Carpel Primordia Research Articles

Flower development and anatomy of Clusia valerioi, a Central American species of Clusiaceae offering floral resin

The present paper is part of a study dealing with various aspects of reproduction of two Costa Rican Clusia species offering resin as a floral reward. It provides data on the floral development and flower (especially stamen and staminode) anatomy of one of the species, Clusia valerioi. In the early stages, both male and female flowers develop in the same manner. The bracts are distinguished by a decussate arrangement from the five sepals and five petals, which emerge in a spiral manner. In the male flowers the apical meristem forms five meristematic mounds (common stamen primordia) that are pentagonally arranged around the apical meristem in epipetalous position. From these mounds, the primordia of the proper stamina emerge in 3–5 whorls. Direction is centrifugal. In the centre, five hemispherical bulges arise which develop into carpel primordia. These, however, cease growth, stay rudimentary and are hidden by the stamens in the mature male flower. The adult stamens consist mainly of a thick angular filament column, while the two anthers situated at the flattened top are very small. One anther is annular and surrounds a second, hemispherical one right in the centre. At the periphery, these two pollen sacs (provided with a distinct wall of customary anatomy) are surrounded by a ring-like protuberance of the filament. The resin canals are situated at the periphery of the filament. Their schizogenous development is documented in cross sections. At anthesis, the resin is released from the ring-like filament protuberance by burst of the single-layered epidermis. In the female flower, the five meristematic mounds produce two whorls of staminode primordia. The development of the staminodes does not essentially differ from that of the fertile stamens, but some staminodes lack the central pollen sac and the other tissues do not develop into pollen grains. An attempt is made to derive the peculiar stamen morphology of Clusia valerioi and similar species from conventional stamens. Three hypotheses are proposed and discussed.

Comparative developmental series of the Mexican triurids support a euanthial interpretation for the unusual reproductive axes of Lacandonia schismatica (Triuridaceae)

The enigmatic monocotyledon family Triuridaceae is composed of inconspicuous mycoheterotrophs, that have been variously interpreted. We present here the first report of a thorough floral developmental series for any member of the Triuridaceae. The two known Mexican Triuridaceae species were studied with anatomical sections and scanning electron microscopy. While both species have ephemeral and reduced radially symmetric flowers arising in a counterclockwise spiral on racemose inflorescences, Lacandonia schismatica is hermaphroditic with a central androecium and Triuris brevistylis is dioecious. Tepals are connately fused at their bases, and during development the subapical caudal tips continue to elongate, while at maturity the tepals are reflexed. Carpel primordia develop centrifugally from compound primordia in both species, with contrasting androecium development. In Lacandonia schismatica stamen and carpel primordia arise from a common precursor. The two species differ in tepal and carpel number and timing of organ development. This paper provides a developmental framework to understand floral characters in the Triuridaceae. Notably, we addressed if L. schismatica and T. brevistylis bear true flowers or pseudanthia, and our data support the former. The role of particular genes in determining the floral developmental patterns studied and the evolutionary significance of these patterns are discussed.

Floral organogenesis of Handeliodendron bodinieri (Sapindaceae) and its systematic implications

Floral organogenesis and development of Handeliodendron bodinieri (Sapindaceae), a species endemic to China, were studied under scanning electron microscope and optical microscope. The results are as follows: the inflorescence primordium initiates at first, forming 2 unequal flower primordia. The sepal primordia initiate in spiral order and are not synchronous. Four to five petal primordia initiate in whorls simultaneously. The petal and stamen primordia initiate separately, and there is no petal-stamen complex. Seven to eight stamen primordia initiate nearly at the same time and grow more quickly than the petal primordia. At last, 3 carpel primordia appear simultaneously and close up gradually to form the ovary. The flowers of H. bodinieri are unisexual. In the female flower, the ovary bulges and the stamens degenerate, whereas the male flower, the stamens grow normally but the ovary degenerates. The systematic implications of floral organogenesis and development of H. bodinieri are discussed.

