Assessing the role of Salicaceae xylem endophytes on the establishment and progression of Xylella fastidiosa or Brenneria salicis

Xylella fastidiosa and Brenneria salicis are xylem-restricted bacterial pathogens responsible for devastating diseases in a wide range of host plants, including key members of the Salicaceae family. Yet, a puzzling phenomenon persists: some infected plants remain symptomless. The drivers behind this contrasting symptomatology remain largely unknown, but increasing evidence points to the resident xylem microbiota as a key player influencing pathogen establishment and disease progression. In this study, we explore the functional interplay between xylem-associated endophytes and vascular pathogens, aiming at identifying microbial interactions that may suppress infection and support plant health. From Salicaceae hosts, we isolated over 400 endophytic bacterial strains, complemented with targeted metagenomic amplification to better capture the xylem microbiome’s composition and diversity. Initial confrontation assays revealed a subset of isolates capable of antagonizing either X. fastidiosa strain KLN59.3 or B. salicis LMG2698 in vitro. These strains were selected for whole-genome sequencing and further phenotypic characterization. Several are currently being engineered for fluorescent proteins expression, facilitating direct visualization of their spatial dynamics, colonization behaviour, and interactions with pathogens within plant xylem vessels. Co-inoculation experiments in Populus, Salix, and Nicotiana tabacum hosts are being used to validate in planta antagonistic effects and unravel potential protective mechanisms. By integrating classical microbiology, plant pathology, genomics, and imaging tools, our work contributes to a better understanding of plant-microbe-pathogen tripartite interactions within the xylem niche. These findings highlight promising candidates for the development of endophyte-based biocontrol strategies against xylem-inhabiting pathogens, and more broadly, emphasize the importance of microbiome-informed approaches for sustainable disease management.