Surgical Experiments on the Differentiation of Vascular Tissue in the Shoot Apex of Carrot (Daucus carota L.)

Surgical techniques were applied to the shoot apex of carrot(Daucus carota L.) to test the interpretation that provasculartissue is the initial stage of vascular differentiation andto localize the sources of the influences that control its differentiation.If the apex is isolated laterally by vertical incisions leavingit at the summit of a plug of pith tissue, vascular differentiationproceeds normally and an independent vascular system is formedin the pith plug. If all leaf primordia are systematically suppressed,provascular tissue continues to differentiate as an acropetalextension of the pre-existing vascular system but no furtherdifferentiation occurs. When the apex is isolated laterallyand all leaf primordia are suppressed, provascular tissue continuesto be formed acropetally and is extended basipetally into thepith plug by redifferentiation of pith cells, but no furtherdifferentiation occurs. This tissue reacts positively to histochemicaltests for esterase indicating its vascular nature. If only oneleaf primordium is allowed to develop on an isolated shoot apex,its vascular system develops normally and extends basipetallyinto the pith plug, but there is no extension of provasculartissue into the pith plug. These results support the interpretationthat the initial stage of vascular differentiation is controlledby the apical meristem but that further maturation of vasculartissue depends upon influences from developing leaf primordia.Copyright 2000 Annals of Botany Company Provascular tissue, differentiation, carrot (Daucus carota L.), shoot apex, surgical techniques, leaf primordia     

Phospholipid and phospholipase changes by jasmonic acid during stolon to tuber transition of potato

Potato tuber formation starts with the stolon swelling and is regulated by jasmonates. The cascade of events leading to tuber formation is not completely understood. The aim of this study was to evaluate phospholipid composition and phospholipase activities during four stages of stolon-to-tuber transition of Solanum tuberosum L., cv. Spunta, and involvement of phosphatidic acid (PA) in stolon cell expansion during early stages. Effects of jasmonic acid (JA) treatment on phospholipid content and activation of phospholipase D (PLD) (EC 3.1.4.4) and phosphatidylinositol-4,5-bisphosphate-specific phospholipase C (PIP₂-PLC) (EC 3.1.4.3) were studied in the early stages (first stage, hooked apex stolon; second stage, initial swelling stolon) of tuberization. All the phospholipid species identified, phosphatidylinositol (PI), phosphatidylserine (PS), phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylglycerol (PG), PA, and cardiolipin (CL), decreased as tuber formation progressed. PLD and PLC were activated in control tissues at an early stage. JA treatment caused a decrease of PC and PS in first stage stolons, accumulation of PA in second stage stolons, and modification of PLD and PLC activities. PA increased stolon cell area in the first and second stages. These findings indicate that phospholipid catabolism is activated from the early stages of tuber formation, and that JA treatment modifies the pattern of phospholipid (PC, PS, and PA) composition and phospholipase (PLD and PLC) activity. These phospholipids therefore may play a role in activation of an intracellular mechanism that switches the developmental fate of stolon meristem cells, causing differentiation into a tuber.     

A HISTOCHEMICAL STUDY OF THE SEEDLING SHOOT APICAL MERISTEM OF PINUS LAMBERTIANA

Vernalized seeds of Pinus lambertiana were scarified and planted in perlite. At 5, 8, 10, 13 and 16 days after planting, seedlings were selected for morphological examination and histochemical study. The shoot apical meristem consisted of a relatively homogeneous population of cells at 5 days. Cytohistological zonation was observed in the meristem by the eighth day and needle primordia initiation began at this time. Acid phosphatase (AP) activity was high in the extreme tip of the apex at 5 days. At 8 days AP activity was intense in the peripheral zone but weak in the apical initial and central mother cell zones. The apical meristem of the 10–16-day-old seedlings exhibited high AP activity in the peripheral zone only during the early stages of needle primordia initiation. The distribution of cytoplasmic and nuclear protein-bound SH was correlated with cytohistological zonation. Protein-bound SH was distributed relatively uniformly at 5 days, but by the eighth day the 4 cytohistological zones contained differential quantities. Succinic dehydrogenase (SD) activity was observed throughout the apex at 5 days, but by the eighth day the apical initial and central mother cell zones exhibited differentially greater levels of SD activity. Irradiation with 500 R of X-rays at 7 days after planting completely inhibited needle primordia initiation and disrupted the cytohistological zonation of the apex. Correlated with the inhibition of needle primordia initiation was the loss of SD activity in the apical initial and central mother cell zones. Irradiation also resulted in the gradual loss of protein-bound SH from the cytoplasm of the apical initial, central mother cell and peripheral zone.     

