Sakuranetin, the major
flavonoid phytoalexin in rice, is induced by ultraviolet (UV) irradiation, CuCl(2) treatment,
jasmonic acid treatment, and
infection by phytopathogens. It was recently demonstrated that
sakuranetin has anti-inflammatory activity, anti-mutagenic activity, anti-pathogenic activities against Helicobacter pylori, Leishmania, and Trypanosoma and contributes to the maintenance of
glucose homeostasis in animals. Thus,
sakuranetin is a useful compound as a plant
antibiotic and a potential
pharmaceutical agent.
Sakuranetin is biosynthesized from
naringenin by
naringenin 7-O-methyltransferase (NOMT). In previous research, rice NOMT (OsNOMT) was purified to apparent homogeneity from UV-treated wild-type rice leaves, but the purified
protein, named OsCOMT1, exhibited
caffeic acid O-methyltransferase (COMT) activity and not NOMT activity. In this study, we found that OsCOMT1 does not contribute to
sakuranetin production in rice in vivo, and we purified OsNOMT using the oscomt1 mutant. A crude
protein preparation from UV-treated oscomt1 leaves was subjected to three sequential purification steps, resulting in a 400-fold purification from the crude
enzyme preparation. Using SDS-PAGE, the purest
enzyme preparation showed a minor band at an apparent molecular mass of 40 kDa. Two O-
methyltransferase-like
proteins, encoded by Os04g0175900 and Os12g0240900, were identified from the 40-kDa band by MALDI-TOF/TOF analysis. Recombinant Os12g0240900
protein showed NOMT activity, but the recombinant Os04g0175900
protein did not. Os12g0240900 expression was induced by
jasmonic acid treatment in rice leaves prior to
sakuranetin accumulation, and the Os12g0240900
protein showed reasonable kinetic properties to OsNOMT. On the basis of these results, we conclude that Os12g0240900 encodes an OsNOMT.