Perianth Organs Research Articles

Delphinium as a model for development and evolution of complex zygomorphic flowers.

The complex zygomorphic flowers of the early-diverging eudicot Delphinium provide an opportunity to explore intriguing evolutionary, developmental, and genetic questions. The dorsal perianth organs, consisting of a spurred sepal and the nectar-bearing spurred petal(s) in Delphinium, contribute to the dorso-ventralization and zygomorphic flower morphology. The seamless integration of the two or three dorsal petaloid spurred organs is considered a synorganization, and the resulting organ complex is referred to as a hyperorgan. The hyperorgan shows variability within the tribe due to variation in the number, size, and shape of the spurs. Research in recent decades within this tribe has enhanced our understanding of morphological evolution of flowers. More recently, functional studies using the RNAi approach of Virus-Induced Gene Silencing (VIGS) have unraveled interesting results highlighting the role of gene duplication in the functional diversification of organ identity and symmetry genes. Research in this early-diverging eudicot genus bridges the gaps in understanding the morphological innovations that are mostly studied in model grass and core eudicot clades. This first comprehensive review synthesizes eco-evo-devo research on Delphinium, developing a holistic understanding of recent advancements and establishing the genus as an exceptional model for addressing fundamental questions in developmental genetics, particularly in the evolution of complex flowers. This progress highlights Delphinium's significant potential for future studies in this field.

PeNAC67-PeKAN2-PeSCL23 and B-class MADS-box transcription factors synergistically regulate the specialization process from petal to lip in Phalaenopsis equestris

The molecular basis of orchid flower development involves a specific regulatory program in which MADS-box transcription factors play a central role. The recent ‘perianth code’ model hypothesizes that two types of higher-order heterotetrameric complexes, namely SP complex and L complex, play pivotal roles in the orchid perianth organ formation. Therefore, we explored their roles and searched for other components of the regulatory network.Through the combined analysis for transposase-accessible chromatin with high-throughput sequencing and RNA sequencing of the lip-like petal and lip from Phalaenopsis equestris var.trilip, transcription factor-(TF) genes involved in lip development were revealed. PeNAC67 encoding a NAC-type TF and PeSCL23 encoding a GRAS-type TF were differentially expressed between the lip-like petal and the lip. PeNAC67 interacted with and stabilized PeMADS3, which positively regulated the development of lip-like petal to lip. PeSCL23 and PeNAC67 competitively bound with PeKAN2 and positively regulated the development of lip-like petal to petal by affecting the level of PeMADS3. PeKAN2 as an important TF that interacts with PeMADS3 and PeMADS9 can promote lip development. These results extend the ‘perianth code’ model and shed light on the complex regulation of orchid flower development.

Image recognition-based petal arrangement estimation.

Flowers exhibit morphological diversity in the number and positional arrangement of their floral organs, such as petals. The petal arrangements of blooming flowers are represented by the overlap position relation between neighboring petals, an indicator of the floral developmental process; however, only specialists are capable of the petal arrangement identification. Therefore, we propose a method to support the estimation of the arrangement of the perianth organs, including petals and tepals, using image recognition techniques. The problem for realizing the method is that it is not possible to prepare a large number of image datasets: we cannot apply the latest machine learning based image processing methods, which require a large number of images. Therefore, we describe the tepal arrangement as a sequence of interior-exterior patterns of tepal overlap in the image, and estimate the tepal arrangement by matching the pattern with the known patterns. We also use methods that require less or no training data to implement the method: the fine-tuned YOLO v5 model for flower detection, GrubCut for flower segmentation, the Harris corner detector for tepal overlap detection, MAML-based interior-exterior estimation, and circular permutation matching for tepal arrangement estimation. Experimental results showed good accuracy when flower detection, segmentation, overlap location estimation, interior-exterior estimation, and circle permutation matching-based tepal arrangement estimation were evaluated independently. However, the accuracy decreased when they were integrated. Therefore, we developed a user interface for manual correction of the position of overlap estimation and interior-exterior pattern estimation, which ensures the quality of tepal arrangement estimation.

