Diversity and characterization of Cd-tolerant dark septate endophytes (DSEs) associated with the roots of Nepal alder (Alnus nepalensis) in a metal mine tailing of southwest China

Nepal alder (Alnus nepalensis), being an ideal and fast-growing pioneer species to accelerate ecological restoration of the degraded habitats in eastern Himalayas, have received extensive attention. In the present study, colonization characteristics, diversity and cadmium tolerance of dark septate endophytes (DSEs) colonizing the roots of Nepal alders were assessed along environmental gradients in a Pb, Zn, and Cd mine tailing, southwestern China. All Nepal alder roots surveyed were conspicuously colonized by DSEs with 23.1-35.8% colonization intensity. In total, 72 culturable strains of melanized root-associated fungi were isolated from the 45 root samples. Most strains were closely related to well-known DSE fungi, such as fungi belonging to the genera Phialophora, Leptodontidium, Cladosporium, and Exophiala. DSEs from heavily metal-polluted sites showed a higher Cd2+ tolerance in vitro than those from the slightly polluted plots. These findings stressed that DSE was an integral component of the root system of Nepal alder growing in metal-contaminated soils, and it is more feasible to obtain heavy metal (HM) tolerant DSEs from mining areas than from no HM-polluted environments for their potential application in phytoremediation. We concluded with a discussion of further research for addressing the unresolved roles of DSEs in A. nepalensis restoration during the early stage of plant establishment in the freshly metal-exposed mining soils.