Interactions between xylem-specialized phytopathogenic bacteria and the endophyte community occupying it - The case of Brenneria salicis and Xylella fastidiosa in Salix and Populus genera

Endophytic microbial communities contribute significantly to plant health. If this is widely accepted, the question remains on the mechanisms underlying such protection. This project aims to assess how endophytic microorganisms interfere with phytopathogenic bacteria colonizing the specific niche of the xylem. As a general objective, we propose to study the interactions between two phytopathogenic xylem-specialist bacteria (Brenneria salicis and Xylella fastidiosa) and endophytic communities in the Salix and Populus genera. In the last decades, public awareness of environmental issues, such as air and water pollution, global climate change and soil erosion, has led to a return to the cultivation of willows and poplars. Indeed, particularly in Belgium, Salix and Populus play an important role in the phytoremediation of degraded sites, in the fight against erosion and desertification of fragile ecosystems, and in the restoration of forest landscapes Understanding and modelling the modus operandi of pathogens interaction with the endophytic microbial communities is therefore a major phytosanitary challenge, and will constitute a major breakthrough in the fight against epidemics possibly affecting willows and poplars. B. salicis and X. fastidiosa have caused and still cause much damage. X. fastidiosa spread in the 1800's in America by ravaging crops such as grapevines and citrus. It has more recently established itself in Europe (in Italy, Spain, Portugal and France) where it ravages notably olive trees. B. salicis settled in Belgium in 1980 and has also spread to England and Japan causing ecological and economic damage. However, although pathogenic most of the time, these two bacteria can be found in a latent state where in spite of the infection of their hosts, these do not develop symptoms. No environmental or genetic factors explain this symptomatic-asymptomatic duality. The interactions between these two phytopathogenic bacteria and the endophytes present in the xylem is the focus of our current explanatory hypothesis. After a screening phase by isolation the endophytic communities colonizing the xylem of Salix and Populus genera, interactions with phytopathogenic bacteria such as X. fastidiosa and B. salicis have been looked at, to better unravel the relationships between woody plant endophytes and xylem-specialized pathogenic bacteria of the different pathosystems. Several interactions are studied, such as the relationship between pathogenic bacteria and their hosts, the competition between the two phytopathogenic bacteria in the same host, and the relationship between pathogenic bacteria and endophytes. Understanding microbial interactions in the xylem might also help controlling major bacterial diseases