Glucose-6-phosphate-dehydrogenase activity in the shoot apical meristem,leaf primordia,and leaf tissues of Dianthus chinensis L.

The oxidation of carbohydrate by the pentose-phosphate pathway in the shoot apical meristem and developing leaf primordia of Dianthus chinensis was assessed by measuring the activity of glucose-6-phosphate dehydrogenase (EC 1.1.1.49). On a kg^-1 dry weight h^-1 basis, activity rose from 250 mmol in the apical meristem to 550 mmol in the first two leaf primordia and then declined to 350 mmol in the sixth pair of leaf primordia, and finally to 200 mmol in leaves just emerged from the shoot bud. Measurements of activity in the sixth leaf pair from the apex showed differential distribution in leaf tissues. Epidermal and mesophyll tissue had about the same activity as whole-leaf tissue, but vascular bundles had 70% greater activity. Within the vascular tissue, activity in the phloem was twice as high as in the xylem. When activity was expressed on a per-cell basis, there was a continuous increase from 20 fmol in the apex to 2 pmol in the sixth leaf pair. Activity on a per unit cell volume basis showed that cells of the apical meristem and the epidermis, mesophyll and xylem of the sixth leaf pair had similar values, about 30 amol; only the two youngest pairs of primordia and the phloem had values two or three times this amount.     

The inhibition and stimulation of DNA synthesis in shoot apices ofChenopodium rubrum L. during photoperiodic induction of flowering

Three short-day inductive cycles bring about inhibition followed by transitional enhancement of growth, not only in roots and leaves but also in different zones of shoot apical meristem, as shown by measurement of DNA synthesis using³H-thymidine autoradiography. The first inductive cycle resulted in marked inhibition of the cells of the central zone (CZ), rib meristem (RM), and peripheral zone (PZ). Subsequent enhancement of DNA synthesis occurs in RM during the second inductive cycle, but in CZ only in the third cycle. The growth activation in PZ is counteracted by decrease in apical dominance which results in further inhibition of leaf primordia and increases in bud primordia. In plants induced only by one cycle, which later reverse the vegetative pattern of growth and differentiation, increased DNA synthesis in RM and CZ was not observed. The significance of inhibitory and stimulatory processes in particular zones of the shoot apex is discussed considering flower morphogenesis.     

Surface growth at the reproductive shoot apex of Arabidopsis thaliana pin-formed 1 and wild type

With the aid of a non-destructive replica method and computational protocol, surface geometry and expansion at the reproductive shoot apex are analysed for pin-formed 1 (pin1) Arabidopsis thaliana and compared with the wild type. The observed complexity of geometry and expansion at the pin1 apex indicates that both components of shoot apex growth, i.e. the meristem self-perpetuation and initiation of lateral organs, are realized by the pin1 apex. The realization of the latter component, however, is only occasionally completed. The pin1 apex is generally dome-shaped, but its curvature is not uniform, especially later during apex ontogeny, when bulges and saddle-shaped regions appear on its periphery. The only saddle-shaped regions at the wild-type shoot apex are creases separating flower primordia from the meristem. Surface expansion at the pin1 apex is faster than at the wild type. In both pin1 and wild type the apex surface is differentiated into regions of various areal strain rates. In the pin1 apex, but not in the wild type, these regions correspond to the geometrically distinguished central and peripheral zones. Expansion of the central zone of the pin1 apex is nearly isotropic and slower than in the peripheral zone. The peripheral zone is differentiated into ring-shaped portions of different expansion anisotropy. The distal portion of this zone expands anisotropically, similar to regions of the wild-type apex periphery, which contact older flower primordia. The proximal portion expands nearly isotropically, like sites of flower initiation in the wild type. The peripheral zone in pin1 is surrounded by a 'basal zone', a sui generis zone, where areal strain rates are low and expansion is anisotropic. The possible relationships between the observed regions of different expansion and the various gene expression patterns in the pin1 apex known from the literature are discussed.     

