AN INVESTIGATION OF THE PHYLOGENETIC AND ONTOGENETIC VARIABILITY OF SHOOT APICAL MERISTEMS IN THE CACTACEAE

The sizes, shapes and zonations of the shoot apical meristems of 22 species of cacti were examined. This family was chosen because of its great diversity of habits; the more primitive members are nonsucculent. leafy trees and more advanced members are highly specialized “leaf-less” stem-succulents. By combining these measurements with those already in the literature, a sample of almost 70 species was obtained. Apical meristems range in size from only 80 μm in diam in some species to as much as 1.500 μm in diam in others. The shape ranges from being flat to almost hemispherical. Despite the great range in size and shape of the apical meristems, or the range in the morphologies of the leaves and stems which are produced by the meristems. all apices had the usual zonation: tunica, central mother cells, peripheral zone, and pith-rib meristem. The sizes of each of the zones. expressed either as the number of cells per zone or expressed as a percentage of the whole apex. were highly variable. The variation in apical dimensions and zone sizes occurred both phylogenetically and ontogenetically. and this has been interpreted to indicate that the morphogenetic mechanisms which control apical size and zonation are easily modified, both during the development of individual plants and during the evolution of new species.     

PRIMORDIAL LEAF AND PHYTOHORMONE EFFECTS ON EXCISED SHOOT APICAL MERISTEMS OF COLEUS BLUMEI BENTH.

Coleus blumei Benth. apical meristems and apical meristems +1, +2, +3 primordial leaf pairs were cultured to examine phytohormone influences on development and correlative effects of developing primordial leaves on in vitro responses. The meristem with no phytohormones or low levels of IAA could not develop in vitro. At least 0.1 mg/l IAA and optimumly 1-2 mg/l IAA were required for development into complete plants. IAA from 0.1 to 3 mg/l also resulted in root development with no apparent leaf or shoot formation. Levels of IAA higher than 3 mg/l were inhibitory to development. Kinetin, as a substitute for naturally occurring cytokinins, alone (0.0003 to 3 mg/l) resulted in development of rosettes of leaves. In the presence of IAA (***1 mg/l) and kinetin (0.003 mg/l) plants, rosettes, individual leaves with roots, and roots developed from isolated meristems. Glutamine and adenine sulfate both appeared inhibitory to meristem development. With +1, +2, +3 developing primordial leaf pairs left attached to the apical dome, three pairs were required for plant formation in the absence of phytohormones. In the presence of IAA, two pairs of primordial leaves resulted in plant formation; whereas, with IAA and low levels of kinetin one pair of primordial leaves was enough. Higher levels of kinetin were inhibitory to plant development with primordial leaves present. ABA appeared to be inhibitory to development of meristems and meristems +1, +3 primordial leaves at low concentrations and resulted in death at ***1 mg/l. Developing primordial leaves appear to supply the apical meristem with a balance of phytohormones during growth. Meristem development into a plant first involved formation of leaf primordia. Establishment of a bipolar axis with root formation followed.     

SHOOT APICAL MERISTEMS AND MUTATION: FIXATION OF SELECTIVELY NEUTRAL CELL GENOTYPES

Shoot apical meristems are interpreted as either structured, that is having a permanent set of apical initials, or stochastic, having apical initials which represent “... momentary representatives of the continuous meristematic residue at the apex of the relevant layer or zone” (Newman, 1965). The two main parameters of stochastic growth are the average number of apical initials (α) and the number of mitotic cycles (r) of the initials and their daughter cells prior to the random selection of subsequent initials. Mathematical analysis and computer simulation studies of stochastic growth have shown that if one starts with 1 mutant initial and α-1 nonmutant initials, eventually a mosaic plant results. The frequency of shoot apices composed of mutant cells is 1/α and the frequency of shoot apices composed of only nonmutant cells is (1 – α)/α. These asymptotics are only attained after considerable growth, thus mericlinal chimeras can persist for many nodes and give the appearance that a permanent set of initials is present.     

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.     

SOME CASES OF DELAYED OR INDUCED DEVELOPMENT OF AXILLARY BUDS FROM PERSISTING DETACHED MERISTEMS IN CONIFERS

In the apparently “empty” axils of the needles of Taxus baccata, Sequoia sempervirens, Sequoiadendron giganteum, Cryptomeria japonica, Thuja occidentalis, and Thujopsis dolabrata persisting detached meristems were found, which are derived from superficial layers of the apical eumeristem. In T. baccata delayed development of minute axillary buds occurs from these meristems after 1–4 yr on the intact plant. In the other conifers, development of additional axillary buds from these meristems was induced by natural frost damage or by artificial pruning and disbudding. The discovery of these detached meristems is discussed with regard to the regenerative capacity of the conifers in comparison to other plants.     

