New Ustilaginoidins from Rice False Smut Balls Caused by <i>Villosiclava virens</i> and Their Phytotoxic and Cytotoxic Activities

Abstract

Ustilaginoidins are a class of bis-naphtho-γ-pyrones, typically produced by Villosiclava virens, the pathogen of the rice false smut (RFS), which has been one of the most destructive rice fungal diseases. Previously, we found that ustilaginoidins identified from the culture of V. virens on rice medium were less polar than those reported from the RFS balls in general. In this study, we reinvestigated the high-performance liquid chromatography with diode array detection and high-resolution mass spectrometry (HPLC-DAD-HRMS) profile of the ethyl acetate (EtOAc) extract of the RFS balls and found several interesting peaks that correspond to new ustilaginoidins. As a result, eight new and polar congeners, named ustilaginoidins Q-T (1-4), 2,3-dihydroustilaginoidin T (5), and ustilaginoidins U-W (6-8), were isolated. In addition, 17 known ustilaginoidins, including ustilaginoidins K-N (9-12), ustilaginoidin P (13), ustilaginoidin E₁ (14), isochaetochromin B₂ (15), and ustilaginoidins A-J (16-25), were re-isolated. The structures of the new compounds were elucidated by comprehensive analysis of the spectroscopic data. Ustilaginoidins Q (1) and R (2) feature an uncommon 2-hydroxypropyl-substituted skeleton and biogenetically incorporate one more acetate unit than common ustilaginoidins. Ustilaginoidin W (8) is a rare formate-containing bis-naphtho-γ-pyrone. Ustilaginoidins R (2), U (6), B (17), and I (24) showed moderate inhibitory activities toward the radicle or germ elongation of rice seeds. Ustilaginoidins R (2), S (3), V (7), W (8), B (17), C (18), and H-J (23-25) were cytotoxic to the tested human cancer cell lines (HCT116, NCI-H1650, BGC823, Daoy, and HepG2), with IC₅₀ values in the range of 4.06-44.1 μM.