Ecological Arsenal and Developmental Dispatcher. The Paradigm of Secondary Metabolism

Abstract

Mankind has been exploiting plant chemicals in the form of potions and poisons for thousands of years. The attitude toward the physiological significance of this plethora of small molecules is reflected in the terminology that was assigned to them: secondary metabolites. Less flattering terms were also assigned: waste products, metabolic leftovers, and excrement. Two schools of thought concerning the function of secondary metabolites had developed into the 1970s. Prominent personalities such as Miriam Rothschild, an amateur naturalist in England, were proponents of a critical ecological function for secondary metabolites such as cardiac glycosides, cannabinoids, anthocyanins, and pyrrolizidine alkaloids. On the other side were followers of Kurt Mothes, a charismatic professor of plant biochemistry in East Germany who held the opinion that substances such as alkaloids have "no special physiological meaning" and that "many people apparently cannot live without the idea that everything in Nature has a purpose." That being said, how far have we come since 1975? SECONDARY METABOLITES CAN BE KEY PLAYERS IN THE INTERACTION BETWEEN PLANTS AND THEIR ENVIRONMENTPioneering work published by the likes of Miriam Rothschild (13), Jeffrey Harborne (The University of Reading), Tom Eisner, and Jerry Meinwald (Cornell University) established a new field of study and coined its name, "chemical ecology." It is now accepted that there is an integral interaction between plants and their environment and that speciesspecific secondary metabolites are key players in this interaction. For example, the pigments that are produced in flower petals, "signatures" of mixtures of anthocyanins or betalains, determine in part how effectively a flower will be pollinated. In another form of interaction, juvenile forms of insects have become specialized to feed on poisonous plants. In fact, some insects can have an out right predilection for poisonous food plants. The protection proffered is 2-fold: Poisonous plants are avoided by large herbivores and the insects accumulate toxic plant secondary metabolites such as cardiac glycosides and pyrrolizidine alkaloids that serve to protect them in later stages of development. To further demonstrate function, the research group of Hans Grisebach (Freiburg University) showed that secondary metabolites have a role in plant defense. Phenylpropanoids were found to accumulate in soybean in response to treatment with pathogens (19). This then linked the field of secondary metabolism to phytopathology.In recent years a discovery of the function of secondary metabolites in the interaction with symbionts, rather than pathogens, was made that was biochemically clearly verifiable. A groundbreaking observation by the group of Sharon Long (Stanford University) demonstrated that the flavone luteolin exuded from roots of alfalfa serves as a signal that activates rhizobial nodulation genes and therefore plays an integral role in root colonization (12). SECONDARY METABOLITES CAN ALSO BE SIGNAL COMPOUNDS INVOLVED IN PLANT DE...

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