Metabolism of Gibberellin A12-7-Aldehyde by Soybean Cotyledons and Its Use in Identifying Gibberellin A7 as an Endogenous Gibberellin

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Abstract
The level of gibberellin(GA)-like material in cotyledons of soybean (Glycine max L.) was highest at mid-pod fill—about 10 nanograms GA3 equivalents per gram fresh weight of tissue, assayed in the immersion dwarf rice bioassay. This amount is about 1000-fold less than levels in Pisum and Phaseolus seed, other legume species whose spectrum of endogenous gibberellins (GAs) is well known. The metabolism of [14C]-GA12-7-aldehyde (GA12ald)—the universal GA precursor—by intact, mid-pod-fill, soybean cotyledons and their cell-free extracts was investigated. In 4 hours, extracts converted GA12ald to two products—[14C]GA12 (42% yield) and [14C]GA15 (7%). Within 5 minutes, intact embryos converted GA12ald to [14C]GA12 and [14C]GA15 in 15% yield; 4 hour incubations afforded at least 22 products (96% total yield). The putative [14C]GA12 was identified as a product of [14C]GA12ald metabolism on the basis of co-chromatography with authentic GA12 on a series of reversed and normal phase high pressure liquid chromatography (HPLC) and thin-layer chromatography (TLC) systems, and by a dual feed of the putative [14C]GA12 and authentic [14C]GA12 to cotyledons of both peas and soybeans. The [14C]GA15 was identified as a metabolite of [14C]GA12ald by capillary gas chromatography (GC)-mass-spectrometry-selected ion monitoring, GC-radiocounting, HPLC, and TLC. By adding the [14C] metabolites of [14C]GA12ald to a different and larger extract (about 0.2 kg fresh weight of soybean reproductive tissue) and purifying endogenous substances co-chromatographing with these metabolites, at least two GA-like substances were obtained and one identified as GA7 by GC-mass spectrometry. Since [14C]GA9 was not found as a [14C]metabolite of [14C]GA12ald, soybean embryos might have a pathway for biosynthesis of active, C-19 gibberellins like that of the cucurbits; GA12ald → GA12 → GA15 → GA24 → GA36 → GA4 → GA7.