Strigolactones - Biology and Applications (pp.1-45)
Strigolactones (SLs) are a group of carotenoid derivatives that act as a hormone regulating plant development and response to environmental stimuli. SLs are also released into soil as a signal indicating the presence of a host for symbiotic arbuscular mycorrhizal fungi and root parasitic weeds. In this chapter, we provide an overview on the enormous progress that has been recently made in elucidating SL biosynthesis and signal transduction. We describe the tailoring pathway from the carotenoid precursor to the central intermediate carlactone, highlighting the stereospecificity of the involved enzymes, the all-trans/9-cis-β-carotene isomerase (D27), the 9-cis-specific CAROTENOID CLEAVAGE DIOXYGENASE 7 (CCD7), as well as CCD8 and its unusual catalytic activity. We then outline the oxidation of carlactone by cytochrome P450 enzymes, such as the Arabidopsis MORE AXILLARY GROWTH 1 (MAX1), into different SLs and the role of other enzymes in generating this diversity, and discuss why plants produce many different SLs. This is followed by depicting hormonal and nutritional factors that regulate SL biosynthesis and release, and by a description of transport mechanisms. In the second part of our chapter, we focus on SL perception and signal transduction, describing the SL receptor DECREASED APICAL DOMINANCE 2 (DAD2)/DWARF14 (D14) and its unique features, the central function of protein degradation mediated by the F-box protein MAX2 and its homologs. We also discuss the latest advances in understanding how SLs regulate the transcription of target genes and the role of SMXL/D53 transcription inhibitors.