Medicinal plants and symbiotic mycorrhizal fungi

Despite advances of the last four decades in utilizing chemical synthetic approaches to drug design and sophisticated structure-activity studies, there is a growing interest for plant compounds with an action in the field of medicine. In the production of pharmaceutical compounds from medicinal plants two parameters are of importance, first the quality and content of the bioactive compounds and secondly, the compound yield as the product of content and the production of biomass. The arbuscular mycorrhizal symbiosis is a symbiotic association between plant roots and fungi. Arbuscular mycorrhizal fungi (AMF) colonize roots and improve plant nutrition. This, in general results in a higher yield of the AM crop, especially in poor soils. Moreover, in plants colonized by mycorrhizal fungi a range of biological and chemical parameters are altered, such as the accumulation of secondary plant compounds. Data on an altered accumulation pattern of a range of secondary plant compounds from different chemical groups (e.g. flavonoids, cyclohexanone derivatives, apocarotenoids, phytoalexins, phenolic compounds, triterpenoids, and glucosinolates) in mycorrhizal plants are available. Several studies report that flavonoids are increased in different mycorrhizal plants. Flavonoids, present in the extracts of several medicinal plant e.g. Primula veris, are bioactive compounds (with a mucolytic, antioxidant, antiviral and antibacterial activity). This would mean that an encrease of flavonoid levels in Primula veris by AMF enhances the pharmaceutical effect of the extract of this plant. The general idea of the presented proposal is to increase the production of bioactive compounds in medicinal plants through colonization of the plant by symbiotic arbuscular mycorrhizal fungi. In large scale production of medicinal plants even relatively small increases of the pharmaceutical compound content through mycorrhization on the single plant level can translate in a total product increment of economic interest. In our approach an enhanced compound production can be obtained through i) an increased content of bioactive compounds through mycorrhization ii) an increased production of biomass through mycorrhization and/or iii) through the combination of i) and ii) To summarize, a positive effect on the production of bioactive compounds in medicinal plants through mycorrhization is not only of economic interest (through the enhanced compound amount per hectare) but has also an environmental aspect as AMF are an important factor in sustainable agriculture.

The arbuscular mycorrhizal (AM) symbiosis refers to an association between plants and AM fungi. Roots of almost all herbaceous plants are colonized by the widespread soil-borne AM fungi under natural conditions. The plant delivers carbon to the obligate biotrophic fungus, while the fungal partner acquires phosphorus (P) from soil more efficiently than plant roots and provides it to the plant. Due to this interaction root colonization sometimes leads to increased plant growth and changes in the biosynthesis of secondary plant metabolites. Since long time humans know that many of these compounds show beneficial effects on their health and wellness. Plants with any type and a certain amount of pharmacologically active compounds used by man to cure diseases are called medicinal plants. Large scale production of medicinal plants for the pharmaceutical industry requires drugs with stable compound compositions and minimum concentrations of the active compounds. Moreover, an ecological and environmentally friendly agricultural practice is more and more appreciated by the consumer. Thus, the selective application of AM fungi in medicinal plant production could be an attractive strategy to meet these requirements since even relatively small increases of the compound concentration or an enhancement of the biomass could be of economic interest. The aim of this study was to find out whether AM fungal root colonization shows an effect on plant growth, on the quantity and quality of pharmacologically active compounds and on the compound yields in medicinal plants. The project included the medicinal plants oregano, garden sage, valerian, liquorice, archangel, marigold, rhubarb and feverfew. The different AM fungal treatments included single inoculation with two fungi as well as a mixture of 6 different Glomus species. Besides a standard control, a treatment with P was added to differentiate between a P nutritional effect and a direct fungus-plant interaction on growth, biosynthesis or yields. All AM treatments improved the P supply for the plants. The AM fungal colonization resulted in diverse effects on the drug biomass ranging from a growth increase in archangel and oregano to a growth reduction in valerian and no effect at all in garden sage and liquorice. For the levels of the pharmacologically active compounds and their yields either increases or no effects were observed. Oregano showed in general a positive effect on the compound levels upon AM inoculation, valerian plants clearly accumulated the desired pharmacological constituents whereas garden sage, archangel and liquorice were not at all affected. In none of the experiments the improved phosphorus supply of the mycorrhizal plants was responsible for the effects; it rather seems to be due to specific responses of the plant to AM fungi. Overall, the inoculation with AM fungi seems to be a sustainable and beneficial way to enhance the agricultural and pharmaceutical productivity of medicinal plant production. However, depending on the interactions between the plant and AM fungus it should be kept in mind that no general predictions on the AM effect for a species and also not for specific compound groups can be made.