Sepal Initiation Research Articles

DRMY1 promotes robust morphogenesis in Arabidopsis by sustaining the translation of cytokinin-signaling inhibitor proteins.

Robustness is the invariant development of phenotype despite environmental changes and genetic perturbations. In the Arabidopsis flower bud, four sepals robustly initiate and grow to a constant size to enclose and protect the inner floral organs. We previously characterized the mutant development-related myb-like 1 (drmy1), where 3-5 sepals initiate variably and grow to different sizes, compromising their protective function. The molecular mechanism underlying this loss of robustness was unclear. Here, we show that drmy1 has reduced TARGET OF RAPAMYCIN (TOR) activity, ribosomal content, and translation. Translation reduction decreases the protein level of ARABIDOPSIS RESPONSE REGULATOR7 (ARR7) and ARABIDOPSIS HISTIDINE PHOSPHOTRANSFER PROTEIN 6 (AHP6), two cytokinin-signaling inhibitors that are normally rapidly produced before sepal initiation. The resultant upregulation of cytokinin signaling disrupts robust auxin patterning and sepal initiation. Our work shows that the homeostasis of translation, a ubiquitous cellular process, is crucial for the robust spatiotemporal patterning of organogenesis.

Tradeoff between speed and robustness in primordium initiation mediated by auxin-CUC1 interaction

Robustness is the reproducible development of a phenotype despite stochastic noise. It often involves tradeoffs with other performance metrics, but the mechanisms underlying such tradeoffs were largely unknown. An Arabidopsis flower robustly develops four sepals from four precisely positioned auxin maxima. The development related myb-like 1 (drmy1) mutant generates noise in auxin signaling that disrupts robustness in sepal initiation. Here, we find that increased expression of CUP-SHAPED COTYLEDON1 (CUC1), a boundary specification transcription factor, in drmy1 underlies this loss of robustness. CUC1 surrounds and amplifies stochastic auxin noise in drmy1 to form variably positioned auxin maxima and sepal primordia. Removing CUC1 from drmy1 provides time for noisy auxin signaling to resolve into four precisely positioned auxin maxima, restoring robust sepal initiation. However, removing CUC1 decreases the intensity of auxin maxima and slows down sepal initiation. Thus, CUC1 increases morphogenesis speed but impairs robustness against auxin noise. Further, using a computational model, we find that the observed phenotype can be explained by the effect of CUC1 in repolarizing PIN FORMED1 (PIN1), a polar auxin transporter. Lastly, our model predicts that reducing global growth rate improves developmental robustness, which we validate experimentally. Thus, our study illustrates a tradeoff between speed and robustness during development.

Tradeoff Between Speed and Robustness in Primordium Initiation Mediated by Auxin-CUC1 Interaction.

Robustness is the reproducible development of a phenotype despite stochastic noise. It often involves tradeoffs with other performance metrics, but the mechanisms underlying such tradeoffs were largely unknown. An Arabidopsis flower robustly develops four sepals from four precisely positioned auxin maxima. The development related myb-like 1 (drmy1) mutant generates noise in auxin signaling that disrupts robustness in sepal initiation. Here, we found that increased expression of CUP-SHAPED COTYLEDON1 (CUC1), a boundary specification transcription factor, in drmy1 underlies this loss of robustness. CUC1 surrounds and amplifies stochastic auxin noise in drmy1 to form variably positioned auxin maxima and sepal primordia. Removing CUC1 from drmy1 provides time for noisy auxin signaling to resolve into four precisely positioned auxin maxima, restoring robust sepal initiation. However, removing CUC1 decreases auxin maxima intensity and slows down sepal initiation. Thus, CUC1 increases morphogenesis speed but impairs robustness against auxin noise. Further, using a computational model, we found that the observed phenotype can be explained by the effect of CUC1 in repolarizing PIN FORMED1 (PIN1), a polar auxin transporter. Lastly, our model predicts that reducing global growth rate improves developmental robustness, which we validated experimentally. Thus, our study illustrates a tradeoff between speed and robustness during development.

