Gynoecium Development Research Articles

Inception of maleness: auxin contribution to flower masculinization in the dioecious cactus Opuntia stenopetala

In Opuntia stenopetala, flowers initiate as hermaphrodite; however, at maturity, only the stamens in male flowers and the gynoecium in female flowers become functional. At early developmental stages, growth and morphogenesis of the gynoecium in male flowers cease, forming a short style lacking stigmatic tissue at maturity. Here, an analysis of the masculinization process of this species and its relationship with auxin metabolism during gynoecium morphogenesis is presented. Histological analysis and scanning electron microscopy were performed; auxin levels were immunoanalyzed and exogenous auxin was applied to developing gynoecia. Male flower style-tissue patterning revealed morphological defects in the vascular bundles, stylar canal, and transmitting tissue. These features are similar to those observed in Arabidopsis thaliana mutant plants affected in auxin transport, metabolism, or signaling. Notably, when comparing auxin levels between male and female gynoecia from O. stenopetala at an early developmental stage, we found that they were particularly low in the male gynoecium. Consequently, exogenous auxin application on male gynoecia partially restored the defects of gynoecium development. We therefore hypothesize that, the arrest in male flower gynoecia patterning could be related to altered auxin homeostasis; alternatively, the addition of auxin could compensate for the lack of another unknown factor affecting male flower gynoecium development.

Isolation and characterisation of an HpSHP gene from Hosta plantaginea

Based on genetic and molecular analyses, the ABC model has been proposed to explain the genetic control of floral development. C-class MADS-box genes play crucial roles in Arabidopsis thaliana development by regulating the organ identities of stamens and gynoecium. The present research reports for the first time the cloning of an HpSHP gene from Hosta plantaginea (Lam.) Aschers. Phylogenetic analysis shows that HpSHP is a member of the C-class MADS-box genes that is closely related to C-lineage SHP homologues from monocot species. Semi-quantitative and real-time polymerase chain reaction analyses show that HpSHP expression is stamen and gynoecium specific. HpSHP also has spatial and temporal expression patterns in the reproductive organs of H. plantaginea. A functional analysis is carried out in Arabidopsis by overexpression of HpSHP. Homeotic transformations of sepals into carpelloid organs, bent ovaries, and prematurely shattering fruits are observed in 35S::HpSHP transgenic plants. All these results show that HpSHP plays a crucial role in gynoecium development.

INDEHISCENT and SPATULA Interact to Specify Carpel and Valve Margin Tissue and Thus Promote Seed Dispersal inArabidopsis

Structural organization of organs in multicellular organisms occurs through intricate patterning mechanisms that often involve complex interactions between transcription factors in regulatory networks. For example, INDEHISCENT (IND), a basic helix-loop-helix (bHLH) transcription factor, specifies formation of the narrow stripes of valve margin tissue, where Arabidopsis thaliana fruits open on maturity. Another bHLH transcription factor, SPATULA (SPT), is required for reproductive tissue development from carpel margins in the Arabidopsis gynoecium before fertilization. Previous studies have therefore assigned the function of SPT to early gynoecium stages and IND to later fruit stages of reproductive development. Here we report that these two transcription factors interact genetically and via protein-protein contact to mediate both gynoecium development and fruit opening. We show that IND directly and positively regulates the expression of SPT, and that spt mutants have partial defects in valve margin formation. Careful analysis of ind mutant gynoecia revealed slight defects in apical tissue formation, and combining mutations in IND and SPT dramatically enhanced both single-mutant phenotypes. Our data show that SPT and IND at least partially mediate their joint functions in gynoecium and fruit development by controlling auxin distribution and suggest that this occurs through cooperative binding to regulatory sequences in downstream target genes.

SPATULA and ALCATRAZ, are partially redundant, functionally diverging bHLH genes required for Arabidopsis gynoecium and fruit development

