Cell Division and Expansion and Resultant Tissue 3

By following the movement of carbon particle markers on theexposed surface of a cultured tomato apex it has been shownthat a leaf primordium is formed by growth on the flank of theapex raising the tissue upwards and outwards to form the leafbuttress. The whole of the apical surface is in an active stateof cell division and expansion except in the axillary regionabove the primordium. The data provide direct estimates of therates of division in the outer layer of cells. The distribution of blocked metaphase figures following treatmentwith colchicine, shows that in the early stages of primordiumformation cell divisions are concentrated in what appears tobo a ‘growth centre’ in the corpus to one side ofthe apical dome. As the bulge of the primordium develops, thegrowth centre spreads out and splits into two parts continuingthe growth of the dome (proximal side) and the primordium (distalside). Between these two regions of active division there arisesa small pocket of cells in the axil, whose rate of divisionrapidly declines. Cuts made in the apical surface in the early stages of primordiumformation immediately gape widely, apparently as a result ofpressure exerted on the outer layers from within by divisionsin the corpus. Once the upper surface of the primordium becomesraised above the dome, the axillary cells seem to become compressedbetween the two zones of active division. In the axil at thisstage (a) cuts do not gape but close up after exuding cell sapand (b) the carbon particle markers move slightly together.     

The induced sector Arabidopsis apical embryonic fate map

Creation of an embryonic fate map may provide insight into the patterns of cell division and specification contributing to the apical region of the early Arabidopsis embryo. A fate map has been constructed by inducing genetic chimerism during the two-apical-cell stage of embryogenesis to determine if the orientation of the first anticlinal cell division correlates with later developmental axes. Chimeras were also used to map the relative locations of precursors of the cotyledon and leaf primordia. Genetic chimeras were induced in embryos doubly heterozygous for a heat shock regulated Cre recombinase and a constitutively expressed beta-glucuronidase (GUS) gene flanked by the loxP binding sites for Cre. Individual cells in the two-apical-cell stage embryo responding to heat shock produce GUS-negative daughter cells. Mature plants grown from seed derived from treated embryos were scored for GUS-negative sector extent in the cotyledons and leaves. The GUS-negative daughters of apical cells had a strong tendency to contribute primarily to one cotyledon or the other and to physically adjacent true leaf margins. This result indicated that patterns of early cell division correlate with later axes of symmetry in the embryo and that these patterns partially limit the fates available for adoption by daughter cells. However, GUS-negative sectors were shared between all regions of the mature plant, suggesting that there is no strict fate restriction imposed on the daughters of the first apical cells.     

Developmental study onHypolepis punctata (Thunb.) Mett.

The origins of the first and second petiolar buds ofHypolepis punctata were clarified in relation to the early development of the leaf primordium, which arises from a group of superficial cells of the shoot apical meristem. One of these superficial cells produces a two-sided leaf apical cell which subsequently cuts off segments to make a well-defined cell group, called here the leaf apical cell complex, on the distal part of the leaf primordium. Meanwhile, cells surrounding the leaf apical cell complex also divide frequently to form the basal part of the leaf primordium. Two groups of basal cells of the leaf primordium located on the abaxial and the adaxial sides initiate the first and the second petiolar buds, respectively. The initial cells are usually contiguous to the leaf apical cell complex, constructing the abaxial and adaxial flanks of the very young leaf primordium. However, the first petiolar bud sometimes develops from cells located farther from the leaf apical cell complex. These cells are derived from those originally situated in the peripheral region of the shoot apical meristem. This study was supported by a Grant-in-Aid for Encouragement of Young Scientists by the Ministry of Education, Science and Culture, of Japan No. 474322 in 1979.     

