Identification of 6-epi-heliolactone as a biosynthetic precursor of avenaol in Avena strigosa

Strigolactones (SLs) known as rhizosphere signaling molecules and plant hormones regulating shoot architecture, are classified into two distinct groups, canonical and non-canonical SLs based on their structures. Avenaol, a non-canonical SL found in the root exudates of black oat (Avena strigosa), has a characteristic bicyclo[4.1.0]heptane skeleton. Elucidating the biosynthetic mechanism of this peculiar structure is a challenge for further understanding the structural diversification of non-canonical SLs. In this study, a novel non-canonical SL, 6-epi-heliolactone in black oat root exudates was identified. Feeding experiments showed that 6-epi-heliolactone was a biosynthetic intermediate between methyl carlactonoate and avenaol. Inhibitor experiments proposed the involvement of 2-oxoglutarate-dependent dioxygenase in converting 6-epi-heliolactone to avenaol. These results provide new insights into the stereochemistry diversity of non-canonical SLs and a basis to explore the biosynthetic pathway causing avenaol.