Biosynthesis of GA(73) methyl ester in Lygodium ferns

Biosynthesis of GA(73) methyl ester (GA(73)-Me), the principal antheridiogen in Lygodium ferns, was investigated. From the methanol extract of prothallia of Lygodium circinnatum, GA(25), GA(73), GA(73)-Me, GA(88)-Me, and a few unknown GA(73) derivatives were detected by GC-MS. Because the presence of GA(25) suggests that GA(24), a direct precursor of GA(25), could also be present in L. circinnatum protharllia, we used feeding experiments to investigate the possibility that GA(24) is a precursor of GA(73)-Me. In L. circinnatum prothallia, [H-2(2)]GA(24) was converted into [H-2(2)]GA(73)-Me and a trace amount of [H-2(2)]GA(9)-Me, whereas [H-2(3)]GA(9) was converted into [H-2(3)]GA(9)-Me and [H-2(3)]monohydroxy-GA(9)-Me. Because GA(73)-Me, GA(9)-Me, and their monohydroxy derivatives had been identified by GC-MS from the culture medium of L. circinnatum prothallia, our results suggest that GA(73)-Me is biosynthesized from GA(24) via GA(73), and that neither GA(9) nor GA(9)-Me is a precursor of GA(73)-Me. Though the possibility had been suggested that GA(73)-Me is biosynthesized from 9,15-cyclo-GA(9) (GA(103)), [H-2(2)]GA(103) was not converted into [H-2(2)]GA(73)-Me.