Transgenic medicinal plants: Agrobacterium-mediated foreign gene transfer and production of secondary metabolites.

Agrobacterium-Ti/Ri plasmids are natural gene vectors, by which a number of attempts have been made in genetic engineering of secondary metabolism in pharmaceutically important plants in the last few years. Opines are biosynthesized by transformed crown galls and hairy roots integrated with T-DNAs of Ti/Ri plasmids. These opines are classified into five families according to their structures and biogenesis. The production of opines is a natural example of genetic engineering of the biosynthetic machinery of plant cells for the benefit of the bacterial pathogen. One recent advance in transgenic technology of potential value to pharmacognosy is an application of transgenic organ cultures such as hairy roots and shooty teratomas to over-production and biotransformation of secondary metabolites. The hairy roots induced by Ri plasmid of Agrobacterium rhizogenes have been proved to be an efficient means of producing secondary metabolites that are normally biosynthesized in roots of differentiated plants. So far the specific metabolites produced by hairy root cultures and/or plants regenerated from hairy roots of 63 species have been analyzed and reported. As an alternative means of producing metabolites normally produced in leaves of plants, the shooty teratomas incited by the tumor-forming Ti plasmid or a shooty mutant of Agrobacterium tumefaciens have been used for the de novo biosynthesis and biotransformation of some specific secondary products. A second and more direct way to manipulate secondary pathways is performed by transferring and expressing specifically modified genes into medicinal plant cells with Agrobacterium vector systems. The genes encoding neomycin phosphotransferase and beta-glucuronidase have been used as model genes under the transcriptional control of appropriate promoters. Recently some specific genes that can eventually modify the fluxes of secondary metabolism have been integrated and expressed in medicinal plant cells. Future prospects are also discussed.