Perianth phyllotaxis and symmetry in Lamiales

ABSTRACT Genetic studies of floral development have essentially focused on events within the floral meristem. By contrast, phyllotaxis models have shown that sepal arrangement is determined by the structure of the inflorescence. In this study, we assess the interplay between the genetic program underlying floral symmetry and phyllotaxic determination, which together shape the flower. We used a phyllotaxic model for the initiation of sepals and petals in six species of Lamiales and studied the impact of the zygomorphic genetic program on their pattern at anthesis. The initiation pattern of floral organs is determined by hormone signaling and contact pressure acting during organogenesis, while the genetic program underlying zygomorphy forces the bilateral symmetry of the initiation of organ primordia. The integration of phyllotaxic and genetic studies allows to deepen our understanding of the mechanisms producing actinomorphic or zygomorphic flowers. In any case, these mechanisms respect Curie’s symmetry principle which states that “when certain causes (here, expression of CYC-like genes) produce certain effects, the elements of symmetry of the causes must be found in the effects produced (here, perianth symmetry)”.

Unstable patterns of flower structure and development in the almost dioecious species Acer negundo (Sapindaceae, Sapindales)

Abstract Acer negundo L. is a wind-pollinated, dioecious tree that occasionally produces flowers with rudiments of the opposite sex. Both the male (staminate) and female (pistillate) flowers possess only two whorls: sepals and stamens or carpels, the arrangement of which is disputed. Here we present data on inflorescence and floral development, pollen fertility in staminodes and the diversity of male and female flowers. We found that the merism of male flowers is unstable, and the numbers of stamens and sepals vary independently. The different floral groundplans always occur within the inflorescences of the same generative shoot. The development of a flower begins with the initiation of sepals, but in female flowers, sepals are initiated sequentially and remain of different sizes, whereas in male flowers, sepals are initiated almost simultaneously and are equal. After the appearance of stamen primordia or carpel primordia, a part of the undifferentiated meristem remains. This unused meristem can be involved in producing staminodes or pistillodia. Both unisexual and (partly) bisexual flowers are found in the same inflorescence. Staminodes are either completely sterile or produce some amount of viable pollen. These features indicate the possibility of functional bisexuality in A. negundo.

Reflections on the Triptych of Meristems That Build Flowering Branches in Tomato.

Branching is an important component determining crop yield. In tomato, the sympodial pattern of shoot and inflorescence branching is initiated at floral transition and involves the precise regulation of three very close meristems: (i) the shoot apical meristem (SAM) that undergoes the first transition to flower meristem (FM) fate, (ii) the inflorescence sympodial meristem (SIM) that emerges on its flank and remains transiently indeterminate to continue flower initiation, and (iii) the shoot sympodial meristem (SYM), which is initiated at the axil of the youngest leaf primordium and takes over shoot growth before forming itself the next inflorescence. The proper fate of each type of meristems involves the spatiotemporal regulation of FM genes, since they all eventually terminate in a flower, but also the transient repression of other fates since conversions are observed in different mutants. In this paper, we summarize the current knowledge about the genetic determinants of meristem fate in tomato and share the reflections that led us to identify sepal and flower abscission zone initiation as a critical stage of FM development that affects the branching of the inflorescence.

Hofmeister’s Rule’s Paradox: Explaining the Changeable Carpel Position in Caryophyllaceae