Characteristics of Dormancy of June-Bearing Strawberry(Fragaria Xananassa Duch. cv. Elsanta)

The development of Fragaria xananassa Duch. cv. Elsanta was analyzed during summer and fall in order to define the sequence of growth alteration and dormancy with regard to inflorescence initiation. The leaf growth as well as the initiation of the inflorescence buds were followed in the fall by conditions imposed in a climate chamber, with plants then being transfered to conditions favorable for growth. Results pointed to clear changes in leaf growth characteristics over the late summer-early fall (September-October) period while the leaf emergence rate remained constant up to mid-October when it stopped. The first sign of inflorescence initiation started in early October, and the differentiation of the terminal flower reached the stamen initiation stage within 2 weeks. From early November, the terminal flower had initiated the carpel primordia and no further differentiation was detected later on. The plant growth potential, expressed by the size, at full growth, of the leaf or inflorescence axis emerging from the terminal bud, decreased progressively from early September and reached a minimum between October and November. By mid-December, the growth capacity of the plant was restored to a situation similar to that described for early September.

The geneFLORAL ORGAN NUMBER1regulates floral meristem size in rice and encodes a leucine-rich repeat receptor kinase orthologous toArabidopsisCLAVATA1

The regulation of floral organ number is closely associated with floral meristem size. Mutations in the gene FLORAL ORGAN NUMBER1 (FON1) cause enlargement of the floral meristem in Oryza sativa (rice), resulting in an increase in the number of all floral organs. Ectopic floral organs develop in the whorl of each organ and/or in the additional whorls that form. Inner floral organs are more severely affected than outer floral organs. Many carpel primordia develop indeterminately, and undifferentiated meristematic tissues remain in the center in almost-mature flowers. Consistent with this result, OSH1, a molecular marker of meristematic indeterminate cells in rice, continues to be expressed in this region. Although floral meristems are strongly affected by the fon1-2 mutation, vegetative and inflorescence meristems are largely normal, even in this strong allele. We isolated the FON1 gene by positional cloning and found that it encodes a leucine-rich repeat receptor-like kinase most similar to CLAVATA1 (CLV1) in Arabidopsis thaliana. This suggests that a pathway similar to the CLV signaling system that regulates meristem maintenance in Arabidopsis is conserved in the grass family. Unlike CLV1, which is predominantly expressed in the L3 layer of the shoot meristem, FON1 is expressed throughout the whole floral meristem, suggesting that small modifications to the CLV signaling pathway may be required to maintain the floral meristem in rice. In addition, FON1 transcripts are detected in all meristems responsible for development of the aerial part of rice, suggesting that genes sharing functional redundancy with FON1 act in the vegetative and inflorescence meristems to mask the effects of the fon1 mutation.

Ectopic expression of OsYAB1causes extra stamens and carpels in rice

Members in the YABBY gene family of proteins are plant-specific transcription factors that play critical roles in determining organ polarity. We have isolated a cDNA clone from rice that encodes a YABBY protein. This protein, OsYAB1, is similar to Arabidopsis YAB2 (50.3%) and YAB5 (47.6%). It carries a zinc-finger motif and a YABBY domain, as do those in Arabidopsis . A fusion protein between OsYAB1 and GFP is located in the nucleus. RNA gel-blot analysis showed that the OsYAB1 gene is preferentially expressed in flowers. In-situ hybridization experiments also indicated that the transcript accumulated in the stamen and carpel primordia. Unlike the Arabidopsis YABBY genes, however, the OsYAB1 gene does not show polar expression pattern in the tissues of floral organs. Our transgenic plants that ectopically expressed OsYAB1 were normal during the vegetative growth period, but then showed abnormalities in their floral structures. Spikelets contained supernumerary stamens and carpels compared with those of the wild types. These results suggest that OsYAB1 plays a major role in meristem development and maintenance of stamens and carpels, rather than in determining polarity.

EgLFY, the Eucalyptus grandis homolog of the Arabidopsis gene LEAFY is expressed in reproductive and vegetative tissues

The EgLFY gene cloned from Eucalyptus grandis has sequence homology to the floral meristem identity gene LEAFY (LFY) from Arabidopsis and FLORICAULA (FLO) from Antirrhinum. EgLFY is preferentially expressed in the developing eucalypt floral organs in a pattern similar to that described previously for the Arabidopsis LFY. In situ hybridization experiments have shown that EgLFY is strongly expressed in the early floral meristem and then successively in the primordia of sepals, petals, stamens and carpels. It is also expressed in the leaf primordia of adult trees. The expression of the EgLFY coding region under control of the Arabidopsis LFY promoter could complement strong lfy mutations in transgenic Arabidopsis plants. These data suggest that EgLFY plays a similar role to LFY in flower development and that the basic mechanisms involved in flower initiation and development in Eucalyptus may be similar to those occurring in Arabidopsis.

