Hydroxyl carlactone derivatives are predominant strigolactones in Arabidopsis

Strigolactones (SLs) regulate important aspects of plant growth and stress responses. Many diverse types of SL occur in plants, but a complete picture of biosynthesis remains unclear. In Arabidopsis thaliana, we have demonstrated that MAX1, a cytochrome P450 monooxygenase, converts carlactone (CL) into carlactonoic acid (CLA), and that LBO, a 2-oxoglutarate-dependent dioxygenase, converts methyl carlactonoate (MeCLA) into a metabolite called [MeCLA+16] Da. In the present study, feeding experiments with deuterated MeCLAs revealed that [MeCLA+16] Da is hydroxymethyl carlactonoate (19-HO-MeCLA). Importantly, this LBO metabolite was detected in plants. Interestingly, other related compounds, methyl 4-hydroxycarlactonoate (4-HO-MeCLA) and methyl 16-hydroxycarlactonoate (16-HO-MeCLA) were also found to accumulate in lbo mutants. 3-HO-, 4-HO- and 16-HO-CL were detected in plants, but their expected corresponding metabolites, HO-CLAs, were absent in max1 mutants. These results suggest that HO-CL derivatives are predominant SLs in Arabidopsis, produced through MAX1 and LBO.