Reversible Conjugation of Gibberellins In Situ in Maize

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Abstract
Gibberellins [3H]GA4 (1.33 Curies per millimole) and [3H]GA20 (2.36 Curies per millimole) were injected into the shanks of maize (Zea mays L.) cobs during rapid grain filling and mature seeds were subsequently harvested. Extracts of mature, dry seeds from 1980 feeds yielded only 20 to 30% of the 3H radioactivity in acidic, ethyl acetate-soluble form, and this was principally associated with the precursor, with lesser amounts of the major metabolite, [3H]GA1 (putative identification based on sequential SiO2 partition, and gradient-eluted reverse-phase C18 high performance liquid chromatography [HPLC]). Most of the radioactivity in the dry seeds was associated with compounds having partition characteristics of, and co-chromatographing on, sequential SiO2 partition and reverse-phase HPLC with glucosyl conjugates of the precursors (GA4 or GA20) and their probable major metabolite (GA1). The majority of conjugate associated with the precursor GA4 eluted coincidental with GA4 glucoside. Subsequent acid or enzymic hydrolysis (β-glucosidase or cellulase) yielded the free GAs, putative identification being based on isocratic HPLC of each 3H-labeled conjugate → hydrolysis → isocratic HPLC of the 3H-labeled hydrolysate. Upon imbibition of the seeds, radioactivity associated with the conjugate fraction decreased; concomitantly, statistically significant increases in levels of free [3H]GA-like compounds were observed. Although the specific ratios of GA-like and GA-glucosyl conjugate-like substances varied substantially across years, hybrids, and even, in different plants from the same hybrid, this `reversible conjugation' (i.e. apparent conjugation during seed maturation followed by release of the GA moiety during germination), was reproducible for [3H]GA20 in seed from two maize hybrids produced over 2 years.