Don't be fooled: false flowers in Asteraceae

Putting heads together: Developmental genetics of the Asteraceae capitulum

Inflorescence architecture is highly variable across plant lineages yet is critical for facilitating reproductive success. The capitulum-type inflorescence of the Asteraceae is marked as a key morphological innovation that preceded the family's diversification and expansion. Despite its evolutionary significance, our understanding of capitulum development and evolution is limited. This review highlights our current perspective on capitulum evolution through the lens of both its molecular and developmental underpinnings. We attempt to summarize our understanding of the capitulum by focusing on two key characteristics: patterning (arrangement of florets on a capitulum) and floret identity specification. Note that these two features are interconnected such that the identity of florets depends on their position along the inflorescence axis. Phytohormones such as auxin seemingly determine both pattern progression and floret identity specification through unknown mechanisms. Floret morphology in a head is controlled by differential expression of floral symmetry genes regulating floret identity specification. We briefly summarize the applicability of the ABCE quartet model of flower development in regulating the floret organ identity of a capitulum in Asteraceae. Overall, there have been promising advancements in our understanding of capitula; however, comprehensive functional genetic analyses are necessary to fully dissect the molecular pathways and mechanisms involved in capitulum development.

The hidden diversity of vascular patterns in flower heads

Vascular systems are intimately related to the shape and spatial arrangement of the plant organs they support. We investigate the largely unexplored association between spiral phyllotaxis and the vascular system in Asteraceae flower heads. We imaged heads of eight species using synchrotron-based X-ray micro-computed tomography and applied original virtual reality and haptic software to explore head vasculature in three dimensions. We then constructed a computational model to infer a plausible patterning mechanism. The vascular system in the head of the model plant Gerbera hybrida is qualitatively different from those of Bellis perennis and Helianthus annuus, characterized previously. Cirsium vulgare, Craspedia globosa, Echinacea purpurea, Echinops bannaticus, and Tanacetum vulgare represent variants of the Bellis and Helianthus systems. In each species, the layout of the main strands is stereotypical, but details vary. The observed vascular patterns can be generated by a common computational model with different parameter values. In spite of the observed differences of vascular systems in heads, they may be produced by a conserved mechanism. The diversity and irregularities of vasculature stand in contrast with the relative uniformity and regularity of phyllotactic patterns, confirming that phyllotaxis in heads is not driven by the vasculature.

South America holds the greatest diversity of native daisies (Asteraceae) in the world: an updated catalogue supporting continental-scale conservation

Asteraceae is the world's richest plant family and is found on all continents, in environments ranging from the coast to the highest mountains. The family shows all growth forms and, as in other angiosperm families, species richness is concentrated in tropical regions. South America has the highest diversity of Asteraceae in the world, yet taxonomic and distributional knowledge gaps remain. This study compiles an updated catalog of Asteraceae native to South America, based on national and regional checklists and ongoing large-scale flora projects. The resulting checklist includes a total of 6,940 species and 564 genera native to South America to date, which represent about a quarter of the family's global diversity. Countries already considered to be megadiverse show the greatest diversity, such as Brazil with 2,095 species, followed by Peru (1,588), Argentina (1,377), and Colombia (1,244), with this diversity mainly focused on the Brazilian Highlands and the Andes. Species endemism also peaks in Brazil, but Sørensen distances reveal the Chilean flora to be eminently different from the rest of the continent. Tribes better represented in the continent are Eupatorieae, Senecioneae and Astereae, also with a remarkably presence of entirely South American subfamilies representing earliest diverging lineages of the Asteraceae, such as Barnadesioideae, Wunderlichioideae, Famatinanthoideae, and Stifftioideae. It is estimated that the discovery and description curves have not yet stabilized, and the number of species is likely to increase by 5 to 10% in the coming years, posing major challenges to continental-scale conservation.

