Central Mother Cells Research Articles

Modeling of growth in shoot apical dome

The vectorial field of displacement velocities V in growing apical dome is calculated from the scalar field adjusted to the geometry of the dome and to different variants of the distribution of linear growth rate along axis. The distribution of growth rate in volume and the temporal course of cell wall net deformation in the apical dome calculated from the V fit within the range of empirical data when the linear growth rate along the axis increases with distance from the tip. The distribution of volume growth rate attains the minimum inside the distal region of the dome where the zone of central mother cells occurs.

Histocytological characteristics of Eucalyptus urophylla × Eucalyptus grandis shoot apical meristems of different physiological ages

Histocytological characteristics of Eucalyptus urophylla × Eucalyptus grandis shoot apical meristems (SAMs) were described, comparing five outdoor and in vitro sources of akin genotypes differing in their physiological age. The size and the number of cells of the five zones identified within each SAM, i.e. the two tunica layers (L1 and L2), the central mother cells (CMC), the peripheral zone (PZ) and the combination of these four zones (4CZ) varied according to physiological age and plastochron phase. These five zones were significantly larger with higher numbers of cells for SAMs from mature and juvenile trees than for those from physiologically rejuvenated, in vitro mature and in vitro juvenile plants. However, these origin-related differences were not significant for SAMs in their early plastochron phase, to become obvious in a more advanced plastochron stage. Individual cell and nuclear measurements confirmed the rationale of distinguishing within SAM zones, characterized by specific cell and nuclear sizes liable to vary according to physiological age. The various histocytological investigations carried out established that SAM cell characteristics appeared to be the more reliable indicators of phase change. This was particularly true for the nucleoplasmic ratio and for more qualitative differences observed also at the nuclear level. SAM nuclei of the two in vitro origins were more evenly stained by naphtol blue-black, uniformly light for the juvenile source, whereas the mature source showed also darker nuclei. In contrast, SAM nuclei from outdoor origins had more chromocenters, darker and diffusely spread for the mature source than for the rejuvenated and the juvenile origins, where they were more peripherally distributed and where the nucleoli appeared more clearly. These results were discussed with respect to physiological ageing and in vitro culture influence, and suggest a determining influence of SAM cell nuclei on phase change phenomenon of arborescent species.

Programmed cell death during terminal bud senescence in a sympodial branching tree, Eucommia ulmoides *

Abstract Eucommia ulmoides Oliv. is a typical sympodial branching tree. The apical bud of the branch ages and dies every year, replaced by the nearby axillary bud in the second year. Structural assays and a series of biochemical analyses were performed to analyze the senescence mechanism in the apical bud. It was revealed that most cells of the apical bud underwent the programmed cell death (PCD) during the senescence: the chromosomes were congregated and the nuclear contents were condensed, as shown by 4′, 6-diamidino-2-phenylindole (DAPI) fluorescence. DNA fragmentation was detected during senescence using the terminal deoxynucleotidyl transferase (TdT)-mediated dUTP nick end in situ labeling (TUNEL) method, coincident with the appearance of a DNA ladder. Moreover, a 20 kD DNase related to fragmentation was found. PCD was initiated first in the young leaves, leaf primordia and peripheral zone cells, then in the central mother cells and initial layer cells in the apical meristem. The terminal buds remain in vegetative growth during senescence, in contrast to buds of many annual plants.

Exogenously applied jasmonic acid induces changes in apical meristem morphology of potato stolons.

Hooked apex stolons and initial swelling stolons of potato plants were treated with 3 x 10-8 mol l-1 jasmonic acid (JA) to study the effect of this compound on histology, cell expansion and tissue differentiation. In hooked apex stolons, JA application increased the meristem thickness and reduced the length of the leaf primordia, whereas in initial swelling stolons narrowing of the apical region, absence of leaf primordia and swelling of the subapical meristem were evident. Early vascular tissue differentiation was observed in response to JA treatment, especially of xylem elements from regions proximal to the tunic. Protoxylem elements, such as tracheal elements, were present with thin primary cell walls. The cell area was measured in two zones: zone I, central mother cells situated immediately under the tunic; and zone II, rib meristem cells. JA caused a four- and six-fold increase in cell area in both zones in hooked apex stolons and initial swelling stolons, respectively. Thus, tuber formation is concluded to occur as a consequence of increased cell expansion, a reduction in the length of leaf primordia, enlargement of meristems, and early vascular tissue differentiation.

