In vitro elicitation of olive calli with methyl jasmonate and Xylella fastidiosa lysate: effects on secondary metabolism and gene expression

In the last ten years, the diffusion of new pathogens such as Xylella fastidiosa subsp. pauca (Xfp) has increased interest in more accurate knowledge of plant defense mechanisms and genes involved in resistance. Our focus is the study of olive secondary metabolites involved in the olive defense responses, such as phenolic and terpene compounds. In particular, the metabolites produced in the callus of the Xfp-susceptible olive cultivar Ogliarola Salentina, after treatment with methyl jasmonate (MeJa), and a Xylella fastidiosa subsp. pauca lysate (XFL) as elicitors, at different exposure times and doses, were analyzed. The total phenols analysis was performed by the COI method, while the individual phenols and terpenoids analysis was performed by LC-MS/MS. Metabolite analysis showed the production of polyphenols such as hydroxytyrosol, oleuropein and verbascoside, and terpenes such as oleanolic, maslinic and corosilic acids in both treatments. High doses of MeJA resulted in a later induction of the total phenolic content. The analysis of the individual compounds showed the induction of verbascoside and its derivative at 72 h after the treatment, whereas hydroxytyrosol was induced at 5 h after the treatment. Finally, real-time PCR analysis revealed variations in the expression of three target genes (OeMYC2, OeBAS and OePAR) involved in the polyphenols and terpenes biosynthesis, highlighting the influence of the two treatments on the metabolic activity of the olive callus. As the defense mechanisms against Xfp are still unknown, the study aims to set up a rapid screening method for susceptibility/resistance to Xylella fastidiosa, based on simplified systems such as olive callus treated with MeJa and XFL, and on evaluating post-elicitation metabolic and gene responses.