Developmental control of cell division patterns in the shoot apex

The shoot apical meristem is a group of rapidly dividing cells that generate all aerial parts of the plant. It is a highly organised structure, which can be divided into functionally distinct domains, characterised by specific proliferation rates of the individual cells. Genetic studies have enabled the identification of regulators of meristem function. These factors are involved in the formation and maintenance of the meristem, as well as in the formation of the primordia. Somehow, they must also govern cell proliferation rates within the shoot apex. Possible links between meristem regulators and the cell cycle machinery will be discussed. In order to analyse the role of cell proliferation in development, cell cycle gene expression has been perturbed using transgenic approaches and mutation. The effect of these alterations on growth and development at the shoot apex will be presented. Together, these studies give a first insight into the regulatory networks controlling the cell cycle and into the significance of cell proliferation in plant development.     

Mitotic activity and cell elongation in geostimulated roots of Zea mays

Mitotic activity was investigated in the primary meristem of horizontally oriented excised root tips of Zea mays during the first six hours of their georeaction. The only statistically significant change that could be detected in the meristem was a decrease of the length of its upper half. No significant difference in mitotic activity was found between the upper and lower halves of roots kept continuously horizontal for 6 h. Cell proliferation thus seems relatively insensitive to changes in the redistribution of endogenous growth regulators that are believed to occur within the meristem during the onset of geotropism. In the zone of bending proximal to the meristem cell length was significantly greater in the upper half than in either the lower half or in the equivalent position in vertical control roots. Thus, cell elongation seems to be promoted in the upper half of the horizontal root. Thus, The differences in cell length were not accompanied by any change in the proportion of nuclei synthesising DNA in these elongating, non-meristematic cells.     

The essential gene EMB1611 maintains shoot apical meristem function during Arabidopsis development

The Arabidopsis thaliana genome contains hundreds of genes essential for seed development. Because null mutations in these genes cause embryo lethality, their specific molecular and developmental functions are largely unknown. Here, we identify a role for EMB1611/MEE22 , an essential gene in Arabidopsis, in shoot apical meristem maintenance. EMB1611 encodes a large, novel protein with N-terminal coiled-coil regions and two putative transmembrane domains. We show that the partial loss-of-function emb1611-2 mutation causes a range of pleiotropic developmental phenotypes, most dramatically a progressive loss of shoot apical meristem function that causes premature meristem termination. emb1611-2 plants display disorganization of the shoot meristem cell layers early in development, and an associated stem cell fate change to an organogenic identity. Genetic and molecular analysis indicates that EMB1611 is required for maintenance of the CLV-WUS stem cell regulatory pathway in the shoot meristem, but also has WUS -independent activity. In addition, emb1611-2 plants have reduced shoot and root growth, and their rosette leaves form trichomes with extra branches, a defect we associate with an increase in endoreduplication. Our data indicate that EMB1611 functions to maintain cells, particularly those in the shoot meristem, roots and developing rosette leaves, in a proliferative or uncommitted state.     

On the constancy of cell division rate in the root meristem

This review examines under what circumstances the rate of cell division among cells of the root meristem is known to vary. First, methods are compared that have been used to quantify cell division rate. These can be grouped as being either cytological, in which the rate of accumulation of cells in a particular phase of the cell cycle is determined based on some form of cytological labeling, or kinematic, in which the rate of cell accumulation is determined from the net movement of cells. Then, evidence is reviewed as to whether cell division rates vary between different tissues or cell types, between different positions in the root, or finally between different environments. The evidence is consistent with cells dividing at a constant rate, and well documented examples where cell division rate changes substantially are rare. The constancy of cell division rate contrasts with the number of dividing cells, which varies extensively, and implies that a major point for cell cycle control is governing the exit from the proliferative state at the basal boundary of the meristem.     

