The raw material for producing herbal medicine originates increasingly from cultivated rather than from wild-sourced plants as medicinal plant species and their native ecologies have become endangered from over-harvesting and habitat loss. Our understanding of reciprocal influences that medicinal plant roots and soil fungi have on each other in their native ecosystem is woefully inadequate. Earlier research (Tims and Bautista, 2007, ) provided the first evidence that medicinally active alkaloids from goldenseal (Hydrastis canadensis) root influenced an a close relationship with the ubiquitous pathogenic soil fungus Fusarium.
Generally soils are low nutrient environments and most fungi remain dormant if not in proximity to plant root or decaying plant tissue. The root soil complex, known as the rhizosphere, stimulates microbial growth 50-100 greater than in soil not in the rhizosphere zone. The density of fungal pathogens, such as Fusarium oxysporum, is higher in the rhizosphere of a host plant than a non-host plant and their distribution in these native forest settings is greatly influenced by specific plant community associations. The host plant tolerates the presence of a nonpathogenic strain resulting in a latent period of infection referred to as an “endophytic phase”. The fungus grows inside the plant but does not destroy plant cells. Similar to the relationship with symbiotic, mycorrhizal fungi, Fusarium often grow among root border cells during first contact between plant root and fungus. Rather than harm the plant, this endophytic state can promote plant growth and protect the plant from attack from other more aggressive fungi. Although not a symbiotic relationship, the endophytic phase may be considered an evolutionary characteristic that allows the fungus to associate with a wide host range, and increase their distribution.
If more of our herbal medicine is being “farmed”, how do these rhizosphere fungi, both symbiotic and endophytic, alter the ratio and makeup of medicinally important plant root compounds; And how do the compounds present in root exudates (a complex mix of chemicals excreted by plant root cells) selectively effect the composition of the fungal rhizosphere community? How do plant and fungus “agree to terms”? Is better medicine produced from these native environments where complex rhizosphere relationships are the norm?