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Competition for in Vitro [³H]Gibberellin A₄ Binding in Cucumber by Gibberellins and Their Derivatives ¹

Department of Biological Sciences, Simon Fraser University, Burnaby, British Columbia, Canada V5A 1S6

The gibberellin (GA) binding properties of a cytosol fraction from hypocotyls of cucumber (Cucumis sativus L. cv National Pickling) were examined using a DEAE filter paper assay, [³H]GA₄, and over 20 GAs, GA derivatives and other growth regulators. The results demonstrate structural specificity of the binding protein for -lactonic C-19 GAs with a 3 -hydroxyl and a C-6 carboxyl group. Additional hydroxylations of the A, C, or D ring of the ent-gibberellane skeleton and methylation of the C-6 carboxyl impede or abolish binding affinity. Bioassay data are generally supported by the in vitro results but significantly GA₉ and GA₃₆, both considered to be precursors of GA₄ in cucumber, show no affinity for the binding protein. The results are discussed in relation to the active site of the putative GA₄ receptor in cucumber.