The effect of cadmium on shoot apical meristems of barley

We studied the effects of cadmium acetate at various concentrations (200 to 800 mg/kg substrate) on growth and development of shoot apical meristem of the barley plants (Hordeum vulgare L.) under the conditions of vegetative experiment. It was shown that in the presence of increasing cadmium concentrations in the soil substrate, the apex length and number of inflorescence elements decreased and the rate of organogenesis slowed down, thus affecting the spike potential productivity and morphological parameters of barley plants at the flowering stage. It is possible that the negative effect of cadmium on the shoot apical meristem is associated with its influence on division of the apex cells.     

Regulation of extent of vegetative development of the maize shoot meristem

In maize plants ( Zea mays L.), the extent of vegetative development in the shoot is precisely regulated such that the apical meristem produces a predictable number of leaves before converting to tassel development. In previous experiments using shoot apex culture, we showed that the developmental program that limits vegetative development in maize is not intrinsic to the shoot apical meristem. Rather, the meristem receives information from elsewhere in the plant and responds by either continuing leaf initiation or becoming determined for determinate growth and forming an inflorescence, the tassel. Here we examine leaf primordia as potential sources for that information using shoot apex culture. Our results show that the presence of the four to six youngest leaf primordia on the shoot apex is sufficient to provide such information. The ability to reset shoot development by meristem culture also allows us to examine the basis for expression of a specific phenotype at a particular developmental stage. We found that the mutation hcf106 , which is typically expressed only during seedling stages, is not re-expressed when the shoot morphogically has regained a juvenile phase.     

Normal and Abnormal Development in the Arabidopsis Vegetative Shoot Apex

Vegetative development in the Arabidopsis shoot apex follows both sequential and repetitive steps. Early in development, the young vegetative meristem is flat and has a rectangular shape with bilateral symmetry. The first pair of leaf primordia is radially symmetrical and is initiated on opposite sides of the meristem. As development proceeds, the meristem changes first to a bilaterally symmetrical trapezoid and then to a radially symmetrical dome. Vegetative development from the domed meristem continues as leaves are initiated in a repetitive manner. Abnormal development of the vegetative shoot apex is described for a number of mutants. The mutants we describe fall into at least three classes: (1) lesions in the shoot apex that do not show an apparent alteration in the shoot apical meristem, (2) lesions in the apical meristem that also (directly or indirectly) alter leaf primordia, and (3) lesions in the apical meristem that alter meristem size and leaf number but not leaf morphology. These mutations provide tools both to genetically analyze vegetative development of the shoot apex and to learn how vegetative development influences floral development.     

Shoot apical meristem and cotyledon formation during Arabidopsis embryogenesis: interaction among the CUP-SHAPED COTYLEDON and SHOOT MERISTEMLESS genes

The shoot apical meristem and cotyledons of higher plants are established during embryogenesis in the apex. Redundant CUP-SHAPED COTYLEDON 1 (CUC1) and CUC2 as well as SHOOT MERISTEMLESS (STM) of Arabidopsis are required for shoot apical meristem formation and cotyledon separation. To elucidate how the apical region of the embryo is established, we investigated genetic interactions among CUC1, CUC2 and STM, as well as the expression patterns of CUC2 and STM mRNA. Expression of these genes marked the incipient shoot apical meristem as well as the boundaries of cotyledon primordia, consistent with their roles for shoot apical meristem formation and cotyledon separation. Genetic and expression analyses indicate that CUC1 and CUC2 are redundantly required for expression of STM to form the shoot apical meristem, and that STM is required for proper spatial expression of CUC2 to separate cotyledons. A model for pattern formation in the apical region of the Arabidopsis embryo is presented.     

The pattern of histone H4 expression in the tomato shoot apex changes during development

Two histone H4 cDNA clones were isolated from a tomato (Lycopersicon esculentum Mill.) shoot-tip cDNA library using a heterologous probe from barley (Hordeum vulgare L.). Both cDNAs, which are 81% identical in the coding region, are polyadenylated and belong to a small gene family in the tomato genome. Histone H4 message is abundant in young tissues and rare in older tissues. In the shoot apical meristem, the distribution of H4-expressing cells changes during development. In a juvenile vegetative apex, H4 message is detectable in the central region and the peripheral parts of the meristem. In a mature vegetative apical meristem, H4-expressing cells are localized in the peripheral zone extending into the provascular strands and the rib meristem whereas the central zone is almost devoid of H4 mRNA. After floral transition, H4 mRNA is found throughout the floral meristem, indicating a second change in the pattern of H4 expression. The observed changes in H4 expression are indicative of changes in the distribution of mitotic activity in the shoot apical meristem during plant development. In addition, H4-expressing cells were found to occur frequently in clusters, which may indicate a partial synchronization of cell divisions in the shoot apex.     

