Xylella fastidiosa subsp. pauca and multiplex elicit differential responses in a susceptible olive cultivar

The emergence of Xylella fastidiosa in Europe revealed the occurrence of bacterial strains belonging to different subspecies and unraveled the vulnerability of some typical European plant species. Among these, olive was found infected in different European outbreaks, with infections caused by strains harboring different sequence types (STs) and belonging to subspecies pauca and multiplex.
In this work, we compared under greenhouse conditions, the response of the olive cultivar “Cellina di Nardò” to infections caused by the strains De Donno (subsp. pauca, ST53) and ESVL (subsp. multiplex, ST6), as this cultivar is one of the most susceptible in the Italian outbreak. The data showed that: (i) strain De Donno systemically colonized the plants faster than the strain ESLV; (ii) 1.5 years post-inoculation, the bacterial population detected in the plants systemically infected was ~25 times higher for strain De Donno than ESLV; (iii) a severe shoot dieback were detected on the De Donno-infected plants, whereas up to 3 years post-inoculation no dieback and desiccations have been recorded on the ESLV-infected plants. This pathogenicity test was complemented with an RNA-seq analysis. Nine libraries, three for each condition (ESVL, De Donno, mock inoculated), were sequenced by Ilumina technology and sequences mapped against the Olea europeae cultivar Picual genome for the analysis of differential gene expression (DEG). The output of this study showed that, respect to the 6,200 genes differentially expressed (FDR<1.00E-05) in De Donno-infected plants, only 550 (FDR<1.00E-05) genes had an altered expression in the ESVL-infected plants.
The present data showed that strains of the two subspecies elicit differential host responses, which were biologically and molecularly attenuated in the ESVL infections with respect to the aggressive De Donno infections and provide useful insights for exploring the molecular basis of Xylella-olive interaction.