GIBBERELLIN-INDUCED GROWTH AND REGULATION OF THE CELL-DIVISION CYCLE IN DEEP-WATER RICE

Excised stem sections of deepwater rice (Oryza sativa L., cv. Pin Gaew 56) containing the highest internode were used to study induction of rapid internodal elongation by gibberellin. It has been shown previously that this growth response is based on an increased cell production rate in the intercalary meristem and on increased cell elongation. Our investigations were aimed at establishing the temporal sequence of these GA-regulated processes. Cell sizes were determined by scanning electron microscopy, the phases of the cell cycle by flow cytometry, and DNA synthesis by [H-3]thymidine incorporation. The lag time for the onset of gibberellic acid (GA3)-induced growth was 40 min. Treatment with GA3 promoted cell elongation in the intercalary meristem within 2 h. After 4 h of treatment with GA3. the fraction of meristematic cells in the G2 phase had declined, indicating that cells in the G2 phase had entered mitosis. Subsequent activation of DNA replication led to an overall increase in the cell-production rate. This was evident from an increase in the percentage of cells in the S phase and from enhanced incorporation of [H-3]thymidine into DNA between 4 and 7 h of GA3 treatment. An increase in the final cell length contributed to the growth response after 7 h of GA3 application. Our results are consistent with the hypothesis that gibberellin first promotes cell elongation in the intercalary meristem and that cell division is stimulated as a result of cell-growth.