Phenometabolomics of olive quick decline syndrome using nuclear magnetic resonance, high resolution mass spectrometry, hyperspectral reflectance, and integrative chemometrics analysis

The olive quick decline syndrome (OQDS) is the disease that, in the past decade, caused severe damages to the ancient olive trees of Apulia region, in Southern Italy. Xylella fastidiosa (Xf) was ascertained as the main agent of OQDS. During the infection period, until the complete decline, the trees go through phenotypic and metabolic changes, as the latter was demonstrated by a non-targeted metabolomics study on Xf-infected olive plants. Such a study used nuclear magnetic resonance (NMR) spectroscopy and high-resolution mass spectrometry (HRMS) to propose a set of metabolites playing a possible role as biomarkers for Xf-infections, in which higher amounts of malic acid, formic acid, mannitol, and sucrose were higher in Xf-infected plants than in Xf-uninfected ones. In contrast, Xf-infected plants revealed slightly lower amounts of oleuropein. As a follow up, the Xf-infected olive plants were studied using hyperspectral reflectance (HSR) aiming at selecting a wavelength region(s) diagnostic for Xf-infections. This integrative approach, successfully applied before, was applied encompassing NMR, HRMS, HSR, and chemometrics. The susceptible variety Cellina di Nardò was studied for artificial infection by X. fastidiosa subsp. pauca ST53, also known as De Donno strain. Plants were grown in a thermally-controlled environment and co-inoculated with some xylem-inhabiting fungi known to occur with OQDS. Leaves were subjected to HSR acquisition while their extracts were analysed by both NMR and HRMS and the acquired spectra were analysed using chemometric techniques. Then, the covariance matrices between those methods were used as a guide to connect HSR spectral features with some diagnostic NMR and HRMS signals. Furthermore, such matrices revealed different ranges of wavelength-regions with different association levels to the corresponding metabolites, which were of few regions within the visible range of 420-520 nm, 570-720 nm, and more within the near-infrared range of 1000-1830 nm.