Influence of leaves and roots on meristem development inNicotiana tabacum L. cv. Wisconsin 38

The terminal, apical shoot meristem ofN. tabacum cv. Wisconsin 38 normally differentiates into a flower after producing 30 to 40 nodes. The influence of leaves and roots on the regulation of flowering was evaluated by counting the number of nodes produced after removal of leaves or the induction of adventitious roots. Leaf removal has no effect on the number of nodes produced before flower formation. Root induction significantly increases the number of nodes produced before flower formation. The plant behaves as if it were measuring the number of nodes between the meristem and the roots as a means of regulating meristem conversion from vegetative to floral differentiation.     

Mechanisms and function of flower and inflorescence reversion

Flower and inflorescence reversion involve a switch from floral development back to vegetative development, thus rendering flowering a phase in an ongoing growth pattern rather than a terminal act of the meristem. Although it can be considered an unusual event, reversion raises questions about the nature and function of flowering. It is linked to environmental conditions and is most often a response to conditions opposite to those that induce flowering. Research on molecular genetic mechanisms underlying plant development over the last 15 years has pinpointed some of the key genes involved in the transition to flowering and flower development. Such investigations have also uncovered mutations which reduce floral maintenance or alter the balance between vegetative and floral features of the plant. How this information contributes to an understanding of floral reversion is assessed here. One issue that arises is whether floral commitment (defined as the ability to continue flowering when inductive conditions no longer exist) is a developmental switch affecting the whole plant or is a mechanism which assigns autonomy to individual meristems. A related question is whether floral or vegetative development is the underlying default pathway of the plant. This review begins by considering how studies of flowering in Arabidopsis thaliana have aided understanding of mechanisms of floral maintenance. Arabidopsis has not been found to revert to leaf production in any of the conditions or genetic backgrounds analysed to date. A clear-cut reversion to leaf production has, however, been described in Impatiens balsamina. It is proposed that a single gene controls whether Impatiens reverts or can maintain flowering when inductive conditions are removed, and it is inferred that this gene functions to control the synthesis or transport of a leaf-generated signal. But it is also argued that the susceptibility of Impatiens to reversion is a consequence of the meristem-based mechanisms controlling development of the flower in this species. Thus, in Impatiens, a leaf-derived signal is critical for completion of flowering and can be considered to be the basis of a plant-wide floral commitment that is achieved without accompanying meristem autonomy. The evidence, derived from in vitro and other studies, that similar mechanisms operate in other species is assessed. It is concluded that most species (including Arabidopsis) are less prone to reversion because signals from the leaf are less ephemeral, and the pathways driving flower development have a high level of redundancy that generates meristem autonomy even when leaf-derived signals are weak. This gives stability to the flowering process, even where its initiation is dependent on environmental cues. On this interpretation, Impatiens reversion appears as an anomaly resulting from an unusual combination of leaf signalling and meristem regulation. Nevertheless, it is shown that the ability to revert can serve a function in the life history strategy (perenniality) or reproductive habit (pseudovivipary) of many plants. In these instances reversion has been assimilated into regular plant development and plays a crucial role there.     

Reversion of flowering

Reversion from floral to vegetative growth is under environmental control in many plant species. However the factors regulating floral reversion, and the events at the shoot apex that take place when it occurs, have received less attention than those associated with the transition to flowering. Reversions may be categorized as flower reversion, in which the flower meristem resumes leaf production, or inflorescence reversions, in which the meristem ceases to initiate bracts with flowers in their axils and begins instead to make leaves with vegetative branches in their axils. Related to these two types of reversion, but distinct from them, are examples of partial flowering, when non-floral meristems grow out so that the plant begins to grow vegetatively again. Anomalous or proliferous flowers may form as a result of unfavourable growth conditions or viral infection, but these do not necessarily involve flower reversions.     

