Aggressiveness of Subspecies and STs of <i>Xylella fastidiosa</i> and Plant Response in <i>Nicotiana benthamiana</i>

The aggressiveness of seventeen <i>X. fastidiosa</i> strains, representing different subspecies and sequence types (STs), was studied in the surrogated host <i>N. benthamiana</i> by the analysis of the population levels and symptoms dynamics, dose-response relationships and the transcriptomic response of the plant. Colonization of all strains was observed after 7 days post inoculation (dpi), and the first symptoms appeared after 14 dpi. Differences in patterns of population dynamics and symptoms development were observed between strains and there was neither a relationship between population growth and symptoms severity, nor between strains of the same subspecies and STs. Strains IVIA 5387, DeDonno, CN28 and GP18 showed a typical S-shaped dose-effect curve, and the minimum infective dose in <i>N. benthamiana</i> was established at 300 UFC/plant for all strains. The plant response to the infection by the subsp. <i>pauca</i> ST53 strains DeDonno, CN28 and GP18 was studied in relation to the expression of defense genes. A strain-dependent modulation depending on time was observed, where CN28 infection showed the highest gene overexpression (12 out of 19 genes), and DeDonno the lowest (8 of 19 genes). At 4 dpi, all strains upregulated the genes <i>PR1</i>, <i>PR1a</i> and <i>ERF1</i> and downregulated the <i>PDF1.2</i> gene, while at 30 dpi most of the genes were downregulated, especially the pathogenesis related (PR) genes, suggesting an immune evasion by the pathogen. Our findings are expected to provide valuable insights into the interaction between <i>X. fastidiosa</i> and its hosts, highlighting the importance of considering differences in aggressiveness among strains and in plant response.