negative bacterium is responsible for important re-emerging plant disease, initially discovered in the late 1800’s and causing outbreaks over the past decade around the Mediterranean basin with hot spots in Italy, France and Spain, and more recently in Israel. X. fastidiosa is transmitted via xylem-sap feeding insects, directly delivered into the xylem, where it colonizes the vessels, formed by dead cells, and systematically infects the whole plant.

As a species, X. fastidiosa can infect more than 600 plant species, including crops, forest, herbal and ornamental plants. In hundreds of these species, X. fastidiosa does not cause macroscopic disease symptoms and occurs as an endophyte but in many important crops the bacterium causes some of the most destructive plant bacterial diseases. For example, Xylella fastidiosa is the causal agent of Pierce’s Disease (PD) in grapevine and the Olive Quick Decline Syndrome (OQDS) in olive trees, ultimately it results in tree mortality. Given the lack of disease control by agrochemicals and the devastating threat to olive production, there is an urgent need to develop resistant crop varieties and new, curative/protective pathogen control treatments.

When infected with X. fastidiosa, grapevine, olive and other host species develop macroscopic disease symptoms, yet other infected plant species remain symptomless. The underlying molecular mechanisms, including the interaction of X. fastidiosa with the highly diversified immune systems across plant species remains largely unexplored. MultiX seeks to provide insight into the mechanisms underlying xylem infection of X. fastidiosa, exploring genetic model plants and crops. More strategically, MultiX aims to deliver information that could pave the way for disease control of X. fastidiosa, through genetic elevation of immune receptor capacities in a cell file-dependent manner.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

We have better defined the conditions of X. fastidiosa infection in the genetic model plants Arabidopsis thaliana and Nicotiana spp., showing colonization of the bacteria in the xylem but no macroscopic leaf scorching symptomology. This condition benefits the investigation of immune responses to X. fastidiosa, likely having little overlap with drought stress responses. We completed our transcriptional profiling of asymptomatic A. thaliana plants infected with X. fastidiosa over time. Interestingly, most differentially genes were upregulated at early stages of infection and downregulated at the late infection stage. We identified around 40 immune-related genes, including several immune receptors and susceptibility genes. We have started to characterize respective mutant plants and to date, six immune receptors were shown to control X. fastidiosa infection.

Pathogens secrete virulence molecules to interfere with host immunity. We developed a computational pipeline to predict X. fastidiosa secreted proteins as candidate virulence molecules. The pipeline integrates 103 genomes of X. fastidiosa isolated from different hosts, resulting in the prediction of 421 candidate virulence molecules, which could target the plant nucleus, chloroplasts and mitochondria. We focussed on predicted nuclear localized virulence molecules. Our findings on one selected candidate suggest that X. fastidiosa could secrete molecules localizing to plant nuclei and suppressing the pathogen-associated molecular pattern (PAMP)-induced oxidative burst, a prototypic immune response. These results provide evidence that X. fastidiosa encodes immunomodulatory molecules. We are now screening several candidate virulence molecules for secretion, their ability to target plant organelles and interference with prototypic immune responses.

We developed an excellent toolbox of several cell file-specific expressed genetically encoded biosensors, having most lines available homozygous and in T3 generation. We used imaging methods and established image analysis pipelines, initially recording the spatiotemporal response to defined PAMPs and are now testing the pipeline to describe vascular defence responses to X. fastidiosa infection and bacterial-derived extracellular vesicles in the xylem.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

We are pleased to report that we have obtained:

A transcriptional framework revealing components of the plant’s immune system against X. fastidiosa. Gene homology analysis is being used to obtain an understanding of species-specific and common transcriptional signatures, a step-change considering that most research in plant-microbe interactions is investigated at the single-host/single-pathogen scale.

To date, six immune receptors were shown to control X. fastidiosa infection in the genetic model plant Arabidopsis thaliana. Genetic transfer of these receptors could improve resistance to X. fastidiosa in susceptible crops.

New virulence molecules predicted from 103 genomes of X. fastidiosa isolated from different hosts. Insights into the molecular mechanisms of X. fastidiosa virulence will aid the development of resistant traits.

A genetic toolbox and resource of cell file-specific expressed biosensors. The analysis of these reporter lines will improve our understanding of the spatiotemporal defence responses to vascular pathogens.

Sterile explants of olive cultivars as a new model system to investigate X. fastidiosa infection in a relevant crop. This model system adds to current field studies by reducing the complexity of immune responses from the crosstalk with other environmental stresses.

Susceptible (right) but not grafted resistant olive cultivar (left) shows X. fastidiosa infection