The response of plants to the spaceflight environment and microgravity is still not well understood, although research has increased in this area. Even less is known about plants’ response to partial or reduced gravity levels. In the absence of the directional cues provided by the gravity vector, the plant is especially perceptive to other cues such as light. Here, we investigate the response of Arabidopsis thaliana 6-day-old seedlings to microgravity and the Mars partial gravity level during spaceflight, as well as the effects of red-light photostimulation by determining meristematic cell growth and proliferation. These experiments involve microscopic techniques together with transcriptomic studies. We demonstrate that microgravity and partial gravity trigger differential responses. The microgravity environment activates hormonal routes responsible for proliferation/growth and upregulates plastid/mitochondrial-encoded transcripts, even in the dark. In contrast, the Mars gravity level inhibits these routes and activates responses to stress factors to restore cell growth parameters only when red photostimulation is provided. This response is accompanied by upregulation of numerous transcription factors such as the environmental acclimation-related WRKY-domain family. In the long term, these discoveries can be applied in the design of bioregenerative life support systems and space farming.

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From Spaceflight to Mars g-Levels: Adaptive Response of A. Thaliana Seedlings in a Reduced Gravity Environment Is Enhanced by Red-Light Photostimulation.

Hormonal storage patterns and morphological heterogeneity of porcine gonadotrope cells during postnatal development

Calcium effect on number, dimension and activity of nucleoli in cortex cells of pea (Pisum sativum L.) roots after treatment with heavy metals

Root meristem nucleoli from soybean (Glycine max. cv. Aldana) seedlings germinated for 3 days at 25 ̊C and then for 4 days at 10 ̊C or still at 25 ̊C (control) were examined. Chill was observed to reduce root meristematic zone growth 15-fold. Nucleoli doubled their volume at 10 ̊C. Autoradiographic studies showed that after 20 min H-uridine incubation at 10 ̊C, incorporation of this precursor (postincubation time 0) into nucleoli of chilled seedlings was 4.7 times weaker than in the control. After 80 min postincubation in nonradioactive medium, the cytoplasm became the most intensely labelled cell area in the control material, while in chilled roots the nucleoli were still most intensely labelled and the cytoplasm was 11 times less labelled than in the control. The increase in nucleoli volume at 10 ̊C is suggested to result from greater cold-induced inhibition of the dynamics of maturation and transport of ribosome subunits than of rRNA synthesis dynamics. Ultrastructural studies of chilled seedling nucleoli showed a significant decrease in the fibrillar component and an increase in the granular component, forming characteristic clusters. They are supposed to correspond to shortened and condensed pre-rRNA transcription complexes (compacted \"Christmas trees\").

Autoradiographic and ultrastructural studies of the effect of chilling on soybean root meristem nucleoli

In the present study, the combination of the micronucleus test with analysis of the activity of the rRNA genes in mutagen-treated Hordeum vulgare (barley) by maleic hydrazide (MH) cells was performed. Simultaneously fluorescence in situ hybridization (FISH) with 25S rDNA as probes and an analysis of the transcriptional activity of 35S rRNA genes with silver staining were performed. The results showed that transcriptional activity is always maintained in the micronuclei although they are eliminated during the next cell cycle. The analysis of the transcriptional activity was extended to barley nuclei. MH influenced the fusion of the nucleoli in barley nuclei. The silver staining enabled detection of the nuclear bodies which arose after MH treatment. The results confirmed the usefulness of cytogenetic techniques in the characterization of micronuclei. Similar analyses can be now extended to other abiotic stresses to study the response of plant cells to the environment.

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Transcriptional Activity of rRNA Genes in Barley Cells after Mutagenic Treatment

The evolution of nuclear and nucleolar sizes throughout interphase have been studied in synchronous caffeine-labeled binucleate cells of onion root meristems by using silver impregnation and stereological methods over semithin sections. Nucleus and nucleolus grow independently, since nucleolus enlarges at its fastest rate in G 1, while nucleus grows mostly in two periods: onset of replication and G 2. Nucleolar size in the cycle seems to be a genecontrolled function, hardly affected by protein synthesis inhibition. Hence, there is a biphasic response to cycloheximide (CHM) in the fast growing nucleoli of both early and late G 1 with an initial stimulation later counterbalanced by a depressed rate, so that nucleolar size in S was similar to control shortly afterwards the start of the CHM treatment. The initial enlargement under CHM was due to an increase of all nucleolar structural components, i.e., fibrillar, granular, vacuolar, and lacunar regions. No cycloheximide effect whatsoever was detected in S and G 2 nucleoli.

F. Guerrero

Papers

Control of nucleolar growth during interphase in higher plant meristem cells

Condensation of chromatin in the interphase of meristem cells depends on labile proteins