SHOOT APICAL MERISTEMS AND MUTATION: FIXATION OF SELECTIVELY NEUTRAL CELL GENOTYPES

Shoot apical meristems are interpreted as either structured, that is having a permanent set of apical initials, or stochastic, having apical initials which represent “... momentary representatives of the continuous meristematic residue at the apex of the relevant layer or zone” (Newman, 1965). The two main parameters of stochastic growth are the average number of apical initials (α) and the number of mitotic cycles (r) of the initials and their daughter cells prior to the random selection of subsequent initials. Mathematical analysis and computer simulation studies of stochastic growth have shown that if one starts with 1 mutant initial and α-1 nonmutant initials, eventually a mosaic plant results. The frequency of shoot apices composed of mutant cells is 1/α and the frequency of shoot apices composed of only nonmutant cells is (1 – α)/α. These asymptotics are only attained after considerable growth, thus mericlinal chimeras can persist for many nodes and give the appearance that a permanent set of initials is present.     

EMBRYOGENY AND HISTOGENESIS IN NERIUM OLEANDER II. ORIGIN AND DEVELOPMENT OF THE NON-ARTICULATED LATICIFER

Mahlberg , P. G. (U. Pittsburgh, Pittsburgh, Pa.) Embryogeny and histogenesis in Nerium oleander. II. Origin and development of the non-articulated Iaticifer. Amer. Jour. Bot. 48(1): 90–99. Illus. 1961.—Laticifer initials, collectively considered as a laticifer system, are differentiated in the globular embryo from meristematic cells which occupy a position within the potential procambial tissue. A total of usually 28 initials, in Nerium oleander, arise as an irregular ring of cells directly below the embryonic shoot apex, during initiation of the cotyledonary primordia. No anastomoses occur between laticifer initials. During subsequent development of the embryo, the laticifer initials grow in a bi-directional manner and penetrate into the root, cotyledons and toward the shoot apex. Upon enlargement the initials bifurcate repeatedly, many branches penetrate into the cotyledons, others grow into the cortex of the hypocotyl or penetrate between cells of the procambium. Repeated nuclear divisions within each initial result in the formation of a multinucleated protoplast in this cell type. The tips of laticifers occupy intercellular spaces during their growth; they do not penetrate into or through adjacent cells. A plexus of laticifer branches is formed within the cotyledonary node of the mature embryo. No new initials are formed during subsequent growth of the plant, rather certain branches from the cotyledonary nodal plexus penetrate into the enlarging shoot system. The nature of their growth habit and branching suggests that the tips of laticifer initials exhibit an intrusive form of growth.     

Cell lineage patterns in the shoot meristem of the sunflower embryo in the dry seed

We mapped the fate of cells in the shoot meristem of the dry-seed embryo of sunflower, Helianthus annuus L. cv. Peredovic, using irradiation-induced somatic sectors. We analyzed 249 chlorophyll-deficient or glabrous (hairless) sectors generated in 236 plants. Most sectors observed in the inflorescence extended into vegetative nodes. Thus cell lineages that ultimately gave rise to reproductive structures also contributed to vegetative structures. No single sector extended the entire length of the shoot. Thus the shoot is not derived from one or a few apical initials. Rather, the position, vertical extent, and width of the sectors at different levels of the shoot suggest that the shoot is derived from three to four circumferential populations of cells in each of three cell layers of the embryo meristem. Sectors had no common boundaries even in plants with two or three independent sectors, but varied in extent and overlapped along the length of the shoot. Thus individual cells in a single circumferential population behaved independently to contribute lineages of different vertical extents to the growing shoot. The predicted number of circumferential populations of cells as well as the apparent cell number in each population was consistent with the actual number of cells in the embryo meristem observed in histological sections.     