Signaling pathways maintaining stem cells at the plant shoot apex

The above ground organs of plants are generated by the shoot apical meristem. Cellular characteristics and molecular markers indicate that the shoot meristem is patterned into domains with different functions, with stem cells residing in the outer three cell layers of the central zone of the meristem. The boundaries of the domains are determined by positional signals. Here we will discuss our current understanding of the signaling network involved in determining stem cell fate and in setting the boundaries of the stem cell niche at the plant shoot apex.     

INITIATION AND EARLY DEVELOPMENT OF THE INFLORESCENCE IN PINEAPPLE (ANANAS COMOSUS, ‘SMOOTH CAYENNE') TREATED WITH ACETYLENE

Pineapple plants ‘Smooth Cayenne’ were made to flower by treatment with acetylene. The organization of the vegetative shoot apex is similar to that of many investigated angiosperms in that it shows a zonate pattern, viz., apical zone, peripheral zone, and central-core rib meristem. The latter zone is weakly developed. Cytological changes at the shoot apex occur as early as 3 days after treatment; these involve nuclear changes and an increase in ribonucleic acid (RNA) in the cytoplasm of cells of the apical zone. A marked increase in the height of the apex occurs by the 9th day; this is preceded by rib meristem activity in the central core. All component parts of the inflorescence are present and in various stages of development by the 21st day at which time vegetative scales and “crown” leaves are initiated.     

CELL DIVISION KINETICS IN THE SHOOT APICAL MERISTEM OF CERATOPTERIS THALICTROIDES BRONG. WITH SPECIAL REFERENCE TO THE APICAL CELL

The duration of mitosis and the cell cycle were determined for defined cell populations of the shoot apical meristem of Ceratopteris thalictroides Brong. by using the colchicine-induced metaphase accumulation technique. The results indicate that the apical cell is mitotically active and cycles at an apparently greater frequency than the cells of subjacent populations. Duration of mitosis was similar for all cells of the meristem. These results are correlated with mitotic indices of control apices, the geometry of the apex, and the mean number of cells in the meristem. Shoot apices from adult plants were examined to determine mitotic indices within the meristem; mitotic activity was again noted for the apical cell. These results contradict recent proposals that the pteridophyte apical cell serves as a unicellular quiescent center which lacks histogenic potential and offer experimental support for the classical concept of apical cell function in those fern shoot meristems which terminate in a single apical cell.     

Anatomy and development of the fern sporophyte

Recent research on the developmental anatomy and morphology of the fern sporophyte is reviewed. Detailed histological and experimental studies of the organization of the fern shoot apical meristem have reconfirmed the recently controversial role of the shoot apical cell as the single apical initial of the meristem. The shoot apical meristem is nevertheless an anatomically and functionally complex structure with a strongly zoned cytohistological organization. Fern shoot apex organization can be compared with that of seed plants. The control of leaf initiation and phyllotaxy remains poorly understood. Studies differ as to whether leaf initiation in ferns involves one leaf mother cell or a multicellular region of the shoot apex. The concept of non-appendicular fronds is refuted for living ferns. The later developmental changes in the determinate leaf apical and marginal meristems of the leaf primordium form an area that is still largely unexplored but could be investigated by methods similar to those used to study shoot and root apices. Branching in ferns is morphologiclaly and developmentally diverse. There is apparently more than one developmental mode of dichotomous branching, and several modes of lateral bud formation have been described, including the phyllogenous initiation of branches at the base of leaf primordia. Developmental changes in bud meristems related to apical dominance, inhibition, and bud activation is another major area for continued study. The traditional concept of the role of the root apical cell has been reestablished by studies similar to those made of the shoot apex. Detailed ultrastructural investigations of the root ofAzolla have given a sophisticated new picture of developmental processes in that organ. Fern roots show remarkably precise patterns of histogenesis in relation to apical segmentation. The formation of secondary vascular tissue inBotrychium suggests that the Ophioglossales may be related to the seed plants. The causal relationship of leaf (and branch and root) formation and the initiation of vascular tissue in the shoot needs more study. Although still poorly understood, protoxylem systems in ferns are variable and may have morphological and systematic significance. Recent investigations of hydraulic conductance in fern stems have found possible correlations of conductance levels with growth forms. The anatomical diversity of ferns makes comparative functional anatomy a promising field for future study.     

