Unravelling xylem sap chemical traits mediating spittlebug-olive interaction through a metabolomics approach

Xylem sap chemistry is crucial for host plant acceptance and settling by xylem sap feeders such as the meadow spittlebug Philaenus spumarius, the main European vector of Xylella fastidiosa subsp. pauca to olive. Knowledge of specific metabolites underlying host plant suitability for vectors is limited. We combined spittlebug probing and feeding behavior characterization using Electropenetrography and UHPLC-Q-Exactive-MS/MS analysis of xylem sap of four olive varieties differing in their susceptibility to X. fastidiosa (Coratina, FS-17, Leccino and Ogliarola) to identify compounds putatively affecting host plant acceptance. Thereafter, we extended probing behavior analysis to 14 olive varieties (including the previously studied), calculated the xylem ingestion index, quantified the compounds selected in the first experiment (biomarkers), and inferred their role in shaping plant suitability. Partial least squares regression (PLSR) analysis revealed the compounds with the greatest influence on plant suitability for the spittlebug, namely glutamine, malic acid, and daucic acid acting as potential phagostimulants, and gluconic acid, salidroside, oleuropein diglucoside, and D-manno-2-heptulose discouraging feeding. Additionally, regardless of the effects of individual metabolites on plant suitability for spittlebugs, the least suitable olive varieties exhibited higher concentrations of all quantified organic acids selected as biomarkers. Our findings represent a crucial step for managing the spread of X. fastidiosa through either the selection of germplasm resistant to the vector, or agronomic practices that reduce plant suitability and vector activity by manipulating xylem sap chemistry.