​Strigolactones (SLs) are a class of phytohormones and rhizosphere signaling compounds with high structural diversity. Three enzymes, carotenoid isomerase ​DWARF27 and carotenoid cleavage dioxygenases ​CCD7 and CCD8, were previously shown to convert ​all-trans-β-caroteneto ​carlactone (​CL), the SL precursor. However, how ​CL is metabolized to SLs has remained elusive. Here, by reconstituting the SL biosynthetic pathway in Nicotiana benthamiana, we show that a rice homolog of Arabidopsis ​MORE AXILLARY GROWTH 1 (​MAX1), encodes a cytochrome P450 CYP711 subfamily member that acts as a CL oxidase to stereoselectively convert ​CL into ent-2′-epi-5-deoxystrigol (B-C lactone ring formation), the presumed precursor of rice SLs. A protein encoded by a second rice ​MAX1 homolog then catalyzes the conversion ofent-2′-epi-5-deoxystrigol to ​orobanchol. We therefore report that two members of CYP711 enzymes can catalyze two distinct steps in SL biosynthesis, identifying the first enzymes involved in B-C ring closure and a subsequent structural diversification step of SLs.