Initial Differentiation of Vascular Tissue in the Shoot Apex of Carrot (Daucus carotaL.)

Although an initial stage of vascular differentiation precedingprocambium has been demonstrated in ferns, its presence in seedplants has not been accepted generally. In the shoot apex ofcarrot, a short cylinder of provascular tissue is recognizedas the initial stage of vascular differentiation. This firstbecomes apparent through the enlargement and vacuolation ofpith and cortical tissue rather than as a result of specificchanges in the provascular tissue itself. Procambium in discretestrands differentiates acropetally in the provascular tissuein relation to developing leaf primordia. Provascular tissueis not recognized above the axil of the youngest leaf primordiumbut it is distinct at or above the level at which the traceof the youngest primordium diverges. Support for the recognitionof provascular tissue is provided by a positive reaction tohistochemical tests for carboxylesterases in this tissue aswell as in procambium and later stages of vascular differentiation.Copyright1999 Annals of Botany Company. Provascular tissue, carboxylesterase, shoot apex, vascular differentiation, carrot (Daucus carotaL.).     

DIFFERENTIATION OF LATERAL SHOOTS AS THORNS IN ULEX EUROPAEUS

Ulex europaeus is a much-branched shrub with small, narrow, spine-tipped leaves and axillary thorn shoots. The origin and development of axillary shoots was studied as a basis for understanding the changes that occur in the axillary shoot apex as it differentiates into a thorn. Axillary bud primordia are derived from detached portions of the apical meristem of the primary shoot. Bud primordia in the axils of juvenile leaves on seedlings develop as leafy shoots while those in the axils of adult leaves become thorns. A variable degree of vegetative development prior to thorn differentiation is exhibited among these secondary thorn shoots even on the same axis. Commonly the meristems of secondary axillary shoots initiate 3–9 bracteal leaves with tertiary axillary buds before differentiating as thorns. In other cases the meristems develop a greater number of leaves and tertiary buds as thorn differentiation is delayed. The initial stages in the differentiation of secondary shoot meristems as thorns are detected between plastochrons 10–20, depending on vigor of the parent shoot. A study of successive lateral buds on a shoot shows an abrupt conversion from vegetative development to thorn differentiation. The conversion involves the termination of meristematic activity of the apex and cessation of leaf initiation. Within the apex a vertical elongation of cells of the rib meristem initials and their immediate derivatives commences the attenuation of the apex which results in the pointed thorn. All cells of the apex elongate parallel to the axis and proceed to sclerify basipetally. Back of the apex some cortical cells in which cell division has persisted longer differentiate as chlorenchyma. Although no new leaves are initiated during the extension of the apex, provascular strands are present in the thorn tip. Fibrovascular bundles and bundles of cortical fibers not associated with vascular tissue differentiate in the thorn tip and are correlated in position with successive incipient leaves in the expected phyllotactic sequence, the more developed bundles being related to the first incipient leaves. Some secondary shoots displayed variable atypical patterns of meristem differentiation such as abrupt conversion of the apex resulting in sclerification with limited cell elongation and small, inhibited leaves. These observations raise questions concerning the nature of thorn induction and the commitment of meristems to thorns.     