β-glucuronidase as a marker for clonal analysis of tomato lateral roots

In higher plants, the root-shoot axis established during embryogenesis is extended and modified by the development of primary and lateral apical meristems. While the structure of several shoot apical meristems has been deduced by combining histological studies with clonal analysis, the application of this approach to root apical meristems has been limited by a lack of visible genetic markers. We have tested the feasibility of using a synthetic gene consisting of the maize transposable elementActivator (Ac) inserted between a 35S CaMV promoter and the coding region of a -glucuronidase (GUS) reporter gene as a means of marking cell lineages in roots. The GUS gene was activated in individual cells byAc excision, and the resulting sectors of GUS-expressing cells were detected with the histochemical stain X-Gluc. Sectors in lateral roots originated from bothAc excision in meristematic cells and from parent root sectors that bisect the founder cell population for the lateral root initial. Analysis of root tip sectors confirmed that the root cap, and root proper have separate initials. Large sectors in the body of the lateral root encompassed both cortex and vascular tissues. The number of primary initial cells predicted from the size and arrangement of the sectors observed ranged from two to four and appeared to vary between roots. We conclude that transposon-based clonal analysis using GUS expression as a genetic marker is an effective approach for deducing the functional organization of root apical meristems.     

THE ROOT APEX OF MALVA SYLVESTRIS. I. STRUCTURAL DEVELOPMENT

The apical organization of the primary root of Malva sylvestris was analyzed at several growth stages, beginning with the embryo, to determine the structural changes that occurred during growth. Seeds were germinated, and plants were grown under controlled conditions. There were three discrete groups of initials in the embryonic root: those of the central cylinder, cortex, and secondary columella. The secondary columella initials consisted of a plate of cells flanked by a ring of cortical initials. The lateral portion of the rootcap shared a common origin with the epidermis. During growth both the initials of the secondary columella and outer cortex produced rootcap cells. The first indication of the outer cortical initials participating in rootcap formation was observed in roots 3 cm long. In 6-, 9-, and 16-cm roots the cellular continuity between the outer cortex and rootcap was marked, but in 23- and 33-cm roots the histogenic continuity between the outer cortex and rootcap was not evident. In all growth stages the initials of the central cylinder and inner cortex retained their histogenic integrity.     

IMPERMANENT INITIALS OF THE SHOOT APEX AND DIPLONTIC SELECTION IN A JUNIPER CHIMERA

Shoot meristems of Juniperus davurica cv. Expansa Variegata possess an apical zonation pattern similar to that found in some angiosperms. Anticlinal divisions predominate in the outer layer, the tunica. The underlying core of cells, the corpus, has cell divisions oriented in all directions. Typically, this variety exists as a periclinal chimera, the outer layer genotypically albino and the inner core composed of normal, chlorophyll-producing cells. In this condition a shoot appears green. Occasionally a tunica cell divides periclinally and displaces an initial in the apical region of the corpus. This event subsequently is expressed during ontogeny as an albino sector on the stem and leaves. Frequent variation in the width of an albino sector throughout ontogeny suggests a temporary nature of cells in the position of apical initials. A correlation was documented between the position of axillary bud release and a subsequent increase or decrease in the width of an albino sector. A model based upon stochastic processes and diplontic selection is proposed to account for the dynamic nature of chimeric patterns observed in this plant.     

ONTOGENY OF THE SHOOT APEX OF SEEDLINGS OF PINUS PONDEROSA

Dormant seeds of ponderosa pine (Pinus ponderosa) were stratified and then planted in the greenhouse. Changes occurring in the shoot apex during germination and growth of the seedlings were observed and are described. Cell divisions in germinating embryos are first noted on the flanks of the apex near the cotyledons. From these loci, mitotic reactivation proceeds up the flanks of the apex and inward toward cells of the rib meristem. Cells are dividing actively in all regions of the shoot apex of seedlings 12 days after planting, but no cytohistological zonation is evident due to differences in staining intensity and in cell and nuclear size. In 64-day-old seedlings the mitotic rate is reduced and the characteristic zonation of pine apices begins to appear. Zonation is even more evident in older seedlings. These observations are discussed in relation to the concept of initials and to theories concerning factors which regulate growth in the shoot apex.     

