Insights on Paraburkholderia phytofirmans PsJN behaviour as biocontrol agent of Xylella fastidiosa in olive

Numerous research efforts are currently underway to identify novel biocontrol agentsfor the management of diseases associated with Xylella fastidiosa. Despite the encouraging premises, most of these studies proved unable to provide long-term effects or failed once they were examined at the field-scale. An intriguing exception was that of the rhizosphere-colonizing bacterium Paraburkholderia phytofirmans PsJN, which appears efficacious to suppress Pierce’s Disease development when inoculated in grapevine infected by Xf ‘Temecula’. Having verified that PsJN did not exhibit any antagonistic in vitro effect towards Xf ‘De Donno’, we wanted to determine if it might colonize and protect olive from OQDS progress. We tested different inoculation approaches, before concluding that pin-prick method ensures the highest success rate. Motile populations of PsJN could be consistently detected distant from the inoculation point, with a newly developed SYBRÒ ­Green qPCR assay, and their long-term viability was demonstrated by successful isolation on TSA medium. We set field trials in the Apulian area affected by OQDS to investigate prophylactic and therapeutic effects of PsJN inoculation in plants naturally exposed to Xf infection. Current observations, limited to a single season, did not evidence any statistical correlation between the population dynamics of Xf and PsJN, but still, require validation on a larger time-frame. Although we collected evidence that PsJN could multiply and move in olive xylem vessels, we also observed that its concentration decreased significantly over time, suggestive of a plant response that limits PsJN expansion. We attempted a WGS metagenomic study to further investigate if PsJN might prime a host response in turn leading to a knock-on effect on the population indices of Xf and the residential microbial communities