Cold nights impair leaf growth and cell cycle progression in maize through transcriptional changes of cell cycle genes

Low temperature inhibits the growth of maize (Zea mays) seedlings and limits yield under field conditions. To study the mechanism of cold-induced growth retardation, we exposed maize B73 seedlings to low night temperature (25 degrees C /4 degrees C, day/night) from germination until the completion of leaf 4 expansion. This treatment resulted in a 20% reduction in final leaf size compared to control conditions (25 degrees C/18 degrees C, day/night). A kinematic analysis of leaf growth rates in control and cold-treated leaves during daytime showed that cold nights affected both cell cycle time (+65%) and cell production (-22%). In contrast, the size of mature epidermal cells was unaffected. To analyze the effect on cell cycle progression at the molecular level, we identified through a bioinformatics approach a set of 43 cell cycle genes and analyzed their expression in proliferating, expanding, and mature cells of leaves exposed to either control or cold nights. This analysis showed that: (1) the majority of cell cycle genes had a consistent proliferation-specific expression pattern; and (2) the increased cell cycle time in the basal meristem of leaves exposed to cold nights was associated with differential expression of cell cycle inhibitors and with the concomitant down-regulation of positive regulators of cell division.     

Developmental, cytological and transcriptional analysis of autotetraploid Arabidopsis

An autopolyploid that contains more than two sets of the same chromosomes causes apparent alterations in morphology, development, physiology and gene expression compared to diploid. However, the mechanisms for these changes remain largely unknown. In the present study, cytological observations of mature embryos and growing cotyledons demonstrated that enlarged organ size of an autotetraploid Arabidopsis was caused by cell size and not by cell number. Quantitative real time PCR (qRT-PCR) analysis of 34 core cell cycle genes revealed a subtle but stable increase in the expression of ICK1, ICK2 and ICK5 in autotetraploid seedlings. Autotetraploid Arabidopsis plants were found to be more sensitive to glucose treatment than diploid with decreased number of rosette leaves and suppressed root elongation. Cytological observations demonstrated that both cell proliferation and cell expansion of autotetraploid were dramatically suppressed under glucose treatment. Expression levels of ICK1, ICK5 together with Cyclin D and Cyclin B was increased under glucose treatment in both diploid and autotetraploid plants. These results suggest that ICK1 and ICK5 may be involved in developmental delay and that the suppressed growth under glucose treatment probably resulted from disturbed mitotic and endoreduplication cycle in autotetraploid Arabidopsis.     

Exit from proliferation during leaf development in Arabidopsis thaliana: a not-so-gradual process

Early leaf growth is sustained by cell proliferation and subsequent cell expansion that initiates at the leaf tip and proceeds in a basipetal direction. Using detailed kinematic and gene expression studies to map these stages during early development of the third leaf of Arabidopsis thaliana, we showed that the cell-cycle arrest front did not progress gradually down the leaf, but rather was established and abolished abruptly. Interestingly, leaf greening and stomatal patterning followed a similar basipetal pattern, but proliferative pavement cell and formative meristemoid divisions were uncoordinated in respect to onset and persistence. Genes differentially expressed during the transition from cell proliferation to expansion were enriched in genes involved in cell cycle, photosynthesis, and chloroplast retrograde signaling. Proliferating primordia treated with norflurazon, a chemical inhibitor of retrograde signaling, showed inhibited onset of cell expansion. Hence, differentiation of the photosynthetic machinery is important for regulating the exit from proliferation.     

Response of cassava leaf area expansion to water deficit: cell proliferation, cell expansion and delayed development

BACKGROUND AND AIMS: Cassava (Manihot esculenta) is an important food crop in the tropics that has a high growth rate in optimal conditions, but also performs well in drought-prone climates. The objectives of this work were to determine the effects of water deficit and rewatering on the rate of expansion of leaves at different developmental stages and to evaluate the extent to which decreases in cell proliferation, expansion, and delay in development are responsible for reduced growth. METHODS: Glasshouse-grown cassava plants were subjected to 8 d of water deficit followed by rewatering. Leaves at 15 developmental stages from nearly full size to meristematic were sampled, and epidermal cell size and number were measured on leaves at four developmental stages. KEY RESULTS: Leaf expansion and development were nearly halted during stress but resumed vigorously after rewatering. In advanced-stage leaves (Group 1) in which development was solely by cell expansion, expansion resumed after rewatering, but not sufficiently for cell size to equal that of controls at maturity. In Group 2 (cell proliferation), relative expansion rate and cell proliferation were delayed until rewatering, but then recovered partially, so that loss of leaf area was due to decreased cell numbers per leaf. In Group 3 (early meristematic development) final leaf area was not affected by stress, but development was delayed by 4-6 d. On a plant basis, the proportion of loss of leaf area over 26 d attributed to leaves at each developmental stage was 29, 50 and 21 % in Group 1, 2 and 3, respectively. CONCLUSIONS: Although cell growth processes were sensitive to mild water deficit, they recovered to a large extent, and much of the reduction in leaf area was caused by developmental delay and a reduction in cell division in the youngest, meristematic leaves.     