Rates of Cell Division in the Shoot Apical Meristem of Pisum

The relative rates of cell division in different regions ofthe pea shoot apical meristem were obtained by measuring theincrease in the numbers of metaphases following applicationof colchicine to the plants. Absolute values for the rates ofcell division could be calculated since the average rate ofcell division for the whole apex was known. Measurements ofthe rates of cell division were obtained at defined intervalsduring the course of a single plastochron. Within each regionof the apex the rate of cell division did not change more thanabout two-fold throughout the plastochron. There was very littleor no increase in the rate of cell division associated withleaf initiation. The formation of a leaf primordium and thesubsequent growth of the apical dome apparently result fromchanges in the direction of growth rather than changes in therates of growth. Three main regions were discernible withinthe apical meristem: a region with a slow rate of cell divisionin the apical dome, a region of a faster rate of cell divisionat the base of the apical dome and at the site of initiationof procambial strands, and a region of an intermediate rateof cell division in the newly initiated leaf primordium andthe adjacent part of the shoot axis.     

CLONAL ANALYSIS OF CELL LINEAGE PATTERNS IN PLANT DEVELOPMENT

Somatic sectors induced by ionizing radiation provide a great deal of information about cell lineage patterns in both plants and animals. Somatic sectors arise when the dominant allele of a mutation with a visible, cell-autonomous phenotype is lost as a result of a deletion or somatic recombination. In addition to marking the fate of cells in a primordium at different stages of development and in different tissues, this technique also provides information about the distribution, orientation, rate, and duration of cell division. The technology and underlying assumptions of this method, termed clonal analysis, are described in this paper.     

MORPHOGENETIC ASPECTS OF A LEAFLESS MUTANT IN TOMATO I. GENERAL PATTERNS IN DEVELOPMENT

The embryo of the reduced form of the lanceolate mutant in tomato fails to undergo the heart-shaped stage of development. Cells in the shoot apical region of this leafless mutant lose their meristematic character and develop into mature parenchyma during embryogenesis. This early loss of meristem tissue leads to the determinate growth which is evident in the seedling. In contrast to normal, starch grains are visible with the light microscope in cells of the shoot tip of the mutant hypocotyl from early embryogeny up to and including the seedling stage, and protein bodies are abundant in the same tissue of fully developed mutant embryos. The shoot apical region in homozygous mutant embryos with cotyledons or cotyledon-like structures exhibits some cytochemical and morphological similarity with the normal shoot apex. Morphological variation in these forms appears to be in a continuous pattern. The extent of their development and consequent longevity is related to possible differences in rates of cell expansion and variation in environmental factors during the early stages of embryogenesis.     

Developmental anatomy of the three-dimensional leaf ofBotrychium ternatum (Thunb.) Sw.

The ophioglossaceous leaf exhibits a unique morphology, three-dimensional constitution. This examination clarified the developmental manner of the leaf ofBotrychium ternatum (Thunb.) Sw. The leaf primordium develops as an ordinary appendicular leaf which shows a typical hyponastic growth curvature, though it soon takes a conical shape with a tetrahedral apical cell. The leaf primordium forms a sporophyll primordium first, then forms vegetative primary pinnae acting as the trophophyll primordium. The sporophyll primordium also forms sporogenous primary pinnae. The sporophyll initiation begins with establishment of a new apical cell (sporophyll apical cell) near the original leaf apical cell. Through activity of the sporophyll apical cell most of the sporophyll primordium is formed later. In contrast, the vegetative or sporogenous primary pinna begins to develop as a low mound of surface cells on the trophophyll or sporophyll primordium respectively. The apical cell is later established on the summit of each pinna primordium. In the developmental point of view, the sporophyll primordium is not equivalent to the primary pinnae on the trophophyll primordium. The sporophyll may not represent two fused basal pinnae of a leaf, but represents an organ independent of and equivalent to the trophophyll.     