Development and evolution of the Asteraceae capitulum

Asteraceae represent one of the largest and most diverse families of plants. The evolutionary success of this family has largely been contributed to their unique inflorescences, capitula that mimic solitary flowers but are typically aggregates of multiple florets. Here, we summarize the recent molecular and genetic level studies that have promoted our understanding of the development and evolution of capitula. We focus on new results on patterning of the enlarged meristem resulting in the iconic phyllotactic arrangement of florets in Fibonacci numbers of spirals. We also summarize the current understanding of the genetic networks regulating the characteristic reproductive traits in the family such as floral dimorphism and differentiation of highly specialized floral organs. So far, developmental studies in Asteraceae are still limited to a very narrow selection of model species. Along with the recent advancements in genomics and phylogenomics, Asteraceae and its relatives provide an outstanding model clade for extended evo-devo studies to exploit the morphological diversity and the underlying molecular networks and to translate this knowledge to the breeding of the key crops in the family.

Pseudanthia in angiosperms: a review

BACKGROUND

Pseudanthia or 'false flowers' are multiflowered units that resemble solitary flowers in form and function. Over the last century the term 'pseudanthium' has been applied to a wide array of morphologically divergent blossoms, ranging from those with easily noticeable florets to derived, reduced units in which individual flowers become almost indistinguishable. Although initially admired mostly by botanists, the diversity and widespread distribution of pseudanthia across angiosperms has already made them a fascinating topic for evolutionary and developmental comparative studies.

SCOPE

This review synthesizes historical and current concepts on the biology of pseudanthia. Our first aim is to establish a clear, operational definition of pseudanthium and disentangle common terminological misconceptions surrounding that term. Our second aim is to summarize knowledge of the morphological and developmental diversity of pseudanthia and embed it within a modern phylogenetic framework. Lastly, we want to provide a comprehensive overview on the evolution and ecological importance of pseudanthia and outline perspectives for future studies.

CONCLUSIONS

The understanding of pseudanthia has changed multiple times and reflects three different interpretations of their 'flower-like' qualities: developmental (similarity in structure), figural (similarity in form and function) and phylogenetic (homology between angiosperm flowers and monoecious reproductive shoots in gymnosperms). Here, we propose to narrow the term pseudanthium to multiflowered blossoms resembling zoophilous flowers in form, i.e. in being structurally subdivided in a showy periphery and a reproductive centre. According to this definition, pseudanthia sensu stricto evolved independently in at least 41 angiosperm families. The recurrent acquisition of pseudanthia sensu stricto in all major lineages of flowering plants indicates repeated interactions between developmental constraints (smallness of flowers, meristematic conditions) and selective pressures, such as demands of pollinators and/or environmental conditions.

Generic concepts and species diversity within the Gynoxyoid clade (Senecioneae, Compositae)

The Gynoxyoid clade of the Senecioneae (Asteraceae) until now included the five genera Aequatorium, Gynoxys, Nordenstamia, Paracalia and Paragynoxys as diagnosed using selected morphological characters. In their pre-phylogenetic circumscription, the genera Aequatorium and Paragynoxys were considered to inhabit the northern Andes in contrast to Nordenstamia and Paracalia that occur in the central Andes. The most species-rich genus, Gynoxys, was believed to be distributed throughout the Andes. We use a recently established plastid phylogenomic framework that rendered Gynoxys paraphyletic to further evaluate the delimitation of genera in the Gynoxyoid clade. We examine the morphological variation of all members of the Gynoxyoid to identify characters potentially informative at genus level. This results in a matrix of eleven, mostly multistate characters, including those originally used to diagnose these genera. The ancestral character state inference displays a high level of homoplasy, but nevertheless supports the recognition of four genera. Aequatorium is characterised by white radiate capitula. Paracalia and Paragynoxys share white flowers and floral characteristics, such as flower opening and length of disc flowers lobes, as plesiomorphic states, but differ in habit (scandent shrubs vs. trees). Paracalia also retained white flowers, but its two species are characterised by the absence of outer phyllaries. The genera Gynoxys and Nordenstamia comprise species with yellow capitula which appear to be a derived feature in the Gynoxyoids. The genus Nordenstamia, with eight species, is synonymised under Gynoxys since molecular evidence shows its species nested within various parts of the Gynoxys subclade and the morphological variation of Nordenstamia falls well within that of Gynoxys. With the goal to assign all species to four genera (Aequatorium, Gynoxys, Paracalia and Paragynoxys), we assess the states for the eleven characters for all members of the Gynoxyoids and generate new ETS and ITS sequences for 171 specimens belonging to 49 species to further support their generic placement. We provide a taxonomic treatment for the four genera recognised here including amended diagnoses and morphological descriptions. Furthermore, a species-level taxonomic backbone is elaborated for all genera using electronic tools that list 158 currently accepted names and synonyms (209 names in total) with the respective protologue and type information, as well as notes on the current understanding of species limits. Eleven names are newly synonymised, two are lectotypified and eight are newly transferred to other genera.