The homeotic protein AGAMOUS controls microsporogenesis by regulation of SPOROCYTELESS.

The Arabidopsis homeotic gene AGAMOUS (AG) is necessary for the specification of reproductive organs (stamens and carpels) during the early steps of flower development. AG encodes a transcription factor of the MADS-box family that is expressed in stamen and carpel primordia. At later stages of development, AG is expressed in distinct regions of the reproductive organs. This suggests that AG might function during the maturation of stamens and carpels, as well as in their early development. However, the developmental processes that AG might control during organogenesis and the genes that are regulated by this factor are largely unknown. Here we show that microsporogenesis, the process leading to pollen formation, is induced by AG through activation of the SPOROCYTELESS gene (SPL, also known as NOZZLE,NZZ), a regulator of sporogenesis. Furthermore, we demonstrate that SPL can induce microsporogenesis in the absence of AG function, suggesting that AG controls a specific process during organogenesis by activating another regulator that performs a subset of its functions.

KNUCKLES (KNU) encodes a C2H2 zinc-finger protein that regulates development of basal pattern elements of the Arabidopsis gynoecium.

Flowers of the parthenocarpic knuckles mutant are conditionally male sterile and contain ectopic stamens and carpels that originate from placental tissue within developing gynoecia. The mutation was mapped to a 123 Kb interval on chromosome 5 using molecular markers. All aspects of the knuckles phenotype could be complemented by a genomic fragment from the region which contained the annotated MAC12.2 gene. A guanine to adenine transition within a predicted C2H2 zinc finger-encoding region of MAC12.2 causes the second of two critical zinc-binding cysteine residues to be replaced by a tyrosine. Transgenic plants in which translational fusions of the GUS reporter to KNUCKLES were driven by the presumptive KNUCKLES promoter indicate that the gene is expressed first in developing carpel primordia, and later in stamens and ovules of flower buds. In situ hybridization experiments showed a broader pattern of transcript localization, suggesting that post-transcriptional regulatory mechanisms may limit KNUCKLES protein accumulation and localization. Based on genetic evidence and the presence of a carboxy-terminal motif demonstrated by others to function as an active repression domain, we propose that KNUCKLES might function as a transcriptional repressor of cellular proliferation that regulates floral determinacy and relative size of basal pattern elements along the proximo-distal axis of the developing Arabidopsis gynoecium.

Studies in the Alismataceae. XII. Floral organogenesis in Damasonium alisma and Baldellia ranunculoides, and comparisons with Butomus umbellatus

Floral organogenesis of Damasonium alisma Mill. occurs at first in an alternating trimerous pattern typical of Alismataceae, with the formation of three sepals, then three bulges, corresponding to the CA (common perianth–androecium) primordia described in some other Alismataceae, alternating with the sepals. A petal is initiated on each bulge, and a pair of stamens is initiated either on or close to it. Three carpels are initiated in positions alternating with the petals and stamen pairs, and three further carpels then arise above and between the first three. At maturity and in fruit the carpels lie in a whorled arrangement. Floral development in Baldellia ranunculoides (L.) Parl. is identical up to the initiation of the six stamens. After this, six carpel primordia are formed alternating with the stamens, and further carpel primordia arise alternating with those previously formed. In Butomus, up to the initiation of the first six stamens, the general plan of development resembles that of the two Alismataceae. Three further whorls of organs arise in alternation: a whorl of three stamens arises over the stamen pairs followed by two whorls each of three carpel primordia. It is argued that the trimerous appearance of the whorl of sepals (or outer perianth in Butomus) arises de novo and represents a genuine expression of trimery. However, most of the subsequent features of development in these flowers can be seen as arising from phyllotactic mechanisms that cause new primordia to arise between and above pre-existing ones. Consequently the appearance of trimerous or hexamerous whorls above the first whorl of perianth does not represent a fundamental feature of development. The nature of variations in the positional relationships of inner perianth, stamen, and carpel primordia in various Alismataceae and Butomus strengthen the case that there is a significant developmental association between inner perianth members and associated pairs of stamens, which may be connected with the evolution of the flowers from pseudanthial structures.Key words: Baldellia, Butomus, Damasonium, Alismatidae, flower, organogenesis.