ON THE STEM APEX,LEAF INITIATION AND EARLY LEAF ONTOGENY IN FILICALEAN FERNS

A general account of the stem apex organization in ferns is presented in support of the classical single apical cell concept. The range in variation of apical cells and of their modes of division are described. Evidence is brought out to indicate probable directing effects of the apical cell on modes of division of surrounding cells and on the leaf mother cell. Initiation of and eventual establishment of a stabilized apex in fern leaves is described. Of the more than 50 genera studied, the leaves of all are traceable to a single mother cell from which the leaf apical cell is cut out. Apical dichotomies are described in a number of genera as well as their effect on early leaf development. Results are discussed in a phylogenetic and morphogenetic context of leaf appendicularization.     

Planes of Cell Division and Growth in the Shoot Apex of Pisum

The planes of cell division and growth were examined in thecourse of a single plastochron in the shoot apical meristemby observing the orientations of mitotic spindles. In the I₁region of the apical dome, cell divisions were at first anticlinalbut 30 h before a leaf primordium emerged at this site 20 percent of the cell divisions became periclinal. These periclinaldivisions were found only in the corpus. Periclinal divisionsin the tunica were coincident with the appearance of the primordiumas a bulge. The change in the direction of growth in I₁ at thesite of the incipient leaf primordium occurred without any changein the rate of growth in this region of the meristem.     

Recent advances in the genetic regulation of the shape of simple leaves

Leaves are major photosynthetic organs, and their diverse shapes and sizes allow adaptation to the natural environment. The early control of leaf shape and size depends on the control of the rate and plane of cell division at the shoot apical meristem and the polarity-dependent cell differentiation in the leaf primordium. In this review, we first summarize knowledge regarding several genes that control the initial stages of leaf formation and leaf polarity (e.g. adaxial–abaxial polarity, symmetry, and flat morphology). Formation of the lateral leaf morphology involves co-ordination of the rates of division and enlargement of leaf cells. Thus, we also summarize information on a number of genes that control these stages of two-dimensional lateral leaf growth (e.g. polarized cell expansion, specific control of cell proliferation, and integration of cell proliferation and expansion). In addition, we discuss several recently identified microRNAs, which are important factors affecting the development of leaf shape via control of spatial and temporal expression of target gene families. We focus on the genetic regulation of leaf shape in the model plant Arabidopsis thaliana from the perspective of spatial and temporal balance among cell proliferation, enlargement, and differentiation, with special emphasis on the results of our own studies.     

植物叶发育调控机理研究的进展

在植物的营养生长阶段,叶原基从植物地上部分顶端分生组织的周边区形成,在一系列细胞分裂和分化程序的指导下,最终发育成叶。近年来,通过遗传学和分子生物学研究已经鉴定和克隆了一批参与叶发育调控的关键基因,植物激素在叶原基的诱导和叶形态建成中也起十分重要的作用。目前这个领域的主要研究工作是鉴定调控叶发育的新基因并且解释叶调控基因之间的相互作用,同时了解基因调控和植物激素作用之间的关系。     

植物叶发育调控机理研究的进展

在植物的营养生长阶段,叶原基从植物地上部分顶端分生组织的周边区形成,在一系列细胞分裂和分化程序的指导下,最终发育成叶。近年来,通过遗传学和分子生物学研究已经鉴定和克隆了一批参与叶发育调控的关键基因,植物激素在叶原基的诱导和叶形态建成中也起十分重要的作用。目前这个领域的主要研究工作是鉴定调控叶发育的新基因并且解释叶调控基因之间的相互作用,同时了解基因调控和植物激素作用之间的关系。     

The Mode of Origin of a Leaf Primordium in the Shoot Apex of the Pea (Pisum sativum)

The movement of carbon-particle markers on the surface of acultured pea apex resembled that previously found for the tomatoapex. In the pea the primordium originated lower down on theside of the apical dome than in the tomato, and its generaldirection of growth was more upright. The results accord wellwith existing data on the rates and directions of cell divisionin the pea apex, and show that the primordium is formed by increasedcell division on the flank of the apex in a growth centre (orregion) analagous to that found in the tomato apex. Becauseof the distichous phyllotaxis of the pea it appears that inlongitudinal section two such growth centres at different stagesare visible, whereas in the tomato, which has spiral leaf arrangement,only one is apparent. It is concluded that, while a change indirection of division inevitably occurs in the primordium asit begins to bulge outwards away from the centre of the apex,its initiation can be traced to a local increase in the rateof division some 2 plastochrons before the bulge is well formed.     