Premise of research. In Caryophyllaceae with a gynoecium isomerous with the other floral whorls, carpels can be in either antesepalous or antepetalous position. Morphologists have long been unable to find a unifying hypothesis to explain the different carpel positions in Caryophyllaceae, as variation in carpel position breaks with Hofmeister’s rule of alternating whorls.Methodology. In this study, we investigate the floral development of six species of Caryophyllaceae with isomerous flowers using scanning electron microscopy to clarify the differences between carpel positions and whether developmental modes are responsible. Special attention is paid to the correlation between the development of carpels and other floral organs.Pivotal results. In Spergula arvensis, Agrostemma githago, and Myosoton aquaticum, carpels are in antepetalous position, while the antesepalous stamens develop more centrally in a sequence inverse to the sepals. In Cerastium glomeratum and Silene diocia, carpels are in antesepalous position, while the antesepalous stamens do not show an inverse development and are inserted almost at the same level as antepetalous stamens. In Lychnis flos-cuculi, the carpels are not clearly in an antesepalous or antepetalous radius, and the development of antesepalous stamens shows a slight unequal growth.Conclusions. Developmental evidence rejects the hypothesis that the developmental modes of carpels are the reason for the different carpel positions. Carpel position is correlated with the developmental sequence of antesepalous stamens and the level of emergence of antepetalous stamens on the floral apex. Space availability and pressure exercised by the sepals is responsible for the unequal development and a more central position of antesepalous stamens, with those opposite the inner sepals developing faster, causing the carpels to shift to an antepetalous position. With greater availability of space, antesepalous stamens initiate simultaneously or continue the sequence of sepal initiation, and all stamens are more or less equidistant from the apex; the antesepalous stamens do not display an inverse development, and the carpels initiate in antesepalous position, which is consistent with Hofmeister’s rule.

Robust organ size requires robust timing of initiation orchestrated by focused auxin and cytokinin signalling.

Organ size and shape are precisely regulated to ensure proper function. The four sepals in each Arabidopsis thaliana flower must maintain the same size throughout their growth to enclose and protect the developing bud. Here we show that DEVELOPMENT RELATED MYB-LIKE1 (DRMY1) is required for both timing of organ initiation and proper growth, leading to robust sepal size in Arabidopsis. Within each drmy1 flower, the initiation of some sepals is variably delayed. Late-initiating sepals in drmy1 mutants remain smaller throughout development resulting in variability in sepal size. DRMY1 focuses the spatiotemporal signaling patterns of the plant hormones auxin and cytokinin, which jointly control the timing of sepal initiation. Our findings demonstrate that timing of organ initiation together with growth and maturation contribute to robust organ size.

Floral development and anatomy of

We investigated the floral anatomy and development of Macarthuria australis Hügel ex Endl., an unusual genus endemic to Australia, in the context of floral evolution of core Caryophyllales. Flower initiation is spiral, with sepals developing quincuncially. The first two petals continue the sequence of sepal initiation, but the remaining petals arise from common stamen–petal primordia. The androecium develops sequentially as three inner antesepalous and five outer antepetalous stamens. The globular ovary is trimerous with a short symplicate zone and two arillate ovules per locule. The rapid emergence of the androecium leads to a partial absorption of the petal primordia within the androecial tissue. The two first-formed petals have more room for development and precede the androecium, supporting the fact that petals are not staminodial in origin. This heterochronic shift correlates with an inversed developmental sequence of the antesepalous stamens. The constraint caused by the spatial occupation of sepals and carpels leads to the loss of two stamens, and the re-arrangement of stamens and petals along the flanks of the carpels. The floral development of Macarthuria anticipates a syndrome of stamen and petal development in other core Caryophyllales and culminating in the Caryophyllaceae.

Floral development of Moraceae species with emphasis on the perianth and androecium

The mulberry and fig family (Moraceae) has tiny flowers with a single whorled or absent perianth and inner whorls with a reduced number of organs. Relevant questions about this reduced floral structure still remain unresolved. Thus, we studied the ontogenic processes that culminate in such a reduced flower, as well as the close relationship between the floral envelope and the androecium in Brosimum gaudichaudii, Castilla elastica, Clarisia ilicifolia, Maclura tinctoria and Morus nigra. Flower buds and flowers were processed for surface (scanning electron microscopy) and anatomical (light microscopy) analyses. The perianth consists of sepals, except the flowers of B. gaudichaudii and C. elastica (staminate), in which the perianth is lacking. The staminate flower of B. gaudichaudii exhibits a bracteole enveloping the stamens. In monochlamydeous flowers the order of sepal initiation is helical or asynchronous. No petals are initiated. The calyx displays different degrees of synorganization being united only basally or along its entire extension. The sepals can have tector and glandular trichomes or only tector trichomes. Laticifers, phenolic and crystalliferous idioblasts occur in the sepal mesophyll. Stamens initiate concomitant with the pistillode in B. gaudichaudii and after initiation of the pistillode in M. tinctoria and M. nigra. In C. ilicifolia there is no pistillode and the stamens initiate after the sepals. The stamens show variation in filament shape and connective structure. The filament is inflexed in M. tinctoria and M. nigra and straight in the other species, although these structures are anatomically similar. The connective is expanded (except in M. nigra and C. ilicifolia) and has phenolic idioblasts. We conclude that the tiny flowers of these species result of two distinct ontogenetic pathways: absence of organs from the inception and by abortion, mechanisms that act to decrease the number of floral whorls as well as the number of organs in each whorl.

