Abstract Background Epichloë endophytes form beneficial symbioses with cool-season grasses, enhancing host tolerance to abiotic stresses such as drought while maintaining normal plant growth. However, the molecular mechanisms underpinning this symbiosis, particularly the role of fungal-secreted protein, remain largely unexplored. Results In this study, we identify EgSPE, a secreted protein from Epichloë gansuensis , as a key regulator of symbiotic establishment and host drought adaptation in drunken horse grass ( Achnatherum inebrians ). Transcriptome profiling during host colonization revealed EgSPE as a strongly induced gene encoding a secreted protein. Functional characterization facilitated by a substantially improved transformation system demonstrates that EgSPE is critical for fungal growth and efficient host colonization, as its deletion severely disrupted symbiotic establishment. Notably, EgSPE activates the host drought-responsive signaling by inducing the marker gene RD29A in a heterologous system ( Nicotiana benthamiana ) and upregulating stress-related genes ( AiRD22 , AiNAC5 , and AiABA1 ) in its native host ( A. inebrians ). Consistently, only the E. gansuensis wild-type and OE- EgSPE strains enhanced host drought resistance, whereas the Δ egspe mutants failed to confer this benefit. Conclusions In summary, our research findings identify EgSPE as a fungal protein that plays an important role in the establishment of symbiosis and in the host’s drought response, providing strong evidence for how E. gansuensis promotes abiotic stress tolerance in grasses.
Zhang et al. (2026) studied this question.