Intercellular signalling in the transition from stem cells to organogenesis in meristems

Meristems continuously produce new cells to sustain plant growth. Stem cells are maintained in the centre of the meristem and provide the precursor cells for the initiation of new organs and tissues in the periphery. The structure of the meristem is maintained while cells are constantly displaced by new divisions. Recent advances have been made in understanding the intercellular signals that maintain meristem structure by adjusting gene expression according to cell position. In addition to refinements in our understanding of how the position and size of the stem-cell population is regulated, there have been advances in understanding how the location of new organ primordia is controlled and how the meristem influences organ polarity.     

Floral morphogenesis in Anagallis: Scanning-electron-micrograph sequences from individual growing meristems before,during, and after the transition to flowering

Non-destructive scanning electron microscopy allows one to visualize changing patterns of individual cells during epidermal development in single meristems. Cell growth and division can be followed in parallel with morphogenesis. The method is applied here to the shoot apex of Anagallis arvensis L. before, during, and after floral transition. Phyllotaxis is decussate; photoperiodic induction of the plant leads to the production of a flower in the axil of each leaf. As seen from above, the recently formed oval vegetative dome is bounded on its slightly longer sides by creases of adjacent leaf bases. The rounded ends of the dome are bounded by connecting tissue, horizontal bands of node cells between the opposed leaf bases. The major growth axis runs parallel to the leaf bases. While slow-growing at the dome center, this axis extends at its periphery to form a new leaf above each band of connecting tissue. Connecting tissue then forms between the new leaves and a new dome is defined at 90° to the former. The growth axis then changes by 90°. This is the vegetative cycle. The first observed departure from vegetative growth is that the connecting tissue becomes longer relative to the leaf creases. Presumably because of this, the major growth axis does not change in the usual way. Extension on the dome continues between the older leaves until the axis typically buckles a second time, on each side, to form a second crease parallel to the new leaf-base crease. The tissue between these two creases becomes the flower primordium. The second crease also delimits the side of a new apical dome with the major axis and growth direction altered by 90°. During this inflorescence cycle the connecting tissue is relatively longer than before. Much activity is common to both cycles. It is concluded that the complex geometrical features of the inflorescence cycle may result from a change in a biophysical boundary condition involving dome geometry, rather than a comprehensive revision of apical morphogenesis.Abbreviation SEM scanning electron microscopy, micrograph Use of the SEM facility of Professor G. Goffinet, Institute of Zoology, University of Liège, is greatly appreciated. We thank Dr. R. Jacques, C.N.R.S., Le Phytotron, Gif-sur-Yvette, France, for providing the experimental material, and Mr. Philippe Ongena for expert photography. Support was from grants from the U.S. Department of Agriculture and National Science Foundation as well as from the Fonds National de la Recherche Scientifique, Fonds de la Recherche Fondamentale et Collective, and the Action de Recherche Concertée of Belgium.     

Revisiting phase transition during flowering in Arabidopsis

Single-phase transition during flowering has been suggested by Hempel and Feldman (1994) [Planta 192: 276]. When early flowering ecotypes of Arabidopsis were microscopically observed, a long day signal simultaneously induced the acropetal (bottom to top) production of flower primordia and the basipetal (top to bottom) differentiation of paraclades (axillary flowering shoots) from the axils of pre-existing leaf primordia. However, this model could not account for the production of an extra number of secondary shoots in the TERMINAL FLOWER 1 overexpressor line or AGL20 overexpressor line in Columbia background with a functional allele of FRIGIDA. We report here that Columbia with a functional allele of FRIGIDA under long days and Columbia under short days show an inflorescence-producing phase between the vegetative and the flower-producing phases, supporting two-step phase transition during flowering. In addition, a late-flowering mutant, fwa shows an inflorescence phase but fca, fy and fve follow a single-phase transition, suggesting flowering time mutations have different effects on phase transition during flowering.     

Biochemical investigations of the nuclear proteins during the transition of root meristems from quiescent to proliferating state in Pisum sativum

Abstract

The electrophoretic analysis of nuclear proteins extracted from root meristems at different times of germination puts in evidence the variations of content of specific proteins. Several nuclear proteins are phosphorylated by endogenous protein kinase and often the maximum rate of phosphorylation it has been observed in proteins present in the nucleus at low concentrations. Moreover also the phosphorylation rate of specific proteins changes at different times of germination. It is interesting the fact that both variations of concentration and phosphorylation in nuclear proteins occurr at the time when root meristems leave the quiescence to enter a proliferating state. We suggest that these variations play a role in this physiological event.     

