Strigolactones: A Cry for Help Results in Fatal Attraction. Is Escape Possible?

During evolution, plants have adapted an ecological balance with their associates, competitors, predators, and pests. Keeping this balance intact is an active process during which the plant needs to respond to many different stimuli in order to survive. For example, plants have developed an array of physiological and biochemical responses to phosphate deprivation. One of these responses is the production of isoprenoid-derived molecules called strigolactones. Strigolactones are used to stimulate the formation of symbiotic associations of plant roots with arbuscular mycorrhizal (AM) fungi. AM fungi colonize the root cortex to obtain carbon from their host while assisting the plant in phosphate acquisition. However, strigolactones also stimulate the germination of root parasitic plant seeds. Only upon perception of the presence of a host through its strigolactone production, seeds of the parasites germinate and attach to the roots of many plant species. In contrast to a mutual symbiotic relationship, where both partners benefit from the affiliation through an exchange of resources, the host is heavily exploited by a parasitic plant and suffers strongly from the interaction because it is robbed from its assimilates, water, and nutrients. In this chapter, we focus on the knowledge about the biosynthetic origin of the strigolactones, their ecological significance, and physiological and biochemical regulation. We finally point at recent scientific developments which may explain why a nonmycorrhized plant like Arabidopsis is still producing strigolactones.