Regulation of essential oil biosynthesis in basil (Ocimum basilicum L.)
Methylchavicol, a volatile phenylpropene, has received attention recently because of its potential application as an antioxidant as well as a potential inhibitor of human carcinoma cell proliferation. In this research, we investigated the developmental regulation of methylchavicol accumulation, the main constituent in the essential oil of basil (Ocimum basilicum L.) EMX-1 chemotype. Methylchavicol accumulation was found to decrease overtime as leaves mature. Accumulation of methylchavicol was significantly correlated to the activity of chavicol O-methyltransferase (CVOMT) and eugenol O-methyltransferase (EOMT), the enzymes that catalyze the final step in its biosynthesis. The CVOMT and EOMT transcript accumulation also followed the same pattern as its respective enzyme activity, indicating that methylchavicol biosynthesis is regulated at the transcriptional level. Furthermore, the CVOMT transcript levels were highest in peltate glands isolated from the youngest leaves compared to mature leaves providing for the first time evidence that CVOMT transcript level is developmentally regulated in basil glandular trichomes, and is not only a result of the trichomes density. In related studies, we observed that methylchavicol biosynthesis was highly inducible and the time response to the chemical elicitors chitosan, methyl jasmonate and methyl salicylate was transient and specific. The CVOMT specific activity and OMT transcript were also significantly higher in the plants treated with these elicitors. Phenylalanine ammonia lyase (PAL) specific activity also increased significantly among elicitor treated plants, suggesting an increase of the substrate flux through the pathway. A transformation system for basil was next developed to allow in the future genetic engineering of basil species for the possible alteration in essential oil content and composition. Transgenic basil (Ocimum spp.) was produced for the first time using an Agrobacterium tumefaciens-mediated transformation system for both O. basilicum and O. citriodorum, a related species included to test the robustness of the transformation system. Inheritance of the transgenes was observed and T1 plants exhibited similar essential oil composition compared with non-transgenic plants. The transformation system developed and initial work toward the isolation of functional specific glandular trichome promoters will assist in the evaluation of the roles of specific enzymes or regulatory factors in the control of essential oil biosynthesis in basil. ^
Major Professor: James E. Simon, Purdue University.
Biology, Molecular|Biology, Plant Physiology