Dark septate endophytes (DSEs) are common, facultative root colonizers of flexible lifestyle, acting as endophytes, free-living saprophytes, parasites, or pathogens, depending on plant and fungal genotype, and environmental conditions. This study represents the first assessment of DSE biodiversity in a polycyclic aromatic hydrocarbon (PAH)-contaminated environment, their PAH-tolerance, catabolic features, and potential to protect a host exposed to PAHs. Metabarcoding analysis and the assessment of DSE colonization of Poa trivialis and Phragmites australis roots revealed that PAH contamination decreased DSE abundance, whereas these fungi were dominant in plants at the uncontaminated site. DSEs isolated from both sites were examined for their PAH-tolerance and saprophytic features. Two isolates of Paraphoma chrysanthemicola (PCH) showing high PAH tolerance and organic nitrogen catabolic features were selected to inoculate Lolium perenne L. Plants and fungi were cultured in vitro, using MSR medium without sugar, and in pots, using sand/bentonite substrate supplemented with organic CPN. Both PCH isolates mitigated PAH toxicity and significantly improved plant growth in pot cultures, while only one of the isolates developed positive interactions with plants in the contaminated MSR medium. In the absence of PAHs, both PCH isolates had no effect on plants in pots, and they negatively affected plants in MSR medium. Obtained results demonstrate that mutualistic plant-DSE interactions require organic CPN supplementation and toxicity stress.
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