A comparative approach reveals diversity of floral developmental processes in Urticaceae

Abstract Most species of Urticaceae, the nettle family, have small and inconspicuous, diclinous flowers, in which the perianth, androecium and gynoecium tend to vary in number. Our objective was to study the morphology of the developing flowers of seven species of Urticaceae to understand the pathways that lead to the different patterns of floral reduction and the complex development of pseudomonomerous gynoecia. Buds and flowers were prepared for electron and light microscopy. Vascularization was studied via high resolution X-ray computed tomography micro-CT. Only one whorl of perianth organs is initiated, except for Phenax sonneratii, the flower of which is achlamydeous; variation in perianth merosity results from absence of organs from inception; dicliny results from the absence of stamens from inception (pistillate flowers) and from pistil abortion at intermediate developmental stages (staminate flowers). The gynoecium results from a primordium that divides partially forming two congenitally united primordia (most species) or from a single primordium that apparently does not divide. The gynoecium is served by a single (four species), or two vascular bundles. This second condition is expected for a pseudomonomerous gynoecium. Pistillode or rudimentary carpels occur in staminate flowers. The comparison among species shows that the developmental processes acting in the floral construction in Urticaceae is diverse.

Conical petal epidermal cells, regulated by the MYB transcription factor MIXTA, have an ancient origin within the angiosperms.

Conical epidermal cells occur on the tepals (perianth organs, typically petals and/or sepals) of the majority of animal-pollinated angiosperms, where they play both visual and tactile roles in pollinator attraction, providing grip to foraging insects, and enhancing colour, temperature, and hydrophobicity. To explore the evolutionary history of conical epidermal cells in angiosperms, we surveyed the tepal epidermis in representative species of the ANA-grade families, the early-diverging successive sister lineages to all other extant angiosperms, and analysed the function of a candidate regulator of cell outgrowth from Cabomba caroliniana (Nymphaeales). We identified conical cells in at least two genera from different families (Austrobaileya and Cabomba). A single SBG9 MYB gene was isolated from C. caroliniana and found to induce strong differentiation of cellular outgrowth, including conical cells, when ectopically expressed in Nicotiana tabacum. Ontogenetic analysis and quantitative reverse transcription-PCR established that CcSBG9A1 is spatially and temporally expressed in a profile which correlates with a role in conical cell development. We conclude that conical or subconical cells on perianth organs are ancient within the angiosperms and most probably develop using a common genetic programme initiated by a SBG9 MYB transcription factor.

The use of plant ontologies in comparative and evolutionary studies should be flexible

The use of plant ontologies in comparative and evolutionary studies should be flexible

Structure, Assembly and Function of Cuticle from Mechanical Perspective with Special Focus on Perianth.

This review is devoted to the structure, assembly and function of cuticle. The topics are discussed from the mechanical perspective and whenever the data are available a special attention is paid to the cuticle of perianth organs, i.e., sepals, petals or tepals. The cuticle covering these organs is special in both its structure and function and some of these peculiarities are related to the cuticle mechanics. In particular, strengthening of the perianth surface is often provided by a folded cuticle that functionally resembles profiled plates, while on the surface of the petal epidermis of some plants, the cuticle is the only integral continuous layer. The perianth cuticle is distinguished also by those aspects of its mechanics and development that need further studies. In particular, more investigations are needed to explain the formation and maintenance of cuticle folding, which is typical for the perianth epidermis, and also to elucidate the mechanical properties and behavior of the perianth cuticle in situ. Gaps in our knowledge are partly due to technical problems caused by very small thicknesses of the perianth cuticle but modern tools may help to overcome these obstacles.

