Document Type



[3H] GA-binding, GA receptor, characteristics and purification; GA biosynthesis and metabolism; steroid receptors; nuclei, isolation, in vitro transcription; GA-insensitive mutants; mode of GA action


Use of tritiated gibberellins ([3H] GA1, [3H] GA4) of high specific activity and purity has allowed the determination of GA-specific binding sites in several plant tissues both in vivo and in vitro. In cucumber hypocotyls and pea epicotyls which have been most investigated the binding of [3H] GA; occurs to soluble proteins. This binding is saturable ('n' =about 30 pmol. Mg-1 soluble protein), of high affinity (KD= about 70 nM), and is competed for by other GA, and derivatives in proportion to their biological activity in these tissues in vivo. Size exclusion and ion-exchange chromatography has yielded several fractions which show specific and exchangeable binding of [3H] GA4, but further purification of these fractions and discrimination whether binding is to a GA metabolizing enzyme or the GA receptor has not yet been possible. Isolated nuclei from cucumber hypocotyls also show specific and exchangeable binding of [3H] GA4. They show run on transcription in vitro. Addition of 100 nM GA4 to the transcription cocktail augments the total RNA produced and, as determined by sensitivity to α-amanitin, dramatically shifts the transcription in favor of RNA polymerase II activity. It also appears that the nuclei contain a soluble inhibitor of GA-induced transcription. GA-insensitive mutants, such as wheat varieties carrying the Rht3 gene, provide a unique tool to study the mechanism of GA action. Preliminary data indicate, however that the D6899 wheat carrying the Rht3 gene, is not a receptor mutant, rather it is a mutant which produces an inhibitor that prevents the GA-induced transcription of α -amylase genes in aleurone tissue. These and other data are reviewed with a backdrop of information about steroid receptors, and gibberellin biosynthesis and metabolism. A model of gibberellin action is presented which is consistent with published data, and some future lines of research are indicated.

Publication Date

June 1991

Journal Title

Journal of the Iowa Academy of Science





First Page


Last Page



© Copyright 1991 by the Iowa Academy of Science, Inc.



File Format