Main conclusion The b-carotene isomerase OsD- WARF27 is stereo- and double bond-specific. It con- verts bicyclic carotenoids with at least one unsubstituted b-ionone ring. OsDWARF27 may con- tribute to the formation of a-carotene-based strigolac- tone-like compounds.
Strigolactones (SLs) are synthesized from all-trans-b-car- otene via a pathway involving the b-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 isomer- ization of all-trans- into 9-cis-b-carotene in vitro. b-car- otene 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 per- formed 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-b-carotene. Substrates isomerized by OsDWARF27 are bicyclic carotenoids, including b-, a- carotene and b,b-cryptoxanthin, that contain at least one unsubstituted b-ionone ring. Accordingly, OsDWARF27 did not produce the abscisic acid precursors 9-cis-violax- anthin 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- a-carotene yielded two different isomers, including 90-cis- a-carotene that might be the precursor of strigolactones with an e-ionone ring, such as the recently identified heliolactone.