Roles for secondary metabolites in plants.

More than about 20,000 secondary metabolites have now been identified and their isolation and characterization continues at an undiminishing rate. Although the production of most secondary metabolites is under genetic control, in relatively few cases have convincing arguments been put forward to rationalize their occurrence in terms of primary metabolic functions. Whatever the initial reason for their evolution, secondary metabolites are now an essential part of the armamentaria used by plants in the battle to survive and propagate, to the extent where the expenditure of energy, photosynthate and nutrients for their production can be demonstrated to be 'cost effective' for that. Their role may centre on defence of the producer against predators (herbivores), pathogens or competitors, on aid to pollination or seed dispersal, or on protection against or adaptation to extrinsic abiotic factors, or on combinations of these functions. Various examples are given in support of this argument.