Root Meristems Of Allium Cepa Research Articles

The effect of colchicine on subchromatid exchanges in root meristem cells of Vicia faba and Allium cepa

The problem of the duplication of subchromatid exchanges has been studied in Vicia faba and Allium cepa root meristems, respectively, with 8-ethoxycaffeine (EOC) and 6-methylcoumarin (6-MC), both effective in the DNA postsynthetic phases (G 2 and early prophase). A high frequency of subchromatid exchanges was induced by the mutagens at 2 h recovery after treatment; both species showed chromatid exchanges at the T 2 divisions and chromosome aberrations at the T 3 divisions after treatments. On the contrary, when colchicine was given to EOC-or 6-MC-treated roots in order to label the cells containing subchromatid exchanges, only chromosome-type aberrations were observed in both species at the T 2 (tetraploid) division. Assuming that sister-strand exchanges are crossover-like spontaneous events, an hypothesis is formulated to explain the presence of chromosome aberrations in T 2 (tetraploid) cells.

Delays in prophase induced by adenosine 2-deoxyriboside and their relation with DNA synthesis.

The rate of DNA synthesis in the course of the division cycle in root meristem ofAllium cepa growing under constant temperature and aeration conditions has been studied by means of treatment with AdR, as a specific inhibitor of the synthesis, as well as by the incorporation of tritiated thymidine. The one-hour treatment with AdR or tritiated thymidine was given at various hours in the course of the interphase of a synchronous population of binucleate cells induced by caffeine. In the case of AdR, sensitivity to the inhibition of DNA synthesis was studied by recording the delays produced by the treatment in the appearance of biprophases and bitelophases. The selection by the use of caffeine, of spontaneously synchronous populations of cells going through the telophase and becoming binucleate and the detection of the first biprophases in the subsequent mitosis provide a highly synchronized system with which to study the incorporation of tritiated thymidine during the interphase. The curves representing sensitivity to the inhibition of DNA synthesis by AdR and the rate of tritiated thymidine incorporation coincide, so that we can regard the delays, under our conditions, as proportional to the rate of DNA synthesis at the moment of the AdR treatment. This rate, in the S period, was found to be variable by both methods, being higher in the first and the last thirds of the S period (S1 and S3) and lower in the middle third (S2).

The chromosome breaking effect of 6-methylcoumarin in Allium cepa in relation to the mitotic cycle

Labelling experiments with [ 3H]thymidine have been carried out to investigate (1) the time of the chromosome breaking action of 6-methylcoumarin (6-MC) in the cell cycle of Allium cepa root meristems and (2) the mitotic delay induced by th compound. A concentration of 0.75 m M of 6-MC induces a very high frequency of chromatid and sub-chromatid breaks in unlabelled anaphase cells scored at 2 to 4 h after treatment, suggesting a higher sensitivity in cells in late G 2. The G 2 phase is delayed by the treatment. The S phase is delayed about 1 h and 20 min by the treatment, and a low frequency of chromatid breaks is induced by action of 6-MC on this phase. The fate of the sub-chromatid changes in T 2 and T 3 cells is discussed.

Duration of the cell division cycle and its dependence on temperature.

Measurement of the B-interphase has afforded a means of calculating the duration of the cell division cycle at various temperatures, from 0° to 40° C, in the root meristems of Allium cepa. For this purpose caffeine labelling was used to produce a small population (1–5%) of highly synchronized binucleate cells. The B-interphase is what we call the period from 0 hour (end of the one-hour caffeine treatment) to the appearance of the first binucleate cells in bi-prophase. The duration of the phases that make up the cycle was calculated from their relative frequency within the meristematic population. Observation shows that at 0° and at 40° the cells do not go through the cycle at all and that from 10° to 30° the speed increases in almost linear proportion. There is no significant change in the relative duration of any of the morphological phases under any of the conditions tested. In discussing these results the authors propose the use of the cell flow (percentage of meristematic cells passing through any point in the cycle in one hour) as an expression in the simplest form of the steady-state kinetics of the meristematic population at a given temperature.

Mitosis and Differentiation in Roots Treated with Actinomycin

In the presence of actinomycin D (90 microg/ml), mitosis in root meristem of Allium cepa L. ceases after a delay of 36 hours. The block occurs at interphase. Differentiation of prevascular tissue is suppressed, and the evidence suggests it begins to fail before mitosis stops. Autoradiograms of roots treated with tritiated uracil show that synthesis of ribonucleic acid is blocked after mitosis fails. The effects of the antibiotic are reversible.

Cytological Reactions Induced by Inorganic Salt Solutions

AN investigation has been carried on for some time in this Laboratory into the immediate cytological effect of salt solutions on root meristems of Allium Cepa. Salts, mostly nitrates, of some forty metals were tested. A dilution series of 10–16 concentrations were studied for each substance, covering the whole range from total lethality down to such weak concentrations that no cytological effect could be detected. Fixations of root tips were made after 4 and 24 hours respectively. All slides were stained with crystal-violet.