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.     

Real-time lineage analysis reveals oriented cell divisions associated with morphogenesis at the shoot apex of Arabidopsis thaliana

Precise knowledge of spatial and temporal patterns of cell division, including number and orientation of divisions, and knowledge of cell expansion, is central to understanding morphogenesis. Our current knowledge of cell division patterns during plant and animal morphogenesis is largely deduced from analysis of clonal shapes and sizes. But such an analysis can reveal only the number, not the orientation or exact rate, of cell divisions. In this study, we have analyzed growth in real time by monitoring individual cell divisions in the shoot apical meristems (SAMs) of Arabidopsis thaliana. The live imaging technique has led to the development of a spatial and temporal map of cell division patterns. We have integrated cell behavior over time to visualize growth. Our analysis reveals temporal variation in mitotic activity and the cell division is coordinated across clonally distinct layers of cells. Temporal variation in mitotic activity is not correlated to the estimated plastochron length and diurnal rhythms. Cell division rates vary across the SAM surface. Cells in the peripheral zone (PZ) divide at a faster rate than in the central zone (CZ). Cell division rates in the CZ are relatively heterogeneous when compared with PZ cells. We have analyzed the cell behavior associated with flower primordium development starting from a stage at which the future flower comprises four cells in the L1 epidermal layer. Primordium development is a sequential process linked to distinct cellular behavior. Oriented cell divisions, in primordial progenitors and in cells located proximal to them, are associated with initial primordial outgrowth. The oriented cell divisions are followed by a rapid burst of cell expansion and cell division, which transforms a flower primordium into a three-dimensional flower bud. Distinct lack of cell expansion is seen in a narrow band of cells, which forms the boundary region between developing flower bud and the SAM. We discuss these results in the context of SAM morphogenesis.     

The effect of a heterochronic mutation, Teopod2, on the cell lineage of the maize shoot

Teopod2 (Tp2) is a semi-dominant mutation of maize that prolongs the expression of characteristics normally confined to the juvenile phase of development. Two of the many dramatic morphological effects of this mutation are an increase in the number of vegetative nodes, and a reduction in the overall size of the shoot. To determine the cellular basis of these phenotypes, the technique of clonal analysis was used to compare the cell division patterns of wild-type and Tp2 plants. Our results indicate that Tp2 increases the number of vegetative nodes produced by the apicalmost cells in the meristem but does not affect the cell lineage of the basal, juvenile, part of the shoot. This result demonstrates that Tp2 does not act uniquely in a 'juvenile' domain of the meristem, but instead causes cells that are normally destined to produce adult structures to express juvenile traits inappropriately. Clonal analysis also demonstrates that Tp2 does not affect the size of the meristem prior to germination, nor does it affect the cell lineage of the basic structural unit of the stem, the phytomer. Thus the effects of this mutation on the size of the shoot are the result of changes in cell fate late in development.     

Cell-lineage patterns in the shoot apical meristem of the germinating maize embryo

A fate map for the shoot apical meristem of Zea mays L. at the time of germination was constructed by examining somatic sectors (clones) induced by -rays. The shoot apical meristem produced stem, leaves, and reproductive structures above leaf 6 after germination and the analysis here concerns their formation. On 160 adult plants which had produced 17 or 18 leaves, 277 anthocyanin-deficient sectors were scored for size and position. Sectors found on the ear shoot or in the tassel most often extended into the vegetative part of the plant. Sectors ranged from one to six internodes in length and some sectors of more than one internode were observed at all positions on the plant. Single-internode sectors predominated in the basal internodes (7,8,9) while longer sectors were common in the middle and upper internodes. The apparent number of cells which gave rise to a particular internode was variable and sectors were not restricted to the lineage unit: a leaf, the internode below it, and the axillary bud and prophyll at the base of the internode. These observations established two major features of meristem activity: 1) at the time of germination the developmental fate of any cell or group of cells was not fixed, and 2) at the time of germination cells at the same location in a meristem could produce greatly different amounts of tissue in the adult plant. Consequently, the developmental fate of specific cells in the germinating meristem could only be assigned in a general way.Abbreviations ACN apparent cell number - LI, LII, LI-LII sectors restricted to the epidermis, the subepidermis, or encompassing epidermis and subepidermis - PCN progenitor cell     

Transformation of plants via the shoot apex

Summary We have transformed petunia byAgrobacterium tumefaciens containing genes for kanamycin resistance and beta-glucuronidase using isolated shoot apices from seedling tissue. Regeneration of transformed plants in this model system was rapid. The technique of shoot apex transformation is an alternative for use inAgrobacterium-mediated transformation of dicotyledonous crop species for which a method of regeneration via protoplasts, leaf disks, or epidermal strips does not exist. This approach offers direct and rapid regeneration of plants and low risk of tissue-culture-induced genetic variation. Texas Agricultural Experiment Station Technical Article No. 23317.     