The Arabidopsis gynoecium is a complex organ that facilitates fertilization, later developing into a dehiscent silique that protects seeds until their dispersal. Identifying genes important for development is often hampered by functional redundancy. We report unequal redundancy between two closely related genes, SPATULA (SPT) and ALCATRAZ (ALC), revealing previously unknown developmental roles for each. SPT is known to support septum, style and stigma development in the flower, whereas ALC is involved in dehiscence zone development in the fruit. ALC diverged from a SPT-like ancestor following gene duplication coinciding with the At-β polyploidy event. Here we show that ALC is also involved in early gynoecium development, and SPT in later valve margin generation in the silique. Evidence includes the increased severity of early gynoecium disruption, and of later valve margin defects, in spt-alc double mutants. In addition, a repressive version of SPT (35S:SPT-SRDX) disrupts both structures. Consistent with redundancy, ALC and SPT expression patterns overlap in these tissues, and the ALC promoter carries two atypical E-box elements identical to one in SPT required for valve margin expression. Further, SPT can heterodimerize with ALC, and 35S:SPT can fully complement dehiscence defects in alc mutants, although 35S:ALC can only partly complement spt gynoecium disruptions, perhaps associated with its sequence simplification. Interactions with FRUITFULL and SHATTERPROOF genes differ somewhat between SPT and ALC, reflecting their different specializations. These two genes are apparently undergoing subfunctionalization, with SPT essential for earlier carpel margin tissues, and ALC specializing in later dehiscence zone development.

CLAVATA Signaling Pathway Receptors of Arabidopsis Regulate Cell Proliferation in Fruit Organ Formation as well as in Meristems

The CLAVATA1 (CLV1), CLV2, and CORYNE (CRN) receptors in Arabidopsis thaliana maintain cell proliferation in shoot apical meristems by restricting expression of the transcription factor WUSCHEL (WUS). Previously characterized receptor mutants generate extra fruit and floral organs that are proposed to arise from enlarged floral meristems (FMs). We identified new alleles in clv1, clv2, and crn and found that most mutants produce only extra fruit organs and generate FMs of similar dimensions as wild type. Characterization of gynoecium development in receptor mutants revealed increased cell proliferation and ectopic fruit organ initiation after FM termination. These regions of increased cell division also display expanded expression of the cell proliferation-promoting transcription factor SHOOTMERISTEMLESS (STM), similar to the expansion of WUS expression in the shoot apical meristems of strong clv1 mutants. We also examined genetic interactions between the ERECTA (ER) and BARELY ANY MERISTEM 1 (BAM1) receptor-like kinases and CLV pathway receptors. Our results suggest a model in which CLV1/BAM1 and CLV2/CRN complexes act in separate, parallel pathways in shoot meristems, while the CLV1, CLV2, and CRN receptors function together in a linear pathway during fruit development. These results demonstrate the importance of regulating cell proliferation in plants that undergo organogenesis throughout their life cycle.

Ontogeny of floral organs in flax (Linum usitatissimum; Linaceae)

Flax (Linum usitatissimum) is an important crop worldwide; however, a detailed study on flower development of this species is lacking. Here we describe the pattern of initiation and a program of key developmental events in flax flower ontogeny. This study provides important fundamental information for future research in various aspects of flax biology and biotechnology. Floral buds and organs were measured throughout development and examined using scanning electron microscopy. Floral organs were initiated in the following sequence: sepals, stamens and petals, gynoecium, and nectaries. The five sepals originated in a helical pattern, followed evidently by simultaneous initiation of five stamens and five petals, the former opposite of the sepals and the latter alternate to them. The gynoecium, with five carpels, was produced from the remaining, central region of the floral apex. Stamens at early stages were dominated by anther growth but filaments elongated rapidly shortly before anthesis. Early gynoecium development occurred predominantly in the ovary, and ovule initiation began prior to enclosure of carpels. A characteristic feature was the twisted growth of styles, accompanied by the differentiation of papillate stigmas. Petal growth lagged behind that of other floral organs, but petals eventually grew rapidly to enclose the inner whorls after style elongation. Flask-shaped nectaries bearing stomata developed on the external surface of the filament bases. This is the first detailed study on flax floral organ development and has established a key of 12 developmental stages, which should be useful to flax researchers.

Isolation and characterization of an AGAMOUS-like gene from Hosta plantaginea

Based on genetic and molecular analyses, the ABC model was proposed to explain the genetic control of floral development. The C-class MADS box gene AGAMOUS (AG) plays crucial roles in Arabidopsis thaliana development through regulation of the organ identity of stamens and gynoecium. The present research reports for the first time the cloning of an AG homologue (HpAG) from Hosta plantaginea Aschers. Phylogenetic analysis shows HpAG is a homologue of AG that is closely related to C-lineage AG homologues from monocot species. Semi-quantitative PCR and real-time PCR analyses show that HpAG is stamen- and gynoecium-specific in expression and has spatial and temporal expression patterns in the reproductive organs of H. plantaginea. The transcriptional activation property of HpAG is also verified by a yeast one-hybrid. Functional analysis is carried out in Arabidopsis by overexpression of HpAG. The homeotic transformations of petals into staminoid organs are observed in 35S::HpAG transgenic plants. All these results show that HpAG1 plays a crucial role in stamen specification and gynoecium development.

