Strigolactones: A New Class of Plant Hormones with Multifaceted Roles

Abstract

Abstract Strigolactones (SLs) are terpenoid lactones produced mainly in plant roots and initially identified as seed germination stimulants for parasitic weeds. In 2005, they were described also as boosters of hyphal branching in arbuscular mycorrhizal fungi, and thereby as promoters of arbuscular mycorrhizal symbiosis. In 2008, they emerged as a new class of plant hormones controlling plant architecture through repression of shoot branching. Since then, several new roles were discovered for SLs: in the adaptive responses to a number of environmental stimuli (including light, osmotic stress, interaction with pathogens and nodulating bacteria), and in several aspects of plant development (including seed germination for nonparasitic plants, hypocotyl elongation, reproduction, leaf senescence and nodulation). The biosynthetic and perception/transduction systems of SLs are being elucidated, and the first mechanistic models presented. Key Concepts: Strigolactones are the key nutrient allocators regulating plant development at the interface between plants, beneficial and detrimental (micro)organisms, and abiotic factors. Strigolactones induce hyphal branching in AM fungi and facilitate the establishment of symbiosis. Strigolactones inhibit shoot branching. Strigolactones affect root architecture and root development depending on nutrients availability. Synthetic SLs (analogues and mimics) are used in pharmacological applications to plants and fungi to decipher the structure–activity relationship. Structure–activity relationship (SAR): Different structures are tested for bioactivity in order to pinpoint which part of the molecule is essential for bioactivity and which can be considered only ‘decoration’.