Reduction in organ\u2013organ friction is critical for corolla elongation in morning glory

In complex structures such as flowers, organ–organ interactions are critical for morphogenesis. The corolla plays a central role in attracting pollinators: thus, its proper development is important in nature, agriculture, and horticulture. Although the intraorgan mechanism of corolla development has been studied, the importance of organ–organ interactions during development remains unknown. Here, using corolla mutants of morning glory described approximately 200 years ago, we show that glandular secretory trichomes (GSTs) regulate floral organ interactions needed for corolla morphogenesis. Defects in GST development in perianth organs result in folding of the corolla tube, and release of mechanical stress by sepal removal restores corolla elongation. Computational modeling shows that the folding occurs because of buckling caused by mechanical stress from friction at the distal side of the corolla. Our results suggest a novel function of GSTs in regulating the physical interaction of floral organs for macroscopic morphogenesis of the corolla.

Floral development and morphology of the mistletoe

Flowers in the Antidaphne genus are among the smallest in family Santalaceae. We traced the development and morphology of flowers in A. viscoidea and compared them with those in other members of the family. Both staminate and carpellate inflorescences proliferate through collateral and serial buds, and become dimorphic when fully elongated. The former are short racemes with a terete axes and a protective, cup-like distalmost bract; the latter are spikes with flattened, furrowed and longer axes with a leafy, non-protective bract. The staminate flowers (traditionally described as apetalous) initiate from transversely flattened primordia, from which 3–5 stamens initiate opposite to variously reduced petals in an abaxial-to-adaxial direction. A massive, intrastaminal, lobed nectariferous disc, likely gynoecium-derived, is formed, with the lobes alternate to the stamens. The carpellate flowers initiate from radially symmetric primordia; they also possess three or four perianth organs. Floral monosymmetry of staminate flowers is likely due to mechanical constraint rather than to floral reduction. Our data support the current phylogenetic relationship between Antidaphne and the neotropical genera Eubrachion and Lepidoceras, all having unisexual flowers, lacking trichomes between the petals and stamens, sessile stigmas, and baccate fruits. The nectariferous disc is likely apomorphic for Santalaceae.

Homeosis and delayed floral meristem termination could account for abnormal flowers in cultivars of Delphinium and Aquilegia (Ranunculaceae)

Abstract Floral mutants display various deviant phenotypes and, as such, they are appropriate material with which to address the origin and the building of morphological variation. To identify the pivotal developmental stages at which floral variation may originate and to infer the putative associated genetic causes, we studied abnormal flowers in nine cultivars of Aquilegia and Delphinium of Ranunculaceae, a family displaying a high range of floral diversity. Wild-type flowers of the two genera are pentamerous and spurred, but they differ in their overall symmetry (actinomorphy vs. zygomorphy). Floral morphology of their cultivars at different developmental stages up to anthesis was observed, and the putative identity of the perianth organs and vascularization was inferred. Our results show that in the floral mutants in both genera, phyllotaxis was generally conserved, whereas floral organization, vascularization and symmetry were modified. Most of the morphological and anatomical deviations impacted the perianth, including organ number, identity and (spur) elaboration, and also led to the formation of mosaic organs. We hypothesized that the phenotypes of floral mutants in Aquilegia and Delphinium result from genetic alterations affecting frontiers between sets of organs of different identity, homeosis and length of floral meristem activity.

Insights into the ancestral flowers of Ranunculales

AbstractThe question of the origin of petals has long been debated in the botanical literature. Ranunculales are characterized by a spectacular floral diversity, particularly at the perianth level. Recent progress in understanding the genetic bases of floral organ identity suggests a single origin for petals in Ranunculaceae, contrasting with the traditional morphological hypothesis of repeated evolution. However, perianth evolution at the ordinal level remains incompletely understood. Recent advances in the elucidation of phylogenetic relationships in the order now provide a new opportunity to study character evolution with model-based methods. We used ancestral state reconstruction methods that take into account various sources of uncertainty to reconstruct the evolution of floral traits at the scale of Ranunculales using a consensus phylogenetic framework of 144 terminal species representing all families in the order. Ancestrally, Ranunculales probably had three trimerous whorls of perianth organs differentiated into two categories of petaloid organs differing in their shape. Each whorl was further lost or duplicated. Moreover, our results support the hypothesis of a single origin of highly specialized (elaborate) nectariferous petals in Ranunculaceae.

