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Applications of gibberellic acid to the mature form of Hedera helix induce morphological reversions to the juvenile form of growth. The juvenile forms produced are stable with time and differ dramatically from the mature in phenotype. DNA-RNA hybridization techniques have been used to study the RNA populations of juvenile, mature and gibberellic acid-treated mature apices. Hybridization competition experiments using RNA extracted by a hot phenol technique and uniformly labeled in vitro with (3)H dimethylsulfate show no qualitative differences between the species of RNA present in juvenile and mature apices. However, differences are observed in the frequency distribution of RNA species using both uniformly labeled or pulse-labeled RNA as a reference. RNA extracted from gibberellic acid-treated mature buds was a less effective competitor than control mature RNA and the difference observed was comparable to that observed between mature and juvenile RNA. These results indicate that at least part of the molecular basis of phase change and gibberellic acid action may involve an alteration in the rate of transcription of certain genes in the apices of the mature form.RNA extracted using the hot phenol procedure contained a fraction of rapidly labeled RNA which was not extractable with cold phenol. When RNA extracted only with cold phenol was used in competition experiments sequences unique to the juvenile were detected and sequences unique to the mature were not detected. Implications of these results in relation to possible post-transcriptional control mechanisms are discussed.

Gibberellic Acid-induced Phase Change in Hedera helix as Studied by Deoxyribonucleic Acid-Ribonucleic Acid Hybridization