In a previous DNA-based microbiome study of grapevines in areas of Pierce’s Disease (PD) pressure, we determined that taxa belonging to the Pseudomonas and Achromobacter genera negatively associated with PD severity and titer of the causal agent, Xylella fastidiosa, leading us to hypothesize that these taxa suppress PD and could be deployed as biocontrols. Here we tested this hypothesis using two bacterial isolates from the grapevine endosphere, Pseudomonas viridiflava and Achromobacter vitis. We demonstrate that pre-treatment with these two isolates significantly reduced PD symptoms and X. fastidiosa titer, comparable to that of a known PD biocontrol agent, Paraburkholderia phytofirmans PsJN. We monitored early spatial transcriptional responses using genome-wide transcriptional profiling in vines that were pre-treated with the biocontrol strains and then inoculated with X. fastidiosa. We coupled this with phenotyping of internal tylose development and external disease symptoms and bacterial titer and determined that grapevines pre-treated with the biocontrols A. vitis and PsJN developed fewer tyloses and PD symptoms and underwent major transcriptional reprogramming in response to X. fastidiosa. These included up-regulation of genes in auxin- and ethylene-signaling pathways linked to tylose development. In contrast, P. viridiflava pre-treatment also resulted in a reduction of tyloses and PD symptoms but did not induce major transcriptional changes in vines, suggesting it likely has a direct inhibitory effect to X. fastidiosa through antibiosis. Using this data, we propose a model that incorporates timely and effective deployment of tyloses driven by induction of ethylene and auxin pathways as a key factor in PD resistance.
Deyett et al. (Thu,) studied this question.