Vascular Differentiation in the Shoot Apex of Matteuccia struthiopteris

Initial vascular differentiation is generally considered tooccur in procambium. In ferns, however, a provascular tissueimmediately subjacent to the promeristem has been suggestedas an initial stage within which the procambium is subsequentlyformed. In contrast to this interpretation, a zonation conceptapplied in ferns recognizes a promeristem consisting of severallayers of cells in which no differentiation takes place. Thisstudy demonstrates that the shoot apex of Matteuccia struthiopterishas one cell layer of promeristem. Immediately subjacent tothe promeristem is the provascular tissue surrounding a centralgroup of pith mother cells. The developmental continuity betweenthe provascular tissue and the mature vascular tissue, and betweenthe pith mother cells and the pith, through transitional stages,indicates that the initial differentiation of vascular tissueand pith takes place in this prestelar tissue. The continuityof vascular differentiation in the area confronting young leavesor incipient leaf positions is interrupted by the formationof leaf gap initials. Developing leaves thus begin to exertinfluence on the vascular system at the prestelar stage. Smallprotoxylem elements with helical cell wall thickening, and distinctiveprotophloem elements are present in the leaf traces, but endabruptly near the junction regions of leaf traces to the meristele.Copyright1994, 1999 Academic Press Initial vascular differentiation, provascular tissue, pith mother cells, shoot apex, Matteuccia struthiopteris     

COMPARATIVE FOLIAR HISTOGENESIS IN ACORUS CALAMUS AND ITS BEARING ON THE PHYLLODE THEORY OF MONOCOTYLEDONOUS LEAVES

A comparative histogenetic investigation of the unifacial foliage leaves of Acorus calamus L. (Araceae; Pothoideae) was initiated for the purposes of: (1) re-evaluating the previous sympodial interpretation of unifacial leaf development; (2) comparing the mode of histogenesis with that of the phyllode of Acacia in a re-examination of the phyllode theory of monocotyledonous leaves; and (3) specifying the histogenetic mechanisms responsible for morphological divergence of the leaf of Acorus from dorsiventral leaves of other Araceae. Leaves in Acorus are initiated in an orthodistichous phyllotaxis from alternate positions on the bilaterally symmetrical apical meristem. During each plastochron the shoot apex proceeds through a regular rhythm of expansion and reduction related to leaf and axillary meristem initiation and regeneration. The shoot apex has a three- to four-layered tunica and subjacent corpus with a distinctive cytohistological zonation evident to varying degrees during all phases of the plastochron. Leaf initiation is by periclinal division in the second through fourth layers of the meristem. Following inception early growth of the leaf primordium is erect, involving apical and intercalary growth in length as well as marginal growth in circumference in the sheathing leaf base. Early maturation of the leaf apex into an attenuated tip marks the end of apical growth, and subsequent growth in length is largely basal and intercalary. Marked radial growth is evident early in development and initially is mediated by a very active adaxial meristem; the median flattening of this leaf is related to accentuated activity of this meristematic zone. Differentiation of the secondary midrib begins along the center of the leaf axis and proceeds in an acropetal direction. Correlated with this centralized zone of tissue specialization is the first appearance of procambium in the center of the leaf axis. Subsequent radial expansion of the flattened upper leaf zone is bidirectional, proceeding by intercalary meristematic activity at both sides of the central midrib. Procambial differentiation is continuous and acropetal, and provascular strands are initiated in pairs in both sides of the primordium from derivatives of intercalary meristems in the abaxial and adaxial wings of the leaf. Comparative investigation of foliar histogenesis in different populations of Acorus from Wisconsin and Iowa reveals different degrees of apical and adaxial meristematic activity in primordia of these two collections: leaves with marked adaxial growth exhibit delayed and reduced expression of apical growth, whereas primordia with marked apical growth show, correspondingly, reduced adaxial meristematic activity at equivalent stages of development. Such variations in leaf histogenesis are correlated with marked differences in adult leaf anatomy in the respective populations and explain the reasons for the sympodial interpretation of leaf morphogenesis in Acorus and unifacial organs of other genera by previous investigators. It is concluded that leaf development in Acorus resembles that of the Acacia phyllode, thereby confirming from a developmental viewpoint the homology of these organs. Comparison of development with leaves of other Araceae indicates that the modified form of the leaf of Acorus originates through the accentuation of adaxial and abaxial meristematic activity which is expressed only slightly in the more conventional dorsiventral leaf types in the family.     

