Identification of a Novel Casbane-Type Diterpene Phytoalexin, ent-10-Oxodepressin, from Rice Leaves

A 70% methanol extract of UV-irradiated rice leaves (400 g) was separated by chromatographic methods to give UV-induced compound 1 (2.1 mg) which showed a possible molecular ion at m/z 300 in the GC/MS analysis. Its structure was determined by NMR and MS methods. The 1H- and 13C-NMR spectra of 1 were identical to those of 10-oxodepressin (2), a casbane-type diterpene derived from the soft coral, Sinularia depressa. The specific rotation of 1 was positive, whereas that of 2 was negative. We therefore established 1 as ent-10-oxodepressin. The accumulation of 1 was also induced by an inoculation of the rice blast fungus. Compound 1 inhibited spore germination (IC50 30 ppm) and germ tube growth (IC50 10 ppm) of the rice blast fungus. We thus concluded that 1 was a novel rice phytoalexin.