Histological and biochemical changes in Pinus spp. seeds during germination and post-germinative growth: triacylglycerol distribution and catalase activity

The spatio-temporal evolution of catalase (CAT) activity and triacylglycerol distribution was investigated in seeds and seedlings from Pinus pinaster Ait., P. pinea L. and P. radiata D. Don during germination and post-germination. The high amount of triacylglycerols contained in the whole dehydrated embryo from the three species was progressively depleted, first, in the radicle and then in hypocotyl and cotyledons during post-germinative growth. In parallel, histological localisation of CAT activity and the quantitative analysis confirmed the involvement of this enzyme in cell detoxification from peroxide released during the intense lipid breakdown. Two isozymes, CAT-1 and CAT-2, were identified during post-germinative growth. Both were particularly active in the hypocotyl and radicle, while CAT-2 was specifically active in the photosynthetic tissues. These results emphasise that CAT activity is also independent from lipid metabolism in certain tissues. The role of each isoenzyme is discussed in connection with the metabolic changes occurring during seed germination and seedling growth. Special attention is given to the role of the shoot apex in triacylglycerol storage and breakdown. Central mother cells have been shown as a specific lipid storage area of the shoot apical meristem, in contrast with the peripheral zone in which lipid reserves were always reduced.

Apical meristem formation during zygotic embryo development of white spruce

The first notable sign of shoot and root meristem development in zygotic embryos of white spruce (Picea glauca (Moench) Voss) was the appearance of starch in the respective poles of the embryo. Starch granules gradually accumulated in the subapical cells of the shoot pole and were soon followed by vacuolation in the subapical cells. Vacuolation reached its highest degree in these cells at the early embryo stage. With the formation of the large vacuolated cells, the surface cell layer at the summit of the shoot pole enlarged and differentiated into the surface initials of the shoot meristem. These cells were large with distinct nuclei. As the embryo matured, the large vacuoles within the subapical cells were replaced by small ones with a concomitant increase in the cytoplasmic density of the subapical cells. After germination, the surface initials remained distinct; however, the subapical cells had different fates depending on their location. The subapical cells located next to the surface initials became the central mother cells of the shoot meristem and those located near the cotyledon junction divided periclinally and gave rise to the epicotyl rib meristem. The remaining subapical cells near the procambium became part of the pith. In the root pole, starch could be found in the cap region terminating underneath the developing procambium at the club-shaped stage of embryo development. Two layers of root initials appeared at the junction between the developing procambium and the root cap. The initials were distinct as they were larger in size than surrounding cells. Cell division activity could not be detected in the layer of initials immediately next to the procambium, while mitotic activity could be seen in the adjoining layer next to the root cap. As the embryo matured, cells surrounding the root initials also took on structural characteristics similar to the root meristem initials. As a result, the region of the root meristem initials expanded in size, and the bilayered configuration became obscure. After germination, mitotic figures could be found in the root initials. As seedling continued to grow, fewer mitotic figures could be found in the root meristem cells adjacent to the procambium pole. In roots of mature plants, a group of isodiametric cells could be discerned between the procambium and the root cap. Mitotic activity was not readily detected within this group of isodiametric cells but mitotic figures could be found in surrounding cells.Key words: apical meristems, Picea glauca, white spruce, zygotic embryogenesis.

Zonation in the shoot apex ofMatthiola incana R. Br.

Zonation in the shoot apex ofMatthiola incana was studied histochemically from the mature embryo to flowering. The plumular apex of the mature embryo shows a single-layered tunica covering a homogeneous mass of cells. The first evidence of zonation is seen in the shoot apex of the 2-day old seedling. Zonation is well established on the fourth day after seed wetting with a single-layered tunica covering a lightly stained central mother cell zone, a subjacent pith meristem and a densely stained peripheral zone. In the transitional apex zonation is less marked and there is increased stratification. At 6-weeks, the apex changes into the indeterminate inflorescence apex showing acropetally developing floral primordia. The axially located cells of the inflorescence apex are lightly stained. The histological and histochemical data indicate that zonation resembling that in the vegetative apex is retained to a lesser degree in the inflorescence apex as well.