Floral development of Gonocaryum with emphasis on the gynoecium

We investigated the floral development of Gonocaryum, a genus of Cardiopteridaceae that was segregated from Icacinaceae s.l., using scanning electron microscopy to clarify its gynoecial structure and facilitate morphological comparisons of Cardiopteridaceae. The key floral developmental characters include sepal initiation that follows a quincuncial spiral sequence; petals that are valvate with inflexed tips and are postgenitally fused at the base; a petal and stamen initiation sequence that is almost simultaneous; a globular protuberance on top of the connective; a gynoecium that is tricarpellate and pseudomonomerous, with the stigma produced by one abaxial lateral carpel; and two ovules that are unitegmic and anatropous with an obturator on the funicle. The floral developmental characters of Gonocaryum are discussed relative to Cardiopteris, which has been well studied and whose gynoecial vasculature is reinterpreted here, and are briefly compared to other members of Aquifoliales and Icacinaceae s.l. The imbricate sepals, initiated in a quincuncial spiral sequence, and the tricarpellate, pseudomonomerous gynoecium are common characters of Cardiopteridaceae. Unisexual flowers are an autapomorphy of Gonocaryum in Cardiopteridaceae.

Developmental Changes in Time and Space Promote Evolutionary Diversification of Flowers: A Case Study in Dipsacoideae.

Diversification in flower shape and function is triggered by the high plasticity of flower meristems. Minute changes in space and time can profoundly affect the formation of adult structures. Dipsacoideae provides an excellent model system to investigate the evolutionary aspects of temporal and spatial changes in flower development due to its small size, the resolved phylogenetic framework, and significant diversity of perianth form and merosity. In the present study, we investigated the sequence of floral organ initiation and quantified the interactions between flower meristem expansion and petal primordium size in eight species representing two major clades of Dipsacoideae. Our quantitative study indicates the plasticity of the flower meristem for the regulation of pentamery either due to a decrease in petal primordium size (Scabiosa) or an increase in flower meristem size (Pterocephalus and Lomelosia) compared to tetramerous flowers. According to our results, temporal shifts of organ initiation during flower evolution contribute to the morphological diversity of perianth. Sepal reduction in members of the Dipknautids is paralleled by a delay in sepal initiation. The multiplication of sepals in Lomelosia and Pterocephalus is correlated with an extension of initiation time. Some heterochronies in early development do not affect adult morphology. The effects of a temporal change in early development can be enhanced, reduced, or eliminated by later changes of the growth rate during development. Our results confirm the hypothesis that the interaction between timing and space plays an important role for evolutionary diversification of flowers.