Structure and Cytogenesis of the Shoot Apex of Syringa oblata var. affinis Lingelsh

The structure, growth and mitotic activity of 211 shoot apices of developing sprouts of Syringa oblata var. affinis Lingelsh. in longitudinal sections and 67 in transverse sections have been studied with the view to understanding the nature of zonation patterns and cytogenesis of the apical meristems during a double plastochron. The external morphology and the anatomical structure of the apices in 4 plastochronic stage-early, middle, late Ⅰ and late Ⅱ stages are described. In the shoot apices examined, especially those at late plastochronic stage, the following zones may be delimited: Zone of tunica initials, zone of corpus initials, peripheral zone and zone of rib meristem. The location and orientation of mitotic figures observed in longisections of the apices in 4 plastochronic stages are plotted in diagrams and the mitotic frequency has been calculated. Information obtained from these investigations reveals that the tunica and corpus inititals constitute an active region of the apex, but their mitotic activity changes periodically within the double plastochron. In the middle plastochronic stage when the apex is at its minimal area and the cells of peripheral zone and rib meristem zone have been completely transformed into constituent parts of foliar primordia and the subjacent tissues of the stem and the pith mother cells respectively, the mitotic frequency of the initials is at its maximium and its intensity of mitotic activity is not much lower than that of any other meristematic zone at any stage. When the apical dome is reformed by the activity of these initials in late plastochronic stages, the mitotic frequency of the initials gradually drops and the region of high mitotic frequency shifts to the flank of the apex, the peripheral zone. Anticlinal divisions are predominant in this zone. On the other hand, those cells directly left behind by the corpus initials, which constitute the rib meristem, are vacuolated and marked by the pre- dominance of transverse divisions. Thus the entire zonation pattern reappears. In the next early plastochronic stage, the mitotic frequency of the tunica and corpus initials drops to its mimimium, but other regions of the apex still maintain a high mitotic frequency. It may be concluded that the tunica and corpus initials form a cytogenerative center of the shoot, and the cytohistological zonation is actually a result of the fact that different regions of apical meristems are different in mitotic activety, different in state of cell differentiation and different in their function in morphogenesis.     

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.     

GROWTH PERIODICITY AND THE SHOOT TIP OF ABIES CONCOLOR

Parke , Robert V. (Colorado State U., Fort Collins.) Growth periodicity and the shoot tip of Abies concolor. Amer. Jour. Bot. 46(2) : 110-118. Illus. 1959.—The terminal shoot of Abies concolor shows a marked seasonal activity or growth periodicity of the meristmatic regions. Hence, the annual developmental sequence may be divided into three growth phases: the rest phase, during which the fully formed telescoped shoot remains in a state of suspended growth; the first growth phase, during which the telescoped shoot elongates rapidly and gives rise to numerous cataphylls; and, the second growth phase, during which shoot elongation is completed and a new unelongated axis bearing many needle primordia is formed. The shoot tip of Abies consists of 4 clearly definable cytohistological zones; the apical initials, the sub-terminal mother cells, the peripheral zone, and the zone of central tissue. The shape of the shoot tip and the volume of its various cytohistological zones change markedly during the annual growth sequence, but the basic zonation pattern remains the same.     

Development of mantle leaves inPlatycerium bifurcatum (Polypodiaceae)

InPlatycerium bifurcatum the leaf primordia emerge alternately right and left below the shoot apical cell on the dorsal surface of the rhizome. They arise from groups of small cells, a single large cell becoming the initial of the leaf apical cell. The longitudinal axis of the leaf apical cell is at a right angle to the rhizome axis and the leaf primordia are arranged longitudinally in two rows. The leaf apical cell gives rise to marginal initials which are responsible for leaf growth in one plain. Early marginal cells are crescent-shaped while the later ones are wedge-shaped. Hairy marginal cells appear in the very early stages of development. The interpretation of these cells as a promeristem and as initials are discussed.     