Cell lineage patterns in the shoot meristem of the sunflower embryo in the dry seed

We mapped the fate of cells in the shoot meristem of the dry-seed embryo of sunflower, Helianthus annuus L. cv. Peredovic, using irradiation-induced somatic sectors. We analyzed 249 chlorophyll-deficient or glabrous (hairless) sectors generated in 236 plants. Most sectors observed in the inflorescence extended into vegetative nodes. Thus cell lineages that ultimately gave rise to reproductive structures also contributed to vegetative structures. No single sector extended the entire length of the shoot. Thus the shoot is not derived from one or a few apical initials. Rather, the position, vertical extent, and width of the sectors at different levels of the shoot suggest that the shoot is derived from three to four circumferential populations of cells in each of three cell layers of the embryo meristem. Sectors had no common boundaries even in plants with two or three independent sectors, but varied in extent and overlapped along the length of the shoot. Thus individual cells in a single circumferential population behaved independently to contribute lineages of different vertical extents to the growing shoot. The predicted number of circumferential populations of cells as well as the apparent cell number in each population was consistent with the actual number of cells in the embryo meristem observed in histological sections.     

A gene regulatory network model for floral transition of the shoot apex in maize and its dynamic modeling

The transition from the vegetative to reproductive development is a critical event in the plant life cycle. The accurate prediction of flowering time in elite germplasm is important for decisions in maize breeding programs and best agronomic practices. The understanding of the genetic control of flowering time in maize has significantly advanced in the past decade. Through comparative genomics, mutant analysis, genetic analysis and QTL cloning, and transgenic approaches, more than 30 flowering time candidate genes in maize have been revealed and the relationships among these genes have been partially uncovered. Based on the knowledge of the flowering time candidate genes, a conceptual gene regulatory network model for the genetic control of flowering time in maize is proposed. To demonstrate the potential of the proposed gene regulatory network model, a first attempt was made to develop a dynamic gene network model to predict flowering time of maize genotypes varying for specific genes. The dynamic gene network model is composed of four genes and was built on the basis of gene expression dynamics of the two late flowering id1 and dlf1 mutants, the early flowering landrace Gaspe Flint and the temperate inbred B73. The model was evaluated against the phenotypic data of the id1 dlf1 double mutant and the ZMM4 overexpressed transgenic lines. The model provides a working example that leverages knowledge from model organisms for the utilization of maize genomic information to predict a whole plant trait phenotype, flowering time, of maize genotypes.     

The liguleless2 gene of maize functions during the transition from the vegetative to the reproductive shoot apex

During a maize plant's (Zea mays) development, the shoot apical meristem (SAM) generates an apex that proceeds through different phases: juvenile vegetative, adult vegetative and reproductive. During each phase the structures produced are distinguishable from structures produced during the other phases. In this paper, we demonstrate that the LIGULELESS2 (LG2) function is required for an accurate vegetative to reproductive phase transition. The maize gene liguleless2 (lg2) has been shown to encode a basic-leucine zipper (bZIP) protein and to function in narrowing the region from which the ligule and auricle develop in a typical maize leaf. Here we show that lg2 mutant plants can have reduced long tassel branches, extra vegetative leaves and extra husk leaves when compared to wild-type siblings. This indicates a role for the lg2 gene in the vegetative to reproductive phase transition of the shoot apex. We also discuss a potential role for the lg2 gene in general phase transition processes.     

Cell lineage patterns in maize embryogenesis: A clonal analysis

The embryonic cell lineage of the shoot meristem in maize (Zea mays L.) has been characterized using clonal analysis. Although there is considerable variability in the size and distribution of somatic sectors induced at a given time in embryogenesis, the fate of meristematic initials is not entirely random. During embryogenesis the number of cells in the presumptive shoot meristem increases and their fate becomes progressively more restricted. Cells toward the periphery of the presumptive meristem give rise to the lower nodes of the plant, while central cells form more distal nodes. Cell lineages become restricted to single nodes starting at the base of the plant, indicating that restriction of cell fate progresses from the periphery towards the center of the meristem. The number of twin shoots produced by X irradiation declines dramatically between 8 and 10 days after pollination, and is preceded by an acropetal progression in the level at which twinning occurs. This phenomenon suggests that the shoot meristem becomes determined gradually and that this process is completed between 8 and 10 days after pollination; i.e., just prior to or during the transition stage of development. At this stage the shoot primordium consists of 100–200 cells encompassing two or more cell layers of the embryo. The orientation of sectors that saddle the shoot (extend from one side of the shoot to the other) demonstrates that the shoot meristem arises from a region of the embryo containing longitudinally oriented cell files and that this cell pattern is at least partially preserved during shoot initation.     

