Changes in cell-cycle duration and growth fraction in the shoot meristem of Sinapis during floral transition

The cell-cycle duration and the growth fraction were estimated in the shoot meristem of Sinapis alba L. during the transition from the vegetative to the floral condition. Compared with the vegetative meristem, the cell-cycle length was reduced from 86 to 32 h and the growth fraction, i.e. the proportion of rapidly cycling cells, was increased from 30–40% to 50–60%. These changes were detectable as early as 30 h after the start of the single inductive long day. The faster cell cycle in the evoked meristem was achieved by a shortening of the G1 (pre-DNA synthesis), S (DNA synthesis) and G2 (post-DNA synthesis) phases of the cycle. In both vegetative and evoked meristems, both-the central and peripheral zones were mosaics of rapidly cycling and non-cycling cells, but the growth fraction was always higher in the peripheral zone.Abbreviations G1 pre-DNA synthesis phase - G2 post-DNA synthesis phase - GF growth fraction - M mitosis phase - PLM percentage-labelled-mitoses method - S DNA synthesis phase - TdR thymidine     

Cytophotometric Study of the 2C Nuclear DNA Content in the Apical Bud of Sinapis alba during Floral Transition

Feulgen cytophotometry was used to detect possible changes inthe 2C DNA content in the various parts of the apical bud ofSinapis alba during floral evocation and flower development.This study showed that there was no significant difference inthe 2C DNA content between the vegetative, evoked or reproductivemeristems. In vegetative plants, the 2C DNA content was lowerin the leaf primordia than in the meristem. This content inthe leaf exhibited a further decrease during the floral transition.In the flower primordia, the 2C value never exceeded the typicalvalue of the meristem. In the flower at anthesis, the DNA contentwas lower in the pistil and stamen than in the meristem. Apical bud, floral transition, 2C DNA content, cytophotometry, Sinapis alba L.     

HISTOCHEMICAL STUDY OF ENZYME ACTIVITY IN THE SHOOT APICAL MERISTEM OF BRASSICA CAMPESTRIS DURING TRANSITION TO FLOWERING. IV. GLUCOSE-6-PHOSPHATASE

Glucose-6-phosphatase (G6P) activity was determined in fresh-frozen, cryostat sections in the shoot apical meristem of Brassica campestris L. Enzymatic activity was differentially distributed in a zonate pattern in the vegetative meristem, but not in the transition and floral meristem. Vegetative apices showed a heterogenous localization with the highest activity in the central zone and the pith-rib meristem zone. At the early transition stage of development, G6P activity in the peripheral zone increased slightly. At the late transitional (prefloral) stage, G6P activity was not localized within the peripheral zone in island-like areas of activity. This is the first demonstration of G6P in shoot apical meristem at the vegetative, transition, and floral stage. The results indicate that G6P activity 1) is an accompanying event of evocation, but 2) does not mark incipient floral primordia. G6P may play an important role in the maintenance of glucose-6-phosphate homeostasis in an evoked shoot apical meristem.     

Overexpression of EVE1, a novel ubiquitin family protein, arrests inflorescence stem development in Arabidopsis

In Arabidopsis, inflorescence stem formation is a critical process in phase transition from the vegetative to the reproductive state. Although inflorescence stem development has been reported to depend on the expression of a variety of genes during floral induction and repression, little is known about the molecular mechanisms involved in the control of inflorescence stem formation. By activation T-DNA tagging mutagenesis of Arabidopsis, a dominant gain-of-function mutation, eve1-D (eternally vegetative phase1-Dominant), which has lost the ability to form an inflorescence stem, was isolated. The eve1-D mutation exhibited a dome-shaped primary shoot apical meristem (SAM) in the early vegetative stage, similar to that seen in the wild-type SAM. However, the SAM in the eve1-D mutation failed to transition into an inflorescence meristem (IM) and eventually reached senescence without ever leaving the vegetative phase. The eve1-D mutation also displayed pleiotropic phenotypes, including lobed and wavy rosette leaves, short petioles, and an increased number of rosette leaves. Genetic analysis indicated that the genomic location of the EVE1 gene in Arabidopsis thaliana corresponded to a bacterial artificial chromosome (BAC) F4C21 from chromosome IV at ～17cM which encoded a novel ubiquitin family protein (At4g03350), consisting of a single exon. The EVE1 protein is composed of 263 amino acids, contains a 52 amino acid ubiquitin domain, and has no glycine residue related to ubiquitin activity at the C-terminus. The eve1-D mutation provides a way to study the regulatory mechanisms that control phase transition from the vegetative to the reproductive state.     

