Fusariumic Acids I and J, Two New Phytotoxic Isocassadiene-Type Diterpenoids from Tomato Fusarium Crown and Root Rot Pathogen Fusarium oxysporum f. sp. radicis-lycopersici.

Fusarium oxysporum f. sp. radicis-lycopersici (Forl) is the etiological agent of tomato Fusarium crown and root rot (FCRR), a devastating soil-borne disease that severely compromises global tomato production. The pathogenicity of Forl has been increasingly linked to its capacity to produce phytotoxic isocassadiene-type diterpenoids. In this study, Forl was cultured in rice medium to obtain Forl cultures, which were used for the separation and identification of secondary metabolites. After removing the known metabolites, two new isocassadiene-type diterpenoid compounds, namely fusariumic acids I (1) and J (2), were isolated from the ethyl acetate extract. Their structures were identified using spectroscopic data analyses and quantum chemical calculations. This is the first report of the fusariumic acid analogs containing a hydroxyl group at position C-1 in the molecule. Fusariumic acids I (1) and J (2) exhibited significantly inhibitory activities on the hypocotyl elongation of tomato (Solanum lycopersicum) and sesame (Sesamum indicum) seedlings, as well as on the coleoptile elongation of rice (Oryza sativa var. japonica) seedlings at concentrations from 10 to 100 µg/mL. The discovery of two new phytotoxic isocassadiene-type diterpenoids expanded the diversity of secondary metabolites of Forl. Meanwhile, it provided critical insights into Forl-tomato interactions and the candidate lead compounds for the development of new herbicides as well.