Growth changes in the shoot apex of Sinapis alba during transition to flowering

The aim of the work was to report morphological changes whichoccur in the shoot apex during the morphogenetic switch to floweringin the model long day (LD) plant, Sinapis alba. During the floraltransition induced by 1 LD the growth rate of all componentsof the shoot apex is modified profoundly. The earliest changes,detected at 24 h after start of LD, include a decrease in plastochronduration and an increase of growth of leaf primordia. One daylater, the meristem dome starts to increase in volume, apicalinternodes have an increased height and there is a precociousoutgrowth of axillary meristems. All these changes precede initiationof flower primordia, which starts at about 60 h after the startof LD. Later changes include meristem doming, a decrease inthe plastochron ratio and a shift to a more complex phyllotaxis.All the changes, except the decreased plastochron ratio, arecharacteristics of an apex with an increased tempo of growth.The stimulation of longitudinal growth (height of apical intemodes)is more marked and occurs earlier than the reduction of radialgrowth (plastochron ratio). Key words: Axillary meristem, internode growth, leaf growth, plastochron ratio, plastochron duration     

Gibberellins in apical shoot meristems of flowering and vegetative sugarcane

Gibberellins A₁, A₃, iso-A₃, A₄, A₁₉, A₂₀, and A₃₆ were identified by gas chromatography-selected ion monitoring in apices of sugarcane (Saccharum spp. hybrids). Flowering apices (i.e., 2–4 cm panicle) contained 8–9 times more (estimated by bioassay) endogenous gibberellins A and iso-GA₃ (ratio of 1:6:8, respectively; in total 51 ng g^–1 fresh weight) than vegetative apices (6.4 ng g^–1 fresh weight). Vegetative apices contained small but significant levels of GA₁₉, which could not be detected in flowering apices; vegetative apices also contained approximately four times more of a GA₃₆-like substance than flowering apices. Since the two apex types developed under the same photoperiod, the increased levels of GA and iso-GA₃ and the reduced levels of GA₁₉ and GA₃₆-like substances are correlated with the flowering state rather than with photoperiod or photoperiod changes per se. Since there were relatively high levels of C₁₉ GAs along with low levels of C₂₀ GAs in flowering apices, and since the converse is true in vegetative apices, metabolism of C₂₀ to C₁₉ GAs may be enhanced in flowering apices.Mention of a trademark, proprietary product, or vendor does not constitute a guarantee or warranty of the product by U.S. Department of Agriculture and does not imply its approval to the exclusion of other products or vendors that may also be suitable.     

Ethylene formation in the leaves of short- and long-day tobacco during transition to flowering

Ethylene formation by loaves of the central stem zone of the short-day tobacco cv. Maryland Mammoth and long-davNicntiana sylvestris was followed for 40 days during in duction and transition to flowering. In SD tobacco Mammoth, ethylene formation rose between days 0-10, remained unchanged for the next 10 davs, rose slightly between days 20 - 30 and sharply within the last 10 days. The time-course of ethylene formation by the leaves of LD tobacco N.silvestris resembled that of Mammoth, but tho changes were less pronounced, especially at the beginning of the period. Generally, ethylene formation is much higher in SD tobacco Mammoth than in LDN. silvestris. Ethephon (0.02 %) application during flower induction significantly reduced flowering in SD tobacco Mammoth (by 47.5 %) and also reduced apical meristem length. In N.silvestris ethephon application did not reduce flowering, but most of the treated plants (62.5 %) did not attain the stage of inflorescence. Apical meristem (or inflorescence) and stem length were also reduced. The possible role of ethylene in regulation of transition to flowering is discussed.     

Changes in density of chromatin in the meristematic cells ofSinapis alba during transition to flowering

Summary Changes in the density of nuclear chromatin in the shoot apical meristem ofSinapis alba L. during floral transition (floral evocation) are described using Feulgen-stained 2 m thick semi-thin sections and scanning cytophotometric techniques. In both G1 and G2 nuclei the chromatin becomes less heterogeneous and less dense in evoked meristems compared to vegetative meristems. When chromatin is resolved into two fractions the dispersed fraction increases relative to the condensed fraction at evocation. This decondensation process occurs earlier in G1 than in G 2 nuclei. These chromatin changes are presumably closely related to the dramatic stimulation of biosynthetic activity and cell division during floral transition.     