Competence to respond to floral inductive signals requires the homeobox genes PENNYWISE and POUND-FOOLISH

The transition from vegetative to reproductive development establishes new growth patterns required for flowering. This switch is controlled by environmental and/or intrinsic developmental cues that converge at the shoot apical meristem (SAM). During this developmental transition, floral inductive signals cause the vegetative meristem to undergo morphological changes that are essential for flowering. Arabidopsis plants containing null mutations in two paralogous BEL1-like (BELL) homeobox genes, PENNYWISE (PNY) and POUND-FOOLISH (PNF), disrupt the transition from vegetative to reproductive development. These double mutants are completely unable to flower even though the SAM displays morphological and molecular changes that are consistent with having received floral inductive signals. These studies establish a link between the competence to receive floral inductive signals and restructuring of the SAM during floral evocation.     

delayed flowering1 Encodes a basic leucine zipper protein that mediates floral inductive signals at the shoot apex in maize

Separation of the life cycle of flowering plants into two distinct growth phases, vegetative and reproductive, is marked by the floral transition. The initial floral inductive signals are perceived in the leaves and transmitted to the shoot apex, where the vegetative shoot apical meristem is restructured into a reproductive meristem. In this study, we report cloning and characterization of the maize (Zea mays) flowering time gene delayed flowering1 (dlf1). Loss of dlf1 function results in late flowering, indicating dlf1 is required for timely promotion of the floral transition. dlf1 encodes a protein with a basic leucine zipper domain belonging to an evolutionarily conserved family. Three-dimensional protein modeling of a missense mutation within the basic domain suggests DLF1 protein functions through DNA binding. The spatial and temporal expression pattern of dlf1 indicates a threshold level of dlf1 is required in the shoot apex for proper timing of the floral transition. Double mutant analysis of dlf1 and indeterminate1 (id1), another late flowering mutation, places dlf1 downstream of id1 function and suggests dlf1 mediates floral inductive signals transmitted from leaves to the shoot apex. This study establishes an emergent framework for the genetic control of floral induction in maize and highlights the conserved topology of the floral transition network in flowering plants.     

A tobacco calcium/calmodulin-binding protein kinase functions as a negative regulator of flowering

A tobacco calcium/calmodulin-binding protein kinase (NtCBK1) was isolated and identified. The predicted NtCBK1 protein has 599 amino acids, an N-terminal kinase domain, and shares high homology with other calmodulin (CaM)-related kinases. Whereas NtCBK1 phosphorylates itself and substrates such as histone IIIS and syntide-2 in the absence of CaM, its kinase activity can be stimulated by tobacco CaMs. However, unlike another tobacco protein kinase designated NtCBK2, NtCBK1 was not differentially regulated by the different CaM isoforms tested. The CaM-binding domain of NtCBK1 was located between amino acids 436 and 455, and this domain was shown to be necessary for CaM modulation of kinase activity. RNA in situ hybridization showed that NtCBK1 was highly regulated in the transition to flowering. Whereas NtCBK1 mRNA was accumulated in the shoot apical meristem during vegetative growth, its expression was dramatically decreased in the shoot apical meristem after floral determination, and in young flower primordia. The expression of NtCBK1 was up-regulated to high levels in floral organ primordia. Fluctuations in NtCBK1 expression were verified by analysis of tobacco plants expressing green fluorescent protein under the control of the NtCBK1 promoter, suggesting a role of NtCBK1 in the transition to flowering. This conclusion was confirmed by overexpressing NtCBK1 in transgenic tobacco plants, where maintenance of high levels of NtCBK1 in the shoot apical meristem delayed the switch to flowering and extended the vegetative phase of growth. Further work indicated that overexpression of NtCBK1 in transgenic tobacco did not affect the expression of NFL, a tobacco homologue of the LFY gene that controls meristem initiation and floral structure in tobacco. In addition, the promotion of tobacco flowering time by DNA demethylation cannot be blocked by the overexpression of NtCBK1.     

