Despite their genetic adaptation to local conditions, plants often achieve ecological success through symbiotic associations with fungal endophytes. However, the habitat-specific functionality of these interactions and their potential to drive plant adaptation to new environments remain uncertain. In this study, we tested this using the vascular flora of the Antarctic tundra (Colobanthus quitensis and Deschampsia antarctica), an extreme environment where fungal endophytes are known for playing important ecological roles. After characterizing the root-associated fungal endophyte communities of both species in two distinct Antarctic terrestrial habitats-hill and coast-we experimentally assessed the contribution of fungal endophytes to plant adaptation in each habitat. The field reciprocal transplant experiment involved removing endophytes from a set of plants and crossing symbiotic status (with and without endophytes) with habitat for both species, aiming to assess plant performance and fitness. The diversity of root fungal endophytes was similar between habitats and mainly explained by plant species, although habitat-specific endophyte community structures were identified in D. antarctica. Endophytes significantly influenced C. quitensis homeostatic regulation, including oxidative stress and osmotic control, as well as plant fitness in both environments. By contrast, the effect of endophytes on D. antarctica was particularly evident in coastal sites, suggesting an endophyte-mediated improvement in local adaptation. Altogether, our results suggest that the two Antarctic vascular plant species follow different strategies in recruiting and developing functional symbiosis with root-associated fungal communities. While C. quitensis is more generalist, D. antarctica establishes specific interactions with habitat-specific microbial symbionts, predominantly in the most stressful environmental context.
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