Phytotoxic activity of grammicin produced by the fungus Xylaria grammica (Ascomycota) recovered from infected lesions on Handroanthus serratifolius (Bignoniaceae) leaves

BACKGROUND We report the bioassay‐guided isolation and identification of phytotoxic compounds from an extract of the fungus Xylaria grammica (Ascomycota), isolated from leaf lesions of seedlings of Handroanthus serratifolius (Bignoniaceae), from the Atlantic Rain Forest in Brazil. From the fungal crude extract, grammicin (1) and xylaric acid methyl ester (2) were purified and identified using one‐ and two‐dimensional nuclear magnetic resonance and high‐resolution mass spectrometry analysis. RESULTS Grammicin exhibited activity against Lactuca sativa (lettuce) and Agrostis stolonifera (bentgrass) at 1 mg mL −1 , resulting in 100% inhibition of seed germination. Compound 2 had no activity against L. sativa but showed moderate activity against A. stolonifera , inhibiting seed germination at 1 mg mL −1 . Against Lemna paucicostata (duckweed), grammicin inhibited growth by 50% (IC 50 ) at 87.7 μM, while compound 2 had no activity. Neither compound had antifungal activity against the plant pathogen Colletotrichum fragariae . Grammicin is structurally similar to the natural compound patulin, which is reported to be phytotoxic by inhibition of photosystem II (PSII). Molecular modeling of the interaction of grammicin with the D1 protein of PSII predicts it to be a strong PSII inhibitor; however, in vivo testing indicated that it is a very weak PSII inhibitor. Patulin was highly cytotoxic to mammalian cells whereas grammicin was not cytotoxic. CONCLUSIONS This is the first report unveiling the phytotoxicity of grammicin, which could serve as a scaffold for developing more potent herbicides with improved physicochemical properties and without any potential for cytotoxicity.