Changes in Nucleic Acids in Phytochrome-dependent Elongation of the Alaska Pea Epicotyl

Red light, which produces the physiologically active form of phytochrome (Pfr), inhibited epicotyl elongation in intact dark-grown Alaska pea seedlings. This red light response was detectable 3 hours after the light treatment and became pronounced after 5 hours. The growth inhibition was completely reversed by far red light applied immediately after the red or by pretreatment of the seedlings with the plant hormone gibberellin A(3).Comparison of the total (32)P-labeled nucleic acids from control and red light-treated Alaska pea epicotyls on methylated albumin-kieselguhr columns revealed a marked alteration of the pattern of nucleic acid synthesis in this plant material with little or no effect on total isotope incorporation into nucleic acids. A single 5-minute red light perturbation caused a 2-fold stimulation of (32)P incorporation into the tRNA fraction while, simultaneously, (32)P incorporation into tenaciously bound RNA was reduced to 50% of control levels. Red light treatment had no effect on (32)P incorporation into the DNA-RNA, rRNA, or mRNA fractions. Far red light reversed the effect of red light on tRNA synthesis but did not restore tenaciously bound RNA levels to the control value. Gibberellin A(3) treatment did not cause reversal of any of the red light effects on RNA synthesis.These light-induced changes in nucleic acids were measurable before any changes in the physiological response (epicotyl elongation) could be detected. These results are consistent with a phytochrome-mediated differential gene activation mechanism in the Alaska pea epicotyl elongation system.