Bi-directional inflorescence development inArabidopsis thaliana: Acropetal initiation of flowers and basipetal initiation of paraclades

In this study we investigated Arabidopsis thaliana (L.) Heynh. inflorescence development by characterizing morphological changes at the shoot apex during the transition to flowering. Sixteen-hour photoperiods were used to synchronously induce flowering in vegetative plants grown for 30 d in non-inductive 8-h photoperiods. During the first inductive cycle, the shoot apical meristem ceased producing leaf primordia and began to produce flower primordia. The differentiation of paraclades (axillary flowering shoots), however, did not occur until after the initiation of multiple flower primordia from the shoot apical meristem. Paraclades were produced by the basipetal activation of buds from the axils of leaf primordia which had been initiated prior to photoperiodic induction. Concurrent with the activation of paraclades was the partial suppression of paraclade-associated leaf primordia, which became bract leaves. The suppression of bract-leaf primordia and the abrupt initiation of flower primordia during the first inductive photoperiod is indicative of a single phase change during the transition to flowering in photoperiodically induced Arabidopsis. Morphogenetic changes characteristic of the transition to flowering in plants grown continuously in 16-h photoperiods were qualitatively equivalent to the changes observed in plants which were photoperiodically induced after 30 d. These results suggest that Arabidopsis has only two phases of development, a vegetative phase and a reproductive phase; and that the production of flower primordia, the differentiation of paraclades from the axils of pre-existing leaf primordia and the elongation of internodes all occur during the reproductive phase.     

CnFL,a FLORICAULA/LEAFY homolog in Chrysanthemum nankingense is dramatically upregulated in induced shoot apical meristems

A FLORICOULA/LEAFY (FLO/LFY) homolog CnFL gene was isolated from the flower bud of a short-day Chrysanthemum nankingense plant during the flowering induction period. The sequence of CnFL contained a 1236 bp open reading frame that encoded a putative protein of 412 amino acids, which shared 68.67% homology with FLO and 60.23% homology with LFY. The spatial expression patterns of CnFL were analyzed using quantitative real-time PCR in different tissues and the apical meristem during short-day flowering induction. The results indicated that CnFL was highly expressed in the flower buds while its expression was also detected in the stems, young leaves, and vegetative apical meristem. During the period of flowering induction, CnFL expression increased remarkably and reached its highest levels after 15 days of induction. The expression of CnFL in the apical shoot after short-day flowering induction indicates that CnFL regulation is controlled primarily by photoperiodicity.     

High frequency early in vitro flowering of <Emphasis Type="Italic">Dendrobium</Emphasis> Madame Thong-In (Orchidaceae)

We have successfully developed a method to induce early in vitro flowering of the self-pollinated seedlings of a tropical orchid hybrid, Dendrobium Madame Thong-In. Transition of vegetative shoot apical meristem to inflorescence meristem was observed when young protocorms were cultured in modified KC liquid medium. In contrast, protocorms cultured on Gelrite-solidified medium only produced axillary shoots and roots. CW was required to trigger the transitional shoot apical meristem and BA enhanced inflorescence stalk initiation and flower bud formation. However, normal flower development was deformed in liquid medium but developed fully upon transferring to two-layered (liquid over Gelrite-solidified) medium. Under optimal condition, in vitro flowering was observed about 5 months after seed sowing. Segregation of flower colours was observed in these seedlings and seedpods formed upon artificial pollination of the in vitro flowers.     

Developmental physiology of floral initiation in Nicotiana tabacum L.

