Endoreduplication and activation of the anaphase-promoting complex during symbiotic cell development

Postembryonic development of plant organs requires a constant interplay between the cell cycle and the developmental programs. Upon endo- and exogenous signals, plant cells can enter, exit or modify the cell cycle. Alteration of mitotic cycles to endoreduplication cycles, where the genome is duplicated without mitosis, is common in plants and may play a role in cell differentiation. The switch from the mitotic to endocycles is regulated by Ccs52A, a plant orthologue of the yeast and animal Cdhl proteins, acting as substrate-specific activator of the anaphase-promoting complex E3 ubiquitin ligase. Here, several aspects of endoreduplication are discussed with special attention on nitrogen-fixing nodule development where endoreduplication is an integral part of symbiotic cell differentiation.     

Potential role of the rice OsCCS52A gene in endoreduplication

In eukaryotes, the cell cycle consists of four distinct phases: G1, S, G2 and M. In certain condition, the cells skip M-phase and undergo endoreduplication. Endoreduplication, occurring during a modified cell cycle, duplicates the entire genome without being followed by M-phase. A cycle of endoreduplication is common in most of the differentiated cells of plant vegetative tissues and it occurs extensively in cereal endosperm cells. Endoreduplication occurs when CDK/Cyclin complex low or inactive caused by ubiquitin-mediated degradation by APC and their activators. In this study, rice cell cycle switch 52 A (OsCCS52A), an APC activator, is functionally characterized using the reverse genetic approach. In rice, OsCCS52A is highly expressed in seedlings, flowers, immature panicles and 15 DAP kernels. Localization studies revealed that OsCCS52A is a nuclear protein. OsCCS52A interacts with OsCdc16 in yeast. In addition, overexpression of OsCCS52A inhibits mitotic cell division and induces endoreduplication and cell elongation in fission yeast. The homozygous mutant exhibits dwarfism and smaller seeds. Further analysis demonstrated that endoreduplication cycles in the endosperm of mutant seeds were disturbed, evidenced by reduced nuclear and cell sizes. Taken together, these results suggest that OsCCS52A is involved in maintaining normal seed size formation by mediating the exit from mitotic cell division to enter the endoreduplication cycles in rice endosperm.     

The mitotic inhibitor ccs52 is required for endoreduplication and ploidy-dependent cell enlargement in plants.

Plant organs develop mostly post-embryonically from persistent or newly formed meristems. After cell division arrest, differentiation frequently involves endoreduplication and cell enlargement. Factors controlling transition from mitotic cycles to differentiation programmes have not been identified yet in plants. Here we describe ccs52, a plant homologue of APC activators involved in mitotic cyclin degradation. The ccs52 cDNA clones were isolated from Medicago sativa root nodules, which exhibit the highest degree of endopolyploidy in this plant. ccs52 represents a small multigenic family and appears to be conserved in plants. Overexpression of ccs52 in yeast triggered mitotic cyclin degradation, cell division arrest, endoreduplication and cell enlargement. In Medicago, enhanced expression of ccs52 was found in differentiating cells undergoing endoreduplication. In transgenic M.truncatula plants, overexpression of the ccs52 gene in the antisense orientation resulted in partial suppression of ccs52 expression and decreased the number of endocycles and the volume of the largest cells. Thus, the ccs52 product may switch proliferating cells to differentiation programmes which, in the case of endocycles, result in cell size increments.     

Mitotic activity of the pars intermedia in the female mouse: age-associated variations in proliferation rate and circadian periodicity

We previously reported daily variations in the mitotic activity of the endocrine cells in the pars intermedia of 21- and 28-day-old male mice. Since cellular proliferation might be affected by factors such as sex and age, we undertook the present experiments to study the mitotic activity of the pars intermedia from 14-, 28-, and 150-day-old female mice. Inbred C3H/S mice, grouped according to age, were housed under standard conditions (12h each of light and dark [LD 12:12]) for periodicity analysis and were killed in lots of 5-11 animals every 4h over a single 24h cycle, with each mouse receiving 2 μg/g of colchicine 4h before decapitation. Pituitaries were excised, extracted, fixed in buffered formaldehyde, embedded in celloidin-paraffin, sectioned at 5 μm, and stained with hematoxylin and eosin. We counted the total number of nuclei to estimate the total number of cells monitored and then calculated the mitotic index (metaphases/1000 nuclei). Differences were analyzed for statistical significance by the Student t test. While the 14-day-old animals manifested no significant changes in mitotic activity during the 24h cycle, the 28- and 150-day-old mice showed higher mitotic indices during the period of darkness. The average mitotic activity over the entire cycle, however, was higher in the two groups of younger animals than in the 150-day-old mice. Moreover, the averages for the 28-day-old females were higher than the corresponding values previously reported by us for male mice of the same age. (Chronobiology International, 17(6), 751-756, 2000)     

The Nuclear Endoreduplication Cycle in Metaxylem Cells of Primary Roots of Zea mays L.

The nuclear DNA content of metaxylem cells in roots of Zea mayscv. Golden Bantam reaches 16C or 32C by successive rounds ofDNA endoreduplication. Each phase of endoreduplication (endo-S)is separated by a non-DNA synthetic phase (endo-G). These phasesseem to occur in zones at fixed distances from the root tip.The duration of the phases in two of the endoreduplication cycles(4C–8C, 8C–16C) has been estimated in two ways.The first makes use of the rate of movement of cells throughthe positions along the root where the different phases of thecycle are occurring, the second uses labelling with methyl-[³H]thymidineand autoradiography. Both methods indicate that the endo-S phaseswhich cause the nuclear DNA content to rise from 4C to 8C andfrom 8C to 16C last 8–10 h, and that the intervening endo-Gphase lasts 8–12 h. DNA endoreduplication keeps pace withthe increase of nuclear volume; cell volume increases at a morerapid rate, however. Comparison of the endoreduplication cyclein the metaxylem with the mitotic cycle in the adjoining filesof parenchyma cells shows that the mitotic cells complete theircycle more slowly. DNA synthesis, endoreduplication cycle, mitotic cycle, root apex, Zea mays     

烟草乙烯转录因子ERF基因NtTOE3的克隆、载体构建及表达分析

ERF（Ethylene-responsive factor）是一类具有AP2特征结构域的乙烯应答转录因子,在响应植物的逆境胁迫应答中起关键作用。为明确ERF家族成员TOE3在烟草中的生物学功能和调控机制,同源克隆普通烟草K326中NtTOE3基因cDNA全长,利用实时荧光定量PCR（qRT-PCR）表征基因表达模式,结合生物信息学分析,对基因功能和调控机制进行初步预测。结果表明,该基因全长1 356 bp,编码451个氨基酸残基,预测分子量为49.84 ku。生物信息学分析表明,该蛋白是一个亲水蛋白,可能定位在细胞核,且与美花烟草（Nicotiana sylvestris）NsTOE3有96%的同源性,故命名为NtTOE3。组织表达分析发现,该基因在烟草根、茎、叶、花中均有表达,其中茎中表达量最高。逆境胁迫试验表明,该基因能快速响应低钾、高盐、干旱、H₂O₂、ABA和4℃等逆境处理。表明NtTOE3转录因子在烟草非生物胁迫中起着重要调节作用。