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Diversity and variability of plant secondary metabolism: a mechanistic view

Based upon a brief historical view, the typical features of plant secondary metabolism and its role in chemical interactions between plants and their environment are discussed. Facts and arguments are presented favouring the hypothesis that secondary metabolism evolved under the selection pressure of a competitive environment. The high ‘degree of chemical freedom’ of secondary metabolism which, in contrast to primary metabolism, allows structural modifications with almost no restrictions, is stressed as mechanistic basis for the generation of chemical diversity. Biochemical and physiological properties of secondary metabolism are in accordance with such a view. It is suggested that the great chemical diversity and intraspecific variability of secondary metabolism is the result of processes of natural selection which act upon highly variable chemical structures. This view is exemplified by the pyrrolizidine alkaloids, a typical class of secondary compounds.