Lateral meristems of higher plants: Phytohormonal and genetic control

Lateral meristems (pericycle, procambium and cambium, phellogen) are positioned in parallel to the lateral surface of the organ, where they are present, and produce concentric layers of undifferentiated cells. Primary lateral meristems, procambium and pericycle, arise during embryogenesis; secondary lateral meristems, cambium and phellogen, — during post embryonic development. Pericycle is most pluripotent plant meristem, as it may give rise to a variety of other types of meristems: lateral meristems (cambium, phellogen), apical meristems of lateral roots, and also shoot meristems during plant in vitro regeneration. Procambium and cambium developing from it give rise to the vascular tissues of the stems and roots, ensuring their thickening. The review considers the genetic control of lateral meristem development and the role of phytohormones in the control of their activities.     

Plant stem cells and their regulations in shoot apical meristems

Stem cells in plants, established during embryogenesis, are located in the centers of the shoot apical meristem (SAM) and the root apical meristem (RAM). Stem cells in SAM have a capacity to renew themselves and to produce new organs and tissues indefinitely. Although fully differentiated organs such as leaves do not contain stem cells, cells in such organs do have the capacity to re-establish new stem cells, especially under the induction of phytohormones in vitro. Cytokinin and auxin are critical in creating position signals in the SAM to maintain the stem cell organizing center and to position the new organ primordia, respectively. This review addresses the distinct features of plant stem cells and focuses on how stem cell renewal and differentiation are regulated in SAMs.     

Specific proteins in stem meristems during transition to flowering

Immunodiffusion tests were used for studying protein composition of apical buds ofRudbeckia bicolor andPerilla nankinensis during their transition from vegetative to reproductive state under inductive photoperiodic conditions or GA₃ treatment. In both species the induced buds differ from the vegetative ones in the presence of specific proteins (P): P1, P2, P3 appear inRudbeckia apical buds 2, 8, 16 d after the start of inductive treatment; P4 appears inPerilla apical buds 6 d after inductive treatment. P1, P2, P4 are revealed in induced buds in the early period of apex development when morphogenetic changes are not yet present. The similarity between antigenic spectra of induced buds and of those treated by GA₃ appears only inRudbeckia. These observations support the hypothesis of a change in gene expression at floral evocation. Presented at the International Symposium “Plant Growth Regulators” held on June 18–22, 1984 at Liblice, Czechoslovakia.     

The tapetum and systematics in monocotyledons

This paper critically reviews the homologies and distribution of tapetum types in monocotyledons, in relation to their systematics. Two main types of tapetum are widely recognised: secretory and plasmodial, although intermediate types occur, such as the “invasive” tapetum described inCanna. In secretory tapeta, a layer of cells remains intact around the anther locule, whereas in the plasmodial type a multinucleate tapetal plasmodium is formed in the anther locule by fusion of tapetal protoplasts. In invasive tapeta, the cell walls break down and tapetal protoplasts invade the locule without fusing to form a plasmodium. When examining tapetum type, it is often necessary to dissect several developmental stages of the anthers. Secretory and plasmodial tapeta are both widely distributed in monocotyledons and have probably evolved several times, although there may be some systematic significance within certain groups. Among early branching taxa,Acorus andTofieldia have secretory tapeta, whereas Araceae and Alismatales are uniformly plasmodial. The tapetum is most diverse within Commelinanae, with both secretory and plasmodial types, and some Zingiberales have an invasive tapetum. Lilianae (Dioscoreales, Liliales, and Asparagales) are almost uniformly secretory.     

Lateral bud development and shoot growth in low-pruned Morus alba as affected by stem orientation

