Transgenic crops for improved medicinal plants and pharmaceutical products

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Phytochemicals of plant origin are being widely examined for their ability to provide health benefits as bioactive compounds and as potential nutritionally active ingredients (Cowan, 1999). As discussed by Dillard and German (2000), the number of possible physiological and biochemical ways through which they could provide health benefits is enormous. Such phytochemicals include terpenoids, phenolic compounds and alkaloids.

Even though the number of synthetically produced drugs has increased enormously during the past hundred years, more than 25 per cent of all prescriptions still contain active ingredients of plant origin (Oksman-Caldentey and Hiltunen, 1996). The amount of natural products is expected to increase, due to the above-mentioned limitations of synthetic industrial processes and also as a consequence of the increasing scientific and public interest in traditional and herbal medicine, e.g. homeopathy and so-called functional foods.

Growing scientific evidence suggests that specific food components can promote human health by reducing disease risk and that these components can be incorporated into food products by the newer methods of molecular genetics. The development of these compositionally enhanced food products has had positive effects on health outcomes beyond the satisfaction of basic nutritional requirements (Young and Jones, 1996).

There is therefore great interest in obtaining plants with desired agronomic traits such as improved resistance to pathogens, increased biomass or improved product quality (Oksman-Caldentey and Hiltunen, 1996).

As discussed earlier, secondary metabolite pathways are complicated, since they require multiple enzymatic steps to the desired end-product used as therapeutic substance or dietary supplement and since these enzymatic pathways are under the control of multifactorial regulatory processes.

Efficient methods of gene characterization and gene cloning are now available, and the expression in regenerated plants of useful genes under the specific control of appropriate tissue-organ-specific promoters is regarded as the most promising technique to obtain plants expressing the desired agronomic traits in the desired plant organ.

Transgenic plants therefore have been obtained from a number of species (Galun and Breiman, 1997), but only few of them from medicinal species (Oksman-Caldentey and Hiltunen, 1996).

Expression of foreign genes in medicinal plants

Hairy-root cultures represent naturally genetically changed organ cultures able to regenerate complete plants. Phenotypical alteration of transformed plants regenerated through in vitro systems might confer new valuable characters for increasing shoot or root yield.

Yang and Choi (2000) obtained transgenic plants from hairy roots of Panax ginseng. Starting from three culture lines, they obtained embryogenic callus induction, somatic embryo formation and their complete germination into adult plants. The transformed ginseng plantlets showed an actively growing root system with abundant lateral roots (Yang and Choi, 2000).

The T-DNA, namely the piece of Agrobacterium rhizogenes plasmid which is transferred from the bacterium and incorporated into host-plant nuclear DNA, can be properly disarmed in order to limit bacterium virulence and reconstructed via a binary vector (Saito et al., 1992a; Saito, 1994) with foreign genes capable of changing plant metabolic activity (Tepfer, 1990; Oksman-Caldentey and Hiltunen, 1996; Giri and Narasu, 2000).

As for most genetically engineered crop species, medicinal plants have been tested for their suitability for gene transfer. The introduction of chimeric reporter genes, whose expression in the plant can be easily assessed by measuring enzymatic activities of gene products, allows the estimation of transformation efficiency (Table 6.3) (Oksman-Caldentey and Hiltunen, 1996).

Transgenic tobacco (Nicotiana tabacum L.), liquorice (Glycyrrhiza uralensis Fisher) and foxglove (Digitalis purpurea L.) plants were obtained with binary vector systems based on a disarmed Agrobacterium tumefaciens or on a virulent A. rhizogenes. The chimeric neomycin phosphotransferase II (NPT-II) gene (kan) and the (^-glucuronidase (GUS) gene (uidA) were introduced and expressed by transgenic plants (Table 6.3) (Saito et al, 1991).

Although reporter genes were not expected to influence expression of plant secondary metabolites, these experiments demonstrated the feasibility of obtaining regenerated plants from hairy-root cultures and laid the technical basis for the development of metabolically engineered medicinal plants.

Attempts have already been made for the transfer and expression in planta of specific

Table 6.3 Transfer and expression of chimeric reporter genes in medicinal plants (from Oksman-Caldentey and Hiltunen, 1996)

Plant species

Reporter genet

Transfer systemb

Effect

Nicotiana rustica

hyg, kan

Ri-plasmid

Nicotine production

Nicotiana tabacum

neo, gus

Ri-plasmid

Increased production of nicotine,

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