Unveiling the complexity of strigolactones: Exploring structural diversity, biosynthesis pathways and signaling mechanisms.

Strigolactone (SL) is the collective name for compounds containing a butenolide as a part of their structure, first discovered as compounds that induce seed germination of root parasitic plants. They were later found to be rhizosphere signaling molecules that induce hyphal branching of arbuscular mycorrhizal (AM) fungi, and finally, they emerged as a class of plant hormones. SLs are found in root exudates, where they display a great variability in their chemical structure. Their structure varies among plant species, and multiple SLs can exist in one species. Over 30 SLs have been identified, yet the chemical structure of the SL that functions as an endogenous hormone and is found in the above-ground parts of plants remains unknown. We discuss our current knowledge of the synthetic pathways of diverse SLs and their regulation as well as recent progresses in identifying SLs as plant hormones. SL is perceived by the D14 receptor, an α/β hydrolase which originated by gene duplication of KARRIKIN INSENSITIVE 2 (KAI2). D14 and KAI2 signaling pathways are partially overlapping paralogous pathways. Progress in understanding the signaling mechanisms mediated by two α/β hydrolase receptors as well as remaining challenges in the field of SL research are reviewed.