Bioactive Bis-naphtho-γ-pyrones from Rice False Smut Pathogen Ustilaginoidea virens

Abstract

Ustilaginoidins were bis-naphtho-γ-pyrones mycotoxins possessing an a<i>R</i> configuration of the chiral axis previously reported from the false smut balls of rice infected by the fungal pathogen Ustilaginoidea virens. To investigate the chemical diversity of these metabolites and their bioactivities, we fermented this fungus on solid rice media, which afforded the isolation of 13 ustilaginoidins, including seven new compounds, namely ustilaginoidins K–P, <b>1</b>–<b>6</b>, and E₁, <b>7</b>, together with the known ustilaginoidins A, <b>8</b>, D, <b>9</b>, E, <b>10</b>, F, <b>11</b>, and G, <b>12</b>, and isochaetochromin B₂, <b>13</b>. The structures of the new compounds were elucidated by using (1D, 2D) NMR, high-resolution mass spectrometry, UV, and circular dichroism, as well as by comparison with the literature data. A plausible biosynthesis pathway was proposed for these dimeric polyketides. The isolated compounds were evaluated for their antibacterial, cytotoxic, and radicle elongation inhibitory activities. Ustilaginoidins K, <b>1</b> and L, <b>2</b> showed cytotoxic activities on the A2780 human ovarian cancer cell line with IC₅₀ values of 4.18 and 7.26 μM, respectively. Ustilaginoidins N, <b>4</b>, D, <b>9</b>, E, <b>10</b>, and G, <b>12</b> were active against the tested pathogenic bacteria with MIC values in the range of 16–64 μg/mL. Ustilaginoidins O, <b>5</b>, E, <b>10</b>, and F, <b>11</b>, and isochaetochromin B_2, <b>13</b> displayed moderate inhibitory activity on the radicle elongation of rice seeds