Preferential Stimulation of the Plant mRNA Synthesis by Gibberellic Acid

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Abstract
1 Treatment of the etiolated maize seedlings with the plant hormone, gibberellic acid results in a significant enhancement of heavy polyribosome formation. 2 This is accompanied by highly increased incorporation of the labelled RNA precursors into RNA engaged in the polyribosotnal complex, as well as by an increased rate of protein synthesis in viva. 3 Determination of the specific radioactivity of particular RNA classes isolated from polyribosomes reveals that gibberellic acid stimulates mostly the synthesis of the rapidly labelled, nonribosomal RNA fraction. 4 A considerable amount of this rapidly labelled RNA fraction, whose synthesis is preferentially stimulated by exogenous gibberellic acid contains poly(A) sequences, as shown by affinity chromatography on oligo(dT)-cellulose, indicating that phytohormone causes an increased transcription of mRNA in etiolated maize seedlings. 5 When [3H]adenosine served as the RNA precursor it was found that the ratio between the heteropolymeric and polyadenylic parts of the poly(A)-RNA chain markedly changed under gibberellin treatment, suggesting that, in addition to an increased rate of mRNA synthesis, the plant hormone also affects the process of post-transcriptional polyadenylation of the newly made mRNA precursors. Possible extension of the polyadenylate segment in the presence of gibberellin may account for a longer functional half-life of the mRNA synthesized in plants treated with the phytohormone, and may explain significantly enhanced heavy polyribosome formation, as well as a higher efficiency of protein synthesis in plants treated with gibberellic acid.