Shoot meristem size is dependent on inbred background and presence of the maize homeobox gene, knotted1

The knotted1 (kn1) gene of maize is expressed in meristems and is absent from leaves, including the site of leaf initiation within the meristem. Recessive mutations of kn1 have been described that limit the capacity to make branches and result in extra carpels. Dominant mutations suggest that kn1 function plays a role in maintaining cells in an undifferentiated state. We took advantage of a Ds-induced dominant allele in order to screen for additional recessive alleles resulting from mobilization of the Ds element. Analysis of one such allele revealed a novel embryonic shoot phenotype in which the shoot initiated zero to few organs after the cotyledon was made, resulting in plants that arrested as seedlings. We refer to this phenotype as a limited shoot. The limited shoot phenotype reflected loss of kn1 function, but its penetrance was background dependent. We examined meristem size and found that plants lacking kn1 function had shorter meristems than non-mutant siblings. Furthermore, meristems of restrictive inbreds were significantly shorter than meristems of permissive inbreds, implying a correlation between meristem height and kn1 gene function in the embryo. Analysis of limited shoot plants during embryogenesis indicated a role for kn1 in shoot meristem maintenance. We discuss a model for kn1 in maintenance of the morphogenetic zone of the shoot apical meristem.     

Genetics of flower initiation and development in annual and perennial plants

Flowering is an integral developmental process in angiosperms, crucial to reproductive success and continuity of the species through time. Some angiosperms complete their life cycle within a year (annual plants), and others have a longer reproductive life, which is characterized by the generation of new flowering and vegetative shoots every year (perennial plants). Despite the differences in their lifespan, the underlying genetics of flower induction and floral organ formation appears to be similar among these plants. Hence, the knowledge gained from the study of flowering mechanism in Arabidopsis thaliana can be used to better understand similar processes in other plant species, especially the perennials, which usually have a long generation time and are not amenable to genetic analysis. Using Arabidopsis as a model, we briefly discuss the current understanding of the transition from vegetative to reproductive growth and the subsequent formation of individual floral organs, and how this knowledge has been successfully applied to the identification of homologous genes from perennial crops. Although annuals appear to share many similarities with perennials in terms of gene function, they differ in their commitment to flowering. Once an annual reaches the reproductive phase, all meristems are typically converted into either floral or inflorescence meristems. In contrast, each year, each meristem of a mature perennial has the choice to produce either a vegetative or a reproductive shoot. The physiology and genetics of flowering in Citrus are used to highlight the complexity of reproductive development in perennials, and to discus possible future research directions.     

DEVELOPMENT OF VACUOLES AND LIPID BODIES IN APICAL MERISTEMS OF PINUS BANKSIANA

Shoot apical meristems of jack pine were examined weekly during the first 8 weeks post-germination with light and electron microscopy. Most of the storage lipids were utilized by the end of the 2nd week. A few lipid bodies, possibly high in phospholipid content, remained in the apical initials and central mother cells and, during the 3rd week, gave rise to vacuoles via lamellar or myelin-like structures which were first seen on their periphery. The inter-lamellar spaces enlarged and eventually a vacuole was formed. At 5 weeks, elongate and spherical osmiophilic inclusions, presumably lipid, were found in the endoplasmic reticulum. Lipid bodies, visible with light microscopy, began to accumulate in the apical initials and central mother cells in the 6th week.     

OCCURRENCE OF INTERCALARY AND UNINTERRUPTED MERISTEMS IN EQUISETUM

Quantitative data on cell length and mitotic index in elongating internodes of seven species of Equisetum are presented as evidence for the occurrence of two patterns of internode development in rhizomes of different species. In rhizomes of three species of subgenus Equisetum (E. arvense, E. diffusum, E. telmateia) uninterrupted meristems are present, characterized by acropetal internode maturation. In rhizomes of four species of subgenus Hippochaetae (E. hyemale, E. variegatum, E. scripoides, and E. laevigatum) intercalary meristems are present. The number and locations of intercalary meristems are described for an aerial shoot of E. diffusum. The absence or diminution of intercalary meristems from rhizomes of a variety of vascular plants with intercalary meristems in aerial shoots is discussed from the standpoint of adaptive significance.     