Perianth Phyllotaxis Is Polymorphic in the Basal Eudicot Anemone and Eranthis Species

Floral organs are clade-specifically arranged to either spiral or whorled (concentric circles) phyllotaxis. The basic number (merosity) of perianth organs within a whorl is limited to three in most monocots and to four or five in most eudicots. Although the Fibonacci number (3, 5) of merosity is well-known to agree with that of the spirals in phyllotaxis, the evolutionary relationship between whorls and spiral phyllotaxis remains unclear. Focusing on aestivation (the relative positioning of margins of flower organs in the bud) to capture phyllotaxis including merosity of whorled flowers, trimerous-whorled flowers and spiral ones coexist within populations with intraspecific variation in organ numbers. In addition, a recent mathematical model showed that tetramerous and pentamerous whorls developed from spiral organ initiation by incorporating a post-meristematic organ displacement, depending on the interaction among organ primordia. Therefore, integrating the variation of aestivation with the spiral-to-whorl development may elucidate the underlying mechanism of the continuous spiral-whorl relationship with the merosity diversification. Here, we showed that the aestivation of perianth organs (tepals) of mature flowers was intra-specifically variable but constrained in wild populations of several Anemone and Eranthis species (Ranunculaceae); the spiral arrangements coexisted within a small population, with dimerous, trimerous, tetramerous, and pentamerous double-whorled arrangements, despite considerable possibilities in the geometry. We determined mathematically that most of these constrained aestivations of 5 to 11-tepaled flowers emerge upon the spiral phyllotaxis with a divergence angle between subsequent organs of 90−102° or 135−144° (known as the Lucas and Fibonacci angles, respectively). Incorporating the post-meristematic organ displacement into the model, double-whorled arrangements work as templates to form multiple whorls, the merosity of which is stabilized to trimery, tetramery, or pentamery depending on the divergence angle. These results demonstrate that spiral phyllotaxis promotes the constrained coexistence of whorl and spiral rather than their interspecific dichotomy. This polymorphic phyllotaxis provides an evolutionary scenario in which the floral bauplans of angiosperms could be differentiated into tri-, tetra- and penta-radial symmetries.

A tetraspanin gene regulating auxin response and affecting orchid perianth size and various plant developmental processes.

The competition between L (lip) and SP (sepal/petal) complexes in P‐code model determines the identity of complex perianth patterns in orchids. Orchid tetraspanin gene Auxin Activation Factor (AAF) orthologs, whose expression strongly correlated with the expansion and size of the perianth after P code established, were identified. Virus‐induced gene silencing (VIGS) of OAGL6‐2 in L complex resulted in smaller lips and the down‐regulation of Oncidium OnAAF. VIGS of PeMADS9 in L complex resulted in the enlarged lips and up‐regulation of Phalaenopsis PaAAF. Furthermore, the larger size of Phalaenopsis variety flowers was associated with higher PaAAF expression, larger and more cells in the perianth. Thus, a rule is established that whenever bigger perianth organs are made in orchids, higher OnAAF/PaAAF expression is observed after their identities are determined by P‐code complexes. Ectopic expression Arabidopsis AtAAF significantly increased the size of flower organs by promoting cell expansion in transgenic Arabidopsis due to the enhancement of the efficiency of the auxin response and the subsequent suppression of the jasmonic acid (JA) biosynthesis genes (DAD1/OPR3) and BIGPETAL gene during late flower development. In addition, auxin‐controlled phenotypes, such as indehiscent anthers, enhanced drought tolerance, and increased lateral root formation, were also observed in 35S::AtAAF plants. Furthermore, 35S::AtAAF root tips maintained gravitropism during auxin treatment. In contrast, the opposite phenotype was observed in palmitoylation‐deficient AtAAF mutants. Our data demonstrate an interaction between the tetraspanin AAF and auxin/JA that regulates the size of flower organs and impacts various developmental processes.

Embryology in Helosis cayennensis (Balanophoraceae): Structure of Female Flowers, Fruit, Endosperm and Embryo.

