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

^a Departments of Plant Science and Biochemistry, University of California, Riverside, California 92502

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 A₃.

Comparison of the total ³²P-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 ³²P incorporation into the tRNA fraction while, simultaneously, ³²P incorporation into tenaciously bound RNA was reduced to 50% of control levels. Red light treatment had no effect on ³²P 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 A₃ 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.