Les critères nucléaires d'une dormance vraie et totale dans le point végétatif du Fraxinus excelsior L.

Summary The cytohistological criteria for the vegetative shoot apex dormancy in Fraxinus excelsior L. have been quantitatively established with the aid of 3 techniques: historadiography after incorporation of [³H]thymidine, mitotic index and nuclear cytophotometry by the two wavelength method. Nuclear DNA content, mitotic activity and DNA synthesis were compared in 3 different zones(apical zone, lateralzone, rib meristem) of the dormant and non-dormant apices. The periodical break in morphogenetic activity, in contrast to the vegetative period (April to July), is characterized by the absence of zonation and by the fact that meristematic cells remain in the G1 phase of the mitotic cycle. In Fraxinus excelsior L., the meristem dormancy is complete (no DNA synthesis, no mitotic activity and no DNA content greater than 2C).

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Abréviations employées ZA zone apicale axiale du point végétatif - ZL zone latérale ou anneau initial - mm meristème médullaire - UA unité arbitraire de quantité de DNA     

Determination of floral initiation in <Emphasis Type="Italic">Malus domestica</Emphasis>: a novel morphogenetic approach

Floral initiation in apple (Malus domestica Borkh) was studied by a novel morphogenetic approach. Developmental stages of apices were evaluated based on the morphology of shoot apical meristem (SAM) from various collection dates. Besides, the frequency of each stage was calculated within apices populations after full blooming (DAFB). Prior to doming of apex, three marked phases were found based on SAM morphology: 1) narrow appearance (vegetative phase), 2) broadened form (transition phase), and 3) prominent shape (commitment phase). A furrow region was formed at the base of leaf primordium during the bract initiation, while significant broadening of SAM was observed. Cell division patterns manifested in modification of anisotropic clusters from isotropic cellular packets, as a result of which profound morphological changes of apices occurred. Based on these findings, we propose that the structural alterations prior to doming may be taken into account for determination of the initial development and reproduction signs in apple trees.     

Isolation and characterization of a rice homebox gene, OSH15

In many eukaryotic organisms including plants, homeobox genes are thought to be master regulators that establish the cellular or regional identities and specify the fundamental body plan. We isolated and characterized a cDNA designated OSH15 (Oryza sativa homeobox 15) that encodes a KNOTTED-type homeodomain protein. Transgenic tobacco plants overexpressing the OSH15 cDNA showed a dramatically altered morphological phenotype caused by disturbance of specific aspects of tobacco development, thereby indicating the involvement of OSH15 in plant development. We analyzed the in situ mRNA localization of OSH15 through the whole plant life cycle, comparing the expression pattern with that of another rice homeobox gene, OSH1. In early embryogenesis, both genes were expressed as the same pattern at a region where the shoot apical meristem would develop later. In late embryogenesis, the expression pattern of the two genes became different. Whereas the expression of OSH1 continued within the shoot apical meristem, OSH15 expression within the shoot apical meristem ceased but became observable in a ring shaped pattern at the boundaries of some embryonic organs. This pattern of expression was similar to that observed around vegetative or reproductive shoots, or the floral meristem in mature plants. RNA in situ localization data suggest that OSH15 may play roles in the shoot organization during early embryogenesis and thereafter, OSH15 may be involved in morphogenetic events around the shoot apical meristem.     