A Sequential Study of Cell Divisions and Expansion Patterns on a Single Developing Shoot Apex of Vinca major

During the growth of a single developing vegetative apex ofVinca major, both the orientation and frequency of cell divisions,and the pattern of cell expansion, were observed using a non-destructivereplica technique. Micrographs taken at daily intervals illustratethat the central region of the apical dome remains relativelyinactive, except for a phase of cell division which occurs after2 d of growth. The majority of growth takes place at the proximalregions of the dome from which develop the successive pairsof leaves. The developing leaf primordia are initiated by aseries of divisions which occur at the periphery of the centraldome and are oriented parallel to the axis of the subsequentleaves. The cells which develop into the outer leaf surfaceof the new leaves undergo expansion and these cells divide allowingfor the formation of the new leaf. This paper describes thefirst high-resolution sequential study of cell patterns in asingle developing plant apex. Sequential development, cell division, expansion patterns, SEM, Vinca major, apical dome, leaf primordium, leaf initiation     

LEAF DEVELOPMENT IN DOXANTHA UNGUIS-CATI (BIGNONIACEAE)

Leaf structure in Doxantha unguis-cati is polymorphic. The usual mature compound leaf is composed of two lanceolate leaflets and a terminal tripartite spine-tendril. Leaf primordia are initiated simultaneously in pairs on opposite flanks of the shoot apical meristem by periclinal cell divisions in the third subsurface layer of the peripheral flank meristem. Two leaflet primordia are the first lateral appendages of the compound leaf. Initiation of these leaflet primordia occurs on the adaxial side of a compound leaf primordium 63–70 μm long. Lamina formation is initiated at the base of a leaflet primordium 70–90 μm long and continues acropetally. Mesophyll differentiation occurs in later stages of development of leaflets. The second pair of lateral appendages of the leaf primordium differentiate as prongs of the tendril. Initiation of the second pair of lateral appendages occurs on the adaxial side of a primordium approximately 168 μm long. Acropetal procambialization and vacuolation of cells extend to the apex of tendrils about 112 μm long, restricting the tendril meristem to the adaxial side of the primordium and resulting in curvature of the tendril. The tendril meristem is gradually limited to a more basipetal position as elongation of apical cells continues. Initiatory divisions and early ontogenetic stages of leaflets and tendrils are similar. Their ontogeny differs when the lateral primordia are approximately 70 μm long. Marginal and submarginal initials differentiate within leaflets but not in tendrils. Apical growth of tendrils ceases very early in ontogeny as compared with leaflets.     

THE SHOOT APEX AND EARLY LEAF DEVELOPMENT IN CLEMATIS

Tepfer , Sanford S. (U. Oregon, Eugene.) The shoot apex and early leaf development in Clematis . Amer. Jour. Bot. 47 (8): 655–664. Illus. 1960.—The high-domed shoot apex comprises a 2-layered tunica and shallow corpus. The rib meristem at times extends to within 5 cells of the summit. The cells of tunica and corpus are uniform cytologically, distinguishable only by the orientation of division planes. No zonation is visible within the corpus. No evidence was found of the existence of a méristème d'attente; mitotic figures appear frequently in the central region of the tunica and corpus. Decussately arranged leaf primordia arise high on the flanks of the apex. Periclinal divisions in the inner tunica and outermost corpus layers mark the site of initiation. Details of the growth and early differentiation of the leaf primordia follow the usual pattern of buttress formation, growth through apical and subapical initials. Apical growth continues beyond the early stages of leaf ontogeny; the blade-forming marginal meristems do not appear until after leaflet primordia are formed. There are 5 primary leaflets, pinnately arranged. Each leaflet is 3- to 5-lobed. In primordium P₃ expansion of the adaxial-lateral margins occurs at the base, but not above. This marks the upper limits of the basal pair of lateral leaflets. In P₄ the upper limits of the upper lateral leaflets become demarcated in similar fashion.     