Cyclin‐dependent kinase activity maintains the shoot apical meristem cells in an undifferentiated state

As the shoot apex produces most of the cells that comprise the aerial part of the plant, perfect orchestration between cell division rates and fate specification is essential for normal organ formation and plant development. However, the inter‐dependence of cell‐cycle machinery and meristem‐organizing genes is still poorly understood. To investigate this mechanism, we specifically inhibited the cell‐cycle machinery in the shoot apex by expression of a dominant negative allele of the A‐type cyclin‐dependent kinase (CDK) CDKA;1 in meristematic cells. A decrease in the cell division rate within the SHOOT MERISTEMLESS domain of the shoot apex dramatically affected plant growth and development. Within the meristem, a subset of cells was driven into the differentiation pathway, as indicated by premature cell expansion and onset of endo‐reduplication. Although the meristem structure and expression patterns of the meristem identity genes were maintained in most plants, the reduced CDK activity caused splitting of the meristem in some plants. This phenotype correlated with the level of expression of the dominant negative CDKA;1 allele. Therefore, we propose a threshold model in which the effect of the cell‐cycle machinery on meristem organization is determined by the level of CDK activity.     

植物干细胞决定基因WUS的研究进展

WUS(WUSCHEL)基因编码一转录因子,它的存在使周围细胞具有干细胞的特征,与之相关的信号系统近年逐步被阐明.在茎尖分生组织内WUS和CLV(CLAVATA)之间形成一个反馈调节环,使得干细胞保持自我更新,维持茎尖的顶端优势.在胚胎分生组织内,CLV3的表达只依赖于WUS的存在,然而在胚以后的发育中,CLV3的表达受到WUS和STM(SHOOTMERISTEMLESS)的双重调节,启动器官发生.在花分生组织中,WUS和LFY(LEAFY)共同激活AG(AGAMOUS)基因的表达,WUS受AG的反馈抑制.由WUS建立的信号体系还参与胚珠的发育.当WUS蛋白和生长素共存时,可以高效启动体细胞胚的发生.细胞对WUS信号的感应性与细胞所处的微环境有关,WUS在不同环境条件下可以启动不同的下游基因表达.     

Defective cell proliferation in the floral meristem of alloplasmic plants of Nicotiana tabacum leads to abnormal floral organ development and male sterility

Flowers of an alloplasmic male-sterile tobacco line, comprised of the nuclear genome of Nicotiana tabacum and the cytoplasm of Nicotiana repanda, develop short, poorly-pigmented petals and abnormal sterile stamens that often are fused with the carpel wall. The development of flower organ primordia and establishment of boundaries between the different zones in the floral meristem were investigated by performing expression analysis of the tobacco orthologs of the organ identity genes GLO, AG and DEF. These studies support the conclusion that boundary formation was impaired between the organs produced in whorls 3 and 4 resulting in partial fusions between anthers and carpels. According to the investigations cell divisions and floral meristem size in the alloplasmic line were drastically reduced in comparison with the male-fertile tobacco line. The reduction in cell divisions leads to a discrepancy between cell number and cell determination at the stage when petal and stamen primordia should be initiated. At the same stage expression of the homeotic genes was delayed in comparison with the male-fertile line. However, the abnormal organ development was not due to a failure in the spatial expression of the organ identity genes. Instead the aberrant development in the floral organs of whorls 2, 3 and 4 appears to be caused by deficient floral meristem development at an earlier stage. Furthermore, defects in cell proliferation in the floral meristem of the alloplasmic male-sterile line correlates with presence of morphologically modified mitochondria. The putative causes of reduced cell number in the floral meristem and the consequences for floral development are discussed.     

Epidermal Patterning Genes Impose Non-cell Autonomous Cell Size Determination and have Additional Roles in Root Meristem Size Control

The regulation of cellular growth is of vital importance for embryonic and postembryonic patterning. Growth regulation in the epidermis has importance for organ growth rates in roots and shoots, proposing epidermal cells as an interesting model for cellular growth regulation. Here we assessed whether the root epidermis is a suitable model system to address cell size determination. In Arabidopsis thaliana L., root epidermal cells are regularly spaced in neighbouring tricho- (root hair) and atrichoblast (non-hair) cells, showing already distinct cell size regulation in the root meristem. We determined cell sizes in the root meristem and at the onset of cellular elongation, revealing that not only division rates but also cellular shape is distinct in tricho- and atrichoblasts. Intriguingly, epidermal-patterning mutants, failing to define differential vacuolization in neighbouring epidermal cell files, also display non-differential growth. Using these epidermal-patterning mutants, we show that polarized growth behaviour of epidermal tricho- and atrichoblast is interdependent, suggesting non-cell autonomous signals to integrate tissue expansion. Besides the interweaved cell-type-dependent growth mechanism, we reveal an additional role for epidermal patterning genes in root meristem size and organ growth regulation. We conclude that epidermal cells represent a suitable model system to study cell size determination and interdependent tissue growth.     