力生长因子E肽对成骨细胞增殖及基因表达的影响

为了探索力生长因子羧基端E结构域的后24个氨基酸组成的短肽(MGF-Ct24E)对成骨细胞生物学活性的影响,通过组织块培养法获得大鼠原代成骨细胞,采用MTT法和流式细胞仪检测细胞的增殖及细胞周期分布情况,基因芯片技术检测细胞基因表达谱,并用定量PCR实验验证芯片检测结果。结果显示MGF-Ct24E组的细胞增殖活性明显高于对照组,且在培养第一天促增殖效果最为显著。细胞周期结果显示MGF-Ct24E显著提高了S期和G2/M期的细胞所占比例。基因芯片检测发现差异表达基因共1397个,其中上调922,下调475,且差异表达的基因主要是关于细胞的增殖分化调节,生长因子结合和活性调节等方面。MGF-Ct24E对成骨细胞的这种增殖分化调控提示MGF-Ct24E在促进骨修复方面有着潜在的应用价值。     

Brassinosteroids control the proliferation of leaf cells of Arabidopsis thaliana

The growth of leaves in the model plant, Arabidopsis thaliana (L.) Heynh., is determined by the extent of expansion of individual cells and by cell proliferation. Mutants of A. thaliana with known defects in the biosynthesis or perception of brassinosteroids develop small leaves. When the leaves of brassinosteroid-related mutants, det2 (de-etiolated2 = cro1) and dwf1 (dwarf1 = cro2) were compared to wild-type plants, an earlier cessation of leaf expansion was observed; a detailed anatomical analysis further revealed that the mutants had fewer cells per leaf blade. Treatment of the det2 mutants with the brassinosteroid, brassinolide, reversed the mutation and restored the potential for growth to that of the wild type. Restoration of leaf size could not be explained solely on the basis of an increase in individual cell volume, thus suggesting that brassinosteroids play a dual role in regulating cell expansion and proliferation.     

Transcriptome analysis of age-related gain of callus-forming capacity in Arabidopsis hypocotyls

Callus-forming capacity is enhanced with hypocotyl maturity in Arabidopsis. However, the genetic regulation of age-related gain in capacity for callus formation is unclear. We used a gene expression microarray assay to characterize the underlying mechanisms during callus formation in young and mature hypocotyl explants of Arabidopsis. As expected, genes involved in photosynthesis and cell wall thickening showed altered expression during hypocotyl maturation. In addition, genes involved in cytokinin perception were enriched in mature hypocotyl tissues. Phytohormone-induced callus formation in hypocotyl explants was accompanied by increased expression of genes mainly related to the cell cycle, histones and epigenetics. The induction level of these genes was higher in mature hypocotyl explants than young explants during callus formation. We identified a number of genes, including those with unknown function, potentially involved in age-related gain in callus formation. Our results provide insight into the effect of hypocotyl age on callus formation. Altered cytokinin signaling components, cell cycle regulation and epigenetics may work in concert to lead to gain of callus-forming capacity in hypocotyls with age.     

Effects of polysaccharides derived from Orostachys japonicus on induction of cell cycle arrest and apoptotic cell death in human colon cancer cells

Crude Orostachys japonicus polysaccharide extract (OJP) was prepared by hot steam extraction. Polysaccharides (OJPI) were separated from OJP by gel filtration chromatography and phenol-sulfuric acid assay. The average molecular weight of the OJPI was 30-50 kDa. The anti-proliferative effect of OJPI on HT-29 human colon cancer cells was investigated via morphology study, cell viability assay, apoptosis assay, cell cycle analysis, and cDNA microarray. OJPI inhibited proliferation and growth of HT29 cells and also stimulated apoptosis in a dose- and time-dependent manner. In cell cycle analysis, treatment with OJPI resulted in a marked increase of cells in the G0 (sub G1) and G2/M phases. To screen for genes involved in the induction of cell cycle arrest and apoptosis, the gene expression profiles of HT-29 cells treated with OJPI were examined by cDNA microarray, revealing that a number of genes were up- or down-regulated by OJPI. Whereas several genes involved in anti-apoptosis, cell proliferation and growth, and cell cycle regulation were down-regulated, expression levels of several genes involved in apoptosis, tumor suppression, and other signal transduction events were up-regulated. These results suggest that OJPI inhibits the growth of HT-29 human colon cancer cells by various apoptosis-aiding activities as well as apoptosis itself. Therefore, OJPI deserve further development as an effective agent exhibiting anticancer activity.     

miR396-targeted AtGRF transcription factors are required for coordination of cell division and differentiation during leaf development in Arabidopsis

In plants, cell proliferation and polarized cell differentiation along the adaxial-abaxial axis in the primordium is critical for leaf morphogenesis, while the temporal-spatial relationships between these two processes remain largely unexplored. Here, it is reported that microRNA396 (miR396)-targeted Arabidopsis growth-regulating factors (AtGRFs) are required for leaf adaxial-abaxial polarity in Arabidopsis. Reduction of the expression of AtGRF genes by transgenic miR396 overexpression in leaf polarity mutants asymmetric leaves1 (as1) and as2 resulted in plants with enhanced leaf adaxial-abaxial defects, as a consequence of reduced cell proliferation. Moreover, transgenic miR396 overexpression markedly decreased the cell division activity and the expression of cell cycle-related genes, but resulted in an increased percentage of leaf cells with a higher ploidy level, indicating that miR396 negatively regulates cell proliferation by controlling entry into the mitotic cell cycle. miR396 is mainly expressed in the leaf cells arrested for cell division, coinciding with its roles in cell cycle regulation. These results together suggest that cell division activity mediated by miR396-targeted AtGRFs is important for polarized cell differentiation along the adaxial-abaxial axis during leaf morphogenesis in Arabidopsis.