The central process in the making of a multicellular organismis the fating of cells and tissues for their terminal phenotypes.The formation of a flower from a shoot apical meristem completesa sequence of fating processes initiated in embryogenesis. Thefating of a vegetative meristem of Nicotiana tabacum L. to initiatea flower involves at least two signals and two developmentalstates. A signal from the roots maintains vegetative growth,or prevents flowering, in the young seedling. As the plant grows,the vegetative meristem gains greater competence to respondto the floral stimulus from the leaves until it is evoked, byfloral stimulus, into a florally determined state. The florallydetermined state is then expressed. These developmental processesnot only establish the time of floral initiation, but also regulateplant size as measured by the number of nodes produced. Key words: Plant size, floral stimulus, competence, floral determination, induction     

Reversion of flowering in Glycine Max (Fabaceae)

Photoperiodic changes, if occurring before a commitment to flowering is established, can alter the morphological pattern of plant development. In this study, Glycine max (L.) Merrill cv. Ransom plants were initially grown under an inductive short-day (SD) photoperiod to promote flower evocation and then transferred to a long-day (LD) photoperiod to delay flower development by reestablishing vegetative growth (SD-LD plants). Some plants were transferred back to SD after 4-LD exposures to repromote flowering (SD-LD-SD plants). Alterations in organ initiation patterns, from floral to vegetative and back to floral, are characteristic of a reversion phenomenon. Morphological features that occurred at the shoot apical meristem in SD, LD, SD-LD, and SD-LD-SD plants were observed using scanning electron microscopy (SEM). Reverted plants initiated floral bracts and resumed initiation of trifoliolate leaves in the two-fifths floral phyllotaxy prior to terminal inflorescence development. When these plants matured, leaf-bract intermediates were positioned on the main stem instead of trifoliolate leaves. Plants transferred back to a SD photoperiod flowered earlier than those left in LD conditions. Results indicated that in plants transferred between SDs and LDs, photoperiod can influence organ initiation in florally evoked, but not committed, G. max plants.     

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.     

植物花发育的分子机理研究进展

花的发育分为开花决定、花的发端和花器官的发育三个阶段。植物开花由多条途径诱导,包括光周期和光质诱导、春化作用、自主途径、赤霉素诱导、碳水化合物诱导等;植物体本身也存在着开花抑制途径。各种开花诱导途径能激活花分生组织特性基因,使茎端分生组织转变为花分生组织。花器官的发育由器官特性基因决定,这些基因的精确表达需要花分生组织特性基因的激活和多个正、负调节因子的调控;另有一类基因控制着花发育的对称性。花发育机理的研究具有重要的理论意义和广泛的应用前景。     

植物花发育的分子机理研究进展

花的发育分为开花决定、花的发端和花器官的发育三个阶段。植物开花由多条途径诱导,包括光周期和光质诱导、春化作用、自主途径、赤霉素诱导、碳水化合物诱导等;植物体本身也存在着开花抑制途径。各种开花诱导途径能激活花分生组织特性基因,使茎端分生组织转变为花分生组织。花器官的发育由器官特性基因决定,这些基因的精确表达需要花分生组织特性基因的激活和多个正、负调节因子的调控;另有一类基因控制着花发育的对称性。花发育机理的研究具有重要的理论意义和广泛的应用前景。     

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.     

Apical Growth and Modification of the Development of Primordia During Re-flowering of Reverted Plants of Impatiens balsamina L

Impatiens balsamina L. was induced to flower by exposure to5 short days and then made to revert to vegetative growth byreturn to long days. After 9 long days reverted plants wereinduced to re-flower by returning them to short days. Petalinitiation began immediately and seven primordia already presentdeveloped into petals instead of into predominantly leaf-likeorgans. However, the arrangement of primordia at the shoot apex,their rate of initiation and size at initiation remained unchangedfrom the reverted apex, as did apical growth rate and the lengthof stem frusta at initiation. The more rapid flowering of thereverted plants than of plants when first induced, and the lackof change in apical growth pattern, imply that the revertedapices remain partially evoked, and that the apical growth patternand phyllotaxis typical of the flower, and already present inthe reverted plants, facilitate the transition to flower formation. Impatiens balsamina, flower reversion, partial evocation, shoot meristem, determination, leaf development     

Changes in Apical Growth and Phyllotaxis on Flowering and Reversion in Impatiens balsamina L.