WOUND RECOVERY BY PITH CELL REDIFFERENTIATION: STRUCTURAL CHANGES

The redifferentiation of tobacco pith cells was examined in two experimental systems: wounds recovering from an incision that severed vascular tissue of the stem, and induced differentiation of excised pith responding to indoleacetic acid supplied locally via pipets inserted into the tissue. In both systems there was an initial period during which cell division was resumed and the pith cells were cleaved into numerous small cells. This was followed by redifferentiation of some of the divided cells as tracheary elements and, especially in the stem, by the formation of a cambial meristem that produced further xylem and phloem. In the stem the size of the wound meristem decreased as the wound was made further from the shoot apex, and in the cultured pith tissue it was demonstrated that the size of the dividing zone increased with the concentration of auxin supplied. Auxin was, therefore, demonstrated to be limiting in the division phase of redifferentiation. The sequence of redifferentiation in the two experimental systems resembled the normal ontogeny of vascular tissues in the intact plant sufficiently that these systems could be used to investigate the relationship between cell differentiation and auxin transport.     

Changes in the anatomical structure of the shoot apex ofSenecio vulgaris L. during ontogenis in relation to the formation of leaves and inflorescence

An investigation was made of the anatomical structure of the shoot apex ofSenecio vulgaris L. a photoperiodically neutral plant, and compared with the formation of successive leaf primordia along the axis up to the initiation of the terminal inflorescence. In the shoot apex of a germinating plant a central zone can first be distinguished from the peripheral zone which is composed of small and intensely stained cells. Later, a rib meristem appears. At the time of the initiation of the middle (the largest) leaves, the shoot apex has a distinct small central zone and a well developed peripheral zone and rib meristem. Between these zones there is a group of cells dividing in all directions, the subcentral zone. At the time of initiation of the last leaves, the central zone extends to the flanks and gradually ceases to be distinguishable. At the same time, the subcentral zone increases in size. This is caused first by cell division and later, with the initiation of the last, most reduced leaves, by enlargement of the cells. Vacuolization in the inner part of the apex and the arrangement of the superficial cells in rows parallel to the surface of the apex, is a preparatory step to the initiation of the inflorescence.     

EARLY HISTOGENESIS AND SEMIQUANTITATIVE HISTOCHEMISTRY OF LEAF INITIATION IN TRITICUM AESTIVUM

The shoot apex of Triticum aestivum cv. Ramona 50 was investigated histologically to describe cell lineages and events during leaf initiation. During histogenesis three periclinal divisions occurred in the first apical layer, with one or two divisions in the second apical layer. This sequence of cell divisions initially occurred in one region and spread laterally in both directions to encircle the meristem. Cells of the third apical layer were not involved in leaf histogenesis. Initially, young leaf primordia were produced from daughter cells of periclinal divisions in the two outer apical layers. Nuclear contents of protein, histone, and RNA in the shoot apex were evaluated as ratios to DNA by means of semiquantitative histochemistry. Daughter cells of periclinal divisions in the outer apical layer which produced the leaf primordia had higher histone/DNA ratios than cells of the remaining meristem. However, protein/DNA and RNA/DNA ratios were similar in both regions. Leaf initial cells had a higher ³H-thymidine labeling index, a higher RNA synthesis rate, and smaller nuclear volumes than cells of the residual apical meristem.     

FOLIAR ONTOGENY AND HISTOGENESIS IN MAGNOLIA GRANDIFLORA L. I. APICAL ORGANIZATION AND EARLY DEVELOPMENT

Foliar ontogeny of Magnolia grandiflora was studied to elucidate possible unique features of evergreen leaves and their development. The apex of Magnolia grandiflora is composed of a biseriate or triseriate tunica overlying a central initial zone, a peripheral zone and a pith rib meristem. Leaf primordia are initiated by periclinal divisions on the apical flank of the tunica in its second layer. This initiation and expansion is seasonal just as in related deciduous magnolias. Following leaf initiation, a foliar buttress is formed and the leaf base gradually extends around the apex. As growth continues, separation of the leaf blade primordium from the stipule proceeds by intensified anticlinal divisions in the surface and subsurface layers near the base. Marginal growth begins in the blade primordium when it reaches approximately 200 μm in height and results in the formation of two wing-like extensions, the lamina. This young blade remains in a conduplicately folded position next to the stipule until bud break.     

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.     