Two phases of bud activity were identified in the new growth of one-year-old erect coppice shoots on 11-year-old low-pruned stumps of mulberry (Morus alba L. cv. Shin-ichinose) in spring, the sprouting phase in which the majority of the buds, including the basal ones, sprout and elongate, and the dominance phase (starting 4–5 weeks after sprouting) during which the upper laterals begin to assert dominance and suppress the growth of lower laterals, becoming new leading shoots. In contrast, arching before sprouting markedly inhibited buds on the under side, leading to poor shoots. By late April, the sprouts on the upper side grew readily into new erect shoots, resulting in considerable dominance over those from the lateral sides. Of these erect shoots, those located closer to the stem base grew more in May and June. The effects of arching made during the sprouting phase (late April) on bud activity and shoot lengths were generally similar to those of earlier archings before spring bud bursting. Separation of the shoots from the upper and under sides by longitudinal, horizontal splitting of the arched stems in late April did not affect the inhibited elongation of the shoots from the under side. These results suggest that in the response to arching before and in late April, the effects are related to spring bud bursting and gravimorphism. In contrast, arching during and after the dominance phase (May) had no gravimorphic effects on growth of the shoots on the upper side, although there was a stimulation of outbreak of the buds on the upper side, which remained dormant during spring bud bursting. Continuous basal applications of abscisic acid in aqueous solution inhibited bud break and shoot growth of the postdormant erect stem segments, and defoliation of the new shoots markedly. In contrast, similar applications of an ethylene-releasing compound, Ethephon, depressed shoot elongation slightly, but enhanced defoliation greatly. Gibberellic acid (GA₃) stimulated shoot elongation, but depressed leaf enlargement.     

Modification of nucleolar components by growth temperature in meristems

Growth temperature modifies the cycle time in Allium cepa L. root meristems, affecting neither the cell size nor the relative duration of the cycle phases. It was to be expected that a similar nucleolus would be found whose activity was merely modified by temperature as any other enzymatic reaction. However, stereology of nucleoli growing at two physiological temperatures (10 and 25 °C) showed that both the fibrillar and granular components were 1.3 and 2 times respectively more abundant at the lower temperature. These nucleoli had also twice as many active nucleolar RNA polymerases as shown after the in situ assay.     

Intercellular signalling in the transition from stem cells to organogenesis in meristems

Meristems continuously produce new cells to sustain plant growth. Stem cells are maintained in the centre of the meristem and provide the precursor cells for the initiation of new organs and tissues in the periphery. The structure of the meristem is maintained while cells are constantly displaced by new divisions. Recent advances have been made in understanding the intercellular signals that maintain meristem structure by adjusting gene expression according to cell position. In addition to refinements in our understanding of how the position and size of the stem-cell population is regulated, there have been advances in understanding how the location of new organ primordia is controlled and how the meristem influences organ polarity.     

Nomenclatural adjustments in some European monocotyledons

New nomenclatural combinations are validated for fifteen taxa belonging to the generaDanthonia, Stipa, Lolium, Phippsia, Elymus, Schoenoplectus andAllium.     

Lateral growth patterns in the Cladoniaceae

Lateral growth from the apex of vertical structures is widespread in cladoniiform lichens. In the family Cladoniaceae, it is accomplished through a developmental shift in the meristem, in which growth orientation changes from isotropous to anisotropous. In anisotropous development, the more or less constant relationship among the axial, radial, and circumferential planes of growth is altered during ontogeny. The result is pronounced lateral elongation of the apical meristem, a departure from the isotropous body plan of early ontogeny. Development that favors radial and circumferential growth over axial growth is an innovation that provides ontogenetic flexibility but which also entails the loss of control from a single centralized meristem to one or more meristems. A shift from the constraints of symmetry to the risks and potential of asymmetry and a subsequent diversity of heritable thallus forms reflect evolutionary processes in the Cladoniaceae. Similar morphogenetic activities, which are apparently highly conserved, are shared by species that are presumably only distantly related.     

Systematics and biology of silica bodies in monocotyledons

Many plants take up soluble monosilicic acid from the soil. Some of these plants subsequently deposit it as cell inclusions of characteristic structure. This article describes the distribution and diversity of opaline silica bodies in monocotyledons in a phylogenetic framework, together with a review of techniques used for their examination, and the ecology, function and economic applications of these cell inclusions. There are several different morphological forms of silica in monocot tissues, and the number of silica bodies per cell may also vary. The most common type is the “druse-like” spherical body, of which there is normally a single body per cell, more in some cases. Other forms include the conical type and an amorphous, fragmentary type (silica sand). Silica bodies are most commonly found either in the epidermis (e.g., in grasses, commelinas and sedges) or in the sheath cells of vascular bundles (e.g., in palms, bananas and orchids). Silica-bearing cells are most commonly associated either with subepidermal sclerenchyma or bundle-sheath sclerenchyma. Silica bodies are found only in orchids and commelinids, not in other lilioid or basal monocots. In orchids, silica bodies are entirely absent from subfamilies Vanilloideae and Orchidoideae and most Epidendroideae but present in some Cypripedioideae and in the putatively basal orchid subfamily Apostasioideae. Among commelinid monocots, silica bodies are present in all palms, Dasypogonaceae and Zingiberales but present or absent in different taxa of Poales and Commelinales, with at least four separate losses of silica bodies in Poales.     