Flowering and apical meristem growth dynamics

The shoot apical meristem generates stem, leaves, and lateralshoot meristems during the entire shoot ontogeny. Vegetativeleaves are generated by the meristem in the vegetative developmentalphase, while in the reproductive phase either bracts subtendinglateral flower primordia (or paraclades), or perianth and strictlyreproductive organs are formed. Meristem growth is fully characterizedby the principal growth rates, directions, volumetric, and arealgrowth rates. Growth modelling or sequential in vivo methodsof meristem observation complemented by growth quantificationallow the above growth variables to be estimated. Indirectly,growth is assessed by cell division rates and other cell cycleparameters. Temporal and spatial changes of growth and geometrytake place at the meristem during the transition from the vegetativeto the reproductive phase. During the vegetative phase, meristemgrowth is generally indeterminate. In the reproductive phaseit is almost always determinate, but the extent of determinacydepends on the inflorescence architecture. In the vegetativephase the central meristem zone is the slowest growing region.The transition from the vegetative to the reproductive phaseis accompanied by an increase in mitotic activity in this zone.The more determinate is the meristem growth, the stronger isthis mitotic activation. However, regardless of the extent ofthe activation, in angiosperms the tunica/corpus structure ofthe meristem is preserved and therefore the mitotic activityof germ line cells remains relatively low. In the case of thethoroughly studied model angiosperm plant Arabidopsis thaliana,it is important to recognize that the flower primordium developsin the axil of a rudimentary bract. Another important featureof growth of the inflorescence shoot apical meristem is theheterogeneity of the peripheral zone. Finally, the role of mechanicalfactors in growth and functioning of the meristem needs furtherinvestigation. Key words: Flower primordium, geometry, growth, inflorescence, shoot apical meristem, transition from vegetative to reproductive phase Received 4 October 2007; Revised 5 November 2007 Accepted 6 November 2007     

A petunia MADS box gene involved in the transition from vegetative to reproductive development.

We have identified a novel petunia MADS box gene, PETUNIA FLOWERING GENE (PFG), which is involved in the transition from vegetative to reproductive development. PFG is expressed in the entire plant except stamens, roots and seedlings. Highest expression levels of PFG are found in vegetative and inflorescence meristems. Inhibition of PFG expression in transgenic plants, using a cosuppression strategy, resulted in a unique nonflowering phenotype. Homozygous pfg cosuppression plants are blocked in the formation of inflorescences and maintain vegetative growth. In these mutants, the expression of both PFG and the MADS box gene FLORAL BINDING PROTEIN26 (FBP26), the putative petunia homolog of SQUAMOSA from Antirrhinum, are down-regulated. In hemizygous pfg cosuppression plants initially a few flowers are formed, after which the meristem reverts to the vegetative phase. This reverted phenotype suggests that PFG, besides being required for floral transition, is also required to maintain the reproductive identity after this transition. The position of PFG in the hierarchy of genes controlling floral meristem development was investigated using a double mutant of the floral meristem identity mutant aberrant leaf and flower (alf) and the pfg cosuppression mutant. This analysis revealed that the pfg cosuppression phenotype is epistatic to the alf mutant phenotype, indicating that PFG acts early in the transition to flowering. These results suggest that the petunia MADS box gene, PFG, functions as an inflorescence meristem identity gene required for the transition of the vegetative shoot apex to the reproductive phase and the maintenance of reproductive identity.     

Expression of fasciation mutation in apical meristems of soybean, Glycine max (Leguminosae)

The phenotype of the apical meristem was used to examine the effect of fasciation mutation at the f locus in different genetic backgrounds in soybean Glycine max (L.) Merr. Comparisons of meristem development in fasciation mutant and wild type were conducted with scanning electron microscope (SEM) on isogenic lines BARC-11-11-ff and BARC-11-11-FF at postgermination and early vegetative stages. Studies of apical meristems of three independently originated fasciation mutants, PI 83945-4, PI 243541, and T173, were carried out at vegetative and early floral transition stages. Corolla Fasciation, the extreme mutant phenotype, was used for comparison of meristem development. Enlargement of the apical meristem and shortened plastochron were observed in the mutant lines 2 d after germination. Similar to Corolla Fasciation, in PI 83945-4, PI 243541, and T173, enlargement of the apical meristem was followed by growth along one axis at the V3 stage and establishment of a ridge-like meristem at the V4 stage. Influence of pedigree on the expression of the fasciation phenotype was demonstrated by different growth patterns (subangular vs. ridge-like) of the apical meristem in BARC-11-11-ff and PI 243541 with the same f gene. During transition of the apical meristem from vegetative to reproductive stage in all mutant lines further production of leaf primordia ceased. The developmental pattern of the apical meristems suggests that the f locus may have the same allele in fasciation mutants of independent origin in soybean.