Early nucleic acid and protein syntheses and mitoses in the primary root tips of germinating Vicia faba

Abstract

1. 1. Primary roots of Vicia faba were pulse labeled during early stages of seed germination. DNA, RNA and protein syntheses were studied in homogenates and autoradiographs of 1 mm root tips. 2. 2. Selection of roots of uniform size reduced the great variation in the rates of DNA synthesis found in randomly sampled roots. 3. 3. Very low levels of DNA and RNA synthesis were observed from 15 to 20 h after the beginning of imbibition. A wave of DNA synthesis between 25 and 40 h was followed by a wave of mitoses between 40 and 50 h. 4. 4. It was shown that 96% of the cells which entered the first post-dormancy cell cycle were in G1 (i.e. before DNA synthesis). 5. 5. The curves describing RNA and DNA syntheses were parallel for the first 50 h. Incomplete imbibition was excluded as a factor limiting the onset of synthesis of both nucleic acids, since imbibition by the 1 mm tip was completed by 8 h. 6. 6. 3H-Leucine incorporation increased gradually from a very low level at 14.5 h until 20 h. Several sharp peaks of high precursor incorporation which preceded the first wave of DNA synthesis were observed. Incorporation experiments with different specific activities of 3H-leucine indicated that these bursts of high incorporation represented changes in the level of protein synthesis, instead of abrupt changes in the amino acid precursor pool only. The leucine; precursor pool decreased in size from 20 to 30 h. 7. 7. Protein synthesis between 30 and 50 h paralleled DNA synthesis. The protein synthesis inhibitor cycloheximide was used to demonstrate that meristematic cells synthesize at least some of the protein essential for DNA synthesis throughout their S period. 8. 8. The cycloheximide induced inhibition of protein synthesis could be separated in time from an indirect inhibition of DNA synthesis caused by suppression of protein synthesis essential for DNA synthesis. Cycloheximide had limited usefulness because its inhibitory effect was irreversible.