Morphology and development of spikelets and flowers in Cyperus and Pycreus (Cyperaceae)

Background : Pycreus, Kyllinga, and Queenslandiella cluster together with Cyperus within the Cyperus s. lat. clade, one of the two large clades in Cypereae. However, in contrast with Cyperus, they have laterally flattened pistils/nutlets. Pycreus, Kyllinga and Queenslandiella form morphologically well circumscribed independent genera. In the context of a broader systematic project to work out a well supported, evolution based taxonomy for Cyperus s. lat., we present in this paper general morphological and developmental data of species of Pycreus in comparison with three species of Cyperus, including C. laevigatus with dorsiventrally flattened nutlets. Approach : Freshly collected material was investigated using scanning electron microscopy (SEM) and light microscopy (LM). Special attention was given to spikelet and gynoecial development. Results : SE micrographs of all species studied show an indeterminate rachilla with distichously arranged glumes, each subtending a bisexual flower. In spikelets of C. capitatus and P pumilus, the proximal glume sometimes subtends a lateral spikelet instead of a flower. In the species of Pycreus studied, each flower sits in a cavity formed by the growth of the rachilla, which is congenitally fused with the wings of the glume of the higher, alternate flower. Glumes appear successively, each soon forming a flower primordium in its axil, which develops according to a general cyperoid ontogenetic pattern. In Pycreus, the stigma branches grow out from dorsiventrally positioned primordia. During gynoecium development, a hypogynous stalklet (gynophore) appears in all species studied. Conclusion : In spikelets of Pycreus, the rachilla and wings of the glumes are congenitally fused and consequently develop with epicaulescent displacements of the glumes resulting in typical spikelets with flowers in cavities. In spikelets of Cyperus, a similar though less pronounced development results in spikelets with zigzagging rachilla. The particular positions of the stigma branches in C. laevigatus and Pycreus are explained by the development of the gynoecium from an annular primordium, which facilitates shifts in localisation of the stigma primordia. Though we consider the combination of the typical spikelet ontogeny and the independently originated laterally flattened nutlets to be strong arguments in favour of a genus Pycreus, a phylogenetic confirmation that the taxon is monophyletic is an absolute, until now unfulfilled, condition. Moreover, the consequences for the giant genus Cyperus must be taken in consideration.

Evolutionary History of the Monocot Flower1

Abstract This paper reviews monocot flower structure and gynoecium development and evaluates these data to clarify the evolutionary history of the monocot flower. Despite some congruence between molecular and morphological data regarding the delimitation and phylogenetic relationships of monocots, there is currently no universally accepted view on the morphology of the ancestral monocot flower, reflecting a high degree of parallelism in monocot floral evolution. We focus on two character suites that encompass the key features of monocot flowers: (1) the typical monocot groundplan of trimerous-pentacyclic flowers, and (2) a character suite related to carpel fusion, including postgenital fusion between carpels and the presence of septal nectaries. It is likely that the trimerous-pentacyclic flower represents a major synapomorphy of monocots; this flower groundplan is virtually absent from the closest relatives of monocots. Such close correlation of a particular groundplan with a phylogenetic group is analog...

The Hormonal Regulation of Flower Development

Homeotic genes comprising the ABCE classes partly detail the genetic networks that control aspects of floral organ initiation, development, and architecture, but less is known about how these gene functions are translated into changes at the cellular level in growth and cellular differentiation that are involved in the formation of diverse floral organs with specific shapes and sizes. Hormones are the principal transducers of genetic information, and due to recent advances in understanding hormone function in floral development, it is timely to review some of these findings. Flower development is the result of a regulated balance between meristem size and coordination and organ initiation. Floral meristem size is regulated by cytokinin, gibberellin, and auxin, and auxin plays a major role in organ initiation and organogenesis. How hormones contribute to the development of each organ is partly known, with stamen development reliant on almost all hormones, petal development is affected by gibberellins, auxin, and jasmonic acid, and gynoecium development is predominantly regulated by auxin. Furthermore, the interconnections between genetic hierarchies and hormones are being elucidated, and as almost all hormone groups are implicated in floral development, points of hormone crosstalk are being revealed.

