natural-products-in-plant-pest-management

Fungal Endophytes 223tumbling rocks and pebbles. These environmental insults created manyportals through which common phytopathogenic oomycetes could enter theplant. Still, the plant population appeared to be healthy, possibly due to theprotection provided by an endophytic product. This was the environmentalbiological clue used to choose this plant for a comprehensive study of itsendophytes.An example of plants with an ethnobotanical history is the snakevineKennedia nigriscans, from the Northern Territory of Australia, which wasselected for study since its sap has traditionally been used as a bush medicine.In fact, this area was selected for plant sampling as it has been hometo the world’s longest standing human civilization, the Australian Aborigines.The snakevine is harvested, crushed and heated in an aqueous brewby local Aborigines in South-west Arnhemland to treat cuts, wounds andinfections. As it turned out, the plant contains an entire suite of streptomycetes(Castillo et al., 2005). One in particular, designated Streptomyces NRRL30562, has unique partial 16S rDNA sequences when compared to those inGenBank (Castillo et al., 2002). It produces a series of actinomycins includingactinomycins D, Xo ß, and X 2. It produces novel, broad-spectrum peptideantibiotics called munumbicins. This seems to be an excellent example illustratingthe potential benefits of knowledge of indigenous peoples andresearchers.Furthermore, some plants generate bioactive natural products and haveassociated endophytes that produce the same natural products. Such is thecase with taxol, a highly functionalized diterpenoid and famed anticanceragent that is found in Taxus brevifolia and other yew species (Taxus spp.) (Waniet al., 1971). In 1993, a novel taxol-producing fungus Taxomyces andreanae wasisolated and characterized from Taxus brevifolia (Strobel et al., 1993).While combinatorial synthesis produces compounds at random, secondarymetabolites, defined as low molecular weight compounds notrequired for growth in pure culture, are produced as an adaptation forspecific functions in nature (Demain, 1981). Schulz (2001) points out thatcertain microbial metabolites seem to be characteristic of certain biotopes,both at an environmental as well as an organism level. Accordingly, itappears that the search for novel secondary metabolites should centre onorganisms that inhabit unique biotopes. Thus, it behoves the investigator tocarefully study and select the biological source before proceeding, ratherthan to take a totally random approach in selecting the source material.Careful study also indicates that organisms and their biotopes that are subjectedto constant metabolic and environmental interactions should produceeven more secondary metabolites (Schulz at al., 2002). Endophytes aremicrobes that inhabit such biotopes, namely higher plants, which is whythey are currently considered as a wellspring of novel secondary metabolitesoffering the potential for exploitation of their medical benefits (Tanand Zou, 2001). Moreover, novel compound-screening experiments havealready proved that endophytes are considerably ahead of soil isolates,with 51% new structure compounds compared with 38%, respectively(Schulz et al., 2002).