Characterization of Stelar Initiation in Shoot Apices of Ferns

Procambium is commonly recognized as a vascular meristem inshoot apices of vascular plants. Prestelar tissue comprisingprovascular tissue (PVT) and pith mother cells (PMCs) immediatelysubjacent to the single cell layer of promeristem has been consideredto represent the initial stage of stelar differentiation precedingprocambium and rib meristem in ferns. In addition to characterizationof PVT and PMCs on the basis of cell morphology, cytologicalfeatures and developmental continuity with procambium and ribmeristem, four lines of evidence from studies of shoot apicesof Matteuccia struthiopteris and Osmunda cinnamomea supportthis interpretation of initial differentiation. (1) Differentialstaining by safranin-fast green and crystal violet-erythrosinshows that PVT and PMCs differ in colour reactions from promeristemand resemble procambium and pith meristem, respectively. (2)Comparative ultrastructural study reveals qualitative differencesin the cell membrane system, nuclei, cytoplasm, vacuoles andplastids between promeristem and PVT but similarity of PVT toprocambium. (3) Large droplets of tannins occur in promeristembut not in PVT, PMCs and procambium. (4) Cytochemical studyof the shoot apex of Osmunda shows that carboxylesterase activityis strongly demonstrated in PVT and procambial cells but notin promeristem cells and PMCs. These observations further substantiatethe interpretation that PVT represents initial vascular differentiationand PMCs reflect a commitment to pith development.Copyright1995, 1999 Academic Press Initial vascular differentiation, provascular tissue, differential staining, ultrastructure, tannins, carboxylesterase, shoot apex, Matteuccia struthiopteris, Osmunda cinnamomea     

APPLICATION OF THE 22.5 MEV DEUTERON MICROBEAM TO THE STUDY OF MORPHOGENETIC PROBLEMS WITHIN THE SHOOT APEX OF OSMUNDA CLAYTONIANA

Excised shoot apices of Osmunda claytoniana were grown under controlled sterile conditions. Histological examination of the normal shoot apex shows that it is comprised of: (1) a promeristem, which possesses 1 or more apical initiating cells at its center; (2) a prestelar tissue consisting of an incipient vascular tissue which flanks the pith-mother-cell zone; the pith-mother-cell zone gives rise to the pith rib meristem and subsequently to the fundamental parenchyma of the pith; (3) the fundamental parenchyma of the cortex and the fundamental parenchyma of the dermal system both arising from flank cells of the promeristem. Apical initial cells of meristems irradiated with a 127,000 rad acute exposure of a deuteron beam having a diameter of 25μ, histologically examined at 7-day intervals for a 12-week period, as early as 3 weeks’ postirradiation, showed the apical initiating cell(s) together with certain of the cells of the pith-mother-cell zone to be destroyed. A wound response develops peripherally to the destroyed initials. In addition, an isolated, organized growth center is observed to develop from normal promeristem cells. Incipient vascular tissue and a new pith-mother-cell zone are also observed to develop in association with the new center of growth. Implications of the role of the interrelationships between apical initiating cell(s) and other cells of the meristem and the role they may play in maintenance of meristematic integrity within the shoot meristem are discussed.     

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.     

Development of maize plants from cultured shoot apices

Excised shoot apices of maize (Zea mays L.), comprising the apical meristem and one or two leaf primordia, have been cultured and can form rooted plantlets. The plantlets, derived from meristems that had previously formed 7–10 nodes, develop into mature, morphologically normal plants with as many nodes as seed-grown plants. These culture-derived plants exhibited the normal pattern of development, with regard to the progression of leaf lengths along the plant and position of axillary buds and aar shoots. Isolation of the meristem from previously formed nodes reinitiates the pattern and number of nodes formed in the new plant. Thus, cells of the meristem of a maize plant at the seedling stage are not determined to form a limited number of nodes.     

