Extraction and Identification of the Bioactive Metabolites Produced by Curvularia inaequalis, an Endophytic Fungus Collected in Iran from Echium khuzistanicum Mozaff

Endophytic fungi (EF) are microorganisms that colonize the internal tissues of host plants, providing a range of benefits to them. In this symbiosis, they act as a reservoir of bioactive metabolites that are important for enhancing the host’s defense mechanisms as a resistance against pathogens. These molecules usually possess antimicrobial properties that can be exploited for application in agriculture and medicine. In this context, the current work was designed to evaluate the phytotoxic and antimicrobial properties of the endophytic fungus Curvularia inaequalis, isolated for the first time from the Iranian medicinal plant Echium khuzistanicum. Culture filtrates, their organic extracts, and isolated metabolites were tested against a series of plants to assess their phytotoxicity, as well as against a wide range of plant and human pathogens to evaluate their antimicrobial activity. The main compounds characterizing the organic extract of C. inaequalis have been identified as (R)-phomalactone, catenioblin A, and (-) asperpentyn (1–3) by using spectroscopic techniques, NMR mainly, and HR-ESI-MS. In the bioactivity evaluation carried out in this study, (R)-phomalactone (1) stood out as the most promising compound, exhibiting significant non-host phytotoxic activity on tomato leaves; potent antibacterial activity against a wide range of human pathogens, including methicillin-resistant Staphylococcus aureus (MRSA) strains; and marked antifungal activity against several economically important phytopathogens. (–)-Asperpentyn (3) also showed robust and selective antifungal activity against phytopathogens, while catenioblin A (2) exhibited only a slight phytotoxic effect and limited overall bioactivity in this study. These findings reveal that the isolated endophytic fungi hold considerable promise as an untapped source of bioactive metabolites with antibacterial, antifungal, and phytotoxic activities.