When plants of Impatiens balsamina L were subjected to 5 shortdays and then re-placed in long days, they began to form a terminalflower and then reverted to vegetative growth at this terminalshoot apex The onset of flowering was accompanied by an increasein the rate of initiation of primordia, an increase in the growthrate of the apex, a change in primordium arrangement from spiralto whorled or pseudo-whorled, a lack of internodes, and a reductionm the size at initiation of the primordia and also of the stemfrusta which give rise to nodal and internodal tissues On reversion,parts intermediate between petals and leaves were formed, followedby leaves, although in reverted apices the size at initiationand the arrangement of primordia remained the same as in thefloweing apex The apical growth rate and the rate of primordiuminitiation were less in the reverted apices than in floral apicesbut remained higher than in the original vegetative apex Sincethe changes in apical growth which occur on the transition toflowering are not reversed on reversion, the development oforgans as leaves or petals is not directly related to the growthrate of the apex, or the arrangement, rate of initiation orsize at initiation of primordia Impatiens balsamina L, flower reversion, evocation, phyllotaxis, shoot meristem     

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.     

Floral development in bolting garlic

Garlic (Allium sativum L.) is a completely sterile plant, propagated only vegetatively. The aim of this research was to study the sequence of morphological processes occurring during floral initiation and development of a number of bolting garlic accessions from the Allium gene bank in Israel by using SEM. The garlic inflorescence is an umbel-like flower arrangement, the branches (flower clusters) of which arise from a common meristem. The numerous flowers have a distinct morphology typical of the genus Allium. Flower-stalk elongation precedes the swelling of the apical meristem and its subdivision into several centers of floral development. Within clusters, floral primordia develop unevenly. Differentiation of topsets begins after floral differentiation on the peripheral part of the apical surface, and their size, number and rate of development vary among genotypes. At least four morphological types differing in flower/topset ratio were distinguished among the 12 clones studied in this investigation. For further studies of flowering physiology and fertility restoration, only clones which can differentiate the greatest proportion of normal flowers and the least of topsets in the apical meristem should be selected. Received: 28 June 2000 / Revision accepted: 6 November 2000     

Aging and flowering of the apex in young Bidens radiata

The response of Bidens radiata Thuill., a short-day plant, to conditions for flower induction changes during the first 96 h after germination: the earlier the induction, the better the flowering response. Flowering is observed without previous vegetative development. An histological study shows that failure to flower is correlated with the development of the primordia of the second node leaves. The antagonism between vegetative and floral development is discussed.     

A TERMINAL FLOWER1-like gene from perennial ryegrass involved in floral transition and axillary meristem identity

Control of flowering and the regulation of plant architecture have been thoroughly investigated in a number of well-studied dicot plants such as Arabidopsis, Antirrhinum, and tobacco. However, in many important monocot seed crops, molecular information on plant reproduction is still limited. To investigate the regulation of meristem identity and the control of floral transition in perennial ryegrass (Lolium perenne) we isolated a ryegrass TERMINAL FLOWER1-like gene, LpTFL1, and characterized it for its function in ryegrass flower development. Perennial ryegrass requires a cold treatment of at least 12 weeks to induce flowering. During this period a decrease in LpTFL1 message was detected in the ryegrass apex. However, upon subsequent induction with elevated temperatures and long-day photoperiods, LpTFL1 message levels increased and reached a maximum when the ryegrass apex has formed visible spikelets. Arabidopsis plants overexpressing LpTFL1 were significantly delayed in flowering and exhibited dramatic changes in architecture such as extensive lateral branching, increased growth of all vegetative organs, and a highly increased trichome production. Furthermore, overexpression of LpTFL1 was able to complement the phenotype of the severe tfl1-14 mutant of Arabidopsis. Analysis of the LpTFL1 promoter fused to the UidA gene in Arabidopsis revealed that the promoter is active in axillary meristems, but not the apical meristem. Therefore, we suggest that LpTFL1 is a repressor of flowering and a controller of axillary meristem identity in ryegrass.