Comparative transcriptomics of biopesticide-treated olives subjected to Xylella infection

The management of Xylella fastidiosa infections relies on integrated strategies of controlling vector populations and removing inoculum sources. The impact of a subspecies pauca strain of this bacterium (XfDD) was particularly severe in olive in Apulia, Italy, where an entire economy was affected, and a territory was devastated. There is an urgent request for strategies to combat the pathogen in planta, as well as for bioinsecticides targeting its vector insect species, and that these should conform to criteria of environmental acceptance. Therefore, this study analyses the Mechanisms of Action (MoA) of three novel biopesticides for pest management of XfDD in olive.

In vitro assays showed that the Onion extract (Onion) inhibits the growth of XfDD while the bacterium Paraburkholderia phytofirmans strain PsJN (PsJN) did not. Using a time course transcriptomics approach, the molecular response of olive to the biopesticides PsJN and Onion, aiming to control XfDD, and to the natural insecticide Sankari^© were observed. As early as 6 to 12 h after the application of PsJN, Onion or Sankari^©, olives showed altered gene expression patterns, which decayed after 15 days. All olives, successively inoculated with XfDD, were analysed 10 days later. All three biopesticides induced an early immune response, including genes related to the control of hormone levels or membrane-associated kinase. In addition, PsJN also modulated ethylene levels, thus showing a potential promotion of plant growth. The expression of a set of early induced genes, monitored by quantitative RT-PCR, confirmed both, the PsJN and Sankari^© transcriptomic response. Moreover, XfDD inoculation stimulated a more intense transcriptomic response in treated compared to untreated and inoculated olives, indicating that the biopesticides induced a priming response.

Dissecting the MoA of a potential biopesticide complements the evaluation of its efficacy in controlling the pathogen. In the case of XfDD, this assessment requires extensive field trials due to the prolonged period before symptoms appear in infected olives and the occurrence of repeated inoculations under natural conditions.