CELL DIVISION STUDIES OF THE SHOOT APEX OF DATURA STRAMONIUM DURING TRANSITION TO FLOWERING

Seedlings of Datura stramonium L., although not photoperiodically sensitive, are useful for floral transition studies when raised in a growth chamber at a constant temperature of 25 C with a photoperiod of 8 hr of light (1,600-2,000 ft-c) and 16 hr of darkness. A terminal flower is formed after the seventh or eighth leaf primordium is produced. A constant rate of leaf initiation up to the time of flowering enables specific apical stages to be obtained and studied. Changes in the mitotic index, substantiated with calculated rates of cell division (measured by the accumulation of metaphases following treatment with colchicine) were studied in shoot apical zones during transition to flowering. Fluctuations in the mitotic index of each zone in the vegetative and transition apex with respect to apical stage as well as time of day were not statistically significant. The mitotic index of the summit zone of the vegetative apex was significantly lower than in the other zones whose mitotic indices were not significantly different from one another. During floral transition the mitotic index of the summit zone as well as the central zone (just below the summit zone) significantly increased while no significant changes were detected in the flank zones. It was shown that the mitotic index could be considered representative of the rates of cell division in Datura.     

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.     

CHANGES IN GLYCERALDEHYDE 3-PHOSPHATE DEHYDROGENASE ACTIVITY IN SHOOT APICAL MERISTEMS OF BRASSICA CAMPESTRIS DURING TRANSITION TO FLOWERING

An assay system has been developed for the histochemical determination of glyceraldehyde 3-phosphate dehydrogenase (G3PD) activity to indicate glycolytic pathway capacity during evocation in the shoot apical meristem of Brassica campestris L. G3PD activity was differentially distributed in a zonate pattern within the meristems at the vegetative, the transition, and the floral stages. The activity of G3PD changed in all apical zones of evoked apices, but especially in the central and the peripheral zones of apices at the prefloral stage. In the prefloral stage of development heavy enzyme activity was localized in island-like areas within the peripheral zone. These results indicate that 1) the capacity of glycolysis fluctuates during evocation, 2) during floral evocation the capacity of the glycolytic pathway parallels the capacity of the citric acid cycle and the electron transport system only at the prefloral stage, and 3) G3PD activity marks incipient floral primordia. It is proposed that the enzymic marking of an incipient floral primordium indicates the end of evocation in Brassica.     

Floral transition as a sequence of growth changes in different components of the shoot apical meristem ofChenopodium rubrum

The changes in cell division rate were studied in different components of the shoot apex ofChenopodium rubrum during short-day photoperiodic induction and after the inductive treatments. Induced and vegetative apices were compared. Accumulation of metaphases by colchicine treatment was used to compare the mean cell cycle duration in different components of the apex. A direct method of evaluating the increase in cell number obtained by anticlinal or periclinal divisions was applied if the corresponding components of induced and non-induced apices had to be compared. The short-day treatment prolonged the cell cycle more in the peripheral zone than in the central zone and still more in the leaf primordia. The importance of changing growth relations for floral transition was shown particularly if the induced plants were compared with the vegetative control with interrupted dark periods. Induced plants transferred to continuous light showed further changes in the rates of cell division. The cell cycle was shortened more in the central zone than in the peripheral zone,i.e. there was a further shift in growth relations within the apical dome. The cell cycle in the leaf and bud primordia was also shortened if compared with the vegetative control, the acceleration being stronger in the bud primordia. There was a subsequent retardation in cell division in the leaf primordia formed during and after the inductive treatment if the plants were fully induced. An inhibition of the oldest bud primordia was observed in fully induced apices, as well.     

DETERMINATION OF NUMBER AND MITOTIC ACTIVITY OF SHOOT APICAL INITIAL CELLS BY ANALYSIS OF MERICLINAL CHIMERAS

The dimensions of mericlinal sectors in periclinal chimeras resulting from replacement-displacement phenomena have been used to determine the number of shoot apical initial cells and their mitotic activity. Narrow sectors were always short, extending an average of less than three nodes. All long sectors were wide, involving 1/3 or 1/2 of the circumference of the stem. These observations define the origin of all primary growth as from 1–3 apical initial cells in each of the apical layers. The sectors reveal a surprising stability of cellular position at the center of growth, with a specific initial cell maintaining its position during formation of over 100 nodes. During vigorous vegetative growth of Ligustrum ovalifolium the initials themselves divide only about once in 12 days during the formation of three nodes. The mitotic index of the initials in privet shoot apices is 1.4, and this rate of division is sufficient for them to be the ultimate source of all cells composing the shoot.     