Helosis cayennensis (Balanophoraceae s.str.) is a holoparasite characterised by aberrant vegetative bodies and tiny, reduced unisexual flowers. Here, we analysed the development of female flowers to elucidate their morpho-anatomy and the historical controversy on embryo sac formation. We also studied the developmental origin of inflorescences and the ontogeny of fruits, embryo and endosperm and discussed in a phylogenetic framework. Inflorescences were analysed by optical, fluorescence and scanning electron microscopy. Inflorescences of H. cayennensis arise endogenously. Female flowers lack perianth organs, thus only consist of the ovary, two styles and stigmata. Ovules are undifferentiated; two megaspore mother cells develop inside a nucellar complex. The female gametophyte, named Helosis-type, is a bisporic four-celled embryo sac, provided with a typical egg apparatus and a uni-nucleated central cell. Fertilization was not observed, yet a few-celled embryo and cellular endosperm developed. In sum, results confirm that, among Santalales holoparasites, Helosis is intermediate in the reduction series of its floral organs. Although perianth absence best supports the Balanophoraceae s.str. clade, our literature survey on female flower developmental data across Balanophoraceae s.l. highlights the many gaps that need to be filled to really understand these features in the light of new phylogenetic relationships.

Revised description, systematic affinities, and biogeographic considerations of the Fijian endemic Aristolochia vitiensis (Aristolochiaceae)

The poorly known Aristolochia vitiensis is one of the few species of the genus that inhabits SW Pacific Islands. Here we revisit and complement the description of this Fijian endemic. Our study indicates that the species belongs to the subgenus Aristolochia sect. Diplolobus subsect. Euaristolochia. The trifid apex of the perianth is unique among the species of the subgenus Aristolochia and it indicates the presence of three distinct perianth organs, a trait that very rarely can be observed in anthetic flowers.

Spiral phyllotaxis underlies constrained variation in Anemone (Ranunculaceae) tepal arrangement

Stabilization and variation of floral structures are indispensable for plant reproduction and evolution; however, the developmental mechanism regulating their structural robustness is largely unknown. To investigate this mechanism, we examined positional arrangement (aestivation) of excessively produced perianth organs (tepals) of six- and seven-tepaled (lobed) flowers in six Anemone species (Ranunculaceae). We found that the tepal arrangement that occurred in nature varied intraspecifically between spiral and whorled arrangements. Moreover, among the studied species, variation was commonly limited to three types, including whorls, despite five geometrically possible arrangements in six-tepaled flowers and two types among six possibilities in seven-tepaled flowers. A spiral arrangement, on the other hand, was unique to five-tepaled flowers. A spiral phyllotaxis model with stochasticity on initiating excessive primordia accounted for these limited variations in arrangement in cases when the divergence angle between preexisting primordia was less than 144°. Moreover, interspecific differences in the frequency of the observed arrangements were explained by the change of model parameters that represent meristematic growth and differential organ growth. These findings suggest that the phyllotaxis parameters are responsible for not only intraspecific stability but interspecific difference of floral structure. Decreasing arrangements from six-tepaled to seven-tepaled Anemone flowers demonstrate that the stabilization occurs as development proceeds to increase the component (organ) number, in contrast from the intuition that the variation will be larger due to increasing number of possible states (arrangements).

The Turkish EndemicPseudodelphinium turcicum(Ranunculaceae): An Unusual Population ofDelphiniumwith Peloric Flowers That Has Persisted in the Wild for 20 Years

Premise of research. The Turkish endemic Pseudodelphinium turcicum (Ranunculaceae) was described by Vural et al. (2012) based on a single population found in the salt lake basin of Tuz Gölü, Konya Province. Although the authors noticed morphological similarities between the plants and Delphinium or Garidella, they highlighted unusual features, which led them to describe a new genus consisting of a single species, P. turcicum. The aim of this study is to investigate the morphological particularities and evolution of Pseudodelphinium.Methodology. We carried out morphological, anatomical, and palynological studies on individuals of P. turcicum. We also conducted a molecular phylogenetic analysis to identify the closest relatives of this species.Pivotal results. This combination of approaches shows that the species belongs in the group of Mediterranean species in Delphinium subg. Delphinium. Based on our sampling, P. turcicum is more precisely sister to the Turkish endemic Delphinium venulosum. The perianth of P. turcicum flowers consists of tepals that are, anatomically speaking, similar to the sepals of Delphinium. A second type of perianth organs was evidenced. They are located between the tepals and the stamens, and their development is arrested at an early stage, therefore being comparable to the ventral petals of Delphinium flowers. The pollen of P. turcicum is heteromorphic.Conclusions. We conclude that the population of P. turcicum is an unusual population of Delphinium presenting peloric flowers (through ventralization) that has been maintained in the wild for at least two decades. We provide hypotheses concerning the identity of the floral organs, as well as an explanation for the origin of the particular floral morphology.