Clonal analysis of leaf development in cotton

Clonal analysis has been used to describe the cellular parameters of leaf development in American Pima cotton (Gossypium barbadense). Sectors (clones) induced before leaf initiation indicate that the leaf primordium arises from ~100 cells on the flank of the shoot meristem. An analysis of sector frequency during the period of leaf expansion suggests that the rate of cell division is fairly uniform throughout the length of the leaf, but is lower at the margin of the lamina than in intercalary regions. The shapes of marginal sectors indicate that the orientation of cell division (as defined by the orientation of the new cell wall) in this region is more often parallel to the margin than perpendicular to it, although the degree of polarization varies along the length of the margin. There is a slight gradient in the duration of cell division along the length of of the lamina late in development, with cell division ceasing progressively from the lamina tip to the base over two cell cycles. The parameters of cell division in cotton are therefore similar to those described for tobacco with the notable exception of the behavior of cells at the leaf margin.     

The Relationship Between the Distribution of Periclinal Cell Divisions in the Shoot Apex and Leaf Initiation

Periclinal cell divisions in vegetative shoot apices of Pisumand Silene were recorded from serial thin sections by mappingall the periclinal cell walls formed less than one cell cyclepreviously. The distribution of periclinal divisions in theapical domes corresponded to the distributions subsequentlyoccurring in the apices when the young leaf primordia were forming.In Pisum, periclinal divisions were almost entirely absent fromthe I₁ region of the apical dome for half a plastochron justafter the formation of a leaf primordium and appeared, simultaneouslyover the whole of the next potential leaf site, about half aplastochron before the primordium formed. In Silene periclinaldivisions seemed to always present in the apical dome at thepotential leaf sites and also round the sides of the dome wherethe ensheathing leaf bases were to form. Periclinal divisionstherefore anticipated the formation of leaf primordia by occuring,in Pisum about one cell cycle and in Silene two or more cellcycles, before the change in the direction of growth or deformationof the surface associated with primordial initiation. Pisum, Silene, planes of cell division, orientation of cell walls, leaf primordia, shoot apical meristem, plastochron     

Cell polarity emerges at first cleavage in sea urchin embryos

In protostomes, cell polarity is present after fertilization whereas most deuterostome embryos show minimal polarity during the early cleavages. We now show establishment of cell polarity as early as the first cleavage division in sea urchin embryos. We find, using the apical markers G_M1, integrins, and the aPKC-PAR6 complex, that cells are polarized upon insertion of distinct basolateral membrane at the first division. This early apical-basolateral polarity, similar to that found in much larger cleaving amphibian zygotes, reflects precocious functional epithelial cell polarity. Isolated cleavage blastomeres exhibit polarized actin-dependent fluid phase endocytosis only on the G_M1, integrin, microvillus-containing apical surface. A role for a functional PAR complex in cleavage plane determination was shown with experiments interfering with aPKC activity, which results in several spindle defects and compromised blastula development. These studies suggest that cell and embryonic polarity is established at the first cleavage, mediated in part by the Par complex of proteins, and is achieved by directed insertion of basolateral membrane in the cleavage furrow.     

DNA, RNA, and Protein in the Pea Shoot Apex in Relation to Leaf Initiation

The average amounts of DNA, RNA, and protein per cell measuredhistochemically in each region of the shoot apical meristemremained unchanged during the course of a plastochron and duringthe early development of the leaf primordium. The average contentof DNA, RNA, and protein per cell was the same in all regionsof the shoot apical meristem.