Endophytic microbial communities contribute significantly to plant health. If this is widely accepted, question remains on the mechanisms underlying such interactions. There is a growing interest for endophytic bacteria for diverse practical applications including potential use in agriculture. Unfortunately, because of the lack of knowledge about the endophyte/host-plat interaction, their use is limited. These organisms can be used to improve plant productivity and tolerance against abiotic and biotic stresses. Endophytes can play diverse roles: control of pathogens and stresses through antimicrobial compound production, protection against herbivores, increase of resistance to abiotic stresses, detoxification of organic pollutants in planta and increasing the acquisition of nutrients such as atmospheric nitrogen fixation, phosphate solubilization, phytohormone production, and siderophore production. Although little studied, endophytes are known to be able to spread systemically within the plant and colonize stems and leaves. It is more difficult to specify and measure the various effects of different types of colonization on the plant. This poster aims to present the main known mechanisms involved in increasing plant resistance to biotic and abiotic stresses. Moreover, two thesis topics related to this theme are presented (1) the role of endophytic bacteria is questioned in the increase of osmotic stress tolerance of tomato plants due the addition of humic and fulvic acids; (2) how endophytic microorganisms interfere with phytopathogenic bacteria (Brenneria salicis and Xylella fastidiosa) colonizing the specific niche of the xylem