HD-ZIP Transcription Factors and Brassinosteroid Signaling Play a Role in Capitulum Patterning in Chrysanthemum

Chrysanthemum is a genus in the Asteraceae family containing numerous cut flower varieties with high ornamental value. It owes its beauty to the composite flower head, which resembles a compact inflorescence. This structure is also known as a capitulum, in which many ray and disc florets are densely packed. The ray florets are localized at the rim, are male sterile, and have large colorful petals. The centrally localized disc florets develop only a small petal tube but produce fertile stamens and a functional pistil. Nowadays, varieties with more ray florets are bred because of their high ornamental value, but, unfortunately, this is at the expense of their seed setting. In this study, we confirmed that the disc:ray floret ratio is highly correlated to seed set efficiency, and therefore, we further investigated the mechanisms that underlie the regulation of the disc:ray floret ratio. To this end, a comprehensive transcriptomics analysis was performed in two acquired mutants with a higher disc:ray floret ratio. Among the differentially regulated genes, various potential brassinosteroid (BR) signaling genes and HD-ZIP class IV homeodomain transcription factors stood out. Detailed follow-up functional studies confirmed that reduced BR levels and downregulation of HD-ZIP IV gene Chrysanthemum morifolium PROTODERMAL FACTOR 2 (CmPDF2) result in an increased disc:ray floret ratio, thereby providing ways to improve seed set in decorative chrysanthemum varieties in the future.

Genome-wide identification of the MIKCc-type MADS-box gene family in Chrysanthemum lavandulifolium reveals their roles in the capitulum development

Chrysanthemum ×morifolium is well known throughout the world for its diverse and exquisite flower types. However, due to the complicated genetic background of C. ×morifolium, it is difficult to understand the molecular mechanism of its flower development. And it limits the molecular breeding of improving chrysanthemum flower types. C. ×morifolium has the typical radial capitulum, and many researches showed that the members of the MIKCc-type MADS box gene family play a key role in the formation and development of the capitulum. However, it has been difficult to isolate the important MIKCc and investigate their roles in this process due to the lack of genomic information in chrysanthemum. Here, we identified MIKCc-type MADS box genes at whole genome-wide level in C. lavandulifolium, a diploid species closely related to C. ×morifolium, and investigated their roles in capitulum development by gene expression pattern analysis and protein interaction analysis. A total of 40 ClMIKCc were identified and were phylogenetically grouped into 12 clades. Members of all clades showed different enriched expression patterns during capitulum formation. We speculate that the E-class genes in C. lavandulifolium underwent subfunctionalization because they have a significantly expanded, more diverse expression patterns, and specifically tissue expression than AtSEPs. Meanwhile, we detected the C-class expressed in disc floret corolla, which could be the clue to explore the morphological differences between disc and ray floret corolla. In addition, the potential roles of some MIKCcs in complex inflorescence formation were explored by comparing the number and phylogenetic relationship of MIKCc subfamily members in Asteraceae with different capitulum types. Members of the FLC branch in Asteraceae were found to be possibly related to the differentiation and development of the ray floret.

Flower-like meristem conditions and spatial constraints shape architecture of floral pseudanthia in Apioideae

BACKGROUND

Pseudanthia are multiflowered units that resemble single flowers, frequently by association with pseudocorollas formed by enlarged peripheral florets (ray flowers). Such resemblance is not only superficial, because numerous pseudanthia originate from peculiar reproductive meristems with flower-like characteristics, i.e. floral unit meristems (FUMs). Complex FUM-derived pseudanthia with ray flowers are especially common in Apiaceae, but our knowledge about their patterning is limited. In this paper, we aimed to investigate both the genetic and morphological basis of their development.