Evolution of the SPATULA/ALCATRAZ gene lineage and expression analyses in the basal eudicot, Bocconia frutescens L. (Papaveraceae)

BackgroundSPATULA (SPT) and ALCATRAZ (ALC) are recent paralogs that belong to the large bHLH transcription factor family. Orthologs of these genes have been found in all core eudicots, whereas pre-duplication genes, named paleoSPATULA/ALCATRAZ, have been found in basal eudicots, monocots, basal angiosperms and gymnosperms. Nevertheless, functional studies have only been performed in Arabidopsis thaliana, where SPT and ALC are partially redundant in carpel and valve margin development and ALC has a unique role in the dehiscence zone. Further analyses of pre-duplication genes are necessary to assess the functional evolution of this gene lineage.ResultsWe isolated additional paleoSPT/ALC genes from Aristolochia fimbriata, Bocconia frutescens, Cattleya trianae and Hypoxis decumbens from our transcriptome libraries and performed phylogenetic analyses. We identified the previously described bHLH domain in all analyzed sequences and also new conserved motifs using the MEME suite. Finally, we analyzed the expression of three paleoSPT/ALC genes (BofrSPT1/2/3) from Bocconia frutescens, a basal eudicot in the Papaveraceae. To determine the developmental stages at which these genes were expressed, pre- and post-anthesis carpels and fruits of B. frutescens were collected, sectioned, stained, and examined using light microscopy. Using in situ hybridization we detected that BofrSPT1/2/3 genes are expressed in floral buds, early sepal initiation, stamens and carpel primordia and later during fruit development in the dehiscence zone of the opercular fruit.ConclusionsOur expression results, in comparison with those available for core eudicots, suggest conserved roles of members of the SPT/ALC gene lineage across eudicots in the specification of carpel margins and the dehiscence zone of the mature fruits. Although there is some redundancy between ALC and SPT, these gene clades seem to have undergone some degree of sub-functionalization in the core eudicots, likely by changes in cis regulatory regions and to some extent in coding sequences, at least in Brassicaceae. Our results also indicate that in Bocconia frutescens, paleoSPT/ALC genes may play a role in early floral organ specification that was subsequently lost in core eudicot lineages.

Floral development of Gymnospermium microrrhynchum (Berberidaceae) and its systematic significance in the Nandinoideae

Morphological and molecular analyses of the Berberidaceae have recognized the Nandinoideae clade of four genera: Caulophyllum, Gymnospermium, Leontice, and Nandina. However, generic relationships of the Nandinoideae remain to be explicitly defined with morphological characteristics. In this study we examined the floral ontogeny of Gymnospermium to gain new insights into the evolution of floral structures in the family and generic relationships of the Nandinoideae. The inflorescence of Gymnospermium is racemose with bracts and floral primordia initiated acropetally. Floral parts form in the order of sepal, stamen, staminode, and carpel. Both sepals and stamens are initiated in three series and each series has two sepals/stamens. Stamen and staminode share a common primordium with the former initiated earlier than the latter. In the Berberidaceae the floral trimery is an ancestral trait, while dimery and its variant (such as non-whorled dimery in Gymnospermium) are a derived feature. The non-whorled pattern of the initiation of sepals and stamens and their dimery provide synapomorphies for the close relationship of Gymnspermium and Leontice in the Nandinoideae.

High developmental lability in the perianth ofInga(Fabales, Fabaceae): a Neotropical woody rosid with gamopetalous corolla

Inga is a highly diverse Neotropical genus of Fabaceae with a rapid and recent diversification. Its flowers are actinomorphic with a wide variation in merism and size and length of perianth. Notwithstanding its species richness and economic importance, few floral ontogenetic studies have been carried out in this genus. Thus, we investigated perianth development and morphology of five Inga spp. using scanning electron and light microscopy. Perianth ontogeny is highly labile. Sepal and petal primordium number ranges from four to seven. The sequence of sepal initiation is generally reversed unidirectionally (from adaxial to abaxial side); petals are initiated simultaneously. The position of sepals and petals is variable among and within species. The sepal may be positioned adaxially, abaxially or slightly displaced in relation to the median sagittal plane. The median petal is positioned in the adaxial (I. congesta, I. grandis) or abaxial region (I. bella, I. hispida). Elongated petals touch each other apically through conspicuous papillae. Perianth development in Inga differs from that of other mimosoids. The early developmental stages show a wide variation. However, subsequent ontogenetic processes, known as equalization during intermediate stages, establish uniformity in the floral architecture, thus preventing possible functional disparities that could influence reproductive success.