DEVELOPMENTAL ANATOMY IN ROOTS OF IPOMOEA PURPUREA. I. RADICLE AND PRIMARY ROOT

The developmental anatomy of the primary root of Ipomoea purpurea was studied at several growth stages, beginning with the radicle. The radicle is generally composed of three superimposed tiers of initials, which produce the vascular cylinder, cortex, and columella; and a peripheral band of lateral rootcap-epidermal initials. The radicular cortex contains 16–19 immature laticifers; none of the tissue regions in the radicle contains mature cells. Following germination and during the first 2–3 cm growth of the primary root the apical meristem and its derivative tissues undergo a series of modifications. Root apical diameter decreases as cells in lateral portions of the rootcap elongate; meanwhile, the columella enlarges vertically. The relationship between cortical and columellar initials changes as fewer mitoses occur in the former while the latter remain active. In longer roots the columellar initials are directly in contact with the vascular initials. Cortical size diminishes during early root growth as cortical laticifers and their associated cells cease to be produced by the outer cortical initials and ground meristem. Early procambium, at the level of vascular pattern initiation, decreases in diameter by cellular reorientation, and the vascular cylinder decreases in overall diameter although the tetrarch pattern remains unchanged.     

Evaluation of 515 expressed sequence tags obtained from guard cells of Brassica campestris

The ear shoot of maize (Zea mays L.) consists of the peduncle and reproductive tissues (ear). Genetic mosaics induced by the unstable allele of thech1 locus were used for cell lineage analysis of the ear shoot. The unstablech1-m1 allele, caused by the insertion of a transposable element, gives rise to yellow-green seedlings with many small revertant green stripes. Rare plants with large revertant sectors comprising 30–50% of the plant were selected. Nineteen plants showing large sectors on the main stem were subjected to sector boundary analysis. Sectoring was recorded for the main stem, leaf subtending the ear shoot, peduncle, prophyll and ear. The reproductive part of the ear shoot, the ear, was scored after removal of the husks and subsequent exposure to light. In 18 cases the ear was non-sectored yellow-green or green. In an additional four cases, peduncle cell lineages entered the ear, but only in the proximal part, while the tip of the ear was non-sectored. Two additional ears showed longitudinal sectors which reached the tip of the ear. These observations indicate that in the lateral meristem of the ear shoot two types of cellular clone exist. One will generate the peduncle, the other will found the ear. Sector boundary analysis indicates that for the vegetative part of the ear shoot the number of meristem founder cells is high, whereas only a few initials are recruited for the formation of the ear. The presence of ear sectors not starting in the peduncle and reaching the ear tip, and the finding that the ear is frequently non-sectored, suggest that this organ derives from an apical type of growth.     

Sector analysis and predictive modelling reveal iterative shoot-like development in fern fronds

Plants colonized the terrestrial environment over 450 million years ago. Since then, shoot architecture has evolved in response to changing environmental conditions. Our current understanding of the innovations that altered shoot morphology is underpinned by developmental studies in a number of plant groups. However, the least is known about mechanisms that operate in ferns--a key group for understanding the evolution of plant development. Using a novel combination of sector analysis, conditional probability modelling methods and histology, we show that shoots, fronds ('leaves') and pinnae ('leaflets') of the fern Nephrolepis exaltata all develop from single apical initial cells. Shoot initials cleave on three faces to produce a pool of cells from which individual frond apical initials are sequentially specified. Frond initials then cleave in two planes to produce a series of lateral merophyte initials that each contributes a unit of three pinnae to half of the mediolateral frond axis. Notably, this iterative pattern in both shoots and fronds is similar to the developmental process that operates in shoots of other plant groups. Pinnae initials first cleave in two planes to generate lateral marginal initials. The apical and marginal initials then divide in three planes to coordinately generate the determinate pinna. These findings impact both on our understanding of fundamental plant developmental processes and on our perspective of how shoot systems evolved.     

