Plant defense factors involved in Olea europaea resistance against Xylella fastidiosa infection

Olive quick decline syndrome (OQDS) is a dangerous plant disease, caused by the bacterium Xylella fastidiosa, which targets olive (Olea europaea). Since field observations suggested that some olive cultivars (i.e. Leccino) were more resistant to OQDS than others (i.e. Cellina di Nardò), the plant defense strategies adopted by olive to contrast X. fastidiosa infection were investigated. In the present study, ELISA and genetic approaches were used to confirm plant infection, while microbial colonization mechanism and distribution in host plant tissues and reactive oxygen species (ROS) levels were examined by light, scanning electron and confocal microscopy analyses. Spectrophotometric and chromatographic techniques were performed to measure secondary metabolites content and qPCR assay was carried out for monitoring plant gene expression variation. Our analysis showed that X. fastidiosa caused accumulation of ROS in Leccino samples compared to Cellina di Nardò. Moreover, the infection induced the up-regulation of defense-related genes, such as NADPH oxidase, some protein kinases, pathogen plant response factors and metabolic enzymes. We also found that Leccino plants enhanced the production of specific antioxidant and antimicrobial molecules, to fight the pathogen and avoid its spreading into xylem vessels. We provided new information on OQDS resistance mechanism applied by Leccino cultivar. In particular, we evidenced that high concentrations of ROS, switching on plant defence signalling pathways, may represent a key factor in fighting X. fastidiosa infection.