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The Mediterranean region is famous for its olives, with over 75% of the world’s olive oil produced in Spain, Italy, and Greece. However, olive production in the region is being threatened by the spread of a bacterial disease referred to as the olive quick decline syndrome (OQDS). OQDS causes dieback of the leaves and branches, preventing the production of fruit. There is currently no known treatment for the disease, which has already devastated the olive groves of the Salento region in Southern Italy. There is however some hope as the degree of OQDS symptoms appear to differ among olive cultivars to the extent that some appear to tolerate infection.
Olive quick decline syndrome is caused by the infection of xylem vessels with the gram-negative bacteria Xylella fastidiosa. There is a general agreement that insect vectors hosting cells of X. fastidiosa in their foregut inoculate the bacteria into the xylem while feeding on the leaf petioles of the host plant. From here it spreads into the xylem of branches and stem. A common consequence of the plant–pathogen interaction is the clogging of xylem vessels with aggregates and degradation of the xylem pit membranes. Yet it is unclear whether differences in xylem anatomy among olive cultivars are responsible for varying degrees of tolerance to X. fastidiosa infection.
In their recent study published in AoBP, Petit et al. aimed to characterise the difference in functional xylem anatomy and allocation patterns between two Italian olive (Olea europaea) cultivars (Cellina di Nardò and Leccino) with contrasting responses to X. fastidiosa infection. The two cultivars have different susceptibilities to the Olive Quick Decline Syndrome, with Cellina di Nardò highly susceptible to the disease and Leccino more tolerant to infection.
In the study, infected branches of the susceptible Cellina di Nardò cultivar had fewer and larger xylem vessels and showed a higher occurrence of embolism in the field compared to those of Leccino. These results would suggest that air embolism in xylem vessels likely plays a key role in predisposing the host plant to a more virulent pathogen attack. Petit et al. hope that their study will help to guide future research on the potential relationship between xylem air embolism and the metabolic activity of the xylem-limited pathogen X. fastidiosa.