Changes in the Pattern of Cell Division in the Shoot and Root Meristems of Lolium perenne during the Transition from Vegetative to Floral Growth

For Lolium perenne cv. Cropper, a system which resulted in 100%flowering comprised 90 short days (SD) at 4 ?C (vernalization)and 30 SD at 18 ?C followed by 8 long days (LD). The mitoticindex and G1 and G2 percentages were measured in the shoot androot apices of plants following 2, 5 or 8 LD and in SD controlssampled at the beginning and end of induction. Identical measurementswere made in plants given 48 SD at 18 ?C followed by 2, 5 or8 LD; plants remained vegetative in response to this treatmentlacking vernalization. Significant increases in both mitoticindex and meristem size occurred in the shoot apex in LD followingthe vernalizing, but not the non-vernalizing, treatment. A clusterof mitoses in the apical dome of the shoot apex was unique tothe vernalized plants given 5 or 8 LD. However, an increasein root meristem size occurred regardless of vernalization,but a significant increase in the mitotic index was limitedto vernalized plants given 5 or 8 LD. Whilst the vernalization-LDtreatment resulted in an increase in the G2 percentage in theshoot apex following 2, 5 or 8 LD, no such alteration was observedin the root meristem. Thus, the changes to the cell cycle whichcorrelated with flowering were increased mitotic indices andG2 percentages in the shoot apex at each sampling time and increasedmitotic indices in the root apex following 5 and 8 LD. Key words: Cell division, flowering, Lolium perenne L.     

The rate of cell division in the shoot apical meristem during photoperiodic induction and transition to flowering

Cell division contributing to longitudinal growth of the shoot apex was investigated inChenopodium rubrum in segments marked by the axils of leaf primordia. Plants treated with two short days (16h of darkness and 8h of light) were compared with two non-induced controls (cultivated in continuous light or treated by alternations of 8 h of darkness and 4 h of light for two days). During the short-day treatments the rate of cell division contributing to the longitudinal growth decreases in all segments of the shoot apex irrespective of whether the darkness was given in inductive or non-inductive photoperiods. The rate of cell division contributing to longitudinal growth increases in the upper internodes of the shoot apex after the termination of the photoperiodic treatment and transfer of the plants to continuous light. However, cell division remains inhibited in the lowest segment of the shoot apex. This inhibition in the differentiating parts of the shoot apical meristem is a direct consequence of photoperiodic induction. It is supposed that this inhibition is related to evocation similarly as the well-known phenomenon of stimulation of cell division in the apical dome.     

Experimental and Analytical Studies of Pteridophytes: XXX. Further Investigations of the Formation of Buds and Leaves in Dryopteris aristata Druce

The influence of the shoot apex upon leaf and bud formationin the fern Dryopteris aristata has been investigated by furtherexperiments on puncturing the apical cell. When the apical cellgroup is damaged, leaf primordia, which may be orientated abnormally,continue to be formed on the meristem, but one or more budsmay also arise. The observations reported here indicate thata zone at the periphery of the apical meristem is particularlyreactive when the apical cell group is damaged, the majorityof buds being induced in this region. The extent of damage tothe apex may affect the sequence of organogenesis: when damageis extensive buds tend to be formed immediately, subsequentprimordia developing as leaves; when the damage is confinedto the apical cell, or extends to only a few of its segments,bud formation tends to be delayed. It is concluded that the effect of the apical cell on organformation is exercised through the growth and organization ofthe apex as a whole.     