Changes in RNA levels in the shoot apex of Silene during the transition to flowering

Changes in RNA concentration in the shoot apical meristem during induction and the transition to flowering were measured histochemically in Silene coeli-rosa (L.) Godron, a long-day plant. In the apices of plants induced by 7 long days the RNA concentration increased to about 25 per cent higher than in non-induced plants. Three long days did not induce flowering but resulted in a transient rise in RNA concentration. When plants were given long days interrupted by varying numbers of short days successful induction was accompanied by a sustained increase in RNA concentration but those treatments which were not inductive gave only transient increases in RNA. Gibberellic acid had no effect on induction or apical growth rates but increased the RNA concentration by 50 per cent or more in both induced and non-induced plants. Plants induced to flower at 13° C had the same RNA concentration and growth rate at the apex as in non-induced plants at 20° C. Since changes in RNA concentration in the apex could occur without changes in growth rate and without flowering, and induction could occur without a change in RNA concentration or growth rate, it is suggested that the increase in RNA and growth rate which normally occur at the transition to flowering might not be essential for the formation of a flower but may be more closely related to the rapid growth associated with the formation of the inflorescence.Abbreviations LD long day - SD short-day     

Lateral meristems and stem thickening growth in monocotyledons

Although monocotyledons lack a vascular cambium of the type found in dicotyledons and conifers, lateral meristems still play an important role in the establishment of their growth habits. The presence near the shoot apex of a primary thickening meristem (PTM), which is probably plesiomorphic in monocotyledons, predisposes evolution into the many pachycaul forms. A PTM occurs in virtually all monocotyledons, whereas the secondary thickening meristem (STM), which is morphologically similar, is limited to a few genera of Liliiflorae. these records are reviewed in a systematic context. To a greater or lesser extent in different taxa, the PTM is responsible for primary stem thickening, adventitious root production, and formation of linkages between stem, root and leaf vasculature. The STM largely contributes to the body of the stem. The sometimes obscure distinction between the two meristems, and their relationship with other stem meristems are discussed. For systematic purposes stem thickening in monocotyledons is separated into two characters: diffuse growth (as in palms), and growth by means of lateral meristems. The three states of the second character are represented by the first three of Mangin’s (1882) four categories (two herbaceous, the third arborescent): (1) The lateral meristem is limited in extent, and ceases activity after root formation. (2) It remains active for a limited period after cessation of root formation, contributing to the plant body. (3) It remains active throughout the life of the plant, contributing the bulk of the plant body.     

Growth regulators in changing apical growth at transition to flowering

Reorganization of growth in the shoot apex ofChenopodium rubrum during transition to flowering is described. Growth and morphogenic changes — a rise in cell division rate, changes in leaf and bud formation and changes in directions of cellular growth — are viewed from the aspect of a possible role of growth hormones in controlling these changes. Growth and morphogenic effects of exogenous growth regulators in the shoot apex ofChenopodium are summarized and their floral effects explained in terms of changing apical growth correlations. New evidence concerning the timing of increased cell division rate and showing the limited requirement of axillary cell division and a shift to more vertical direction of growth in the apex in the floral developmental pathway was obtained in experiments with kinetin application and by surgical treatments.     

植物成花转变基因的研究现状

介绍了拟南芥中与开花相关的一系列控制成花的基因,包括花分生组织特异性基因和COPS基因,这些基因相互作用,通过成花抑制路径、自发促进路径、光周期促进路径和春化促进路径来调节植物的成花转变。     

Changes in free IAA level in the leaves of short-and long-day tobacco during flowering and the effect of applied IAA on the transition to flowering