THE INITIATION OF SPOROPHYTES BY OBLIGATE APOGAMY IN CHEILANTHES CASTANEA

The initiation of apogamous sporophytes in Cheilanthes castanea was recorded by daily photography of individual gametophytes. Whereas an ordinary embryo arises from a zygote, apogamous embryos of C. castanea originate from one to three initial cells which occur just behind the apical region of the prothallus. The initial (or initials) produce cells with small chloroplasts behind the sinus of the gametophyte. The appearance of cells with smaller chloroplasts than those normally found in gametophytes is the first indication that apogamy is occurring. The cells with small plastids produce a group of densely-cytoplasmic meristematic cells. The size of the meristematic mass increases until shoot and root apices of the apogamous embryo are organized.     

Surface-viewed shoot apex ofAngiopteris lygodiifolia Ros. (Marattiaceae)

Marattian ferns are thought to be an exception to the rule that a single apical cell is always present in the shoot apex of ferns; the occurrence of plural apical initials has been generally accepted for these ferns. However, a contradicting conclusion was reached in this study which examined the apical organization of the shoot ofAngiopteris lygodiifolia Ros., using fresh materials which had not been fixed. Shoot apices were hand-sectioned transversely into thin sections, including the surface layer of the shoot apex, which were observed by differential interference contrast microscopy without staining. In contrast with the generally accepted view, the shoot apex ofA. lygodiifolia was found to usually possess a single apical cell with three cutting faces. The segments cut off from the apical cell are regularly arranged in a helical sequence. The apical cell seems to actually function as an initial cell of the whole shoot apex. The shoot apices, particularly those of plants cultivated in a greenhouse, sometimes show somewhat irregular organization. In extreme cases, no apical cell is recognizable. However, even in these exceptional cases of such apparently irregular shoot apices, plural apical initials are not found.     

ONTOGENY AND DIFFERENTIATION OF SCLEREIDS IN RAUWOLFIA

An interesting anatomic feature of Rauwolfia is the occurrence of a remarkable type of sclereid in the stem and root. The initials of the sclereids in the stem arise in the ground tissue element of the pith in a region between 50 and 70μ below the surface of the shoot apex. This region of the shoot remains surrounded by a whorl of either 3 or 4 leaves. Sclereids initiate in succession in association with each whorl of leaves. Thus, the sclereids are restricted to the nodes. The sclereids in the stem arise as a primary element of the shoot from the ground tissue of the pith. In the root, they differentiate from the cells of the phelloderm and are secondary in origin. Morphologically, the sclereids in these 2 organs are basically the same, except that the sclereids in the stem are larger in size than those in the root. A solitary cell, or 2 to several cells in a longitudinal cell file (originated from a single mother cell), may differentiate into sclereid initials. The growth of the sclereids through relatively compact ground tissue of the pith is possibly accomplished by a combination of gliding growth and apical intrusive process. The sclereid initials grow rapidly and force their way between the parenchymatous cells. As a result, the neighboring cells lose their original surface contacts. Sclereids increase in size rapidly, and, therefore, very enlarged sclereids with thin primary walls may be observed in the second node. They mature progressively in basipetal direction in the subjacent nodes. In the fifth or sixth node, mature sclereids with massive secondary walls are most common. The secondary walls of sclereids contain much lignin as determined by the phloroglucinol-HCl test. The walls of sclereids at maturity show a variable number of lamellae ranging from 10 to 15 in the lateral walls. A remarkable feature of the sclereids is their canal-like pits in the secondary walls. Two adjacent pits may coalesce uniquely to form a Y-like configuration directed centrifugally from the lumen of the sclereids. The sclereids are ventrically symmetrical, joined end-to-end by their transverse walls like 2 superimposed young fibers.     

