SUMMARYOn germination, maize seeds rapidly become intolerant of desiccation. Germinating seeds exposed to dehydrating conditions for 24 h show a significant rise in lipid peroxidation and the accumulation of an electron paramagnetic resonance-detectable organic radical, in parallel with the development of desiccation intolerance. Throughout the 5-d-long germination phase and the period of loss of desiccation tolerance, the radical scavengers a-and ytocopherol and glutathione accumulate in response to episodes of dehydration. However dehydration suppresses, or delays, increases in activity of superoxide dismutase, peroxidase and glutathione reductase, major components in the protection against activated forms of oxygen. We present evidence of a sequence of events in which activated oxygen plays an early and probably causative role in loss of tolerance of desiccation in these seeds.
~~ ~ ~~~ ~~~~~~~When germinating Zea mays 1. seeds are rapidly desiccated, free radical-mediated lipid peroxidation and phospholipid deesterification is accompanied by a desiccation-induced buildup of a stable free radical associated with rapid loss of desiccation tolerance. Comparison of the eledron paramagnetic resonance and eledron nuclear double resonance properties of this radical with those of the radical in dried, desiccation-intolerant moss showed that the two were identical. At the subcellular level, the radical was associated with the hydrophilic fraction resulting from lipid extraction. lsolated mitochondria subjected to drying were also found to accumulate an identical radical in vitro. When increasing concentrations of cyanide were used, a significant positive correlation was shown between rates of respiration and the accumulation of the radical in desiccation-intolerant tissues. Another positive correlation was found when rates of O2 uptake by radicles at different stages of germination were plotted against free radical content following desiccation. This indicates that free radical production is closely linked to respiration in a process likely to involve the desiccation-induced impairment of the mitochondrial eledron transport chain to form thermodynamically favorable conditions to induce accumulation of a stable free radical and peroxidized lipids. Modulation of respiration using a range of inhibitors resulted in broadly similar modulation of the buildup of the stable free radical. One site of radical generation was likely to be the NADH dehydrogenase of complex I and probably as a dired consequence of desiccation-impaired electron flow at or close to the ubiquinone pool.Most vascular plants can withstand little desiccation except during the later stages of seed development and the early
We studied the fine structural organization of nuclear bodies in the root meristem during germination of maize and Arabidopsis thaliana using electron microscopy (EM). Cajal bodies (CBs) were observed in quiescent embryos and germinating cells in both species. The number and distribution of CBs were investigated. To characterize the nuclear splicing domains, immunofluorescence labelling with antibodies against splicing factors (U2B 00 and m3G-snRNAs) and in situ hybridisation (with U1/U6 antisense probes) were performed combined with confocal microscopy. Antibodies specific to the Arabidopsis SR splicing factor atRSp31 were produced. AtRSp31 was detected in quiescent nuclei and in germinating cells. This study revealed an unexpected speckled nuclear organization of atRSp31 in root epidermal cells where micro-clusters of interchromatin granules were also observed by EM. Therefore, we examined the distribution of green fluorescent protein (GFP)-tagged atRSp31 in living cells after Agrobacterium -mediated transient expression. When expressed transiently, atRSp31-GFP exhibited a speckled distribution in leaf cells. Treatments with a-amanitin, okadaic acid, staurosporine or heat shock induced the speckles to reorganize. Furthermore, we generated stable Arabidopsis transgenics expressing atRSp31-GFP. The distribution of the fusion protein was identical to that of endogenous atRSp31. Three-dimensional time-lapse confocal microscopy showed that speckles were highly dynamic domains over time.
Abstract. We investigated the onset of desiccation tolerance in developing embryos of Brassica campestris seeds and possible correlated ultrastructural modifications in the radiele cells. Since the acquisition of desiccation tolerance is a long asynchronous process which took 9 d to be achieved, we determined criteria allowing us to separate freshly intact harvested seeds into desiccation intolerant and desication tolerant batches that differed in age by only 2 d. No particular structural modifications were found except a strong depletion of intraplastidial starch (‐90%) coincident with the appearance of stachyose and an increase of sucrose (+30%) on the acquisition of desiccation tolerance. As we did not observe an increase of lipid reserves as a consequence, we suggest that these metabolic events can be a key factor towards the acquisition of desiccation tolerance.
Progressive loss of organogenic totipotency appears to be a common event in long-term plant tissue eultme. This loss of totipotency, which has been proposed to be a typical trait of plant neoplastic progression, is compared to some mechanisms thai occur during the establishment of animal differentiation-resistant cancer lines in ~itro. Evidence is presented that alteration in DNA methylation patterns and expression of genes occur during long-term callus culture. An effect of the auxin, 2,4-dichlorophenoxyacetic acid, in the progressive methylation, is moreover suggested. Methylation of genes relevant to cell differentiation and progressive elimination of ceils capable of differentiation is proposed as being responsible for this progressive loss of organogenic potential. Finally, the epigenetie alteration (DNA methylation) that occurs during prolonged periods of culture may induce other irreversible genetic alterations that uhimately make the loss of totipotency irreversible.
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