Synergistic disruptions in seuss cyp85A2 double mutants reveal a role for brassinolide synthesis during gynoecium and ovule development

BackgroundThe Arabidopsis SEUSS (SEU) gene encodes a transcriptional adaptor protein that is required for a diverse set of developmental events, including floral organ identity specification, as well as gynoecium, ovule and embryo development. In order to better understand the molecular mechanisms of SEUSS action we undertook a genetic modifier screen to identify seuss-modifier (sum) mutations.ResultsScreening of M2 lines representing approximately 5,000 M1 individuals identified mutations that enhance the seuss mutant phenotypic disruptions in ovules and gynoecia; here we describe the phenotype of the sum63 mutant and enhanced disruptions of ovule and gynoecial development in the seu sum63 double mutant. Mapping and genetic complementation tests indicate that sum63 is allelic to CYP85A2 (AT3G30180) a cytochrome p450 enzyme that catalyzes the final steps in the synthesis of the phytohormone brassinolide.ConclusionsOur identification of mutations in CYP85A2 as enhancers of the seuss mutant phenotype suggests a previously unrecognized role for brassinolide synthesis in gynoecial and ovule outer integument development. The work also suggests that seuss mutants may be more sensitive to the loss or reduction of brassinolide synthesis than are wild type plants.

A new role for the SHATTERPROOF genes during Arabidopsis gynoecium development

Gynoecium development is a complex process which is regulated by key factors that control the spatial formation of the apical, medial and basal parts. SHATTERPROOF1 (SHP1) and SHP2, two closely related MADS-box genes, redundantly control the differentiation of the dehiscence zone and promote the lignification of adjacent cells. Furthermore, SHP1 and SHP2 have shown to play an important role in ovule identity determination. The present work identifies a new function for these two genes in promoting stigma, style and medial tissue development. This new role was discovered by combining the shp1 shp2 double mutant with the aintegumenta (ant) and crabs claw (crc) mutants. In quadruple mutant flowers, the inner whorl is composed of unfused carpels which lack almost completely apical and medial tissues, a phenotype similar to the previously reported fil ant and lug ant double mutants.

Floral thermogenesis of three species of Hydnora (Hydnoraceae) in Africa

Floral thermogenesis occurs in at least 12 families of ancient seed plants. Some species show very high rates of respiration through the alternative pathway, and some are thermoregulatory, with increasing respiration at decreasing ambient temperature. This study assesses the intensity and regulation of respiration in three species of African Hydnora that represent the Hydnoraceae, an unusual family of holoparasitic plants from arid environments. Long-term respirometry (CO(2) production) and thermometry were carried out on intact flowers of H. africana, H. abyssinica and H. esculenta in the field, and short-term measurements were made on floral parts during the protogynous flowering sequence. For H. africana, there was no temperature elevation in either the osmophores or the gynoecial chamber in any phase, and mass-specific respiration rates of the flower parts were low (maximum 8.3 nmol CO(2) g(-1) s(-1) in osmophore tissue). Respiration tracked ambient and floral temperatures, eliminating the possibility of the inverse relationship expected in thermoregulatory flowers. Hydnora abyssinica flowers had higher respiration (maximum 27.5 nmol g(-1) s(-1) in the osmophores) and a slight elevation of osmophore temperature (maximum 2.8 degrees C) in the female stage. Respiration by gynoecial tissue was similar to that of osmophores in both species, but there was no measurable elevation of gynoecial chamber temperature. Gynoecial chamber temperature of H. esculenta could reach 3.8 degrees C above ambient, but there are no respiration data available. Antheral tissue respiration was maximal in the male phase (4.8 nmol g(-1) s(-1) in H. africana and 10.3 nmol g(-1) s(-1) in H. abyssinica), but it did not raise the antheral ring temperature, which showed that thermogenesis is not a by-product of pollen maturation or release. The exceptionally low thermogenesis in Hydnora appears to be associated with scent production and possibly gynoecial development, but has little direct benefit to beetle pollinators.