INITIATION AND ONTOGENY OF THE MICROSPORANGIATE CONE IN CUPRESSUS ARIZONICA IN RESPONSE TO GIBBERELLIN

Cupressus arizonica, a member of the Cupressaceae, was induced to produce pollen cones in response to gibberellin treatment. All apices remained vegetative during the first 17 days of treatment. At this time many lateral vegetative apices began to undergo a transition to the reproductive state. The transition was marked by changes in apical zonation characterized by increased mitotic activity primarily in the subapical mother-cell and peripheral zones. Also, precocious initiation of branch meristems occurred much higher on the shoot apex than before the transition period. About 22 days after the initial treatment, most apices became distinctly reproductive. The reproductive apex has a zonation pattern similar to the branching apex but is shorter and wider and quite distinct from the vegetative apex. The small subapical mother cells and cells of the peripheral zone form a continuous mantle of mitotically active cells with prominent nucleoli. This mantle encloses a very broad pith region which differentiates nearly to the summit of the apex. Microsporophyll and leaf initiation are similar and the protoderm of the apex remains discrete and does not contribute to deeper tissues. Sporangia do not originate from superficial cells of the microsporophyll. After all microsporophylls are initiated the reproductive apex becomes inactive. A discussion concerns the morphological implication in the origin of the foliar structures and of the similarity of C. arizonica to many angiosperms in the transition of the apex from vegetative to reproductive.     

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     

Photoperiod and Temperature Regulation of Floral Initiation and Anthesis in Soya Bean

Floral development includes initiation of floral primordia andsubsequent anthesis as discrete events, even though in manyinvestigations only anthesis is considered. For ‘Ransom’soya bean [Glycine max (L.) Merrill] grown at day/night temperaturesof 18/14, 22/18, 26/22, 30/26, and 34/30 °C and exposedto photoperiods of 10, 12, 14, 15, and 16 h, time of anthesisranged from less than 21 days after exposure at the shorterphotoperiods and warmer temperatures to more than 60 days atlonger photoperiods and cooler temperatures. For all temperatureregimes, however, floral primordia were initiated under shorterphotopenods within 3 to 5 days after exposure and after notmore than 7 to 10 days exposure to longer photoperiods. Onceinitiation had begun, time required for differentiation of individualfloral primordia and the duration of leaf initiation at shootapices increased with increasing length of photoperiod. Whileproduction of nodes ceased abruptly under photoperiods of 10and 12 h, new nodes continued to be formed concurrently withinitiation of axillary floral primordia under photoperiods of14, 15 and 16 h. The vegetative condition at the main stem shootapex was prolonged under the three longer photoperiods and issuggestive of the existence of an intermediate apex under theseconditions. The results indicate that initiation and anthesisare controlled independently rather than collectively by photoperiod,and that floral initiation consists of two independent steps—onefor the first-initiated flower in an axil of a main stem leafand a second for transformation of the terminal shoot apex fromthe vegetative to reproductive condition. Apical meristem, intermediate apex, floral initiation, anthesis, photoinduction, Glycine max(L.) Merrill, soya bean, photoperiod, temperature     

Responses of the rice shoot apex to irradiation with red and far-red light

Summary Determinations of cell-doubling times using the technique of colchicine-induced metaphase accumulation showed that after 40 h exposure to red or far-red light the rates of cell division in young rice (Oryza sativa L. cv. Ballila) shoot apices were faster than in dark controls. In red light, the increase was already taking place after 16 h of irradiation but in far-red the rates at this time were slower than in the dark controls. Seedlings became more responsive to far-red light as they continued to grow in darkness between 2 1/2 and 4 1/2 days. Mitotic activity at the apex increased in the leaf primordium and decreased in the sub-summit corpus between the 4th and 5th days of growth in darkness at 30° C.     

CELL DIVISION KINETICS IN THE SHOOT APICAL MERISTEM OF CERATOPTERIS THALICTROIDES BRONG. WITH SPECIAL REFERENCE TO THE APICAL CELL

The duration of mitosis and the cell cycle were determined for defined cell populations of the shoot apical meristem of Ceratopteris thalictroides Brong. by using the colchicine-induced metaphase accumulation technique. The results indicate that the apical cell is mitotically active and cycles at an apparently greater frequency than the cells of subjacent populations. Duration of mitosis was similar for all cells of the meristem. These results are correlated with mitotic indices of control apices, the geometry of the apex, and the mean number of cells in the meristem. Shoot apices from adult plants were examined to determine mitotic indices within the meristem; mitotic activity was again noted for the apical cell. These results contradict recent proposals that the pteridophyte apical cell serves as a unicellular quiescent center which lacks histogenic potential and offer experimental support for the classical concept of apical cell function in those fern shoot meristems which terminate in a single apical cell.     

