Cytogenetical and ultrastructural effects of copper on root meristem cells of Allium sativum L

Different copper concentrations, as well as different exposure times, were applied to investigate both cytogenetical and ultrastructural alterations in garlic (Allium sativum L.) meristem cells. Results showed that the mitotic index decreased progressively when either copper concentration or exposure time increased. C-mitosis, anaphase bridges, chromosome stickiness and broken nuclei were observed in the copper treated root tip cells. Some particulates containing the argyrophilic NOR-associated proteins were distributed in the nucleus of the root-tip cells and the amount of this particulate material progressively increased with increasing exposure time. Finally, the nucleolar material was extruded from the nucleus into the cytoplasm. Also, increased dictyosome vesicles in number, formation of cytoplasmic vesicles containing electron dense granules, altered mitochondrial shape, disruption of nuclear membranes, condensation of chromatin material, disintegration of organelles were observed. The mechanisms of detoxification and tolerance of copper are briefly discussed.

The G2 checkpoint activated by DNA damage does not prevent genome instability in plant cells

Root growth, G2 length, and the frequency of aberrant mitoses and apoptotic nuclei were recorded after a single X-ray irradiation, ranging from 2.5 to 40 Gy, in Allium cepa L. root meristematic cells. After 72 h of recovery, root growth was reduced in a dose-dependent manner from 10 to 40 Gy, but not at 2.5 or 5 Gy doses. Flow cytometry plus TUNEL (TdT-mediated dUTP nick end labeling) showed that activation of apoptosis occurred only after 20 and 40 Gy of X-rays. Nevertheless, irrespective of the radiation dose, conventional flow cytometry showed that cells accumulated in G2 (4C DNA content). Simultaneously, the mitotic index fell, though a mitotic wave appeared later. Cell accumulation in G2 was transient and partially reversed by caffeine, thus it was checkpoint-dependent. Strikingly, the additional G2 time provided by this checkpoint was never long enough to complete DNA repair. Then, in all cases, some G2 cells with still-unrepaired DNA underwent checkpoint adaptation, i.e., they entered into the late mitotic wave with chromatid breaks. These cells and those produced by the breakage of chromosomal bridges in anaphase will reach the G1 of the next cell cycle unrepaired, ensuring the appearance of genome instability.

In vitro cloning and homestead cultivation of primitive Musa cultivars.

Two primitive diploid Musa cultivars, Matti and Chemmatti from the extreme southern part of the Western Ghats were multiplied by in vitro culture of sucker-derived shoot apices. Decontaminated corm explants (1 cm x 1 cm) having shoot apex (approximately 0.3 cm) cultured for 1 month in Murashige and Skoog basal agar medium was cut vertically into eight segments and each segment having a part of shoot meristem was cultured in presence of 6-benzylaminopurine (BAP) and combinations of BAP and indole-3-acetic acid (IAA) or indole-3-butyricacid (IBA) to produce multiple shoots. After 12 weeks of culture, maximum number of shoots (32) in both the cultivars were produced in approximate 60% of the explants in presence of BAP and IAA each at 1.5 mg/l(-1) (Matti) and 40% of the explants in 2.5 mg/l(-1) of BAP and 1.5 mg/l(-1) of IAA (Chemmatti). Buds were formed from the base of the subcultured shoots and somewhat more number (34) of shoots were obtained in Matti than in Chemmatti (31) after 8 weeks. Difference in the concentration of cytokinin required for shoot initiation and multiplication, persistence of exudation through the subculture and red colouration of the early formed sheathing leaf bases in the shoots in Chemmatti indicated possible genotypic differences between the two cultivars. Multiple shoot proliferation achieved through five subcultures of the isolated shoots without any decline. Transfer of shoots (4-5 cm) into MS basal medium favoured rooting in 4 weeks and rooted plants (9 cm) were hardened and established (80-95%). Mericlones of Matti cultivated in homesteads produced bunches of uniform characters in 13 months.