Termination of stem cell maintenance in Arabidopsis floral meristems by interactions between WUSCHEL and AGAMOUS.

Floral meristems and shoot apical meristems (SAMs) are homologous, self-maintaining stem cell systems. Unlike SAMs, floral meristems are determinate, and stem cell maintenance is abolished once all floral organs are initiated. To investigate the underlying regulatory mechanisms, we analyzed the interactions between WUSCHEL (WUS), which specifies stem cell identity, and AGAMOUS (AG), which is required for floral determinacy. Our results show that repression of WUS by AG is essential for terminating the floral meristem and that WUS can induce AG expression in developing flowers. Together, this suggests that floral determinacy depends on a negative autoregulatory mechanism involving WUS and AG, which terminates stem cell maintenance.     

Hetrogeneity of nuclear volumes and interphase nuclear growth in cells of root apical meristems

Nuclear volume distributions from proliferative- and stationary-phase meristems of pea roots have been compared and shown to be almost identical. A wide range of nuclear volumes is also found within cells of the same interphase age, specifically early G1 cells and late G2 cells, further indicating that nuclear volume cannot be used to estimate interphase age. The average increase in nuclear volume is approx. 2.5-fold, and all nuclei of the meristem, whatever their starting volume, increase by the same factor. Sister nuclei have very similar volumes in early G1, indicating that asymmetric mitoses within the meristem do not contribute significantly to the heterogeneity found.     

Triaperturate pollen in the monocotyledons: configurations and conjectures

Triaperturate pollen are known in at least twenty seven genera of monocotyledons. Differences between aperture type and polarity indicate that the development of three apertures has occurred a number of times. Mode of cytokinesis during microsporogenesis is compared with differences in aperture configuration, to assess the extent to which this appears to influence aperture arrangement. Triapertury in monocot pollen tends to fall into one or another of three situations: 1) it is the normal state, 2) it is fairly common, but pollen with more or less apertures also occur in the taxon or sample, 3) it is a rare, or abnormal state for pollen which usually has less than three apertures. The various forms of triaperturate pollen are described, as well as monosulcate pollen of the orchid genera Cypripedium and Paphiopedilum, often misinterpreted as tri-sulcate, and the unusual extended trichotomosulcate pollen of Agrostocrinum (Hemerocallidaceae). Monosulcy, trichotomosulcy, and zonasulcy, with unusual and rare exceptions of zonasulcy in the eudicots, are aperture states shared exclusively with the basal dicots. Furthermore, to some extent all have links with the triaperturate condition in monocots and basal dicotyledons. This is discussed, as well as the association of tripory with polypory in monocots and basal dicots. The fossil pollen record is considered.This paper is dedicated to Klaus Kubitzki in recognition, not only for his extensive contribution to systematic botany, but also for his firm belief that pollen characteristics contribute to a better understanding of plant systematics and evolution.     

Density and the commitment of apical meristems to clonal growth and reproduction in Hieracium pilosella

Summary We examined responses to population density in the commitment of apical meristems to reproduction and clonal growth in a rosette-forming, stoloniferous herb (Hieracium pilosella). Despite close physiological coupling between the evocation of the terminal inflorescence bud and the development of one or more axillary buds into stolons, the allocation of meristems was extremely plastic.Genets at the higher sowing densities showed density-dependent mortality consistent with self-thinning along a-3/2 trajectory. The probability of inflorescence evocation and associated stolon development was negatively dependent on surviving density. The proportinal distribution of primary stolons amongst genets became strikingly more unequal (expressed as the Gini coefficient) with increasing density. Clonal growth was resolved into the number of primary stolons per stoloniferous genet and the extent of stolon branching (i.e. number of apices per primary stolon); both showed strongly negative density-dependence. Reproduction, expressed as the mean number of flowering capitula per stoloniferous genet, declined 15-fold with increasing density; although theoretically expected to be unity, greater values resulted from capitulum production by attached secondary rosettes and lower values reflected the increasing abortion rate of inflorescence buds with increasing density.Both the total number of apices produced per unit area and the corresponding number of reproductive apices were maximal at intermediate surviving densities (700–1,000 m^-2). The balance between reproductive and clonal growth may be expressed as the probability of an apical meristem producing a capitulum, that also peaked sharply at intermediate density. This finding does not conform with linear models that predict a shift from vegetative growth to sexual reproduction with increasing population density.