The ancestral flower of angiosperms and its early diversification

Recent advances in molecular phylogenetics and a series of important palaeobotanical discoveries have revolutionized our understanding of angiosperm diversification. Yet, the origin and early evolution of their most characteristic feature, the flower, remains poorly understood. In particular, the structure of the ancestral flower of all living angiosperms is still uncertain. Here we report model-based reconstructions for ancestral flowers at the deepest nodes in the phylogeny of angiosperms, using the largest data set of floral traits ever assembled. We reconstruct the ancestral angiosperm flower as bisexual and radially symmetric, with more than two whorls of three separate perianth organs each (undifferentiated tepals), more than two whorls of three separate stamens each, and more than five spirally arranged separate carpels. Although uncertainty remains for some of the characters, our reconstruction allows us to propose a new plausible scenario for the early diversification of flowers, leading to new testable hypotheses for future research on angiosperms.

Differences in DNA Binding Specificity of Floral Homeotic Protein Complexes Predict Organ-Specific Target Genes.

Floral organ identities in plants are specified by the combinatorial action of homeotic master regulatory transcription factors. However, how these factors achieve their regulatory specificities is still largely unclear. Genome-wide in vivo DNA binding data show that homeotic MADS domain proteins recognize partly distinct genomic regions, suggesting that DNA binding specificity contributes to functional differences of homeotic protein complexes. We used in vitro systematic evolution of ligands by exponential enrichment followed by high-throughput DNA sequencing (SELEX-seq) on several floral MADS domain protein homo- and heterodimers to measure their DNA binding specificities. We show that specification of reproductive organs is associated with distinct binding preferences of a complex formed by SEPALLATA3 and AGAMOUS. Binding specificity is further modulated by different binding site spacing preferences. Combination of SELEX-seq and genome-wide DNA binding data allows differentiation between targets in specification of reproductive versus perianth organs in the flower. We validate the importance of DNA binding specificity for organ-specific gene regulation by modulating promoter activity through targeted mutagenesis. Our study shows that intrafamily protein interactions affect DNA binding specificity of floral MADS domain proteins. Differential DNA binding of MADS domain protein complexes plays a role in the specificity of target gene regulation.

The ABCs of flower development: mutational analysis of AP1/FUL-like genes in rice provides evidence for a homeotic (A)-function in grasses.

The well-known ABC model describes the combinatorial interaction of homeotic genes in specifying floral organ identities. While the B- and C-functions are highly conserved throughout flowering plants and even in gymnosperms, the A-function, which specifies the identity of perianth organs (sepals and petals in eudicots), remains controversial. One reason for this is that in most plants that have been investigated thus far, with Arabidopsis being a remarkable exception, one does not find recessive mutants in which the identity of both types of perianth organs is affected. Here we report a comprehensive mutational analysis of all four members of the AP1/FUL-like subfamily of MADS-box genes in rice (Oryza sativa). We demonstrate that OsMADS14 and OsMADS15, in addition to their function of specifying meristem identity, are also required to specify palea and lodicule identities. Because these two grass-specific organs are very likely homologous to sepals and petals of eudicots, respectively, we conclude that there is a floral homeotic (A)-function in rice as defined previously. Together with other recent findings, our data suggest that AP1/FUL-like genes were independently recruited to fulfil the (A)-function in grasses and some eudicots, even though other scenarios cannot be excluded and are discussed.