From Plant Press, Vol. 5, No. 3 from July 2002.
By Robert DeFilipps
The scope of variation and potentiality of certain plant chemicals, the molecules, are gradually becoming better known. “Taxol,” a molecule developed for the treatment of breast cancer, was originally synthesized from bark of the Pacific yew tree (Taxus). And many people hovering over chili bowls are aware that chili peppers (Capsicum) contain a molecule known as “capsaicin,” which arouses the nerves that respond to painful (i.e., “hot”) stimuli. As neuroscientist Barry Green recently explained, long-term exposure to capsaicin can reduce the sensitivity (to pain) of those nerves, which has led to capsaicin being used in ointments for inflammatory diseases such as arthritis. In another area, the relative similarities and differences in secondary metabolic molecules such as alkaloids, aside from their function in the plants as natural protective mechanisms against herbivorous predators and decay-causing microorganisms, are being studied by conservationists for their predictive value in finding species that may be of potential use for medicines and drugs.
In fact, botanical molecules pervade the very air we breathe, to the extent that, as reported by S.A. Russell, plants can and do communicate with each other by means of, as it were, “cries for help, invitations, even warnings, each in the form of odor molecules that float past human noses unnoticed” (Discover 23(4): 46-51. 2002). Another interesting major group, the macromolecules, is functionally unlike all the micromolecules mentioned above. They occur in the genetic material (DNA) in the nuclei and chloroplasts of cells. Molecules comprising strands of helically coiled DNA (deoxyribonucleic acid) are composed of numerous tiny groupings or coded sequences of proteins. Investigations of the variability in the homologous arrangement or coding of these proteins in the chloroplast DNA plasmids, mitochondrial DNA, or ribosomal DNA (and in isozymes), can reveal the hereditary proximity of one line of organisms to another. That is the basis for understanding evolutionary relationships and processes such as plant introgression, polyploidy and other phenomena. The discipline is termed “Plant Molecular Systematics,” and studies in this complex field are becoming ever more helpful towards reducing the guesswork inherent in some traditional botanical pursuits. Scoping out the difference between molecular systematics and regular taxonomy is like comparing a ballet to a barn dance.
Smithsonian botany curators are participating in molecular systematics studies encompassing various major groups such as algae, lichens and flowering plants. Paula DePriest, in collaboration with colleagues at the University of Manitoba (Winnipeg, Canada), has found that switching of algal and fungal partners can occur in the symbioses of some cladoniaceous lichens; this was done by comparing their nuclear internal transcribed spacer (ITS) phylogenies to test aspects of coevolution, cospeciation and parallel cladogenesis. DePriest, with colleagues in Canada, Russia, Wisconsin and Tanzania, has also worked with amplified ribosomal DNA sequences from subfossils of an Umbilicaria lichen in a Greenland glacier. Some of the fungus groups they found were the same as others previously detected in DNA extracted from the grass clothing of a Tyrolean Iceman who had been frozen for 3,000 years. With colleagues in Austria and Sweden, DePriest analyzed small subunit ribosomal DNA sequences, and in an article that made the cover of Science magazine (9 June 1995) revealed that lichen symbioses have originated multiple times during fungal evolution in disparate groups of ascomycetes and basidiomycetes.
