Other potential antimicrobial bioactivity metabolites
According to the Green et al. (2020) study on E. festucae -L. perenne symbiont, Epichloëcyclins (A-E) were the most abundant class of DMs between symbionts and non-symbionts. Epichloëcyclins (cyclic peptides) were composed of lysine, tyrosine, proline, methionine, phenylalanine, and arginine and might have a bioactive role (Johnson et al. 2015). As epichloëcyclins information was not obtained in our metabolite database, we did not detect epichloëcyclins in EI plants in this study. However, endophytes induced the accumulation of lysine, arginine, and proline, the main constitutes of epichloëcyclins. Therefore, Epichloëcyclins might occur in A. sibiricum and possess antimicrobial activities. Moreover, Aspergillomarasmine A and 4-Methyl-5-thiazoleethanol were highly accumulated in the endophyte-infected host. Aspergillomarasmine A is a rapid and effective enzyme inhibitor that restores the sensitivity of carbapenems to resist bacteria (Meziane-Cherif and Courvalin, 2014). Thiazoles with fungicidal effects have been widely used in disease prevention and crops control (Cui et al., 2012). Therefore, Aspergillomarasmine A and 4-methyl-5-thiazoleethanol might play a role in the endophyte-associated pathogen resistance of the host.
In summary, We first reported thatE. sibirica endophytes activated the ISR in the host by JA and ET signaling pathways which might play an essential role in enhancing host disease resistance. In addition, we discovered that the antimicrobial phenolic metabolites and potential bioactive metabolites were accumulated in large quantities in endophyte-infected leaves. So far, achievements have been made in breeding Epichloë -grass symbionts that do not produce toxic alkaloids to mammals but have toxic alkaloids to insect herbivores (Bharadwaj et al., 2020). Once the metabolites responsible for pathogen resistance are further tested, the endophyte-grass symbionts will become crucial resources of biological pesticides.