The YABBY gene DROOPING LEAF regulates carpel specification and midrib development in Oryza sativa.

In this article, we report that carpel specification in the Oryza sativa (rice) flower is regulated by the floral homeotic gene DROOPING LEAF (DL) that is distinct from the well-known ABC genes. Severe loss-of-function mutations of DL cause complete homeotic transformation of carpels into stamens. Molecular cloning reveals that DL is a member of the YABBY gene family and is closely related to the CRABS CLAW (CRC) gene of Arabidopsis thaliana. DL is expressed in the presumptive region (carpel anlagen), where carpel primordia would initiate, and in carpel primordia. These results suggest that carpel specification is regulated by DL in rice flower development. Whereas CRC plays only a partial role in carpel identity, DL may have been recruited to have the more essential function of specifying carpels during the evolution of rice. We also show that DL interacts antagonistically with class B genes and controls floral meristem determinacy. In addition, severe and weak dl alleles fail to form a midrib in the leaf. The phenotypic analysis of dl mutants, together with analyses of the spatial expression patterns and ectopic expression of DL, demonstrate that DL regulates midrib formation by promoting cell proliferation in the central region of the rice leaf.

Whorl-Specific Expression of the SUPERMAN Gene of Arabidopsis Is Mediated by cis Elements in the Transcribed Region

The SUPERMAN (SUP) gene of Arabidopsis is involved in controlling cell proliferation in stamen and carpel primordia and in ovules during flower development [1–5]. The SUP gene encodes a transcription factor with a C2H2-type zinc finger motif, a serine/proline-rich domain, a basic domain, and a leucine-zipper-like domain and is expressed in a very limited region in stamen primordia and in the developing ovary during flower development [3, 5]. The SUP gene is susceptible to methylation, resulting in epigenetic gene silencing [6–11]. To understand how the SUP gene is expressed spatially and temporally in its restricted domain, and why methylation of the transcribed region affects early-stage SUP expression, we have identified the SUP cis regulatory elements by characterizing SUP gene fusions. These studies show that the SUP gene has discrete upstream promoter elements required for expression in stamen primordia in early stages and in the ovary in later stages. The promoter activity for stamen primordia is modulated by several positive and negative elements located in the transcribed and translated regions. Several regulatory elements in the transcribed region correlate with the areas of the gene that are heavily methylated in epigenetic alleles; these data provide a possible explanation of how methylation of the transcribed region represses transcription.

Separation of genetic functions controlling organ identity in flowers.

Comparative studies on the ABC model of floral development have revealed extensive conservation of B and C class genes, but have failed to identify similar conservation for A class genes. Using a reverse genetic approach, we show that the previous inability to obtain Antirrhinum mutants corresponding to the A class gene AP2 of Arabidopsis reflects greater genetic redundancy in Antirrhinum . Antirrhinum has two genes corresponding to AP2, termed LIP1 and LIP2, both of which need to be inactivated to give a mutant phenotype. Analysis of interactions between LIP and class B/C genes shows that unlike AP2 in Arabidopsis, LIP genes are not required for repression of C in outer whorls of the flower. However, like AP2, LIP genes play a role in sepal, petal and ovule development, although some of their detailed effects are different, reflecting the diverse morphologies of Antirrhinum and Arabidopsis flowers. The dual functions for which AP2 is required in Arabidopsis are therefore separate in Antirrhinum, showing that the genetic basis of some aspects of organ identity have undergone major evolutionary change.

Separable roles of UFO during floral development revealed by conditional restoration of gene function.