Carpel development in a floral mutant of dioecious Silene latifolia producing asexual and female-like flowers

The genes SlSTM1 and SlSTM2 (orthologs of Arabidopsis SHOOT MERISTEMLESS) and SlCUC (an ortholog of CUP-SHAPED COTYLEDON1 and CUC2) of the dioecious species Silene latifolia have been proposed to control the gynoecium suppression pathway in developing flowers. In a mutant of S. latifolia (K034) that produces no males but only asexual and imperfect female (female-like) flowers, both on the same individual, gynoecia are completely suppressed in asexual flowers and partially suppressed in female-like flowers. To determine whether these two epigenetic phenotypes in gynoecium development are caused by changes in SlSTM and SlCUC expression, we performed in situ hybridization with probes of SlSTM and SlCUC. We found two different pattern of gene expression in flower buds prior to the onset of phenotypic differentiation, which were similar to the reciprocal expression of the two genes described in male and female wild-type plants. In young K034 flower buds, 14.3% of developing structures showed female and the rest male determination. This ratio corresponds to the ratio of female-like to asexual flowers eventually produced by the K034 plants. The same ratio (7-16%) was not only found in the original mutants but also in the first and second backcross generations and in vegetative clones of the original mutant line. Hence, the switch-like and reciprocal SlSTM and SlCUC expression patterns in K034 correspond to the gynoecium suppression patterns in the wild type, suggesting that the mutation(s) responsible for the two mutant genotypes acts upstream of SlSTM and SlCUC.

Comparative Gynoecium Structure and Development in Calycanthaceae (Laurales)

Calycanthaceae, sister to all other Laurales, are the most distinct family of the order in their gynoecium structure (lateral placentation, presence of two ovules). Gynoecium structure was studied in a representative of each genus of Calycanthaceae and gynoecium development in a representative of each of the two subfamilies (Calycanthoideae and Idiospermoideae). Newly found shared features are postgenital coherence between the free carpels (extragynoecial compitum), abortion of the upper of the two ovules, and lobation of the outer integument. Differences in reproductive structures between the two subfamilies are reviewed. Newly found differences include carpel primordium shape, contribution of the outer integument to micropyle formation, epidermis differentiation of the rim of the integuments, and mode of formation of the compitum. Unexpectedly, at anthesis, ovary and ovules of Idiospermum are not larger than those of Calycanthoideae, despite the conspicuous difference in fruit. The identity of “staminod...

Auxin can act independently of CRC, LUG, SEU, SPT and STY1 in style development but not apical‐basal patterning of the Arabidopsis gynoecium

Patterning of the Arabidopsis thaliana gynoecium is dependent on the localization and concentration of the plant hormone auxin and it has been previously reported that STYLISH1 (STY1) activates transcription of the auxin biosynthesis gene YUCCA4 (YUC4) and affects gynoecium development. Here, the relationship between auxin, STY1 and other regulators of gynoecium development was examined. Exogenous auxin in droplets of lanolin paste were applied to young gynoecia; auxin biosynthesis rate was measured and STY1 overexpression or chemically mediated polar auxin transport (PAT) inhibition were induced in various mutants. The style phenotype of sty1-1sty2-1 mutants was restored by exogenous application of auxin, and STY1 over-activation resulted in an elevated auxin biosynthesis rate. Both over-activation of STY1 and inhibition of PAT restored the stylar defects of several unrelated mutants, but with regard to gynoecium apical-basal patterning the mutants responded differently to inhibition of PAT. These results suggest that reduced auxin concentrations cause the sty1-1 sty2-1 phenotype, that STY1 induces auxin biosynthesis, that elevated apical auxin concentrations can compensate for the loss of several style-promoting factors, and that auxin may act downstream of, or in parallel with these during style development but is dependent on their action in apical-basal patterning.

SEUSSandAINTEGUMENTAMediate Patterning and Ovule Initiation during Gynoecium Medial Domain Development

The Arabidopsis (Arabidopsis thaliana) gynoecium, the female floral reproductive structure, requires the action of genes that specify positional identities during its development to generate an organ competent for seed development and dispersal. Early in gynoecial development, patterning events divide the primordium into distinct domains that will give rise to specific tissues and organs. The medial domain of the gynoecium gives rise to the ovules, and several other structures critical for reproductive competence. Here we report a synergistic genetic interaction between seuss and aintegumenta mutants resulting in a complete loss of ovule initiation and a reduction of the structures derived from the medial domain. We show that patterning events are disrupted early in the development of the seuss aintegumenta gynoecia and we identify PHABULOSA (PHB), REVOLUTA, and CRABS CLAW (CRC) as potential downstream targets of SEUSS (SEU) and AINTEGUMENTA (ANT) regulation. Our genetic data suggest that SEU additionally functions in pathways that are partially redundant and parallel to PHB, CRC, and ANT. Thus, SEU and ANT are part of a complex and robust molecular system that coordinates patterning cues and cellular proliferation along the three positional axes of the developing gynoecium.