Shoot apex, leaf development and unifacial tip inAgave wightii drumm. et prain (agavaceae)

The shoot apex has one tunica layer enclosing a mass of corpus which is differentiated cytohistologically into central mother cell zone, flank zone, rib zone and a ‘cambium-like’ zone. Occurrence of ‘cambium-like’ zone during minimal phase is considered as an expression of nodal region. Agave wightii shows spirodistichous arrangement of leaves which have an expanded photosynthetic surface with a reduced unifacial tip. Leaves are initiated by periclinal divisions in the second layer. Vertical growth in the leaves is by subapical initials and lateral growth is by marginal and submarginal initials in their early stages of development. The unifacial tip is formed by the extension of adaxial meristematic activity. The derivatives thus formed are pushed to the abaxial side of the primordiuj. Hence the unifacial part of the leaf is regarded as equivalent to a phyllode.

Distribution of relative growth rates and variation of cytohistological zonation in apical meristems of seedlings of two contrasting open-pollinated jack pine (Pinusbanksiana) families

A general method is developed to obtain relative growth rates in different parts of an apical meristem, bare apical dome, internode, and whole apical dome. Expressions are given for transverse, longitudinal, and volume growth. It is shown that these growth rates depend on only four parameters, radius (Ro) and height (Ho) of the bare apical dome and transverse (r) and longitudinal (h) plastochron ratios. A method is described to obtain relative volumes of different zones in apices showing cytohistological zonation using relative area measurements on median longitudinal sections. Both methods are applied to a practical example, on ontogenetical variation of seedlings originating from two open-pollinated families of jack pine (Pinusbanksiana Lamb.) that exhibited a contrasting pattern of height growth in previous studies. Both families differ in their plastochron duration (PD) and this difference can be related to different distributions of growth rates in the apical meristem. PD values range, between 16–6 h in the first family, and between 12–4 h in the second one. Most family differences appear on bare apical dome growth rates and on allometric coefficients between bare apical dome growth and intermode growth. As an example, bare apical dome radial relative growth rates per PD varies during ontogeny from 0.004 to 0.008 and then decreases to −0.011 during the first 70 days after germination for the first family. Corresponding values for the second family are 0.001, 0.007, and −0.004. There are also ontogenetical variations in cytohistological zonation. Both families show an increase in the apical initial plus central mother cell zone relative volume following the decrease of plastochron duration. However, no significant differences between families are obtained for relative volumes of cytohistological zones. Relationships between distributions of growth rates and changes of cytohistological zonation are discussed.

Bud development in Picea engelmannii. I. Vegetative bud development, differentiation, and early development of reproductive buds

Vegetative buds of Engelmann spruce (Picea engelmannii Parry) from the Prince George Forest District (British Columbia) were collected and studied. In mid-April, dormancy ended as determined from mitotic divisions within the leaf primordia; 2 weeks later mitotic activity occurred in the bud apices. Bud-scale initiation began in terminal buds by late May followed by that in axillary buds 2 weeks later. Shoot elongation began in late May, bud burst occurred in late June, and both shoot elongation and bud-scale initiation were complete by late July. Terminal buds began to differentiate by the initiation of leaf primordia, into vegetative buds early in August whereas axillary buds began to differentiate 1 week later. Leaf initiation was completed in all vegetative buds by late September and buds were dormant by mid-October. Pollen cones initiated microsporophylls after bud-scale initiation. Microsporangial initiation began in late August. Microsporangial enlargement began in mid-September and continued until dormancy when pollen mother cells were observed in a premeiotic stage. Seed cones initiated bracts directly after bud-scale initiation. In mid-August, ovuliferous scales began to be initiated. Two ovule primordia formed adaxially, one on each side of the median longitudinal axis of each ovuliferous scale. Each ovule formed one large central megaspore mother cell which overwintered in a premeiotic stage.

Shoot Apical Organization in Acacia nilotica (L.) Delile

Summary The vegetative apices have a tunica-corpus organization and no cytohistological zonation. The corpus is divided into a central mother cell zone (CMZ), peripheral zone (PZ) and pith meristem (PM) based on analysis of the cell net. The tunica is 2-layered, the clarity of the layers depending on the plastochronic stage. The CMZ shows a decrease in depth from the minimal to the maximal stage of the plastochron. The inflorescence and floret apices show a mantle-core organization. In the young reproductive apex the axial cells of the mantle are ligthly stained. The complete inflorescence apex is used up in the production of bracts and florets.