CHANGE IN EPILOBIUM PHYLLOTAXY DURING REPRODUCTIVE TRANSITION

The ontogeny of Epilobium hirsutum grown under natural summer photoperiod in a glasshouse was divided into vegetative, early transitional, transitional, and floral stages. Bijugate phyllotaxy, common to both the vegetative and early transitional stages, is transformed into spiral phyllotaxy during the transitional stage by an initial change in the divergence angle of a single primordium inserted at a unique level on the shoot. Leaf primordia subsequently are inserted in a spiral arrangement in the indeterminate floral shoot apex. The early transitional shoot apical meristem is about 1.5 times the volume of the vegetative meristem but expands at about two-thirds the relative plastochron rate of volume increment of the vegetative meristem. There are progressive decreases in the plastochron and relative plastochron rates of radial and vertical shoot growth through ontogeny. Relative chronological rates of shoot growth, however, are not altered during ontogeny. Spiral transformation results from changes in the relative points of insertion of leaf primordia on the shoot meristem. These changes are accompanied by an increased rate of primordia initiation on a more circular shoot meristem. The change in phyllotaxy during ontogeny is similar to that which was artificially induced by chemical modification of auxin concentration gradients in the shoot apex, with the additional feature that there is an initial increase in the volume of the shoot meristem prior to the natural spiral transformation. Size of the shoot apical meristem, however, appears to have little influence on Epilobium phyllotaxy; but the geometric shape of the meristem is well correlated with bijugate to spiral transformations. This suggests that geometric parameters of the shoot meristem should be considered in theoretical models of phyllotaxy.     

Coordination of cell proliferation and cell fate decisions in the angiosperm shoot apical meristem.

A unique feature of flowering plants is their ability to produce organs continuously, for hundreds of years in some species, from actively growing tips called apical meristems. All plants possess at least one form of apical meristem, whose cells are functionally analogous to animal stem cells because they can generate specialized organs and tissues. The shoot apical meristem of angiosperm plants acts as a continuous source of pluripotent stem cells, whose descendents become incorporated into organ primordia and acquire different fates. Recent studies are unveiling some of the molecular pathways that specify stem cell fate in the center of the shoot apical meristem, that confer organ founder cell fate on the periphery, and that connect meristem patterning elements with events at the cellular level. The results are providing important insights into the mechanisms through which shoot apical meristems integrate cell fate decisions with cellular proliferation and global regulation of growth and development.     

Apical Growth and Modification of the Development of Primordia During Re-flowering of Reverted Plants of Impatiens balsamina L

Impatiens balsamina L. was induced to flower by exposure to5 short days and then made to revert to vegetative growth byreturn to long days. After 9 long days reverted plants wereinduced to re-flower by returning them to short days. Petalinitiation began immediately and seven primordia already presentdeveloped into petals instead of into predominantly leaf-likeorgans. However, the arrangement of primordia at the shoot apex,their rate of initiation and size at initiation remained unchangedfrom the reverted apex, as did apical growth rate and the lengthof stem frusta at initiation. The more rapid flowering of thereverted plants than of plants when first induced, and the lackof change in apical growth pattern, imply that the revertedapices remain partially evoked, and that the apical growth patternand phyllotaxis typical of the flower, and already present inthe reverted plants, facilitate the transition to flower formation. Impatiens balsamina, flower reversion, partial evocation, shoot meristem, determination, leaf development     

COMPARISON OF SHOOT APEX DYNAMICS AND EARLY LEAF ONTOGENY IN TWO SPECIES OF SARACA (LEGUMINOSAE)

Shoot apices of Saraca indica produce adult leaves that have 4 to 6 pairs of leaflets, whereas those of S. bijuga usually have only 2 pairs. In both species one leaflet pair is found during the juvenile phase. Juvenility lasts many plastochrons in S. bijuga but is restricted to a few in S. indica. The shoot apical meristems of these two taxa are similar in structure, cell number, and cell size; however, the shoot apex of Saraca bijuga is slightly more stratified, having 2–3 tunica layers as opposed to 1–2 in S. indica. For most of the plastochron the apical meristem in both species is situated laterally at the base of the most recently formed leaf. A newly forming primordium and its internode shift the apical meristem upward unilaterally; the meristem passes through a brief apical dome stage and becomes positioned 180° from its origin at the beginning of the plastochron. Hence, there is a true pendulum meristem in both species. In S. bijuga the maximum length of the youngest leaf primordium, just prior to the formation of its successor, is twice that of S. indica. The internodes immediately below the shoot apex and the axillary buds develop more rapidly in S. bijuga than in S. indica. It is suggested that the bijugate leaf of S. bijuga represents a case of neoteny in plants.     

Establishment of the embryonic shoot apical meristem in <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis>

In higher plants, shoot organs such as leaves, branches, and flowers are generated from the shoot apical meristem (SAM), a small group of undifferentiated cells located at the tip of the shoot. The SAM maintains its pluripotency and simultaneously produces lateral organs at its periphery. The SAM arises during embryogenesis and its positioning requires axis-dependent embryo patterning and compartmentalization of the embryo apex. Here, we introduce major factors involved in these processes in Arabidopsis thaliana and discuss how the embryonic SAM is established.