Evolution of shoot apical meristem structures in vascular plants with respect to plasmodesmatal network

Vascular plants have evolved shoot apical meristems (SAMs), whose structures differ among plant groups. To clarify the evolutionary course of the different structural types of SAMs, we compared plasmodesmatal networks in the SAMs for 17 families and 24 species of angiosperms, gymnosperms, and pteridophytes, using transmission electron microscopy (TEM). The plasmodesmata (PD) in almost all cell walls in median longitudinal sections of SAMs were counted, and the PD density per unit area was calculated for each cell wall. Angiosperm and gymnosperm SAMs have low densities, with no difference between stratified (tunica-corpus) and unstratified structures. SAMs of ferns, including Psilotum and Equisetum, have average densities that are more than three times higher than those of seed plants. Interestingly, microphyllous lycopods have both the fern and seed-plant types of PD networks; Selaginellaceae SAMs with single apical cells have high PD densities, while SAMs of Lycopodiaceae and Isoetaceae with plural initial cells have low PD densities, equivalent to those of seed plants. In summary, PD networks are strongly correlated to SAM organizations-SAMs with single and plural initial cells have the fern and seed-plant types of PD, respectively. The two SAM organizations may have evolved separately in lycophytes and euphyllophytes and may be associated with gain or loss of the ability to form secondary PD.     

Organization of the root apical meristem in angiosperms

Although flowers, leaves, and stems of the angiosperms have understandably received more attention than roots, the growing root tips, or root apical meristems (RAMs), are organs that could provide insight into angiosperm evolution. We studied RAM organization across a broad spectrum of angiosperms (45 orders and 132 families of basal angiosperms, monocots, and eudicots) to characterize angiosperm RAMs and cortex development related to RAMs. Types of RAM organization in root tips of flowering plants include open RAMs without boundaries between some tissues in the growing tip and closed RAMs with distinct boundaries between apical regions. Epidermis origin is associated with the cortex in some basal angiosperms and monocots and with the lateral rootcap in eudicots and other basal angiosperms. In most angiosperm RAMs, initials for the central region of the rootcap, or columella, are distinct from the lateral rootcap and its initials. Slightly more angiosperm families have exclusively closed RAMs than exclusively open RAMs, but many families have representatives with both open and closed RAMs. Root tips with open RAMs are generally found in angiosperm families considered sister to other families; certain open RAMs may be ancestral in angiosperms.     

CHANGES IN GLYCERALDEHYDE 3-PHOSPHATE DEHYDROGENASE ACTIVITY IN SHOOT APICAL MERISTEMS OF BRASSICA CAMPESTRIS DURING TRANSITION TO FLOWERING

An assay system has been developed for the histochemical determination of glyceraldehyde 3-phosphate dehydrogenase (G3PD) activity to indicate glycolytic pathway capacity during evocation in the shoot apical meristem of Brassica campestris L. G3PD activity was differentially distributed in a zonate pattern within the meristems at the vegetative, the transition, and the floral stages. The activity of G3PD changed in all apical zones of evoked apices, but especially in the central and the peripheral zones of apices at the prefloral stage. In the prefloral stage of development heavy enzyme activity was localized in island-like areas within the peripheral zone. These results indicate that 1) the capacity of glycolysis fluctuates during evocation, 2) during floral evocation the capacity of the glycolytic pathway parallels the capacity of the citric acid cycle and the electron transport system only at the prefloral stage, and 3) G3PD activity marks incipient floral primordia. It is proposed that the enzymic marking of an incipient floral primordium indicates the end of evocation in Brassica.     

The Handlebars gene is required with Phantastica for dorsoventral asymmetry of organs and for stem cell activity in Antirrhinum

In angiosperms, individual lateral organs and whole flowers may develop asymmetrically along their dorsoventral axes. Dorsoventral asymmetry of Antirrhinum leaves requires activity of the Phantastica gene and other factors acting redundantly with it. We describe the effects of a mutation in the Handlebars gene, identified as an enhancer of the phantastica mutant phenotype. Genetic analysis suggests that Handlebars functions redundantly with Phantastica to promote dorsal fate in lateral organs and to maintain activity of stem cells within shoot apical meristems. Handlebars appears dispensable in vegetative development but is needed for asymmetry of petals along the dorsoventral axis of the flower as a whole. This suggests that common mechanisms may control dorsoventral asymmetry in lateral organ primordia and in floral meristems.     

Comparative anatomy of embryogenesis in three species of Podostemaceae and evolution of the loss of embryonic shoot and root meristems

