The potential bioproduction of the pharmaceutical agent sakuranetin, a flavonoid phytoalexin in rice

Abstract

Sakuranetin, the major flavonoid phytoalexin in rice, can be induced by ultraviolet (UV) irradiation, treatment with CuCl 2 or jasmonic acid (JA), or phytopathogenic infection. In addition to sakuranetin's biological significance on disease resistance in rice, its broad bioactivities have recently been described. Results from these studies have shown that sakuranetin is a useful compound as a plant antibiotic and a potential pharmaceutical agent. Sakuranetin is biosynthesized from naringenin, a precursor of sakuranetin, by naringenin 7-O-methyltransferase (NOMT), but the relevant gene has not yet been identified in rice. Recently, we identified the OsNOMT gene, which is involved in the final step of sakuranetin biosynthesis in rice. In previous studies, OsNOMT was purified to apparent homogeneity from UV-treated wild-type rice leaves; however, the purified protein, termed OsCOMT1, exhibited caffeic acid 3-O-methyltransferase (COMT) activity, but not NOMT activity. Based on the analysis of an oscomt1 T-DNA tagged mutant, we determined that OsCOMT1 did not contribute to sakuranetin production in rice in vivo. Therefore, we took advantage of the oscomt1 mutant to purify OsNOMT. 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 with a minor band at an apparent molecular mass of 40 kDa in the purest enzyme preparation. Matrix-assisted laser desorption/ionization time of flight/time of flight analysis showed that the 40 kDa protein band included two O-methyltransferase-like proteins, but one of the proteins encoded by Os12g0240900 exhibited clear NOMT activity; thus, this gene was designated OsNOMT. Gene expression was induced by treatment with jasmonic acid in rice leaves prior to sakuranetin accumulation, and the recombinant protein showed reasonable kinetic properties to NOMT. Identification of the OsNOMT gene enables the production of large amounts of sakuranetin through transgenic rice and microorganisms. This finding also allows for the generation of disease-resistant and sakuranetin biofortified rice in the future.

Figure 1. The final step in the biosynthetic pathway for sakuranetin, a flavonoid phytoalexin in rice, catalyzed by OsNOMT. Sakuranetin is a major rice phytoalexin and a potential pharmaceutical agent.

Figure 2. Multiple alignment of OsCOMT1 and two OMTL proteins. Blue and red lines indicate the dimerization and conserved domain of O-methyltransferase family proteins, respectively. Os12 g0240900 was confirmed to be rice NOMT (OsNOMT).

Figure 3. Possible application for sakuranetin bioproduction. (A) Generation of transgenic plants expressing the OsNOMT gene. Sakuranetin accumulation could be expected in transgenic plants with enhanced disease resistance. (B) Fermentative sakuranetin production from tyrosine by recombinant E. coli harboring four biosynthetic pathway genes for naringenin and OsNOMT. PAL, phenylalanine ammonia-lyase; 4CL, 4-coumarate: CoA ligase; CHS, chalcone synthase; CHI, chalcone isomerase.