EFFECTS OF GIBBERELLIC ACID ON STEM APICES OF VERNALIZABLE GRASSES

Koller , D. (Hebrew U., Jerusalem, Israel), H. R. Highkin , and O. H. Caso . Effects of gibberellic acid on stem apices of vernalizable grasses. Amer. Jour. Bot. 47(6) : 518–524. Illus. 1960.—Gibberellic acid was found to have distinct morphogenic effects on the stem apices of 3 vernalizable but unvernalized grasses: Hordeum vulgare var. ‘Kentucky,’ Hordeum bulbosum and Secale cereale var. ‘Winter Petkus.’ In these 3 species, GA caused the activation of lateral meristems on the embryonic nodes of the stem apices, under both short- and long-day conditions. These lateral meristems differentiated into either flower primordia or vegetative shoots. The flower primordia developed almost invariably in Hordeum, while in Secale they seldom did, the apical meristem usually resuming normal vegetative growth after treatment. Despite the occurrence of flowering, it is concluded that the role of GA in this phenomenon is restricted to the activation of lateral meristems in the apex.     

THE SHELL ZONE: ITS DIFFERENTIATION AND PROBABLE FUNCTION IN SOME DICOTYLEDONS

Thirty-five species belonging to various dicotyledonous families were investigated to study the origin, development, and probable function of the shell zone, which is defined as an arcuate zone of cambiform cells delimiting the early axillary bud meristem. It is present in the majority of the investigated plants and five intergrading patterns of origin are described: (i) from the parenchymatized derivatives of the cells of the peripheral meristem of the shoot apex, adaxial to the bud meristem, (ii) from the peripheral meristem of the shoot apex along with the initiation of the early bud meristem, (iii) from the adaxial cells of the bud meristem, (iv) from the derivatives of the cells of the bud meristem at its base, and (v) partly from the parenchymatized cells of the peripheral meristem adaxial to the bud and partly from the adaxial derivatives of the bud meristem. The shell zone loses its identity at different stages of bud development in various species. Its cells ultimately contribute to the ground meristem, procambium, and pith cells of the axis. In Cuminum cyminum and lpomoea cairica the shell zone contributes in bringing about the axillary position of the bud from its early lateral position. In Solarium melongena, derivatives of the shell zone initiate the internodal elongation between the flower or inflorescence and the shoot apex, ultimately shifting the bud to an extra-axillary position on the internode.     

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.     

Interaction of Auxin-TIBA in Lycopersicon esculentum

[5-³H] indoleacetic acid (IAA) was applied to the apical budof young tomato plants (Lycopersicon esculentum Mill.) treatedor not with 2,3,5-triiodobenzoic acid (TIBA). Chromatographicanalysis showed that treatment with TIBA increased the quantityof breakdown products in the apical 1.5 mm segment of the stem.Autoradiographs of ultra-thin sections of this segment wereprepared after treatment with glutaraldehyde. The inhibitionof polar auxin transport by TIBA caused an increase in the densityof labelling of all the cellular types in the shoot apex. TheTIBA treatment provoked a modification of the structure of theshoot apex in which the greatest increases in density of labellingwere detected in the superficial tunical layer and in the pithmeristem. The increase in radioactivity of the superficial tunicallayer could be explained by the presence of polyphenols in thevacuoles. For the pith meristem, the TIBA treatment intensifiedthe high labelling previously observed at this level in thecontrol plants. This could indicate a role for auxin in cellularelongation. It could also point to the importance of the pithmeristem in the functioning of the shoot apex. Key words: [³H] IAA, Lycopersicon esculentum, TIBA     

A MORPHOMETRY STUDY OF THE ULTRASTRUCTURE OF ECHINOCEREUS ENGELMANNII (CACTACEAE). II. THE MATURE,ZONATE SHOOT APICAL MERISTEM

The shoot apical meristems of adult Echinocereus engelmannii plants are zonate and have a tunica, central mother cells, a peripheral zone, and a pith-rib meristem. An ultrastructural, stereological study showed that each zone has its own distinct ultrastructure, but that the differences between the zones are quite small, both on a protoplasmic basis and on a cytoplasmic basis. Furthermore, the ultrastructure present in the adult apices differed only slightly from that which had been found in seedling apices, demonstrating a long-term stability of structure. The standard deviations found in the sample were small, indicating little variability from one plant to the next and suggesting that there are little or no cyclic changes during the plastochron or a 24-hr photoperiod. The ultrastructures found in the shoot apical meristems differed significantly and markedly from mature tissues of the same plants.     

