Sesquiterpene Lactones: More Than Protective Plant Compounds With High Toxicity

Sesquiterpene lactones (STLs) constitute a large group of secondary metabolites that are widely distributed in several angiosperm plant families and a few bryophytes, including liverworts. These metabolites are particularly diversified in the family Asteraceae, in which more than 5,000 compounds have been reported so far. In addition to their pharmacological importance and potential therapeutic applications, most STLs display a wide range of protective activities in plants, including acting as anti-herbivory and antimicrobial substances or inhibiting growth of competing plants. These activities are mainly related to their characteristic α,β-unsaturated structural elements, which can participate in Michael-type additions with biological nucleophiles that contain sulfhydryl groups. Supporting the protective roles of STLs, they are mainly located in glandular trichomes of aerial parts because the highly nonspecific toxicity of such compounds necessitates compartmentalization to prevent autotoxicity. However, STLs have also been reported in other aerial and underground organs, where they are assumed to exhibit other biological activities. Recent studies have suggested that these metabolites not only display protective activities due to toxicity but also play key physiological roles in mediating rhizosphere communication among plants, soil microorganisms and plant parasites. STLs have been directly implicated in plant phototropism, resulting in differential growth of a plant organ due to auxin inhibition when accumulated in response to a light stimulus. This review therefore not only highlights the protective roles of STLs in producing plants but also explores the physiological roles of these metabolites, thus providing insights for new research approaches for understanding the roles of STLs in plants and their potential future applications.