The pattern of histone H4 expression in the tomato shoot apex changes during development

Two histone H4 cDNA clones were isolated from a tomato (Lycopersicon esculentum Mill.) shoot-tip cDNA library using a heterologous probe from barley (Hordeum vulgare L.). Both cDNAs, which are 81% identical in the coding region, are polyadenylated and belong to a small gene family in the tomato genome. Histone H4 message is abundant in young tissues and rare in older tissues. In the shoot apical meristem, the distribution of H4-expressing cells changes during development. In a juvenile vegetative apex, H4 message is detectable in the central region and the peripheral parts of the meristem. In a mature vegetative apical meristem, H4-expressing cells are localized in the peripheral zone extending into the provascular strands and the rib meristem whereas the central zone is almost devoid of H4 mRNA. After floral transition, H4 mRNA is found throughout the floral meristem, indicating a second change in the pattern of H4 expression. The observed changes in H4 expression are indicative of changes in the distribution of mitotic activity in the shoot apical meristem during plant development. In addition, H4-expressing cells were found to occur frequently in clusters, which may indicate a partial synchronization of cell divisions in the shoot apex.     

Cell division and morphological changes in the shoot apex of Arabidopsis thaliana during floral transition

Eight-week-old vegetative plants of Arabidopsis thaliana, ecotype Columbia, were induced to flower by a single long day (LD). In this experimental system, it is known that the last component of the floral stimulus moves from the leaves to the apex 24-36 h after the start of the LD, and the first floral meristem is initiated by the shoot apical meristem (SAM) at 44-56 h (Corbesier et al., 1996, The Plant Journal 9: 947-952). Here we show that the rate of cell division is increased at floral transition in all SAM parts but not in the sub-apical pith cells. Mitotic activity starts to increase 24 h after the start of the LD and is two- to three-fold higher at peak times than that in non-induced plants. This activation is followed by the start of SAM enlargement at 44 h, SAM doming at 48 h, and the elongation of apical internodes (bolting) at 52 h.     

INFLORESCENCE DEVELOPMENT IN BRASSICA CAMPESTRIS L.

Vegetative seedlings of the Ceres strain Brassica campestris L., a quantitative, long-day plant, were induced to flower by exposure to a 16-hr, long-day cycle. Cytohistological and cytohistochemical changes associated with inflorescence development were examined. Developing shoot apices were classified in vegetative, transitional, and reproductive stages. The vegetative apex possessed a biseriate tunica, central zone, peripheral zone and pith-rib meristem. The transitional stage at 48 hr was marked by an increase in size and by a stratification of the upper cell layers of the shoot apex with a concurrent decrease of apical cytohistochemical zonation. The reproductive stage was initiated at 58 hr by periclinal cell divisions in the 3rd and 4th cell layers of the flank region. Cytohistochemical zonation in the vegetative apical meristem was restored in the floral apex. An “intermediate developmental” phase was not observed between the vegetative and reproductive stage.     

Universal florigenic signals triggered by FT homologues regulate growth and flowering cycles in perennial day-neutral tomato

The transition from vegetative to floral meristems in higher plants is programmed by the coincidence of internal and environmental signals. Classic grafting experiments have shown that leaves, in response to changing photoperiods, emit systemic signals, dubbed 'florigen', which induce flowering at the shoot apex. The florigen paradigm was conceived in photoperiod-sensitive plants: nevertheless it implies that although activated by different stimuli in different flowering systems, the signal is common to all plants. Tomato is a day-neutral, perennial plant, with sympodial and modular organization of its shoots and thus with reiterative regular vegetative/reproductive transitions. SINGLE FLOWER TRUSS a regulator of flowering-time and shoot architecture encodes the tomato orthologue of FT, a major flowering integrator gene in Arabidopsis. SFT generates graft-transmissible signals which complement the morphogenetic defects in sft plants, substitute for light dose stimulus in tomato and for contrasting day-length requirements in Arabidopsis and MARYLAND MAMMOTH tobacco. It is discussed how systemic signals initiated by SFT interact with the SELF PRUNING gene to regulate vegetative to reproductive (V/R) transitions in the context of two flowering systems, one for primary apices and the other for sympodial shoots.     