Ontogeny of the Fertile Flower in Platycrater arguta (Hydrangeaceae)

The monotypic genus Platycrater has had varying systematic placements. Recent phylogenetic analyses place Platycrater in the Hydrangeaceae, tribe Hydrangeeae. The ontogeny of fertile flowers of Platycrater arguta is examined using scanning electron microscopy to provide further insights into the floral morphological evolution in the poorly resolved tribe Hydrangeeae. Sepals are initiated successively. Petals are initiated nearly simultaneously. The gynoecium is initiated before the androecium. Stamen initiation in the polystemonous androecium is somewhat irregular but is centrifugal overall, which is distinctive among the otherwise centripetal members of Hydrangeeae. It is similar to that of Deinanthe in having stamens initiated on an extended hypanthium. Gynoecial development in the genus resembles that of Hydrangea and Cardiandra, in which the styles remain separate. The pattern of floral development supports the taxonomic placement of Platycrater in Hydrangeeae rather than in Philadelpheae, in which polystemonous members have common androecium primordia. In addition, the results indicate that Platycrater may be closely related to Hydrangea, Deinanthe, and Cardiandra.

Floral Ontogeny of the Afro-Madagascan Genus Mitrasacmopsis with Comments on the Development of Superior Ovaries in Rubiaceae

Members of Rubiaceae are generally characterized by an inferior ovary. However, Mitrasacmopsis is cited in the literature as having a semi-inferior to superior ovary. It has previously been hypothesized that the gynoecial development of Rubiaceae with semi-inferior to superior ovaries takes place in the same way as in Gaertnera, one of the most commonly cited rubiaceous genera with a superior ovary. To test this hypothesis, a floral ontogenetic study of Mitrasacmopsis was carried out with special attention paid to the gynoecial development. Floral ontogeny and anatomy of Mitrasacmopsis were examined using scanning electron and light microscopy. At an early developmental stage, a concavity becomes visible in the centre of the floral apex simultaneously with the perianth initiation. A ring primordium surrounding this concavity expands vertically forming the corolla tube (early sympetaly). Stamen primordia develop inside the corolla. From the bicarpellate gynoecium only two carpel tips are visible because the ovary is formed by a gynoecial hypanthium. In the basal part of each carpel, a placenta primordium is initiated. Two septa divide the ovary into two locules. In each locule, the placenta becomes mushroom shaped and distinctly stalked. Eventually, the inferior ovary of Mitrasacmopsis develops into a beaked capsule. Only very late in the fruiting stage, the continuously expanding ovary is raised above the insertion point of the persistent calyx, changing the ovary position of Mitrasacmopsis from basically inferior to secondarily semi-inferior. Mitrasacmopsis follows an epigynous pattern of floral development. However, the presence of a prominent beak in the fruiting stage gives the whole ovary a semi-inferior appearance. This kind of secondarily semi-inferior ovary is shown to be different from the secondarily superior ovary observed in Gaertnera.

Floral meristem indeterminacy depends on flower position and is facilitated by acarpellate gynoecium development in Impatiens balsamina

* Floral meristems are generally determinate. Termination of their activity varies with species, occurring after carpel or ovule development, depending on the placentation type. In terminal flowering Impatiens balsamina (cv. Dwarf Bush Flowered) some flowers exhibit meristem indeterminacy; they produce organs from the placenta after ovule development. * Here we provide a detailed description of gynoecium development in this line and explore the basis of the indeterminate nature of some of its floral meristems. * We find that the placenta is sometimes established without complete carpel fusion. Proliferative growth derives from meristematic remnants of the placenta and is more common in the terminal inflorescence. RNA in situ hybridization reveals that IbLFY (Impatiens LFY homologue) is expressed in all meristem states, even in proliferating meristems. Expression of IbAG in axillary flowers is as expected in the meristem, stamens and carpels but absent from the proliferating meristem. * We conclude that I. balsamina has cauline placentation. Incomplete suppression of inflorescence identity in flowers of the terminal inflorescence leads to floral meristem proliferation after ovule development in this species.