The beneficial effect of Epichloë endophytes on the growth of host grasses was affected by arbuscular mycorrhizal fungi, pathogenic fungi and nitrogen addition

Native grasslands are often limited by nitrogen, so even a low level of nitrogen deposition may significantly impact the growth of plants as well as plant-associated microbes. Epichloë endophytes inhabit the aboveground tissues of healthy plants, and their beneficial effects on the growth of host grasses have been widely reported. In this study, the abundance of the endophyte and its effect on the host Achnatherum sibiricum when cooccurring with arbuscular mycorrhizal fungi (AMF) or pathogens was investigated with increasing nitrogen supply. The results demonstrated that endophyte abundance increased significantly with increasing leaf nitrogen concentration and plant biomass under low nitrogen conditions, but there was no such correlation for AMF or pathogen abundance. The individual effect of endophytes on plant performance was positive under both low (P = 0.002) and high (P = 0.013) nitrogen conditions; only the promotion degree was even greater under low nitrogen conditions (P = 0.004). When endophytes and AMF were both present, the combined effects of the two symbionts on plant growth were antagonistic under low nitrogen levels (P = 0.048), while the combined effects of the two symbionts on plant growth were independent under high nitrogen levels (P = 0.109). When endophytes and pathogens were both present, the net effect on plants was greater than the sum of the separate effects of the two microbes and was not affected by environmental nitrogen. Endophytes could inhibit pathogen abundance by increasing total phenolic content of the host A. sibiricum. Overall, the impact of endophytes on host plants was affected not only by abiotic environmental nitrogen but also by biotic factors AMF and pathogens. Therefore, clarifying the impact factors of endophytes on host growth is important for predicting the population dynamics and community diversity of host plants.