Seasonal variation of organogenetic activity and reserves allocation in the shoot apex of Pinus pinaster Ait

BACKGROUND AND AIMS: To understand better the basic growth characteristics of pines and the fundamental properties of the shoot apical meristem (SAM), variations within the shoot apex of buds were studied. METHODS: A detailed structural comparison of meristem dimensions, organogenetic activity, and the presence of lipids, starch grains and tannins was performed on shoot apices of juvenile, and male and female adult Pinus pinaster at five different times in the annual growth cycle. KEY RESULTS: There were significant correlations among traits and differences in the pattern for juvenile and adult shoots. In juvenile shoots, peaks of organogenesis were present in spring and autumn, but not in summer. In adult shoots, one peak, characterized by an increase in meristem dimensions, was present in summer. The accumulation of starch grains beneath the SAM and of tannin in sub-apical pith parenchyma were at their maximum when organogenetic activity was high in spring and autumn in juvenile plants, and in summer in adult plants. In juvenile and adult plants, lipids were stored within the SAM in autumn, filling a large part of the bud in winter, and were depleted in the cortical parenchyma and then in the pith during shoot elongation. CONCLUSIONS: Depending on the sites of accumulation within the SAM and on the stage of the annual growth cycle, lipids, starch and tannins may be involved in different processes. In spring, energy and structural materials released by lipid hydrolysis may contribute to stem elongation and/or cell-to-cell communication. During organogenesis, energy and structural materials released by starch hydrolysis may influence developmental programmes in the SAM and adjacent cells. Tannins may be involved in cellular detoxification. At the end of the growing season, accumulation of lipid and starch is positively correlated with the onset of dormancy.     

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.     

FASCIATION AND DICHOTOMOUS BRANCHING IN ECHINOCEREUS (CACTACEAE)

In Echinocereus reichenbachii dichotomous branching and fasciation (cresting) are rare events. Both were found together in only a few of many populations investigated and are interpreted as variants of a single phenomenon. They may occur at any stage of shoot development, but crest meristems arise most commonly on young branches among clusters of normal shoots. Sometimes they appear on unbranched young plants or seedlings, very rarely on older shoots. Dichotomy results from the division of an apical meristem into equal parts each of which functions independently, producing a forked shoot. Fasciation involves the extension of a single meristem into an apical ridge. The product is a flabellate shoot that becomes undulate if growth along the summit continues. In longisection linear meristems appear similar to radial sections of normal shoots; in median sagittal section they have a much extended central mother cell zone within which the cell pattern resembles a rib meristem. Although crest meristems become sluggish or even inactive with age, localized renewed growth may occur spontaneously or be induced by injury. In this species the random production of normal shoots from crest meristems (defasciation) was not observed, but if much or all of such a meristem is removed, branches may arise from lateral areoles, and these are always normal. It seems, therefore, that whatever induces fasciation in E. reichenbachii originates in and is restricted to the apical meristem and its immediate vicinity.     

TERMINAL FLOWER1 is a mobile signal controlling Arabidopsis architecture

Shoot meristems harbor stem cells that provide key growing points in plants, maintaining themselves and generating all above-ground tissues. Cell-to-cell signaling networks maintain this population, but how are meristem and organ identities controlled? TERMINAL FLOWER1 (TFL1) controls shoot meristem identity throughout the plant life cycle, affecting the number and identity of all above-ground organs generated; tfl1 mutant shoot meristems make fewer leaves, shoots, and flowers and change identity to flowers. We find that TFL1 mRNA is broadly distributed in young axillary shoot meristems but later becomes limited to central regions, yet affects cell fates at a distance. How is this achieved? We reveal that the TFL1 protein is a mobile signal that becomes evenly distributed across the meristem. TFL1 does not enter cells arising from the flanks of the meristem, thus allowing primordia to establish their identity. Surprisingly, TFL1 movement does not appear to occur in mature shoots of leafy (lfy) mutants, which eventually stop proliferating and convert to carpel/floral-like structures. We propose that signals from LFY in floral meristems may feed back to promote TFL1 protein movement in the shoot meristem. This novel feedback signaling mechanism would ensure that shoot meristem identity is maintained and the appropriate inflorescence architecture develops.     

Formation and maintenance of the shoot apical meristem

Development in higher plants is characterized by the reiterative formation of lateral organs from the flanks of shoot apical meristems. Because organs are produced continuously throughout the life cycle, the shoot apical meristem must maintain a pluripotent stem cell population. These two tasks are accomplished within separate functional domains of the apical meristem. These functional domains develop gradually during embryogenesis. Subsequently, communication among cells within the shoot apical meristem and between the shoot apical meristem and the incipient lateral organs is needed to maintain the functional domains within the shoot apical meristem.     

