Accumulation of Azelaic Acid in Xylella fastidiosa-Infected Olive Trees: A Mobile Metabolite for Health Screening.

Abstract

Monitoring Xylella fastidiosa is critical for eradicating or at least containing this harmful pathogen. New low-cost and rapid methods for early detection capability are very much needed. Metabolomics may play a key role in diagnosis; in fact, mobile metabolites could avoid errors in sampling due to erratically distributed pathogens. Of the various different mobile signals, we studied dicarboxylic azelaic acid (AzA) which is a key molecule for biotic stress plant response but has not yet been associated with pathogens in olive trees. We found that infected Olea europaea L. plants of cultivars Cellina di Nardò (susceptible to X. fastidiosa) and Leccino (resistant to the pathogen) showed an increase in AzA accumulation in leaf petioles and in sprigs by approximately seven- and sixfold, respectively, compared with plants negative to X. fastidiosa or affected by other pathogens. No statistically significant variation was found between the X. fastidiosa population level and the amount of AzA in either of the plant tissues, suggesting that AzA accumulation was almost independent of the amount of pathogen in the sample. Furthermore, the association of AzA with X. fastidiosa seemed to be reliable for samples judged as potentially false-negative by quantitative polymerase chain reaction (cycle threshold [C t ] > 33), considering both the absolute value of AzA concentration and the values normalized on negative samples, which diverged significantly from control plants. The accumulation of AzA in infected plants was partially supported by the differential expression of two genes (named OeLTP1 and OeLTP2) encoding lipid transport proteins (LTPs), which shared a specific domain with the LTPs involved in AzA activity in systemic acquired resistance in other plant species. The expression level of OeLTP1 and OeLTP2 in petiole samples showed significant upregulation in samples positive to X. fastidiosa of both cultivars, with higher expression levels in positive samples of Cellina di Nardò compared with Leccino, whereas the two transcripts had a low expression level (C t > 40) in negative samples of the susceptible cultivar. Although the results derived from the quantification of AzA cannot confirm the presence of the erratically distributed X. fastidiosa, which can be definitively assessed by traditional methods, we believe they represent a fast and cheap screening method for large-scale monitoring.

Plant phenotyping relevance

植物体内のアゼライン酸蓄積を感染状態の指標として評価し、PCRや他病原体対照との比較を通じて、Xylella fastidiosa感染植物の迅速スクリーニング法として検証している。単なる代謝測定ではなく、病害状態の判定法が中心である。

Code and data availability

The article describes HPLC-ESI-TOF quantification of azelaic acid and RT-PCR gene expression in olive samples, but no public phenotype dataset, image/sensor data, author analysis code, or trained model is deposited. The only URLs mentioned (arabidopsis.org, NCBI, ExPASy, CDD, ClustalW) are generic third-party databases

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