The UNUSUAL FLORAL ORGANS (UFO) gene is required for several aspects of floral development in Arabidopsis including specification of organ identity in the second and third whorls and the proper pattern of primordium initiation in the inner three whorls. UFO is expressed in a dynamic pattern during the early phases of flower development. Here we dissect the role of UFO by ubiquitously expressing it in ufo loss-of-function flowers at different developmental stages and for various durations using an ethanol-inducible expression system. The previously known functions of UFO could be separated and related to its expression at specific stages of development. We show that a 24- to 48-hour period of UFO expression from floral stage 2, before any floral organs are visible, is sufficient to restore normal petal and stamen development. The earliest requirement for UFO is during stage 2, when the endogenous UFO gene is transiently expressed in the centre of the wild-type flower and is required to specify the initiation patterns of petal, stamen and carpel primordia. Petal and stamen identity is determined during stages 2 or 3, when UFO is normally expressed in the presumptive second and third whorl. Although endogenous UFO expression is absent from the stamen whorl from stage 4 onwards, stamen identity can be restored by UFO activation up to stage 6. We also observed floral phenotypes not observed in loss-of-function or constitutive gain-of-function backgrounds, revealing additional roles of UFO in outgrowth of petal primordia.

Aspects of inflorescence and floral development in the putative basal angiospermAmborella trichopoda(Amborellaceae)

Amborella has woody axes whose development is intrinsically plagiotropic and determinate. The tree habit is achieved through overtopping of older axes by basally produced younger axes, as in Mangenot's model. Inflorescence units, which are produced in the axils of distal leaves, may be described as extended cymes, each branch ending in a flower. Basal bracteoles have a decussate arrangement, which is modified to an alternate phyllotaxis distally. Flowers produce one or more additional bracteoles with a rapid transition to the spiral phyllotaxis of the broad overlapping tepals. In this transition the initially conical floral apex becomes invaginated to form a floral cup, with subsequent appendages appearing on its inner margin. The floral apex then forms the flat base of the cup but retains a discrete single tunica layer. The receptacular bowl is deepened and narrowed by the basal meristem of each appendage, the last formed floral organs usually consuming the floral meristem. Sexual parts are more numerous in male than female flowers, accounting for their size difference, but primordia of stamens and carpels are initially very similar. Floral symmetry is largely a consequence of close packing of appendages within the floral cup. In its initial stages of development the flower does not conform to any conventional floral model in angiosperms and is better regarded as highly specialized rather than ancestral in its synorganization. This is not unexpected in a lineage of such long independent evolution.Key words: Amborella, basal angiosperm, development, inflorescence, primitive flower.

Developmental study on the inflorescence and flower of Caldesia grandis Samuel (Alismataceae)

The inflorescence and floral development of Caldesia grandis Samuel is reported for the first time in this paper. The basic units of the large cymo-thyrsus inflorescence are short panicles that are arranged in a pseudowhorl. Each panicle gives rise spirally to three bract primordia also arranged in a pseudowhorl. The branch primordia arise at the axils of the bracts. Each panicle produces spirally three bract primordia with triradiate symmetry (or in a pseudowhorl) and three floral primordia in the axils of the bract primordia. The apex of the panicle becomes a terminal floral primordium after the initiations of lateral bract primordia and floral primordia. Three sepal primordia are initiated approximately in a single whorl from the floral primordium. Three petal primordia are initiated alternate to the sepal primordia, but their subsequent development is much delayed. The first six stamen primordia are initiated as three pairs in a single whorl and each pair appears to be antipetalous as in other genera of the Alismataceae. The stamen primordia of the second whorl are initiated trimerously and opposite to the petals. Usually, 9-12 stamens are initiated in a flower. There is successive transition between the initiation of stamen and carpel primordia. The six first-initiated carpel primordia rise simultaneously in a whorl and alternate with the trimerous stamens, but the succeeding ones are initiated in irregular spirals, and there are 15-21 carpels developed in a flower. Petals begin to enlarge and expand when anthers of stamens have differentiated microsporangia. Such features do not occur in C. parnassifolia. In the latter, six stamen primordia are initiated in two whorls of three, carpel primordia are initiated in 1-3 whorls, and there is no delay in the development of petals. C. grandis is thus considered more primitive and C. parnassifolia more derived. C. grandis shares more similarities in features of floral development with Alsma, Echinodorus, Luronium and Sagittaria. (C) 2002 The Linnean Society of London.

