On the substrate specificity of the rice strigolactone biosynthesis enzyme DWARF27
The β-carotene isomerase OsDWARF27 is stereo- and double bond-specific. It converts bicyclic carotenoids with at least one unsubstituted β-ionone ring. OsDWARF27 may contribute to the formation of α-carotene-based strigolactone-like compounds.
Strigolactones (SLs) are synthesized from all-trans-β-carotene via a pathway involving the β-carotene isomerase DWARF27, the carotenoid cleavage dioxygenases 7 and 8 (CCD7, CCD8), and cytochrome P450 enzymes from the 711 clade (MAX1 in Arabidopsis). The rice enzyme DWARF27 was shown to catalyze the reversible isomerization of all-trans- into 9-cis-β-carotene in vitro. β-carotene occurs in different cis-isomeric forms, and plants accumulate other carotenoids, which may be substrates of DWARF27. Here, we investigated the stereo and substrate specificity of the rice enzyme DWARF27 in carotenoid-accumulating E. coli strains and in in vitro assays performed with heterologously expressed and purified enzyme. Our results suggest that OsDWARF27 is strictly double bond-specific, solely targeting the C9–C10 double bond. OsDWARF27 did not introduce a 9-cis-double bond in 13-cis- or 15-cis-β-carotene. Substrates isomerized by OsDWARF27 are bicyclic carotenoids, including β-, α-carotene and β,β-cryptoxanthin, that contain at least one unsubstituted β-ionone ring. Accordingly, OsDWARF27 did not produce the abscisic acid precursors 9-cis-violaxanthin or -neoxanthin from the corresponding all-trans-isomers, excluding a direct role in the formation of this carotenoid derived hormone. The conversion of all-trans-α-carotene yielded two different isomers, including 9′-cis-α-carotene that might be the precursor of strigolactones with an ε-ionone ring, such as the recently identified heliolactone.
KeywordsApocarotenoids Carotenoids Carotenoid cleavage dioxygenase Carotene isomerase Carlactone Strigolactones
Carotenoid cleavage dioxygenase
Maltose binding protein
We thank Dr. Peter Beyer, University of Freiburg, Germany, for valuable discussions and Dr. Hansgeorg Ernst, BASF, Germany, for providing the synthetic apocarotenoid substrates. The research reported in this publication was supported by funding from King Abdullah University of Science and Technology (KAUST) and the EU (METAPRO; FP7 KBBE-2009-3-1-01).
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