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. 2023 Nov 9;28(22):7512.
doi: 10.3390/molecules28227512.

Non-Targeted Spectranomics for the Early Detection of Xylella fastidiosa Infection in Asymptomatic Olive Trees, cv. Cellina di Nardò

Elhussein Ahmed  1   2 , Biagia Musio  1 , Stefano Todisco  1 , Piero Mastrorilli  1   3 , Vito Gallo  1   3 , Maria Saponari  4 , Franco Nigro  5 , Stefania Gualano  2 , Franco Santoro  2
Affiliations

Non-Targeted Spectranomics for the Early Detection of Xylella fastidiosa Infection in Asymptomatic Olive Trees, cv. Cellina di Nardò

Elhussein Ahmed et al. Molecules. .

Abstract

Olive quick decline syndrome (OQDS) is a disease that has been seriously affecting olive trees in southern Italy since around 2009. During the disease, caused by Xylella fastidiosa subsp. pauca sequence type ST53 (Xf), the flow of water and nutrients within the trees is significantly compromised. Initially, infected trees may not show any symptoms, making early detection challenging. In this study, young artificially infected plants of the susceptible cultivar Cellina di Nardò were grown in a controlled environment and co-inoculated with additional xylem-inhabiting fungi. Asymptomatic leaves of olive plants at an early stage of infection were collected and analyzed using nuclear magnetic resonance (NMR), hyperspectral reflectance (HSR), and chemometrics. The application of a spectranomic approach contributed to shedding light on the relationship between the presence of specific hydrosoluble metabolites and the optical properties of both asymptomatic Xf-infected and non-infected olive leaves. Significant correlations between wavebands located in the range of 530-560 nm and 1380-1470 nm, and the following metabolites were found to be indicative of Xf infection: malic acid, fructose, sucrose, oleuropein derivatives, and formic acid. This information is the key to the development of HSR-based sensors capable of early detection of Xf infections in olive trees.

Keywords: chemometrics; fingerprint; hyperspectral reflectance (HSR); metabolomics; non-targeted; nuclear magnetic resonance (NMR); olive quick decline syndrome (OQDS); plant stress; xylem-inhabiting fungi.

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Conflict of interest statement

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

Figures

Figure 1
Figure 1
A typical 1D 1H NOESY spectrum of an aqueous extract of an olive leaf sample. The main classes of metabolites identified via comparison with reference compounds are indicated by increasing numbering. The full chemical shift assignment is reported in Table 2. “W” refers to the residual water signal. The chemical shift scale is referenced to the TSP-d4 singlet at 0 ppm. The metabolites’ assignment is in agreement with data reported in the literature [28,35].
Figure 2
Figure 2
Hyperspectral reflectance (HSR) spectra of the analyzed samples with a wavelength in the range of 400–1830 nm and with a spectral resolution of 1 nm (interpolated). For each sample, the spectrum is indicated with a different color.
Figure 3
Figure 3
Multivariate data analysis of the NMR data in relation to Xf infection. Xf: samples from plants infected indicated with +; A: samples from non-infected plants indicated with ∆. (a) Scores plot obtained from the PCA showing the first two components, PC1 and PC2; (b) scores plot obtained from the partial least squares discriminant analysis (PLS-DA). (c) Ten-fold cross-validation of the PLS-DA model; Acc = accuracy. The red star indicates the component with the highest value of accumulative Q2. (d) Variable importance in projection (VIP) plot derived from the PLS-DA. The relative concentrations of the metabolites in each belonging class are indicated in blue and red colors for leaf extracts from X. fastidiosa-infected plants (Xf) and non-infected ones (A), respectively.
Figure 4
Figure 4
Significant correlations between hyperspectral reflectance (HSR; 10 nm wavebands; x-axis) wavebands and diagnostic NMR signals (0.04 ppm buckets; y-axis) related to Xylella fastidiosa infection. A positive correlation is indicated in red while a negative correlation is in blue. Kendall’s τ correlation coefficient indicated the degree of the relationship, while the p-value indicated the significance of such a relationship.
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