INITIATION AND ONTOGENY OF THE MICROSPORANGIATE CONE IN CUPRESSUS ARIZONICA IN RESPONSE TO GIBBERELLIN

Cupressus arizonica, a member of the Cupressaceae, was induced to produce pollen cones in response to gibberellin treatment. All apices remained vegetative during the first 17 days of treatment. At this time many lateral vegetative apices began to undergo a transition to the reproductive state. The transition was marked by changes in apical zonation characterized by increased mitotic activity primarily in the subapical mother-cell and peripheral zones. Also, precocious initiation of branch meristems occurred much higher on the shoot apex than before the transition period. About 22 days after the initial treatment, most apices became distinctly reproductive. The reproductive apex has a zonation pattern similar to the branching apex but is shorter and wider and quite distinct from the vegetative apex. The small subapical mother cells and cells of the peripheral zone form a continuous mantle of mitotically active cells with prominent nucleoli. This mantle encloses a very broad pith region which differentiates nearly to the summit of the apex. Microsporophyll and leaf initiation are similar and the protoderm of the apex remains discrete and does not contribute to deeper tissues. Sporangia do not originate from superficial cells of the microsporophyll. After all microsporophylls are initiated the reproductive apex becomes inactive. A discussion concerns the morphological implication in the origin of the foliar structures and of the similarity of C. arizonica to many angiosperms in the transition of the apex from vegetative to reproductive.     

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.     

A SURVEY OF THE VEGETATIVE SHOOT APICES IN THE FAMILY MALVACEAE

The present study compares the structure of the vegetative shoot apex in 40 species of the Malvaceae. There is a wide range of size, shape, and zonation within the apices of the family. Although many of the apices are domed, some are flat-topped and do not extend above the axil of the youngest leaf primordium. Also, most of the species investigated are recorded as having a more or less marked cytohistological zonation superimposed on the tunica-corpus configuration. The tunica is single-layered in a majority of species, but stratification of the upper corpus is common. In an effort to give a more accurate concept of apical structure and activity, the apex is described as the metrameristem and its derivatives: the flanking meristem, and the pith rib meristem or pith mother cells. The metrameristem, consisting of the tunica initials and the co pus initials, is the focal point of the study of the zoned apices. Data are presented for the measurements of the metrameristem, measurements of the apical dome, type of flanking meristem, origin of the pith, and growth habit of the plant. There appears to be a correlation between growth habit and the distinctness with which the metrameristem is marked off from the surrounding tissue. Most of the herbaceous species have an indistinctly marked metrameristem, whereas the shrubby trees and trees have a distinctly marked metrameristem. Zonation in shrubs and suffrutescent plants may be of either type.     

Histochemical Studies of Sclereid Induction in the Shoot of Rauwolfia Species: I. CYTOCHROME OXIDASE

Results of histochemical tests for cytochrome oxidase activityin four species of Rauwolfia have been reported. The cells beneaththe terminal shoot meristem constitute the pith. Histochemicaltests showed intensified enzymatic activity in those cells ofthe pith which would differentiate as sclereid initials. Similarcytochrome oxidase activity also occurred in the sclereid initialsand the developing sclereids. The cytochrome oxidase activitywas associated with two types of particulate formation, thegranular and rod-shaped. The ground parenchymatous cells ofthe pith and the leaf-base showed very little enzymatic activityof cytochrome oxidase. The characteristic indophenol blue colorationdue to cytochrome oxidase activity did not appear in controlsections. Physiological changes correlated with morphogeneticexpression of some pith cells demonstrate that the physiologicalchanges occur before the initiation of sclereids in the morphologicallyhomogeneous parenchymatous cells of the pith. Intensified cytochromeoxidase activity was also recorded in the meristematic tissuesof shoot apex, procambium, axillary buds and the laticiferouscells of Rauwolfia.     

On the use of morphactin and triiodobenzoic acid in apical dominance studies

Summary Application of either CFM or TIBA to bean plants caused four main effects: (i) inhibition of main shoot and leaf growth; (ii) abscission of young leaflets and internodes; (iii) limited outgrowth of lateral buds below the point of application of the substances followed by abscission of these buds; (iv) abscission of all other lateral buds. Although the chemical pruning effects of CFM and TIBA may be the result of their action in blocking auxin transport, the use of these substances for analysing auxin effects in apical dominance is questionable.