Dark septate endophytes (DSEs) are a polyphyletic assemblage of Ascomycetes that colonize plant roots and are originally characterized by the accumulation of high concentrations of melanin in their hyphae. It has been hypothesized that this trait could be advantageous to both partners in plant-DSE associations in response to a variety of biotic and abiotic stresses. However, evidence for the contribution of high melanization of DSEs to stress mitigation is still lacking. Secondly, we hypothesize that melanin plays a role in root penetration by hyphae and subsequent colonization, because melanization is analogously required for virulent fungal pathogens to successfully infect animal and plant tissues. In this French-German collaborative project, we aim to better understand the melanization process in the DSE model Leptodontidium sp., including the study of regulatory mechanisms modulating melanization. Moreover, complementary genetic, pharmacological, physico-chemical, physiological and omics approaches contributed by the Franco-German partners will be used to decipher the role that melanin might play in the competitiveness of Leptodontidium sp. for plant colonization and in the high tolerance of Leptodontidium sp. to a range of abiotic and biotic stresses. The consortium is composed of researchers from four laboratories having complementary expertise in microbiology, plant-microbe interactions under stress conditions, fungal ecology, multi-omics analyses and bioinformatics. Particular techniques and topics are genetic transformation of DSEs and atomic force microscopy (Université of Lorraine - P1), miRNA analyses and metal stress (Université de Bourgogne Franche-Comté - P2), epigenetics and RNAseq analysis (Friedrich Schiller University Jena - P3), and interactions between fungi and mycoparasites (Wismar University of Applied Sciences - P4). Therefore, answering the questions and testing the hypotheses concerning the role of melanin for DSEs and for DSE-plant interactions can only be achieved by the combined activities of the French-German team. Understanding the mechanisms that mitigate environmental stress for DSEs and for plants colonized by DSEs could contribute to the exploitation of this relevant fungal resource for the sustainable and economically meaningful production of crops that face increasing constraints, including the presence of mycophagous and plant-pathogenic organisms in the rhizosphere, exposure to contaminants, and climate change impacts such as drought and heat. Consequently, we also aim to ensure wide dissemination of the project results to the scientific community, society and stakeholders in agriculture, horticulture and forestry.