Green Foxtail-Derived Endophytic Fungus Chaetomium novozelandicum SV7 Produced Orsellinic Acid Ester Glycosides Active Against Rice Blast Pathogen

Plants encounter various fungi during their life history, with some of them pathogenic, some beneficial, and some neither, forming complex plant–fungi interactions. Endophytic fungi are a type of fungi that live inside the plant, and are often beneficial to the host. In this study, the endophytic fungi of green foxtail (Setaria viridis) were explored for their defensive role against the pathogenic fungus Magnaporthe oryzae, which causes rice blast, one of the most devastating crop diseases. The endophytic fungi were isolated, taxonomically identified, fermented, and chemically extracted. The fungal extracts were screened for their inhibition against M. oryzae using mycelial growth rate assay, of these, Chaetomium novozelandicum SV7 displayed the most inhibition. A scale-up fermentation of this fungus, followed by extraction and chromatographic purification, led to the isolation and identification of eleven orsellinate-types of metabolites (1–11), including four new compounds (1, 2, 6, 8), named chaetorsellide A-D. The structures of the new compounds were elucidated by means of NMR, HRMS, and quantum chemical calculations. All these metabolites were reported from C. novozelandicum for the first time, with five of them being orsellinic acid ester glycosides (1–5). Plant infection assay was carried out for the isolated compounds, among which compounds 8 and 3 displayed significant activities, with 8 being the most potent. These results not only reveal that these compounds have potential to be developed as natural fungicides against the rice blast pathogen, but also add one more example for exploring new antimicrobial substances from plant–microbe interactions.