From Darkness to Light: Emergence of the Mysterious Dark Septate Endophytes in Plant Growth Promotion and Stress Alleviation

Dark septate endophytes (DSEs) are ascomycetous fungi whose structure is characterised by dark melanised hyphae and microsclerotia located in plant roots. Associations with DSEs are commonly found in various biomes and plant taxa. Although DSEs are commonly recorded, the effects of their colonisation on plant growth and fitness are unclear. This chapter summarises the state of knowledge about DSEs from the literature and personal data. The effects of DSEs on plant growth range from parasitism to mutualism. They can promote plant growth by improving nutrition (e.g. solubilisation of minerals, degradation of complex carbon compounds), producing secondary metabolites (e.g. phytohormones, volatile organic compounds) and protecting against phytopathogens. More particularly, the high tolerance of DSEs to abiotic stress and their relatively high abundance in trace element-contaminated and other stressful habitats suggest that they may have an important function for host survival under these conditions. Finally, this chapter outlines why additional research is required in the emerging field of plant–DSE interactions to address future challenges.