Promotion of Growth and Invertase Activity by Gibberellic Acid in Developing Avena Internodes

Gibberellic acid (GA(3)) induces invertase activity within 6 hours in Avena stem segments that are incubated in the dark at 23 degrees . The maximum amount of promotion is about 5 times that of invertase activity in untreated segments. GA(3) causes significant promotion of invertase activity at concentrations as low as 3 x 10(-5) mum GA(3). The increase in invertase activity elicited by GA(3) between 3 x 10(-5) mum and 300 mum closely parallels the growth promotion that is caused by GA(3) over this concentration range. In control segments, invertase activity rises steeply during the first 6 hours of incubation, then decays slowly between 12 and 48 hours. In GA(3)-treated segments, the invertase activity also rises during the first 6 hours, parallel to that in control segments and continues to rise during the next 42 hours. These changes in invertase activity during 48-hour incubation periods do not parallel the changes in growth that occur in control and GA(3)-treated segments. Cycloheximide at 10 mug/ml abolishes all GA(3)-promoted growth and invertase activity in these segments. Actinomycin D at 40 and 80 mug/ml decreases GA(3)-promoted growth by 20% and invertase activity by 38 and 44%, respectively. The data clearly support the idea that protein synthesis is necessary for GA(3)-promoted growth and invertase activity in Avena stem segments.