Structure of the vegetative shoot apex ofCassiope lycopodioides

The structure of the vegetative shoot apex ofCassiope lycopodioides D. Don, which has a decussate leaf arrangement, was analyzed using trans- and longisections to generate a three-dimensional viewpoint. The apical dome of this species is relatively high from the middle to the maximal area phase of a plastochron. Therefore, the initial protrusion of a pair of leaf primordia occurs laterally on an apical dome conspicuously in contrast to the cases ofDaphne pseudo-mezereum andClethra barbinervis whose apices are nearly flat or slightly convex. The structure of the apex ofCassiope, however, may be understood with the concept of “apical sectors” on the same basis asDaphne andClethra (Hara, 1961, 1962, 1971a, b, c).     

Early effects of floral induction on cell division in the shoot apex of Silene

The changes in cell number, the relative proportions of interphase nuclei with different amounts of DNA, mitotic index and labelling index have been investigated in the shoot apex of Silene coeli-rosa L. (a long-day plant) during the first long day of photoinduction, and compared with the corresponding changes in plants in short days. 3 h after the start of induction the proportion of nuclei in the G2 phase of the cell cycle had increased, the mitotic index tended to be higher, and the labelling index was lower than in plants in short days. 8–9 h later the values for plants in the long day had become similar to those for plants in short days. No evidence was obtained for a synchronisation of cells in one phase of the cell cycle as a result of photoinduction. The results obtained were consistent with a temporary shortening of the cell cycle in the induced apices over the first long day which resulted in a greater increase in cell number by the end of the first day of photoinduction than in plants in short days.Abbreviations LD long day - SD short day     

Rates of cell division in the young prefloral shoot apex ofChrysanthemum segetum L.

Summary The rate of cell division was determined by the colchicine induced metaphase-accumulation technique in the young prefloral shoot apex of the quantitative long-day plantChrysanthemum segetum L. growing under conditions favourable to flowering (16-hour photoperiod; 124Em^–2s^–1; 22 °C). Cell cycle duration was evaluated in relation to the location of the cells in the intact apex. The cell cycle durations were 53.5 hours, 47.4 hours, and 97.7 hours in the axial, lateral and subapical central cells respectively. Compared with previous results, these data give evidence of the major role played by the early increase in cell division rate of axial cells in the new pattern of the prefloral shoot apex at its initial stage of development. By comparison with the vegetative shoot apex, the cell cycle duration was preferentially shortened in the axial zone; it was only slightly altered in the lateral zone while it was lengthened in the vacuolating subapical central cells. In the three zones within the prefloral shoot apex, the duration of mitosis was constant (3.2 to 3.3 hours) and the same as in the vegetative shoot apex.     

Live-imaging stem-cell homeostasis in the Arabidopsis shoot apex

A precise spatio-temporal regulation of growth and differentiation is crucial to maintain a stable population of stem cells in the shoot apical meristems (SAMs) of higher plants. The real-time and simultaneous observations of dynamics of cell identity transitions, growth patterns, and signaling machinery involved in cell-cell communication is crucial to gain a mechanistic view of stem-cell homeostasis. In this article, I review recent advances in understanding the regulatory dynamics of stem-cell maintenance in Arabidopsis thaliana and discuss future challenges involved in transforming the static maps of genetic interactions into a dynamic framework representing functional molecular and cellular interactions in living SAMs.     

The cell cycle in the shoot apex ofSilene during the first day of floral induction

Summary The length of the cell cycle was measured in the shoot apical meristem ofSilene coeli-rosa during the first day of an inductive photoperiod. The length of the cell cycle in the shoot apex of vegetative controls (those in short days) was about 18–20 hours. Exposure of plants to the long day resulted in an immediate shortening of the cell cycle to about 13 hours, roughly two thirds of that in short days. Measurements of the component phases of the cell cycle revealed that the shortened cycle in long days was the result of a decrease in the length of G 1 and perhaps S, whilst G 2 and M remained constant.     

The role of the shoot apex in geotropism*

Abstract. The belief that the shoot apex plays a special role in geotropism is shown to be erroneous and the implications of this widely held misconception are discussed.