Floral transition in lisianthus ( Eustoma grandiflorum)

Floral transition was studied in lisianthus [ Eustoma grandiflorum (Raf.) Shinn.] plants grown in a greenhouse at two planting times (winter and summer) under two photoperiod regimes: long day (LD) and short day (SD), 16 h/8 h and 8 h/16 h (light/dark), respectively. Morphological analyses of the flower meristem of the lisianthus cultivar ‘Heidi Deep Blue’ revealed three distinct stages of development: (1) sepal primordia, (2) petal and stamen primordia (almost simultaneously), and (3) carpel primordia. The appearance of sepal primordia was taken to mark the floral transition stage. Floral transition was hastened in summer-grown plants and under LD treatment, as expressed by a reduction in both the number of nodes and the time from planting. Stem length from base of plant to floral meristem, as measured at floral transition, was similar in all treatments, regardless of the number of nodes produced. At both planting dates, SD induced a higher number of flowers per inflorescence than LD. This report thus describes the direct effect of environmental factors on floral transition in lisianthus.

Open Carpels with Ovules in Fabaceae

Young open carpels with developing ovules exposed to view are a regularly observed stage in development in 25 caesalpinioid species and one papilionoid legume studied. In all, the carpel margins later become appressed and sealed. A literature survey revealed examples of open, ovulate carpel primordia in 42 species in 19 additional angiosperm families. The feature of angiospermy, or enclosed ovules, is achieved gradually during ontogeny in the taxa studied, rather than being present ab initio. This heterochronic lag of carpel closure may be viewed as either precocious timing of ovule initiation or as delayed timing of carpel closure, compared to other legumes with closed carpels at ovule initiation.

Sexual development and sex chromosomes in hop.

The stages of floral development in staminate and pistillate plants of hop (Humulus lupulus) were defined using scanning electron microscopy and light microscopy. Vegetative meristems of male and female plants are morphologically indistinguishable. On transition to the reproductive phase, inflorescence apices reduce greatly in size and striking developmental sex differences become apparent. The first sex-specific differences occur extremely early in floral ontogeny. Both male and female plants initiate inflorescence meristems at each leaf node, each meristem being enclosed within a bract. Male secondary inflorescence meristems give rise to clusters of asynchronously developing flowers. Female inflorescence meristems produce flowers arranged in 'cones'. Each male floral meristem initiates a whorl of five sepal primordia, followed by an inner whorl of five stamen primordia. There is no sign of carpel development at any stage. In females, two carpel primordia are initiated, surrounded at their base by a vestigial perianth whorl. No stamen development is observed. Several monoecious lines carry bisexual flowers, either within cymose panicles or within the basal bracts of terminal female inflorescences. Bisexual flowers usually possess perianth, stamen and carpel whorls. The central whorls are often highly variable, and range from a pair of stigmas fused to a thin central filament to a well developed gynoecium. Chimaeric central whorls consisting of fused staminoid-carpelloid structures also occur. Sex differences in unisexual hop flowers are determined at an extremely early stage in ontogeny. The inappropriate set of sex organs is suppressed before it becomes visible or, more probably, it is not initiated at all. Genes directing the development of sex are likely to act at an extremely early stage, well in advance of floral organogenesis. The sex chromosomes of dioecious hop plants are described, as well as the chromosome constitutions of monoecious plants and those carrying bisexual flowers.

A DEFICIENS homolog from the dioecious tree black cottonwood is expressed in female and male floral meristems of the two-whorled, unisexual flowers.

We isolated PTD, a member of the DEFICIENS (DEF) family of MADS box transcription factors, from the dioecious tree, black cottonwood (Populus trichocarpa). In females, in situ hybridization experiments showed that PTD mRNA was first detectable in cells on the flanks of the inflorescence meristem, before differentiation of individual flowers was visually detectable. In males, the onset of PTD expression was delayed until after individual flower differentiation had begun and floral meristems were developing. Although PTD was initially expressed throughout the inner whorl meristem in female and male flowers, its spatial expression pattern became sex-specific as reproductive primordia began to form. PTD expression was maintained in stamen primordia, but excluded from carpel primordia, as well as vegetative tissues. Although PTD is phylogenetically most closely related to the largely uncharacterized TM6 subfamily of the DEF/APETELA3(AP3)/TM6 group, its spatio-temporal expression patterns are more similar to that of DEF and AP3 than to other members of the TM6 subfamily.