Indole acetic acid distribution coincides with vascular differentiation pattern during Arabidopsis leaf ontogeny

We used an anti-indole acetic acid (IAA or auxin) monoclonal antibody-based immunocytochemical procedure to monitor IAA level in Arabidopsis tissues. Using immunocytochemistry and the IAA-driven beta-glucuronidase (GUS) activity of Aux/IAA promoter::GUS constructs to detect IAA distribution, we investigated the role of polar auxin transport in vascular differentiation during leaf development in Arabidopsis. We found that shoot apical cells contain high levels of IAA and that IAA decreases as leaf primordia expand. However, seedlings grown in the presence of IAA transport inhibitors showed very low IAA signal in the shoot apical meristem (SAM) and the youngest pair of leaf primordia. Older leaf primordia accumulate IAA in the leaf tip in the presence or absence of IAA transport inhibition. We propose that the IAA in the SAM and the youngest pair of leaf primordia is transported from outside sources, perhaps the cotyledons, which accumulate more IAA in the presence than in the absence of transport inhibition. The temporal and spatial pattern of IAA localization in the shoot apex indicates a change in IAA source during leaf ontogeny that would influence flow direction and, consequently, the direction of vascular differentiation. The IAA production and transport pattern suggested by our results could explain the venation pattern, and the vascular hypertrophy caused by IAA transport inhibition. An outside IAA source for the SAM supports the notion that IAA transport and procambium differentiation dictate phyllotaxy and organogenesis.     

The Structure of the Plumule of Nelumbo Nucifera Gaertn. and the Nature of its Scale

The structure of the plumule of Nelumbo nucifera Gaertn. and its feature covered with scale are seldom seen in dicotyledon. The fact that the plumule possesses scale is even more uncommon. This particular phenomenon is investigated by observing the differentiation of the plumule apex and the development of the leaf organs. After the seed is formed, the embryo has two young leaves and a terminal bud covered with scale. In the bud it has already differentiated the 3rd and the 4th leaf primordium and a shoot apex, the differentiation of which is very complex. So the structure of the plumule passes through 4 plastochrons altogether. It is made clear through observation and analysis that, before the 4th leaf primordium is formed, the transforma- tions of the shoot apex of the embryo in each plastochron are fundamentally alike. After the 4th leaf primordium is developed, the shoot apex becomes complex and there appear 3 different active cell regions which become the bases of vegetative bud of the seeding apex. The development of these 3 active cell regions will be stated in “The Structure of the Vegetative Bud of Nelumbo nucifera Gaertn. and the Nature of its Scales.” The apices of the plumule are almost slightly domed in structure. As a rule, their width is from 95 to 107 μ. Their height is from 17 to 20 μ during one plastochron. Before the 3rd leaf initiation, the anatomical structure of apices is examined and the fol- lowing zones may be delimited: zone of tunica initials, zone of corpus initials, peripheral zone, and zone of rib meristems. It is frequently observed that the cell of corpus in subapical peripheral zone develops periclinal division, which is the initial cell of leaf primordium; Procambium will appear before the stage of the appearance of leaf buttress. The apex of the plumule is in an apical position, but when the seedling is formed, as the developing leaves are alternate, the directions of the shoot apex are changed, simultaneously the base part of the leaf encloses the axis, and the adaxial meristem also differentiates the scale which encloses the terminal bud, thus placing the bud in axillary of the leaf and forming a zigzag phenomenon of the axis of the seedling. Above the basal adaxial side of the leaf primordium develops the scale of the plumule with meristem periclinal division of closely attached protoderm as its base. So the scale of the plumule of Nelumbo nucifera Gaertn. and the axillary stipule are of the same origin. To sum up, the scale of the embryo of Nelumbo nucifera Gaertn. is differentiated from the adaxial meristem of the basal part of the leaf primordium, and is the derivative part of the leaf. It has the same function as the coleoptile of the monocotyledon. Whether they are homologous organs or not is still to be investigated.     