CHANGES IN MITOTIC ACTIVITY AND CELL SIZE IN THE APICAL MERISTEM OF HELIANTHUS ANNUUS L. DURING THE TRANSITION TO FLOWERING

Cellular changes in the shoot apical meristem of Helianthus annuus L. have been investigated in relation to its progress towards flowering. During the strictly vegetative phase, lasting for 6–7 days from sowing, mitotic divisions were confined to the peripheral zone, while the central mother cells zone proper, together with the distal cells positioned above the mother zone and also the central portion of the tunica, were relatively quiescent. Mitotic activity increased in the distal cells zone on day 8 and reached the level of that in the peripheral zone by day 12. This was accompanied by an enlargement of this zone and the consequent recession of the mother zone away from the central tunica. At the same time there was a substantial increase in the amount of cytoplasm in the cells of the central tunica. Mitotic activity in the central tunica began on day 12 and reached a peak on day 16. This zone then lost its distinct entity and was replaced by a uniform dome‐shaped meristematic layer that became apparent by day 16. The cells of the mother zone remained quiescent during the transition period.

The effects of acute and chronic gamma irradiation on Lupinus albus L.—I. Effects of acute irradiation on the vegetative shoot apex and general morphology

A Plastochron Index (P.I.) was established for lupine that provided a baseline from which dose rate and total dose effects could be evaluated. The P.I. was calculated to be 3.56 days and each control leaf elongated exponentially for about 10 days at approximately equal plastochron intervals. In plants irradiated with 3.3–16 kR, the plastochron interval tended to lengthen slightly during irradiation but returned to control values in 11 days. Statistically significant ( p ≦ 0.01; t test for correlated samples) depressions were observed in the slopes of growth curves at all dosages for leaves growing exponentially at the time of irradiation. Using divergence angle and plastochron ratio it was shown that phyllotaxis in lupine was not changed after irradiation (6.6–32 kR) from the 2,3 arc. An increase in divergence angle did occur, however, after irradiation, indicating that the positions of leaf primordia were moved laterally, but only a change in the radial rates of leaf displacement would elicit a change in phyllotactic pattern. Lupine apices given acute doses of 6.6–32 kR showed both scattered and specific damage. Scattered damage was limited and unimportant since it induced no developmental changes. Specific damage was reflected in an increasing percentage of apices which showed tunica-corpus damage at increasing dosages at or above a threshold dose of 8.5 kR, but no one apex showed all stages in the whole range of localized necrotic damage. Therefore, only the percentage of apices damaged was dose dependent, and the induced cellular changes, from initial vacuolation to complete cell necrosis followed by post-mortem changes, were not dose dependent. At 6.6 kR there was no localized damage to the second tunica layer common to higher dosages, but only slight vacuolation of some apical zone cells (initials and central mother cells). Vacuolation is discussed as the earliest and most obvious radiation response and can best be explained by the passive osmotic theory of Vasil'ev. Localized damage in the second tunica layer, beginning in the initial zone and extending down to and including the region of leaf initiation, could suppress the initiation of a leaf primordium. Corpus cells subjacent to the collapsed second tunica layer then divided to form files of cells which enlarged by vacuolation and then collapsed to obliterate the apical meristem. Primary tissues derived from the apical meristem showed varying radiosensitivities—the pith was most sensitive and then the primary vascular tissues, while the epidermis and cortex showed only slight vacuolation. Other general morphological changes were a fusion of folioles and the occasional (5 of 26) elimination of leaves. The main vegetative radiation response in lupine was suppression of a median leaf bundle and subsequent failure of the leaf to be initiated after doses of 6.6–32 kR and 3–14 days recovery (to allow the apex to continue to produce leaves). This experimental work supports the view of longitudinal induction of patterning of primary vascular differentiation and the associated positioning of putative leaves. Data from the present study support the concept that the phyllotactic pattern of putative leaves found on the apex is a result of the apex and its derivatives interacting with more mature subjacent tissues in a correlative fashion.

