Gibberellic Acid Enhancement of DNA Turnover in Barley Aleurone Cells

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Abstract
When imbibed, deembryonated halfseeds from barley (Hordeum vulgare L. variety Himalaya) are incubated in buffer, the DNA content of the aleurone layer increases 25-40% over a 24 h period. The DNA of isolated aleurone layers declines by 20% over the same time period. GA causes a reduction in DNA levels in halfseed aleurone layers and isolated aleurone layers. GA increases the specific radioactivity of [3H]thymidine-labeled halfseed aleurone layer DNA during the first 12 h of treatment. Pulse-chase studies demonstrated that the newly synthesized DNA is metabolically labile. The buoyant density on CsCl density gradients of hormone-treated aleurone DNA is identical with that of DNA extracted from whole seedlings. After density-labeling halfseed DNA with 5-bromodeoxyuridine, a bimodal absorption profile is obtained in neutral CsCl. The light band (1.70 g/ml) corresponds to unsubstituted DNA, while the heavy band (1.725-1.74 g/ml) corresponds to a hybrid density-labeled species. GA increases the relative amount of the heavy (hybrid) peak in halfseed aleurone layer DNA, further suggesting that the hormone enhances semiconservative replication in halfseeds. DNA methylation was also demonstrated. Over 60% of the radioactivity from [3H-Me]methionine is incorporated into 5-methylcytosine. GA has no effect on the percentage distribution of label among the bases. GA enhances the rate of DNA degradation and DNA synthesis (turnover) in halfseeds, but primarily DNA degradation in isolated aleurone layers. Incorporation by isolated aleurone layers is due to DNA repair. Semiconservative replication apparently plays no physiological role in the hormone response, since both isolated aleurone layers and gamma-irradiated halfseeds respond normally. The hypothesis was advanced that endoreduplication and DNA degradation are means by which the seed stores and mobilizes deoxyribonucleotides for the embryo during germination.