Floral organogenesis in Urophysa rockii, a rediscovered endangered and rare species of Ranunculaceae

Urophysa is an Asian endemic genus in the Ranunculaceae, but data on floral organogenesis, which would be a useful complement to molecular data in clarifying the relationship with closely related taxa (Aquilegia and Semiaquilegia) in Ranunculaceae, are completely lacking. We used scanning electron microscopy and light microscopy to study the floral development of Urophysa rockii Ulbrich, a recently rediscovered species in this genus. The sepals are initiated spirally, whereas other organs are nonsimultaneously whorled; the floral phyllotaxis is whorled. Primordia of the sepals are lunular and truncate, but those of the petals and stamens are hemispherical and rounded. After sepal initiation, there is a delay in development, but the initiation of petals and stamens is continuous. The developmental sequence of the microspores in the stamens is centrifugal, although the stamens are initiated centripetally. The early developmental stages of the staminodes are similar to those of the stamens, although much smaller, so they may be phylogenetically homologous organs. The carpel primordia are lunular in shape and plicate. The mature ovule is anatropous and bitegmic. Urophysa shows similar floral development features as Aquilegia and Semiaquilegia, although with some differences, which supports the relationship inferred by DNA sequence data.

Polarity in the early floral meristem of Arabidopsis

The diversity of angiosperm flowers depends on organ meristy and position. However, the signaling pathways that establish polarity and positional information remain largely unelucidated. Use of the founder-cell marker DORNRÖSCHEN-LIKE (DRNL) in Arabidopsis has recently highlighted the importance of the abaxial–adaxial axis for early floral development. We have extended the use of DRNL::GFP to further characterize floral organogenesis in genotypes that are altered in floral organ meristy or position, including ettin (ett-3) and blade-on-petiole (bop)1–11 bop2–4 double mutants. The creation of supernumery sepals by the splitting of sepal founder-cell populations along an ab-/adaxial axis strengthens the importance of the ab-/adaxial developmental axis in early floral meristem development. Furthermore, we confirm the dependency of the wildtype sequence of sepal initiation on bract suppression and demonstrate that supernumery stamens derive from the imprecise resolution of a ring of DRNL expression. Expression of DRNL in apetala1 (ap1–1) and ap2–8 mutants reflect the altered whorl structure and show that these homeotic genes function upstream of DRNL. Analyzing the dynamism of early floral meristem ontogeny at a fine temporal and spatial resolution in Arabidopsis can reveal mechanisms of organogenesis and is applicable to other species with differing floral body plans in a comparative evolutionary context.

Floral Evolution in the Detarieae (Leguminosae): Phylogenetic Evidence for Labile Floral Development in an Early-Diverging Legume Lineage

Premise of research. We examine floral evolution and phylogenetic relationships in the monophyletic Detarieae and related lineages of Caesalpinioideae. Tribe Detarieae (82 genera) includes nearly half of the genera in subfamily Caesalpinioideae and represents some of the most diverse legumes with respect to floral morphology.Methodology. A total of 67 floral ontogenetic and morphological characters were combined with DNA sequences from the plastid trnL-F and matK regions of 34 Detarieae species and representatives of Cassieae, Cercideae, Caesalpinieae, Papilionoideae, and Mimosoideae, for which we have near-complete ontogenetic series. The morphological and ontogenetic characters were optimized onto the resulting most parsimonious phylogenetic trees and Bayesian topology to study character evolution.Pivotal results. Our study supports previously proposed relationships within the tribe Detarieae and among caesalpinioid lineages and indicates that certain features (bracteole and hypanthium characters, sepal initiation, anther position in bud, overlap in timing of initiation of organ whorls) are phylogenetically informative for particular clades whereas others (reductions in petal and stamen number, sepal and petal initiation patterns) have evolved multiple times in parallel in the Detarieae and other Caesalpinioideae. These analyses suggest that modifications that occur early in ontogeny can be good phylogenetic characters for distinguishing both major taxonomic groups and more closely related taxa and that morphological differences that differentiate species within genera can be caused by changes that occur at all stages of ontogeny. Phylogenetic distribution of character states and ontogenetic evidence suggest that in the Caesalpinioideae, loss or suppression of organs within a whorl, both of which are very common, usually does not affect development in subsequent whorls.Conclusions. Our analyses reveal several switches from zygomorphy to actinomorphy (and vice versa), but in Caesalpinioideae (contrary to Papilionoideae), zygomorphy is likely not clearly associated with higher diversification rates. This study suggests that floral initiation patterns are much more variable in Caesalpinioideae than in the other two subfamilies. Although particular patterns may be canalized in certain lineages of the subfamily (Cercideae, Caesalpinieae), in other clades (Detarieae, Dialiinae) floral development is more labile, explaining the high diversity in floral morphology encountered overall in the paraphyletic Caesalpinioideae.