A Morphometric 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. A RECENT STUDY of the geometry of shoot apical meristems by Niklas and Mauseth (1980) demonstrated that meristem zonation could be produced solely by cell-packing phenomena; that is, a meristem with tunica, central mother cells, peripheral zone, and a pith-rib meristem could be constructed from cells which are all identical to each other. The zonation that is visually apparent would be due only to the patterns in which the cells are packed into the volume of the apex. However, the study did not preclude the possibility that each zone could also be composed of specialized cells, such that central mother cells would have a fine structure different from that of the cells of the peripheral zone or pith-rib meristem. If this were the case, the visibility of the zonation would be due both to cell-packing and differ

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.

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.

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.

CYTOKININ‐ELICITED FORMATION OF THE PITH‐RIB MERISTEM AND OTHER EFFECTS OF GROWTH REGULATORS ON THE MORPHOGENESIS OF ECHINOCEREUS (CACTACEAE) SEEDLING SHOOT APICAL MERISTEMS

Shoot apical meristems of Echinocereus engelmannii have only a tunica‐corpus organization at germination, but the corpus rapidly develops central mother cells, a peripheral zone and a pith‐rib meristem. The manner in which nutrition, darkness and various growth regulators at several concentrations and in several combinations affect the development of zonation was examined by growing derooted seedlings on agar which contained the nutrients or growth regulators. Benzylaminopurine was able to elicit the formation of the pith‐rib meristem in an otherwise non‐zonate corpus. Also, the rate of leaf initiation was greatly increased. Gibberellic acid severely inhibited the formation of corpus zones but had little effect on leaf initiation. Indoleacetic acid had no effects other than mild inhibition of zonation and a slight retardation of leaf initiation. Abscisic acid was strongly inhibitory. Sucrose only slightly increased the rate of leaf formation and did not affect apex size or zonation. To more closely examine the cytokinin‐induced effects on the apical meristem, several growth regulators were applied in combination with the most effective concentration of cytokinin. Certain combinations were able to interfere with several of the cytokinin‐induced responses, while other cytokinin‐induced responses occurred even in the presence of high concentrations of these other growth regulators. Leaf initiation and meristem morphogenesis appeared to be remarkably stable and insensitive to the presence of most hormones except cytokinin and gibberellin.

Cytokinin-Elicited Formation of the Pith-Rib Meristem and Other Effects of Growth Regulators on the Morphogenesis of Echinocereus (Cactaceae) Seedling Shoot Apical Meristems

Shoot apical meristems of Echinocereus engelmannii have only a tunica-corpus organization at germination, but the corpus rapidly develops central mother cells, a peripheral zone and a pith-rib meristem. The manner in which nutrition, darkness and various growth regulators at several concentrations and in several combinations affect the development of zonation was examined by growing derooted seedlings on agar which contained the nutrients or growth regulators. Benzylaminopurine was able to elicit the formation of the pith-rib meristem in an otherwise non-zonate corpus. Also, the rate of leaf initiation was greatly increased. Gibberellic acid severely inhibited the formation of corpus zones but had little effect on leaf initiation. Indoleacetic acid had no effects other than mild inhibition of zonation and a slight retardation of leaf initiation. Abscisic acid was strongly inhibitory. Sucrose only slightly increased the rate of leaf formation and did not affect apex size or zonation. To more closely examine the cytokinin-induced effects on the apical meristem, several growth regulators were applied in combination with the most effective concentration of cytokinin. Certain combinations were able to interfere with several of the cytokinin-induced responses, while other cytokinin-induced responses occurred even in the presence of high concentrations of these other growth regulators. Leaf initiation and meristem morphogenesis appeared to be remarkably stable and insensitive to the presence of most hormones except cytokinin and gibberellin.

The Shoot Apex of Bougainvillea spectabilis

The shoot apex of Bougainvillea spectabilis consists of five zones: A two- or occasionally three-layered tunica, a central mother cell zone, a cambium-like zone, a rib meristem (central meristem), and a peripheral meristem. The presence of a cambium-like zone is somewhat unusual in the apex of vascular plants, having only been reported for a few taxa. In B. spectabilis the cambium-like zone is consistently present throughout the plastochron and all yearly seasonal periods.