Plant breeding: importance of plant secondary metabolites for protection against pathogens and herbivores

Summary Chemical protection plays a decisive role in the resistance of plants against pathogens and herbivores. The so-called secondary metabolites, which are a characteristic feature of plants, are especially important and can protect plants against a wide variety of microorganisms (viruses, bacteria, fungi) and herbivores (arthropods, vertebrates). As is the situation with all defense systems of plants and animals, a few specialized pathogens have evolved in plants and have overcome the chemical defense barrier. Furthermore, they are often attracted by a given plant toxin. During domestication of our crop and food plants secondary metabolites have sometimes been eliminated. Taking lupins as an example, it is illustrated that quinolizidine alkaloids are important as chemical defense compounds and that the alkaloid-free varieties (sweet lupins), which have been selected by plant breeders, are highly susceptible to a wide range of herbivores to which the alkaloid-rich wild types were resistant. The potential of secondary metabolites for plant breeding and agriculture is discussed.