Interspecific hybrids of Nicotiana repanda × N. tabacum die shortly after germination. Seedlings that are hybrids between N. repanda and N. tomentosiformis, which is a progenitor of N. tabacum, express hybrid lethality when they are cultured at 28°C for 90 days after germination, but seedlings that are hybrids between N. repanda and N. sylvestris, another progenitor of N. tabacum, grow normally. We detected several features of programmed cell death (PCD) in hybrid seedlings of N. repanda × N. tomentosiformis expressing hybrid lethality. Nuclear fragmentation was detected in nuclei isolated from dying shoots and roots of hybrid seedlings expressing lethality. In addition, electrophoresis of total DNA extracted from the shoots and roots of hybrid seedlings showed a distinctive DNA ladder pattern, suggesting cleavage of nuclear DNA into oligonucleosomal fragments. These results indicate that hybrid seedlings of N. repanda × N. tomentosiformis undergo PCD in the process of hybrid lethality. However, in hybrid seedlings of N. repanda × N. sylvestris, no features of PCD were detected. These results suggest that PCD might also be expressed in hybrid seedlings of N. repanda × N. tabacum because N. tomentosiformis is a progenitor of N. tabacum.
Features of programmed cell death (PCD), including nuclear fragmentation and DNA ladders were detected in hybrid cells of Nicotiana suaveolensϫN. tabacum expressing hybrid lethality at 28°C, but not in cells kept at 36°C. Heat treatment (HT, 50°C for 15 min) before transfer to 28°C from 36°C temporarily suppressed the increase in the percentage of dead cells. In hybrid cells without HT, the percentage of Sub G1 nuclei, corresponding to those with nuclear fragmentation increased at 6 h after transfer to 28°C and DNA ladders were detected at 9 h after transfer to 28°C. On the other hand, in hybrid cells with HT, the percentage of Sub G1 increased and DNA ladders were detected 15 h after transfer to 28°C. These results suggest that HT temporarily suppresses PCD during expression of hybrid lethality. Key words:Heat treatment, hybrid lethality, Nicotiana, programmed cell death. Short Communication Copyright © 2005 The Japanese Society for Plant Cell and Molecular BiologyAbbreviations: BAP, 6-benzylaminopurine; CTAB, cetyltrimethylammonium bromide; HT, heat treatment; PCD, programmed cell death; TLCC, thin layer cell culture.were maintained under the same conditions on a 7 day subculture cycle and used for experiments 3 days after subculturing.To remove old medium from cells maintained in suspension culture, hybrid cells were sieved through a 200 mm nylon mesh and about 0.5 g (fresh weight) of cells was transferred to culture dishes (f90 mm) at a high temperature (36°C). Three ml of fresh medium was added and cells were placed in order to form a single layer of cells. Then about 2 ml of the extra culture medium was removed to expose the cells to air in order to keep the dishes under observation. For HT, the dishes were placed in a water bath at 40, 50, 60 and 80°C for 15 min. Then, the dishes were placed at 28°C for 24 h.Hybrid cells cultured at 36°C were transferred to 28°C, which is the lethal temperature for hybrid seedlings (Manabe et al. 1989), and maintained at 28°C in a thin layer cell culture (TLCC) system (Masuda et al. 2003). Hybrid cells were sieved through a 200 mm nylon mesh to remove the clustered cells and were resuspended in 50 ml fresh medium after centrifugation (2,000 rpm, 10 min). Ten microliters of this cell suspension was dropped on a glass slide and observed by light or fluorescence microscopy. The progression of lethality in hybrid cells was estimated from the percentage of dead cells after 28°C treatment for different intervals over 24 h. Dead cells were scored under a light microscope after staining with 2.5% (w/v) Evans Blue. At least three independent experiments were performed with more than 500 cells counted per condition.For cytometric analysis, nuclei were isolated from hybrid cells cultured with or without HT by chopping them in ice-cold buffer (Michaelson et al. 1991) and filtering the macerated tissue through 70 and 20 mm nylon mesh. The nuclei were collected from the filtrate by centrifugation for 5 min at 2,500 rpm, suspended in the ice-cold buffer supplemented with 5 mg/m...
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