Stereospecificity in strigolactone biosynthesis and perception.
Plants produce strigolactones with different structures and different stereospecificities which provides the potential for diversity and flexibility of function. Strigolactones (SLs) typically comprise a tricyclic ABC ring system linked through an enol-ether bridge to a butenolide D-ring. The stereochemistry of the butenolide ring is conserved but two alternative configurations of the B-C ring junction leads to two families of SLs, exemplified by strigol and orobanchol. Further modifications lead to production of many different strigolactones within each family. The D-ring structure is established by a carotenoid cleavage dioxygenase producing a single stereoisomer of carlactone, the likely precursor of all SLs. Subsequent oxidation involves cytochrome P450 enzymes of the MAX1 family. In rice, MAX1 enzymes act stereospecifically to produce 4-deoxyorobanchol and orobanchol. Strigol- and orobanchol-type SLs have different activities in the control of seed germination and shoot branching, depending on plant species. This can partly be explained by different stereospecificity of SL receptors which includes the KAI2/HTL protein family in parasitic plants and the D14 protein functioning in shoot development. Many studies use chemically synthesised SL analogues such as GR24 which is prepared as a racemic mixture of two stereoisomers, one with the same stereo-configuration as strigol, and the other its enantiomer, which does not correspond to any known SL. In Arabidopsis, these two stereoisomers are preferentially perceived by AtD14 and KAI2, respectively, which activate different developmental pathways. Thus caution should be exercised in the use of SL racemic mixtures, while conversely the use of specific stereoisomers can provide powerful tools and yield critical information about receptors and signalling pathways in operation.
School of Chemistry and Biochemistry, The University of Western Australia, Western Australia, 6009, Australia.
Gavin R
GR
Flematti
School of Chemistry and Biochemistry, The University of Western Australia, Western Australia, 6009, Australia.
Adrian
A
Scaffidi
School of Chemistry and Biochemistry, The University of Western Australia, Western Australia, 6009, Australia.|Centre of Excellence in Plant Energy Biology, The University of Western Australia, Western Australia, 6009, Australia.
Mark T
http://orcid.org/0000-0001-5657-9005
MT
Waters
School of Biological Sciences, University of Tasmania, Hobart, TAS, 7001, Australia. steven.smith@utas.edu.au.|State Key Laboratory of Plant Genomics, National Center for Plant Gene Research (Beijing), Institute of Genetics and Developmental Biology, Chinese Academy of Sciences, Beijing, 100101, China. steven.smith@utas.edu.au.
Steven M
http://orcid.org/0000-0001-5661-9994
SM
Smith
D053898:Biosynthetic Pathways; D007783:Lactones / Q000138:chemical synthesis / Q000737:chemistry* / Q000378:metabolism; D038821:Mycorrhizae / Q000378:metabolism; D063245:Plant Development; D010937:Plant Growth Regulators / Q000096:biosynthesis / Q000737:chemistry* / Q000378:metabolism; D013237:Stereoisomerism
D007783:Lactones; D010937:Plant Growth Regulators
Carlactone; Carotenoid; Stereochemistry; Strigolactone; α/β-Fold hydrolase