Xylem Core Endophytome and Its Role in Bacterial Diseases of Salicaceae: Interactions with Xylella fastidiosa and Brenneria salicis and Potential for Dysbiosis Mitigation.

Xylella fastidiosa and Brenneria salicis are xylem-limited pathogens responsible for vascular diseases in Salicaceae, yet infected plants often show contrasting outcomes ranging from severe decline to complete asymptomatic persistence. The underlying drivers of this variability remain unclear, but increasing evidence points to the critical role of the xylem core endophytome in modulating disease expression. In this study, we explore the interplay between endogenous xylem-associated bacterial endophytes and these two pathogens, focusing on their capacity to inhibit pathogen colonization and reduce xylem dysbiosis. A total of over 400 cultivable endophytic strains were isolated from willow and poplar species, complemented by metagenomic amplification to refine microbiome profiling. Through in vitro confrontation assays against X. fastidiosa KLN59.3 and B. salicis LMG2698, we identified several antagonistic endophytes with promising biocontrol potential. Selected isolates were subjected to genome sequencing and phenotypic characterization, including transformation with fluorescent markers to enable in planta tracking of colonization and interactions within xylem tissues. Co-inoculation experiments in Salicaceae and Nicotiana tabacum models further elucidate the dynamics between endophytes and pathogens in planta. Our findings highlight the functional role of the xylem core endophytome in shaping host-pathogen outcomes and support its potential application in sustainable disease management strategies targeting vascular pathogens in woody plants.