Herbivory could unlock mutations sequestered in stratified shoot apices of genetic mosaics

Many higher plants have shoot apical meristems that possess discrete cell layers, only one of which normally gives rise to gametes following the transition from vegetative meristem to floral meristem. Consequently, when mutations occur in the meristems of sexually reproducing plants, they may or may not have an evolutionary impact, depending on the apical layer in which they reside. In order to determine whether developmentally sequestered mutations could be released by herbivory (i.e., meristem destruction), a characterized genetic mosaic was subjected to simulated herbivory. Many plants develop two shoot meristems in the leaf axils of some nodes, here referred to as the primary and secondary axillary meristems. Destruction of the terminal and primary axillary meristems led to the outgrowth of secondary axillary meristems. Seed derived from secondary axillary meristems was not always descended from the second apical cell layer of the terminal shoot meristem as is expected for terminal and primary shoot meristems. Vegetative and reproductive analysis indicated that secondary meristems did not maintain the same order of cell layers present in the terminal shoot meristem. In secondary meristems reproductively sequestered cell layers possessing mutant cells can be repositioned into gamete-forming cell layers, thereby adding mutant genes into the gene pool. Herbivores feeding on shoot tips may influence plant evolution by causing the outgrowth of secondary axillary meristems.     

On the potential for evolutionary change in meristematic cell lineages through intraorganismal selection

In the absence of sexual recombination somatic mutations represent the only source of genetic variation in clonally propagating plants. We analyse the probability of such somatic mutations in the shoot apical meristem being fixed in descendant generations of meristems. A model of meristem cell dynamics is presented for the unstratified shoot apical meristem. The fate of one mutant initial is studied for a two- and three-celled shoot apical meristem. The main parameters of the model are the number of apical initials, the time between selection cycles, number of selection cycles and cell viability of the mutant genotype. As the number of mitotic divisions per selection cycle and number of selection cycles increases the chimeric state dissipates and the probability of mutation fixation approaches an asymptote. The value of this fixation asymptote depends primarily on cell viability, while the time to reach it is mainly influenced by the total number of mitotic divisions as well as the number of initials. In contrast to the presumed operation of Muller’s Ratchet in plants the chimeric state may represent an opportunity for deleterious mutations to be eliminated through intraorganismal selection or random drift. We conclude that intraorganismal selection not only can be a substantial force for the elimination of deleterious mutations, but also can have the potential to confer an evolutionary change through a meristematic cell lineage alone.     

Tissue- and development-specific distributions of cytoskeletal elements in growing cells of the maize root apex

ABSTRACT

Indirect immunofluorescence performed using sections of actively growing maize root apices fixed and then embedded in low-melting-point Steedman's wax has proved efficient in revealing the arrangements and reorganizations of motility-related cytoskeletal elements which are associated with root cell development and tissue differentiation. This powerful, yet relatively simple, technique shows that specific rearrangements of both microtubular (MT) and actin microfilament (MF) arrays occur in cells as they leave the meristem and traverse the transitional region interpolated between meristem and elongation region. Cytoskeletal and growth analyses have identified the transition zone as critical for both cell and root development; it is in this zone that cell growth is channelled, by the cytoskeleton, into a strictly polarized mode which enables root tips to extend rapidly through the soil in search of water and nutrients. An integrated cytoskeletal network is crucial for both the cytomorphogenesis of individual cells and the overall morphogenesis of the plant body. The latter process can be viewed as a reflection of the tight control which cytoskeletal networks exert not only over cell division planes in the cells within meristematic apices but also over the orientation of cell growth in the meristem and elsewhere. Endoplasmic MTs interconnecting the plasma membrane with the nucleus are suggested to be involved in cell division control; they may also act as a two-way cytoskeletal communication channel for signals passing to and fro between the extracellular environment and the genome. Moreover, the dynamism of endoplasmic MTs exerts direct effects on chromatin structure and the accompanying nuclear architecture and hence can help exert a cellular level of control over cell growth and cell cycle progression. Because the inherent dynamic instability of MTs depends on the concentration of tubulin dimers within the cytoplasm, we propose that when asymmetric cell division occurs, it will result in two daughter cells which differ in the turnover rates of their MTs. This phenomenon could be responsible for different cell fates of daughter plant cells produced by such cell divisions.     

