Radiosensitivity, RNA, and protein metabolism of leaky and arrested cells in sunflower root meristems (Helianthus-annuus)

Two cell populations in sunflower root meristems are described. Most cells stop in G1 after being cultured in sucrose‐deficient medium, but "leaky" cells continue through DNA synthesis and stop in G2. A comparison of "leaky" and arrested cells is reported on the basis of radiosensitivity, and cytological and biochemical responses to metabolic inhibitors. "Leaky" cells are randomly distributed throughout primary meristematic tissues. They are not inhibited from initiating DNA synthesis by exposure to doses of γ‐irradiation ranging from 300–7200 R; arrested cells, depending upon the dose, are inhibited partially or completely. "Leaky" cells do, however, show a dose‐dependent mitotic delay in G2, which is the same as arrested cells. Treatment with puromycin and actidione does not inhibit "leaky" cells from initiating DNA synthesis but does inhibit them from mitosis. Arrested cells are inhibited from advancing to S and M by both inhibitors. Also, puromycin and actidione cause a decrease in protein and RNA synthesis, demonstrating a possible protein dependent RNA synthesis necessary for cell cycle progression. Actinomycin D (10 μg/ml) inhibits neither "leaky" nor arrested cells from entering S and M. At 30 μg/ml, however, arrested cells are partially inhibited. "Leaky" cell metabolism is unique in preparation for and initiation of DNA synthesis but similar to that of the remaining cells of the meristem in terms of requirements for progression through the rest of the mitotic cycle.