Constitutive Secondary Plant Metabolites and Soil Fungi: Defense Against or Facilitation of Diversity

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In general, we assign the term secondary metabolites to low molecular weight compounds produced by living organisms that apparently lack life-sustaining functions. Instead, they are assumed to contribute to the producing organism’s survival in the ecosystem. By contrast, primary metabolites are practically indispensable (Hartmann 1996). The congruence of apparent accumulation of secondary metabolites in those organisms lacking an immune system, such as plants, fungi, or invertebrates, additionally supports the notion that the main function of these originally classified as waste products metabolites is defense against pathogens and predators. Besides, there exist also morphological defenses, such as cutin armor, thorns, or spines (Rubinstein 1992; Gershenzon 2002). Plant–herbivore interactions represent highly apparent biotic interactions and their exploration has been a starting point to obtain insight into how secondary metabolites may act as agents in chemical defense (Dethier 1954; Fraenkel 1959; Ehrlich and Raven 1964; but see Kerner von Marilaun 1890). Secondary plant metabolites provide various cues for insects to locate their food plants and oviposit on plants that produce chemicals which can be tolerated, detoxified, or even sequestrated by the hatching larvae (Hilker and Meiners 2002; Nishida 2002). Moreover, predators of the herbivore’s larvae may locate their host by plant-produced volatiles induced by components present in the oral secretions of the feeding larvae (Holopainen 2004). The seminal paper by Ehrlich and Raven (1964) on the coevolution between butterflies and their host plants has fundamentally stimulated research in exploring the role of secondary metabolites in biotic interactions (Stamp 2003). Some authors viewed the diversity of secondary metabolites found in plants as a result of a coevolutionary arms race between plants and their herbivore predators