A Methyltransferase for Synthesis of the Flavanone Phytoalexin Sakuranetin in Rice Leaves

Rice (Oryza sativaL.) leaves irradiated with short wave UV light accumulated the major rice phytoalexin, flavanone sakuranetin. The extracts from these leaves catalyzed the methylation of the hydroxyl group at position 7 of naringenin to yield sakuranetin, withS-adenosyl-l-methionine as the methyl donor. Activity of the naringenin 7-O-methyltransferase was not found in healthy rice leaves but increased upon irradiation with UV light in parallel with increase in sakuranetin. The autoradiogram of the enzymatic product on 2D-TLC was found to be sakuranetin. The enzyme did not methylate sakuranetin, but methylated other flavanones, but not isoflavanones.