The decreasing trend of the competitive advantage of endophyte-infected Achnatherum sibiricum over endophyte-free plants under high nitrogen conditions was reversed by pathogenic fungi inoculation

Background and aims According to the nitrogen-disease hypothesis, plant diseases will become more serious with the aggravation of global nitrogen (N) deposition. Numerous studies have indicated that Epichloë endophytes can enhance host plant resistance to pathogens. It is unclear how the competitive ability of endophyte-infected (EI) over endophyte-free (EF) plants changes under the interference of N deposition and plant disease. Methods In this study, Achnatherum sibiricum , native to the Inner Mongolia steppe of China, was used as experimental material. We experimentally manipulated N addition and pathogen inoculation and examined the growth and competition between EI and EF A. sibiricum . Results The results showed that EI plants had a greater competitive advantage than EF plants under low N conditions, and this advantage decreased with the N supply. When high N and pathogens were both present, pathogen inoculation reversed the adverse effects of high N supply on the competitive advantage of EI A. sibiricum . Epichloë endophytes not only reduced the disease of EI plants but also reduced the disease of neighboring EF plants. Meanwhile, Epichloë endophytes changed the response of the host disease to N. The disease index of EF plants increased with increasing leaf N content, while that of EI plants did not change. Conclusion This study highlights that under the dual factors of N deposition and pathogen infection, endophytic fungi improve the competitive ability of host plants. Our results show that considering both biotic and abiotic factors is crucial for predicting the dominance of plant-fungal symbionts in the community.