Elevated CO2 stimulates cells to divide in grass meristems: a differential effect in two natural populations of Dactylis glomerata

In this study, we tested the hypothesis that elevated [CO₂] shortens the cell cycle in meristems of Dactylis glomerata, more in a Portuguese population (38°53′N) than in a Swedish population (63°09′N). In the shoot meristem, the cell cycle shortened to about the same extent (～ 26%) in both populations exposed to the elevated [CO₂] treatment. In the root meristem, the cell cycle shortened by 17% in the Portuguese and by 8% in the Swedish population. However, the proportion of rapidly cycling cells increased in the Portuguese much more than in the Swedish population in both meristems. In the root meristem, there was a 1.86-fold increase in the Portuguese compared with a 1.31-fold increase in the Swedish. In the shoot meristem, the increases were 1.5–3-fold for the Portuguese and 1.2-fold for the Swedish. The data are consistent in showing that a major response to the elevated [CO₂] treatment was an increase in the proportion of cells that were cycling and that this was more marked for the Portuguese population. A more general response to the elevated [CO₂] treatment was a shortening of the cell cycle regardless of population.     

High level histone H2A gene expression during early stages of adventitious bud formation in Norway spruce (Picea abies)

We have demonstrated the correlation between cell division and the expression of a histone H2A-encoding gene, His2A , in Norway spruce. Picea abies (L.) Karst and used a cDNA clone in in situ hybridization experiments to monitor the cytokinin-induced cell division during early stages of adventitious bud development. A general stimulation of division of epidermal and cortical cells followed upon the cytokinin treatment. After two weeks in culture a high mitotic activity was detected only in single cells or small groups of cells in the epidermis and subepidermal cell layers. These cells presumably constitute the early stages of meristem primordia. The small clusters of dividing cells enlarge and subsequently form adventitious buds. Cells of the meristem and needle primordia of adventitious buds divide frequently as do the corresponding cells in vegetative buds. A quiescent center is distinguished within the apical meristem of vegetative buds. These cells, in the summit of the domed meristem, divide with a considerably lower frequency than cells in the flanking region. Differences in the temporal expression pattern of the histone H2A gene in cells of the vascular tissue, detected between embryos germinating in vitro and bud-induced embryos, suggest that the cytokinin treatment affects the timing of cell divisions in the differentiating procambium.     

Nutrient Sensing in Plant Meristems

Plants need nutrient to grow and plant cells need nutrient to divide. The meristems are the factories and cells that are left behind will expand and differentiate. However, meristems are not simple homogenous entities; cells in different parts of the meristem do different things. Positional cues operate that can fate cells into different tissue domains. However, founder/stem cells persist in specific locations within the meristem e.g. the quiescent centre of root apical meristem (RAM) and the lower half of the central zone of the shoot apical meristem (SAM). Given the complexity of meristems, do their cells simply respond to a diffusing gradient of photosynthate? This in turn begs the question, why do stem cell populations tend to have longer cell cycles than their immediate descendants given that like all other cells they are directly in the path of diffusing nutrient? In this review, we have examined the extent to which nutrient sensing might be operating in meristems. The scene is set for sugar sensing, the plant cell cycle, SAMs and RAMs. Special emphasis is given to the metabolic regulator, SnRK1 (SNF1-related protein kinase 1), hexokinase and the trehalose pathway in relation to sugar sensing. The unique plant cell cycle gene, cylin-dependent kinase B1;1 may have evolved to be particularly responsive to sugar signalling pathways. Also, the homeobox gene, STIMPY, emerges strongly as a link between sugar sensing, plant cell proliferation and development. Flowering can be influenced by sucrose and glucose levels and both meristem identity and organ identity genes could well be differentially sensitive to sucrose and glucose signals. We also describe how meristems deal with extra photosynthate as a result of exposure to elevated CO₂. What we review are numerous instances of how developmental processes can be affected by sugars/nutrients. However, given the scarcity of knowledge we are unable to provide uncontested links between nutrient sensing and specific activities in meristems.