Decoding dark septate endophyte symbiosis: a functional and ecological perspective.

Plant root microbial symbioses influence plant performance and ecosystem processes. Among these, dark septate endophyte (DSE) fungi are ubiquitous, non-mycorrhizal root endophytes with broad host ranges that facilitate nutrient acquisition, mitigate biotic and abiotic stress, and elicit positive, neutral, or negative plant growth responses depending on host and environmental context. Comparative genomic and transcriptomic analyses provide a robust framework for evaluating DSE lifestyles. Beyond their direct effects on plants, fungal endophytes contribute to soil microbiome networks, soil health, regenerative agriculture, and One Health. Integrating comparative genomics, transcriptomics, microbiome ecology, soil health, and agricultural studies, we present a systems-level synthesis of DSE biology that identifies key knowledge gaps and advances our understanding of DSE ecology and function. Comparative genomic, transcriptomic, and ecological evidence indicates that DSE share molecular and functional characteristics with ericoid mycorrhizal fungi, consistent with a hypothesized transition from saprotrophic toward specialized root-associated lifestyles. Their melanin production, extensive CAZyme repertoires, and broad ecological distribution suggest important roles in microbiome assembly, nutrient cycling, carbon dynamics, soil aggregation, and plant stress tolerance. We conclude by presenting an integrated framework linking plant-soil-fungus-environment interactions that defines core DSE competencies and highlights their importance for ecosystem functioning and sustainable agriculture.