Ishida_2022_Phytochem.Lett_49_162

Gibberellins (GAs) are a group of diterpenoid plant hormones that control plant growth and development at various stages. Biologically active GAs share the common structures of a 3beta-hydroxy group, a carboxy group at C-6, and a ?-lactone between C-4 and C-10. Hydroxylation at C-2beta is a major deactivation step in many plant species, and hydroxylation at C-13 has been shown to weaken the binding affinity of GAs to their receptor proteins. In rice, bioactive GA4 has also been shown to be deactivated through 16a,17-epoxidation. Moreover, 16,17-dihydro-16a,17-dihydroxy GA4 has been identified as an aglycon of its glucoside from rice. However, our knowledge on the biological activity of 16,17-epoxidized GAs is currently limited to 16,17-dihydro-16a,17-epoxy GA4. Moreover, the bioactivity of 16,17-dihydro-16a,17-dihydroxy GA4 remains unknown. Here, we synthesized 16,17-epoxidized or dihydroxylated GA derivatives and performed a structureactivity relationship study using rice seedlings. 16,17-Epoxidation of bioactive GA1 and GA4 reduced their activity to promote elongation of rice leaf sheaths. Moreover, 16,17-dihydroxylation significantly decreased the activities of 16,17-dihydro-16a,17-epoxy GAs. These results suggest that GAs are deactivated in a stepwise manner via 16,17-epoxidation and hydrolysis of these epoxy groups