Phenotyping and expression profile of clones of the olive cultivar Leccino for the resistance to Xylella fastidiosa

The severe epidemic of Xylella fastidiosa subspecies pauca ST53 in olive in Apulia, southern Italy, prompted to evaluate the adoption of resistant germplasm as a possible strategy of coexistence with the bacterium. To date, resistance traits have been identified in four olive cultivars and, among them, Leccino reacts to infections with limited desiccation and a better physiological response, while hosting low populations of the bacterium. In this study, seven Leccino clones were experimentally inoculated in replicates with the Apulian isolate of X. fastidiosa subsp. pauca and compared over time for the physiological and phenotypic response. At sixteen months after inoculation, the percentages of infected plants ranged from 57.1% to 100% among the clones, whose severity of desiccation was variable, causing in four of them 16% to 40% of dead plants. Measurements over time of stomata conductance and stem water potential indicate that all clones perform better than the susceptible cultivar Cellina di Nardò, although showing a different clone adaptation to the drought stress imposed by the bacterium. Forty-eight hours post-inoculation clones differently perceived the bacterium, responding with the expression of genes related to plant immunity, cell wall remodeling or secondary metabolism. The work addresses the study of Leccino resistance to X. fastidiosa subsp. pauca with different approaches contributing to the knowledge of the mechanisms of resistance and identifying possible molecular targets useful for the screening of olive germplasm and the exploitation of assisted breeding strategies.