QUANTITATIVE STUDIES OF THE VEGETATIVE SHOOT APEX OF EQUISETUM SCIRPOIDES

Equisetum scirpoides Michx., propagated from a single clone, was grown in a controlled growth chamber at 24 ± 1 C under a photoperiod of 16 hr light/8 hr darkness. The apical cell of aerial vegetative shoots gives rise to derivatives (merophytes) in a helical sequence. Each newly formed merophyte divides anticlinally to form two superposed cells that are parallel to a lateral face of the apical cell. Radial longitudinal divisions then take place in the two superposed cells. Shoot tips were fixed every 2 hr for 24 hr to determine the mitotic index of the apical cell, six subjacent cells, and the remaining cells above the level of leaf initiation. Average mitotic indices for the 24-hr period were 3.9%, 3.9%, and 7.0%, respectively. The results indicate that the apical cell is quite active mitotically; there was no clear evidence of endopolyploidy in cells of the shoot apex, young leaves or in the developing cortex, based upon cytophotometric measurements of DNA content.     

THE DEVELOPMENTAL ANATOMY OF LONG-BRANCH TERMINAL BUDS OF PINUS BANKSIANA

A study of the composition of long-branch terminal buds (LBTB) of Pinus banksiana Lamb. and the yearly periodicity associated with their formation, development, and elongation was undertaken. Each LBTB has lateral bud zones and zones of cataphylls lacking axillary buds. When present, staminate cone primordia differentiate from the lowest lateral buds in the lowest lateral bud zone of the LBTB. Ovulate cone primordia and lateral long-branch buds can differentiate from the upper lateral buds in any lateral bud zone. When both types of buds are present, lateral long-branch buds are uppermost. Dwarf-branch buds occur in all lateral bud zones. During spring LBTB internodes elongate, new cataphylls are initiated, dwarf branches elongate, needles form and elongate, pollen forms and is released, and ovulate cones are pollinated. During summer buds form in the axils of the newly formed cataphylls. By early fall the new LBTB are in overwintering condition and the four types of lateral buds are discernable. The cytohistological zonation of the LBTB shoot apex is similar to that of more than 20 other conifer species. Cells in shoot apices of pine are usually arranged in distinct zones: apical initials, subapical initials, central meristem, and peripheral meristem. Periclinal divisions occur in the surface cells of the apex; therefore no tunica is present. At any given time, shoot apex volume and shape vary among LBTB in various positions on a tree. In any one LBTB on a tree, shoot apex shape changes from a low dome during spring to a high dome during summer to an intermediate shape through fall and winter.     

Cell-lineage patterns in the shoot apical meristem of the germinating maize embryo

A fate map for the shoot apical meristem of Zea mays L. at the time of germination was constructed by examining somatic sectors (clones) induced by -rays. The shoot apical meristem produced stem, leaves, and reproductive structures above leaf 6 after germination and the analysis here concerns their formation. On 160 adult plants which had produced 17 or 18 leaves, 277 anthocyanin-deficient sectors were scored for size and position. Sectors found on the ear shoot or in the tassel most often extended into the vegetative part of the plant. Sectors ranged from one to six internodes in length and some sectors of more than one internode were observed at all positions on the plant. Single-internode sectors predominated in the basal internodes (7,8,9) while longer sectors were common in the middle and upper internodes. The apparent number of cells which gave rise to a particular internode was variable and sectors were not restricted to the lineage unit: a leaf, the internode below it, and the axillary bud and prophyll at the base of the internode. These observations established two major features of meristem activity: 1) at the time of germination the developmental fate of any cell or group of cells was not fixed, and 2) at the time of germination cells at the same location in a meristem could produce greatly different amounts of tissue in the adult plant. Consequently, the developmental fate of specific cells in the germinating meristem could only be assigned in a general way.Abbreviations ACN apparent cell number - LI, LII, LI-LII sectors restricted to the epidermis, the subepidermis, or encompassing epidermis and subepidermis - PCN progenitor cell