Arabidopsis floral phytomer development: auxin response relative to biphasic modes of organ initiation.

In the Arabidopsis inflorescence meristem (IM), auxin is considered a prepatterning signal for floral primordia, whereas a centripetal mode of positional information for floral organ identity is inherent to the ABCE model. However, spatio-temporal patterns of organ initiation in each whorl at the earliest initiation stages are largely unknown. Evidence suggests that initial flower development occurs along an abaxial/adaxial axis and conforms to phytomer theory. Use of the founder cell marker DORNRÖSCHEN-LIKE (DRNL) as a tool in leafy, puchi, and apetala 1 cauliflower mutant backgrounds suggests that bract founder cells are marked at the IM periphery. The DRNL transcription domain in the wild-type IM is spatially discrete from DR5 expression, suggesting that bract initiation is independent of canonical auxin response. When bracts develop in lfy and puchi mutant floral primordia the initiation of lateral sepals precedes the specification of medial sepals compared with wild type, showing an interplay between bract and abaxial sepal founder cell recruitment. In the perianthia (pan) mutant background, DRNL expression indicates that a radial outer whorl arrangement derives from splitting of sepal founder cell populations at abaxial and adaxial positions. This splitting of incipient sepal primordia is partially dependent on PRESSED FLOWER (PRS) activity and implies that sepal specification is independent of WUSCHEL and CLAVATA3 expression, as both marker genes only regain activity in stage-2 flowers, when patterning of inner floral organs switches to a centripetal mode. The transition from an initially abaxial/adaxial into a centripetal patterning programme, and its timing represent an adaptive trait that possibly contributes to variation in floral morphology, especially unidirectional organ initiation.

Floral ontogeny in Dipterygeae (Fabaceae) reveals new insights into one of the earliest branching tribes in papilionoid legumes

Flowers of Dipterygeae (Fabaceae, Papilionoideae) exhibit an unusual petaloid calyx. The two adaxial sepals are large and petaloid, and the three abaxial sepals form a three-toothed lobe. The goal of this study was to elucidate the ontogenetic pathways of this peculiar calyx in light of the floral development of the three genera that comprise the tribe. Floral buds of Dipteryx alata, Pterodon pubescens and Taralea oppositifolia were analysed using scanning electron microscopy and light microscopy. The order of bracteole and sepal initiation varies among the species. The androecium is asymmetric. The carpel cleft is positioned to the right or to the left, and is opposite the adaxial antepetalous stamen. The peculiarity of the calyx becomes noticeable in the intermediate stages of floral development. It results from the differential growth of the sepal primordia, in which the abaxial and lateral primordia remain diminutive during floral development, compared with the adaxial ones that enlarge and elongate. Bracteoles, abaxial sepals, petals and anthers are appendiculate, except in T. oppositifolia, in which the appendices were not found in bracteoles or anthers. These appendices comprise secretory canals or cavities. Considering that the ontogenetic pathway for the formation of the petaloid calyx is similar and exclusive for Dipterygeae, it might be a potential synapomorphy for the group, with the presence of secretory canals in the appendices of abaxial and lateral sepals and petals. © 2014 The Linnean Society of London, Botanical Journal of the Linnean Society, 2014, 174, 529–550.