Changes in free IAA level were studied in the leaves of the central stem zone of short-day tobacco (Mcotianatabacum, cv. Maryland Mammoth) and long-day tobacco(Nicotiana silvestris) in inductive photoperiodic regime after 10, 20, 30 and 40 d, respectively. The leaves of SD tobacco Mammoth showed a high free IAA level in vegetative plants kept under long days but it significantly decreased (by ca. 50 %) after 10, 30 and 40 short days, respectively. After 20 short days the IAA level was as high as in the leaves of plants at the beginning of inductive treatment. The changes of freeIAA level in the leaves of LD tobacco N.silvestris were similar to those of SD Mammoth, but the IAA level in this species was significantly lower than that of Mammoth throughout the investigated period. Consequently, the changes observed in N. silvestris were much less pronounced. Plants of both tobacco species were fully induced to flowering by 30 inductive days and this was associated with differentiation of the flower organs. Application of 10 -4 M IAA during the last 10 d of the inductive treatment of 30 d significantly reduced flowering in SD tobacco Mammoth without changing the stem length and apex width. Apex length was slightly reduced. IAA application elicited almost no effect inN. silvestris. The results are discussed with respect to the possible role of IAA in flower induction in SD and LD plants.     

Genesis of cells of apical meristems and realization of gametophytic apomixis in flowering plants

Based on our own and literature data on peculiarities of caryotypical variability, we concluded that gametophytic apomixis is naturally accompanied with phenomena of poly-, aneu-, and mixoploidy and that apomicts have genome instability manifesting at the level of meristematic somatic cells. In this connection, a hypothesis is substantiated that realization of this mode of seed reproduction in flowering plants is caused by modification of systems of cell cycle control, following after acts of hybridogenesis and/or polyploidization. It is concluded that instability of the seed reproduction system by gametophytic apomixis manifests not only at the stage of choice of a seed reproduction pathway (apomeiosis-euspory; apozygosis-zygosis) but also in all the cycles of reproduction of the cells of a germ line in plant ontogenesis.     

Genesis of cells of apical meristems and realization of gametophytic apomixis in flowering plants

Based on our own and literature data on peculiarities of caryotypical variability, we concluded that gametophytic apomixis is naturally accompanied with phenomena of poly-, aneu-, and mixoploidy and that apomicts have genome instability manifesting at the level of meristematic somatic cells. In this connection, a hypothesis is substantiated that realization of this mode of seed reproduction in flowering plants is caused by modification of systems of cell cycle control, following after acts of hybridogenesis and/or polyploidization. It is concluded that instability of the seed reproduction system by gametophytic apomixis manifests not only at the stage of choice of a seed reproduction pathway (apomeiosis—euspory; apozygosis—zygosis) but also in all the cycles of reproduction of the cells of a germ line in plant ontogenesis.     

N content of phloem and xylem exudates during the transition to flowering in Sinapis alba and Arabidopsis thaliana

The involvement of nitrogenous substances in the transition to flowering was investigated in Sinapis alba and Arabidopsis thaliana (Columbia). Both species grown in short days (SD) are induced to flower by one long day (LD). In S. alba, the phloem sap (leaf and apical exudates) and the xylem sap (root exudate) were analysed in LD versus SD. In A. thaliana, only the leaf exudate could be analysed but an alternative system for inducing flowering without day‐length extension was used: the displaced SD (DSD). Significant results are: (i) in both species, the leaf exudate was enriched in Gln during the inductive LD, at a time compatible with export of the floral stimulus; (ii) in S. alba, the root export of amino acids decreased in LD, whereas the nitrate remained unchanged – thus the extra‐Gln found in the leaf exudate should originate from the leaves; (iii) extra‐Gln was also found very early in the apical exudate of S. alba in LD, together with more Glu; (iv) in A. thaliana induced by one DSD, the leaf export of Asn increased sharply, instead of Gln in LD. This agrees with Asn prevalence in C‐limited plants. The putative role of amino acids in the transition to flowering is discussed.     

Changes in the carbohydrate content of the leaf and the apical bud of Sinapis during transition to flowering

Vegetative plants of Sinapis alba L. were induced to flower by a single long day of 20 h or by a single short day of 8 h starting at an unusual time of the 24-h cycle (displaced short day). The soluble sugar and starch contents of the just-expanded leaf and the apical bud were measured at various times after the start of each of these two photoinductive treatments. Associated with the induction of flowering there were temporary increases in the soluble sugar and starch contents of the leaf and of the bud. These increases were apparent 14 h after the start of the long day and 12 h after the start of the displaced short day. The starch content of the bud increased later. These results indicate that an increase of the soluble sugar content of the bud is required for its transition from the vegetative to the reproductive condition.