In situ localization of storage protein mRNAs in developing meristems of Brassica napus embryos

Probes derived from cDNA clones of napin and cruciferin, the major storage proteins of Brassica napus, and in situ hybridization techniques were used to examine changes in the spatial and temporal distribution of storage protein messages during the course of embryogeny, with a special emphasis on the developing apical meristems. Napin mRNAs begin to accumulate in the cortex of the axis during late heart stage, in the outer faces of the cotyledons during torpedo stage and in the inner faces of the cotyledons during cotyledon stage. Cruciferin mRNAs accumulate in a similar pattern but approximately 5 days later. Cells in the apical regions where root and shoot meristems develop do not accumulate storage protein messages during early stages of embryogeny. In the upper axis, the boundary between these apical cells and immediately adjacent cells that accumulate napin and cruciferin mRNAs is particularly distinct. Our analysis indicates that this boundary is not related to differences in tissue or cell type, but appears instead to be coincident with the site of a particular set of early cell divisions. A major change in the mRNA accumulation patterns occurs halfway through embryogeny, as the embryos enter maturation stage and start drying down. Final maturation of the shoot apical meristem is associated with the development of leaf primordia and the accumulation of napin mRNAs in the meristem, associated leaf primordia and vascular tissue. Cruciferin mRNAs accumulate only in certain zones of the shoot apical meristem and on the flanks of leaf primordia. Neither type of mRNA accumulates in the root apical meristem at any stage.     

Effects of Developing Leaves on Stelar Pattern Development in the Shoot Apex of Matteuccia struthiopteris

A developmental study of the normal shoot apex of Matteucciastruthiopteris suggested that patterned stelar differentiationis initiated immediately beneath the single layer of promeristemand occurs prior to the initiation of the youngest leaf primordium.A developmental study in which all leaf primordia were suppressed,with or without lateral isolation of the terminal meristem byvertical incisions, has confirmed this interpretation of stelardifferentiation. Experimentally-induced changes in the tissueimmediately below the promeristem were reflected in the resultingmature structure of the stele. Failure of leaf gap initialsto differentiate, if all leaf primordia were suppressed at theincipient stage, resulted in a mature stele without leaf gaps.Similarly the disappearance of pith mother cells after severalweeks of leaf removal was associated with the formation of astele without pith. Leaf influence was further assessed by allowingone primordium to develop while all others were suppressed.The developing leaf had a small promoting effect on caulinevascular tissue differentiation but its major impact on theexpansion of the parenchymatous tissues of the stele. Characteristicprotoxylem and protophloem failed to differentiate when allleaves were suppressed and, when leaf was allowed to develop,formed only in relation to the leaf.Copyright 1995, 1999 AcademicPress Leaf influence, vascular pattern formation, experimental surgery, shoot apex development, protoxylem, protophloem, Matteuccia struthiopteris     

The seminal root primordia in barley and the participation of their non-meristematic cells in root construction

In addition to the primary seminal primordium, the so-called secondary seminal root primordia are also initiated in a barley embryo. The primary root primordium is developmentally most advanced. It is formed by root meristem covered with the root cap, and by a histologically determined region with completed cell division. On germination, the restoration of growth processes begins in this non-meristematic region of root primordium by cell elongation, with the exception of the zone adjacent to the scutellar node, the cells of which do not elongate but continue differentiating. In the root primordia initiated later, the zone with completed cell division is relatively shorter, in the youngest primordia the non-meristematic cells may be lacking. The root meristem is reactivated after the primary root primordium has broken through the sheath-like coleorrhiza and emerges from the caryopsis as the primary root. The character of root meristem indicates a reduced water content at the embryonic development of root primordium. With progressing growth the root apex becomes thinner, the meristematic region becomes longer, and the differences in the extent of cell division between individual cell types increase. — The primary root base is formed of cells pre-existing in the seminal root primordium. Upon desiccation of caryopsis in maturation, and subsequent quiescent period, their development was temporarily broken, proceeding with the onset of germination. The length of this postembryonically non-dividing basal zone is different in individual cell types. The column of central metaxylem characteristic of the smallest number of cell cycles, has, under the given conditions, a mean length of about 22 mm, whereas the pericycle, as the tissue with most prolonged cell division, has a mean length of about 6 mm. In the seminal root primordia initiated later the non-dividing areas are relatively shorter. The basal region of seminal roots thus differs in its ontogenesis from the increase which is formed “de novo” by the action of root meristem upon seed germination.