Histochemical Study of Photoperiodic and Low Temperature Effects on Floral Induction of Spinacia oleracea

Shoot apices of Spinacia oleracea plants have been induced toflower either by: (a) subjecting leaves to 24 h long day, or(b) exposure to a short photoperiod but displaced by 8 h (displacedshort day) in the usual 24 h short-day cycle, or (c) exposureto low temperature (5 °C) during the dark period of thenormal short day. A quantitative cytochemical assay of pentosephosphate pathway activity during floral induction indicatesan approximate doubling of the rate of activity when comparedto that of vegetative apices (short day) (21 °C). Exposure to either low temperature, or a displaced short photoperiodstimulates pentose phosphate pathway activity in the shoot apexin a manner similar to that seen by long-day induction. Thischange in metabolic activity is accompanied by changes in theshape of the shoot apex which resembles that seen at an earlystage during floral induction. Spinacia oleracea, pentose phosphate pathway, shoot apex, glucose-6-phosphate dehydrogenase, floral induction, chilling, displaced short day     

Growth correlations and rna synthesis in different parts of the shoot apical meristem ofChenopodium rubrunt L. induced to flowering

Uridine-³H incorporation and RNA concentration were investigated in different parts of the shoot apical meristem ofChenopodium rubrum using autoradiography and cytophotometry. A single inductive cycle was sufficient to bring about postinductive first events in the shoot apex but not for complete flower differentiation. The initial activation of RNA synthesis manifested itself in all zones of the apex. The first increase was more conspicuous in the peripheral than in the central zone. The indications of the first events in the apices after a single inductive cycle disappear prior to morphological reversal to the vegetative state. Induction by three short days led to rapid flower differentiation. The increase in RNA synthesis and concentration was most conspicuous in the central zone in this case. The ratio of RNA synthesis and content between bud and leaf primordia (B/L) also change in relation to photoperiodic induction. In vegetative plants the B/L ratio was low while after induction it increased. The shifts in activity of RNA synthesis observed in the shoot apical meristem are related to the changes in growth activity of the different parts of the apex. The growth ratios in the apices bear the character of growth correlations. The change in the growth correlations following photoperiodic induction together with the total activation of RNA synthesis are considered to represent one of the first events of the transition to the reproductive state.     

Involvement of the MADS-box gene ZMM4 in floral induction and inflorescence development in maize

The switch from vegetative to reproductive growth is marked by the termination of vegetative development and the adoption of floral identity by the shoot apical meristem (SAM). This process is called the floral transition. To elucidate the molecular determinants involved in this process, we performed genome-wide RNA expression profiling on maize (Zea mays) shoot apices at vegetative and early reproductive stages using massively parallel signature sequencing technology. Profiling revealed significant up-regulation of two maize MADS-box (ZMM) genes, ZMM4 and ZMM15, after the floral transition. ZMM4 and ZMM15 map to duplicated regions on chromosomes 1 and 5 and are linked to neighboring MADS-box genes ZMM24 and ZMM31, respectively. This gene order is syntenic with the vernalization1 locus responsible for floral induction in winter wheat (Triticum monococcum) and similar loci in other cereals. Analyses of temporal and spatial expression patterns indicated that the duplicated pairs ZMM4-ZMM24 and ZMM15-ZMM31 are coordinately activated after the floral transition in early developing inflorescences. More detailed analyses revealed ZMM4 expression initiates in leaf primordia of vegetative shoot apices and later increases within elongating meristems acquiring inflorescence identity. Expression analysis in late flowering mutants positioned all four genes downstream of the floral activators indeterminate1 (id1) and delayed flowering1 (dlf1). Overexpression of ZMM4 leads to early flowering in transgenic maize and suppresses the late flowering phenotype of both the id1 and dlf1 mutations. Our results suggest ZMM4 may play roles in both floral induction and inflorescence development.     

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     

CHANGES IN MITOTIC ACTIVITY AND CELL SIZE IN THE APICAL MERISTEM OF HELIANTHUS ANNUUS L. DURING THE TRANSITION TO FLOWERING

Cellular changes in the shoot apical meristem of Helianthus annuus L. have been investigated in relation to its progress towards flowering. During the strictly vegetative phase, lasting for 6–7 days from sowing, mitotic divisions were confined to the peripheral zone, while the central mother cells zone proper, together with the distal cells positioned above the mother zone and also the central portion of the tunica, were relatively quiescent. Mitotic activity increased in the distal cells zone on day 8 and reached the level of that in the peripheral zone by day 12. This was accompanied by an enlargement of this zone and the consequent recession of the mother zone away from the central tunica. At the same time there was a substantial increase in the amount of cytoplasm in the cells of the central tunica. Mitotic activity in the central tunica began on day 12 and reached a peak on day 16. This zone then lost its distinct entity and was replaced by a uniform dome-shaped meristematic layer that became apparent by day 16. The cells of the mother zone remained quiescent during the transition period.