RESULTS

We analysed umbel morphogenesis with SEM in six species representing four clades of Apiaceae subfamily Apioideae with independently acquired floral pseudanthia. Additionally, using in situ hybridization, we investigated expression patterns of LEAFY (LFY), UNUSUAL FLORAL ORGANS (UFO), and CYCLOIDEA (CYC) during umbel development in carrot (Daucus carota subsp. carota). Here, we show that initial differences in size and shape of umbel meristems influence the position of ray flower formation, whereas an interplay between peripheral promotion and spatial constraints in umbellet meristems take part in the establishment of specific patterns of zygomorphy in ray flowers of Apiaceae. This space-dependent patterning results from flower-like morphogenetic traits of the umbel which are also visible at the molecular level. Transcripts of DcLFY are uniformly distributed in the incipient umbel, umbellet and flower meristems, while DcCYC shows divergent expression in central and peripheral florets.

CONCLUSIONS

Our results indicate that umbels develop from determinate reproductive meristems with flower-like characteristics, which supports their recognition as floral units. The great architectural diversity and complexity of pseudanthia in Apiaceae can be explained by the unique conditions of FUMs-an interplay between expression of regulatory genes, specific spatio-temporal ontogenetic constraints and morphogenetic gradients arising during expansion and repetitive fractionation. Alongside Asteraceae, umbellifers constitute an interesting model for investigation of patterning in complex pseudanthia.

Co-option of a carotenoid cleavage dioxygenase gene (CCD4a) into the floral symmetry gene regulatory network contributes to the polymorphic floral shape-color combinations in Chrysanthemum sensu lato

Morphological novelties, including formation of trait combinations, may result from de novo gene origination and/or co-option of existing genes into other developmental contexts. A variety of shape-color combinations of capitular florets occur in Chrysanthemum and its allies. We hypothesized that co-option of a carotenoid cleavage dioxygenase gene into the floral symmetry gene network would generate a white zygomorphic ray floret. We tested this hypothesis in an evolutionary context using species in Chrysanthemum sensu lato, a monophyletic group with diverse floral shape-color combinations, based on morphological investigation, interspecific crossing, molecular interaction and transgenic experiments. Our results showed that white color was significantly associated with floret zygomorphy. Specific expression of the carotenoid cleavage dioxygenase gene CCD4a in marginal florets resulted in white color. Crossing experiments between Chrysanthemum lavandulifolium and Ajania pacifica indicated that expression of CCD4a is trans-regulated. The floral symmetry regulator CYC2g can activate expression of CCD4a with a dependence on TEOSINTE BRANCHED1/CYCLOIDEA/PROLIFERATING (TCP) binding element 8 on the CCD4a promoter. Based on all experimental findings, we propose that gene co-option of carotenoid degradation into floral symmetry regulation, and the subsequent dysfunction or loss of either CYC2g or CCD4a, may have led to evolution of capitular shape-color patterning in Chrysanthemum sensu lato.

Evolution and development of inflorescences and floral symmetry in Solanaceae

PREMISE

The inflorescences of Solanaceae are unique and complex, which has led to long-standing disputes over floral symmetry mainly due to different interpretations of the cyme-like inflorescence structure. The main disagreements have been over how the phyllomes associated with the flower were arranged relative to the inflorescence axis especially during early flower initiation.

METHODS

Here we investigated the evolution of inflorescences in Solanaceae by analyzing inflorescence structure in the context of phylogeny using ancestral state reconstruction (ASR) to determine the evolutionary transitions between loosely arranged and tightly clustered inflorescences and between monochasial-like and dichasial-like cymes. We also reconstructed two- and three-dimensional models for 12 solanaceous species that represent both inflorescence and phylogenetic diversity in the family.

RESULTS

Our results indicate that the most recent common ancestor of Solanaceae had a loosely arranged and monochasial-like cyme, while tightly clustered inflorescences and dichasial-like cymes were derived. Compared to the known process of scorpioid cyme evolution, Solanaceae achieved their scorpioid cyme-like inflorescences through a previously undescribed way. Along the pedicel, the two flower-preceding prophylls are not in the typical transverse position of dicotyledonous plants; they frequently have axillary buds, and the main inflorescence axis continues in a sympodial fashion. As a result, the plane of symmetry of the flower is 36° from the median, and the inflorescence axis and the two flower-preceding prophylls are symmetrically located along that plane.

CONCLUSIONS

A better understanding of the morphological evolution of solanaceous inflorescence structure helped clarify the floral symmetry of Solanaceae.