Towards a Better Understanding of Melanins from Dark Septate Endophytes (DSEs): Their Variability, Synthesis Pathways and Biological Roles

Dark septate endophytes (DSEs) are a diverse group of fungi that form mutualistic relationships with plant roots, especially under stressful conditions such as heavy metal exposure, water limitation, and exposure to xenobiotics. DSEs belong to the phylum Ascomycota, and mainly colonize the parenchymal tissue of plant roots either intra- or intercellularly. These fungi synthesize dark pigments called melanins, which are considered to be important in their ability to tolerate extreme or toxic environmental conditions. The aim of this study was to gather information on the current state of knowledge regarding melanins from DSE fungi. While most DSEs synthesize 1,8-dihydroxynaphthalene (DHN)-melanin, some can also synthesize other types of melanin such as 3,4-dihydroxyphenylalanine (DOPA)-melanin, pyomelanin, and heterogeneous ones. Additionally, the genome of some DSEs contains genes that are involved in various melanin synthesis pathways. Understanding the diversity and characteristics of melanins from DSE fungi and their main melanization pathways could contribute to a better understanding of the role of melanins in these mutualistic fungi. The chemical nature and function of many of the dark pigments synthesized by these fungi are not yet known, so further research is needed to elucidate their contribution to stress tolerance and their impact on the interaction of DSE fungi with various plants of agro-forestry importance.