Focus on Alternative and Complementary Therapies
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Focus Alternat Complement Ther©2005 Pharmaceutical Press
Focus Altern Complement Ther 2005; 10: 167–8
More than 80% of the world’s population in developing countries depends primarily on herbal medicine for basic healthcare needs; the use of herbal medicines in developed countries is growing and 25% of the UK population take herbal medicines regularly. Around two-thirds of the 50 000 different medicinal plant species in use are collected from the wild, and in Europe only 10% of the medicinal species used commercially are cultivated. This is leading to diminishing populations, extinctions and habitat degradation. Well-known species with wild populations threatened by wild harvesting include Arcostaphylos uva-ursa (bear-berry), Piper methysticum (kava) and Glycyrrhiza glabra (liquorice). As many as 10 000 medicinal species may be endangered.1,2
Some protection might be achieved through regulation and the introduction of sustainable wild harvesting methods, but domestic cultivation of medicinal plants is an option that could both reduce pressure on wild populations and solve some of the problems inherent in the production of herbal medicines. These include species misidentification, genetic and phenotypic variability, variability and instability of extracts, and toxic components and contaminants. Cultivation offers the opportunity to optimise yield, achieve a uniform, high-quality product and control every stage of the production process. It might also allow us to modify concentrations of biologically important compounds through the manipulation of growing environments, through traditional selective breeding methods and through the application of modern biotechnology.3
Experience gained with commercial food crops suggests that a controlled and scientific approach to plant breeding can overcome problems such as low germination rates, low survival rates and poor seed production, either through selection of particular genotypes or by gaining a better understanding of the environmental conditions required by a species. Similarly, controlling the growing environment makes it feasible to control phenotypic variation in the concentration of medicinally important compounds present at harvest. Aims would be to increase potency, reduce toxin levels or increase the uniformity and predictability of extracts. Target compounds are usually secondary metabolites which, for the plant, serve as adaptations to fluctuating temperature and light conditions (e.g. antioxidants), stress (e.g. proline), infection (e.g. flavanoids) or herbivory (e.g. alkaloids). The control of light, temperature, water availability and soil composition can all therefore feasibly be manipulated to achieve the desired effects at harvest.
There is no reason why traditional crop-breeding methods should or could not be applied to medicinal herbs grown domestically. The basic principles are to select and breed only from individual genotypes showing the desired characteristics and to make crosses between specific genotypes, bearing in mind the desired combination of characters and an understanding of genetics. Modern plant breeding increasingly uses genetic markers to assist the selection process. These are DNA sequences that map closely on the chromosomes to genes controlling target traits. They allow the identification of useful genotypes at an early stage in their life cycle and therefore both accelerate and increase the precision of the selection process. The increasing availability of complete DNA sequences from important commercial species and model plant species will make the identification of key genes and genetic markers in related, but economically less important, species a more viable option.
Examples of this approach specifically targeted at the medicinal properties of plants are presently few but sometimes relevant knowledge is available because the plant has other, non-medicinal, uses. In the case of Cannabis there have been reports since the 1940s of growers, both legitimate and illicit, successfully selecting for high or low tetrahydrocannabinol and cannabidiol. Molecular markers in Cannabis have been developed both for breeding the species as a fibre crop4 and for forensic use.5 Specific markers for codominant alleles thought to code for the two synthases responsible for cannabinoid biosynthesis have also been sequenced6 specifically with a view to their use in breeding pharmaceutically useful lines.
Although no doubt more controversial, direct manipulation of DNA sequences to alter gene expression in medicinal plants is an area ripe for expansion. The primary target for trait manipulation in medicinal plants is the content of active compounds, but to develop them as crops basic agronomic characteristics related to uniformity, stability, growth and development, and resistance to biotic and abiotic stresses also need to be improved. There is already considerable interest in manipulating plant biosynthetic pathways in order to produce drug precursors, food components or pesticides. In Mentha spp. (mint), for example, biosynthetic pathways have been engineered to modify essential oil production in the trichomes and to enhance the plant’s resistance to fungal infection and abiotic stresses.7 We have given numerous other examples in our recent review of the opportunities and problems generated by the use of biotechnology in the development of medicinal plants.3
The commercial viability of bringing medicinal plants into domestic cultivation and the potential for increased use of modern biotechnologies are likely to be strongly influenced by popular perceptions both of herbs and of biotechnology. One of the main attractions of herbs as medicines is their ‘natural’ status and the associated, but erroneous, view that they must therefore be safe and intrinsically good for us. In stark contrast is the popular view of crops bred with the assistance of molecular biology and modern farming methods as highly ‘unnatural’. This is particularly so for transgenic plants, and it is probable that organic growing methods will be received favourably by purchasers of cultivated medicinal plant extracts. Of course it is entirely possible to grow selected or genetically modified varieties using organic growing methods.
There is a danger that, if medicinal herbs were increasingly brought into domestic cultivation, then wild harvested plants would enjoy an increased cachet and commercial value, and non-sustainable harvesting methods would continue. It is perhaps worth considering that any form of cultivation or wild harvesting of plants is bound to involve the application of selective forces, conscious or unconscious, on the part of the grower or harvester. In the case of wild harvesting, continual selection of the largest wild-growing individuals or those with the traits considered desirable from a medicinal point of view, if it involves destruction of the whole plant or its reproductive organs, will inevitably lead to a degradation of the wild population. We cannot use medicinal plants on a large scale without modifying the characteristics of the plant populations available to us, be they wild or domesticated. If medicinal herbs are brought into cultivation, then we can at least attempt to do this in a controlled fashion, and at the same time attempt to conserve wild populations.