During embryogenesis in angiosperms, the embryonic shoot and root meristems are created at opposite poles of the embryo, establishing a vertical body plan. However, the aquatic eudicot family Podostemaceae exhibits an unusual horizontal body plan, which is attributed to the loss of embryonic shoot and root meristems. To infer the embryogenetic changes responsible for the loss of these meristems, we examined the embryogenesis of three podostemads with different meristem characters, that is, Terniopsis brevis with distinct shoot and root meristems, Zeylanidium lichenoides with reduced shoot and no root meristems, and Hydrobryum japonicum with no shoot and no root meristems. In T. brevis, as in other eudicots, the putative organizing center (OC) and L1 layer (=the epidermal cell layer) arose to generate a distinct shoot meristem initial, and the hypophysis formed the putative quiescent center (QC) of a root meristem. Z. lichenoides had a morphologically unrecognizable shoot meristem, because a distinct L1 layer did not develop, whereas the putative OC precursor arose normally. In H. japonicum, the vertical divisions of the apical cells of eight-cell embryo prevented putative OC initiation. In Z. lichenoides and H. japonicum, the putative QC failed to initiate because the hypophysis repeated longitudinal divisions during early embryogenesis. Based on their phylogenetic relationships, we infer that the conventional embryonic shoot meristem was lost in Podostemaceae via two steps, that is, the loss of a distinct L1 layer and the loss of the OC, whereas the loss of the embryonic root meristem occurred once by misspecification of the hypophysis.     

AN INVESTIGATION OF THE MORPHOGENETIC MECHANISMS WHICH CONTROL THE DEVELOPMENT OF ZONATION IN SEEDLING SHOOT APICAL MERISTEMS

The development of zonation in the shoot apical meristems of 28 species of cacti was examined. At germination the embryonic apex may have one of three types of organization: 1) tunica/corpus; 2) tunica/central mother cells/corpus; 3) tunica/central mother cells/peripheral zone/pith-rib meristem. Apices of the third type have complete zonation and undergo little or no further structural development. Both of the other types develop the missing zones. First, the apices become mitotically active, and peripheral zone characters develop in the outer corpus. Simultaneously, or slightly later, the central mother cells differentiate if they are not yet present. The final step is the formation of the pith-rib meristem. The sequence of appearance of the zones was constant throughout all species examined, but the time of appearance of only one zone could be correlated with any other morphogenetic process: the development of peripheral zone characteristics in the outer corpus occurs with or before the beginning of leaf production. The development of zonation appears to be independent of apical size, shape, or age, either real age or plastochron age. This has been interpreted to indicate that the metabolic mechanism controlling the development of zonation in shoot apical meristems is largely autonomous and independent of other aspects of morphogenesis occurring in the seedling. Also, the fact that leaf initiation and shoot axis production can both occur before the development of either the central mother cells or the pith-rib meristem indicates that neither of these zones is essential for these two apical morphogenetic activities.     

CONSTANCY OF RELATIVE VOLUMES OF ZONES IN SHOOT APICAL MERISTEMS IN CACTACEAE: IMPLICATIONS CONCERNING MERISTEM SIZE,SHAPE, AND METABOLISM

The volumes and dimensions of shoot apical meristem zones were determined for 21 species in 10 genera of the Cactaceae. If the volumes of the central-mother-cell zone (CMCZ), the peripheral zone (PZ), and the pith-rib meristem (PRM) are expressed as percentages of the total volume of the corpus, then they are remarkably constant (CMCZ = 11.1% of the volume of the corpus, PZ = 61.2% and PRM = 27.1%) regardless of the relative size of the apical meristems. The relative volume of the tunica decreases, however, as the whole apex increases. The zones have variable shapes, and whereas the PZ and PRM are always trapezoidal in longitudinal section, in apices with large volumes, these zones have a flatter, more flaring shape than they have in smaller apices. Thus large apices are flatter and less hemispherical than are small apices. The CMCZ, however, maintains a constant shape and is always relatively hemispherical regardless of its volume. A hypothesis that relates all of these shape and volume relationships as an integrated growth sequence is proposed.     

茎顶端分生组织在植物发育过程中的保持、转变和逆转

顶端分生组织(shoot apical meristems,SAM)为产生新的器官和组织而不断提供新的细胞,它的活性依赖于平衡分生组织细胞的增殖和器官发生之间关系的调控基因.来自不具备光合能力的顶端分生组织的细胞可形成具有光合能力的营养器官.在从营养生长到生殖发育的转变过程中,茎顶端分生组织,转变为花序分生组织,最终形成花分生组织.在进入开花决定状态以前,SAM的状态很大程度上受到环境信号和转录调控因子的影响.以模式植物拟南芥为主,对在顶端分生组织的保持和转变中复杂同时又有差异的基因调控网络进行讨论.在花和花序分生组织逆转过程中,SAM中的细胞也受到相关基因的调控,且表达方式存在明显的时空差异.因此,具有决定性的和未决定性双重特性的分生组织之间的转变和相互协调,对于器官发生和形态建成起到至关重要的作用.     

Fundamental mechanisms of the stem cell regulation in land plants: lesson from shoot apical cells in bryophytes

Plant Molecular Biology - This review compares the molecular mechanisms of stem cell control in the shoot apical meristems of mosses and angiosperms and reveals the conserved features and evolution...