Warming reduces fungal endophyte dependency for establishment but amplifies stress-response gene expression in the...

Fungal endophytes transmitted through seeds are recognized as key drivers of plant invasion success, yet whether their role persists or shifts under climate warming remains poorly understood. Here we address this question in Poa annua, the only non-native vascular plant with established self-sustaining populations in Maritime Antarctica, using a factorial experiment on King George Island. Combining open-top chamber warming (OTC⁺) and endophyte manipulation (E⁺ vs. E^-), we assessed germination, seedling survival, and expression of two stress-response genes (HSP and LEA1) in this invasive grass. Germination was unaffected by either treatment, while survival was independently improved by both endophyte presence and warming, but their effects did not interact, suggesting that thermal amelioration may reduce reliance on endophyte-mediated buffering for establishment. Under warming, however, endophytes strongly amplified HSP expression through a significant OTC × E interaction, indicating conditional molecular priming of the heat-stress response. LEA1 expression showed the additive effects of both factors. These results suggest that as Antarctic temperatures rise, P. annua may progressively decouple from its seed-borne fungal endophytes for establishment while retaining endophyte-amplified transcriptional responses to thermal and hydric stress.