Carlactone is converted to carlactonoic acid by MAX1 in Arabidopsis and its methyl ester can directly interact with AtD14 in vitro

Strigolactones (SLs) stimulate seed germination of root parasitic plants and induce hyphal branching of arbuscular mycorrhizal fungi in the rhizosphere. In addition, they have been classified as a new group of plant hormones essential for shoot branching inhibition. It has been demonstrated thus far that SLs are derived from carotenoid via a biosynthetic precursor carlactone (CL), which is produced by sequential reactions of DWARF27 (D27) enzyme and two carotenoid cleavage dioxygenases CCD7 and CCD8. We previously found an extreme accumulation of CL in the more axillary growth1 ( max1 ) mutant of Arabidopsis , which exhibits increased lateral inflorescences due to SL deficiency, indicating that CL is a probable substrate for MAX1 (CYP711A1), a cytochrome P450 monooxygenase. To elucidate the enzymatic function of MAX1 in SL biosynthesis, we incubated CL with a recombinant MAX1 protein expressed in yeast microsomes. MAX1 catalyzed consecutive oxidations at C-19 of CL to convert the C-19 methyl group into carboxylic acid, 9-desmethyl-9-carboxy-CL [designated as carlactonoic acid (CLA)]. We also identified endogenous CLA and its methyl ester [methyl carlactonoate (MeCLA)] in Arabidopsis plants using LC-MS/MS. Although an exogenous application of either CLA or MeCLA suppressed the growth of lateral inflorescences of the max1 mutant, MeCLA, but not CLA, interacted with Arabidopsis thaliana DWARF14 (AtD14) protein, a putative SL receptor, as shown by differential scanning fluorimetry and hydrolysis activity tests. These results indicate that not only known SLs but also MeCLA are biologically active in inhibiting shoot branching in Arabidopsis . Significance Strigolactones (SLs) are plant hormones that inhibit shoot branching and are parasitic and symbiotic signals toward root parasitic plants and arbuscular mycorrhizal fungi, respectively. Therefore, the manipulation of SL levels potentially improves the yield of crops. To achieve this goal, the biosynthesis pathway of SLs must be fully understood. SLs are biosynthesized from a precursor, named carlactone (CL), which is derived from carotenoid. However, no downstream pathway of CL has been elucidated. In this study, we show that CL is converted into a carboxylated metabolite, named carlactonoic acid, by Arabidopsis MAX1, the enzymatic function of which had been unknown, and that its methyl ester has the ability to interact with a SL receptor and suppress shoot branching in Arabidopsis .