The Evolution of Diverse Floral Morphologies

Tomato DC1 domain protein SlCHP16 interacts with the 14-3-3 protein TFT12 to regulate flower development

Flower development is of great significance for plant reproductive growth, but the molecular mechanisms underlying flower development remain to be fully understood. In this study, a tomato (Solanum lycopersicum L.) Divergent C1 (DC1) domain protein SlCHP16 was identified as a negative regulator of flower development. Overexpression of SlCHP16 led to the delay of flower bud development and failure of flowers to blossom and bear fruits. Conversely, down-regulation of SlCHP16 transcripts, via RNA interference (RNAi), led to formation of larger flowers in transgenic tomato plants. In SlCHP16-overexpressing plants, floral primordia and floral organs were initiated normally, but their subsequent growth and development were severely arrested. Transcriptome analysis showed that this arrest was associated with the changes in expression levels of a large number of genes involved in cell division and organ development. Tomato 14-3-3 protein 12 (TFT12) was identified as an interacting protein of SlCHP16 by tandem mass spectrometry, and its overexpression in tomato plants led to the formation of enlarged flowers. The presence of SlCHP16 disturbed the stability and homodimerization of TFT12 in plant cells. The results of this study demonstrate an inhibitory role of SlCHP16 in flower development in tomato by interaction with the 14-3-3 protein TFT12. This work provides new insights into the mechanisms that control development of floral organs.

Ontogenetic Mechanisms of Differentiation in Two Salvia Species With Different Pollinators

Shifts between pollinators are a major driver in the evolution and diversification of angiosperms and often involve major changes in flower morphology. These morphological differences typically originate during development, highlighting the importance of integrating ecological and developmental studies. Corolla tube length, in particular, is a key trait in specialized plant-pollinator interactions. Here, we compared flower development in two closely related Salvia species with contrasting corolla tube lengths: Salvia guaranitica, pollinated by hummingbirds, and Salvia stachydifolia, primarily pollinated by bees. We characterized developmental trajectories, floral development duration, and patterns of cell growth and proliferation. Both species shared similar allometric trajectories, differing only in their prolongation, suggesting ontogenetic scaling. However, S. guaranitica exhibited longer and faster corolla tube growth, resulting in a larger final size compared to S. stachydifolia. Corolla tube growth was linked to cell proliferation during the early stages of bud development and rapid anisotropic cell elongation in later stages. Additionally, we observed differences in anisotropic growth rates across basal, middle, and distal regions of the corolla tube. These findings suggest that shifts between pollination syndromes in Salvia species may occur without major changes to basic developmental patterns, but through ontogenetic scaling accompanied by heterochronic changes.

Phenotypic Plasticity and Stability in Plants: Genetic Mechanisms, Environmental Adaptation, Evolutionary Implications, and Future Directions

The phenotypic display, survival, and reproduction of organisms depend on genotype-environment interactions that drive development, evolution, and diversity. Biological systems exhibit two basic but paradoxical features that contribute to developmental robustness: plasticity and stability. However, the understanding of these concepts remains ambiguous. The morphology and structure of plant reproductive organs-flowers and fruits-exhibit substantial stability but display a certain level of plasticity under environmental changes, thus representing promising systems for the study of how stability and plasticity jointly govern plant development and evolution. Beyond the genes underlying organ formation, certain genes may maintain stability and induce plasticity. Variations in relevant genes can induce developmental repatterning, thereby altering stability or plasticity under light and temperature fluctuations, which often affects fitness. The regulation of developmental robustness in plant vegetative organs involves transcriptional and post-transcriptional regulation, epigenetics, and phase separation; however, these mechanisms in the reproductive organs of flowering plants remain poorly investigated. Moreover, genes that specifically determine phenotypic plasticity have rarely been cloned. This review clarifies the concepts and attributes of phenotypic plasticity and stability and further proposes potential avenues and a paradigm to investigate the underlying genes and elucidate how plants adapt and thrive in diverse environments, which is crucial for the design of genetically modified crops.

Well-resolved phylogeny supports repeated evolution of keel flowers as a synergistic contributor to papilionoid legume diversification

The butterfly-shaped keel flower is a highly successful floral form in angiosperms. These flowers steer the mechanical interaction with bees and thus are hypothesized to accelerate pollinator-driven diversification. The exceptionally labile evolution of keel flowers in Papilionoideae (Fabaceae) provides a suitable system to test this hypothesis. Using 1456 low-copy nuclear loci, we confidently resolve the early divergence history of Papilionoideae. Constrained by this backbone phylogeny, we generated a time tree for 3326 Fabales to evaluate the tempo and mode of diversification within a state-dependent evolutionary framework. The first keel flowers emerged c. 59.0 million years ago in Papilionoideae, predating the earliest fossil by 3-4 million years. The Miocene diversification of Papilionoideae coincided with the rapid evolution of keel flowers. At least six independent origins and 32 losses of keel flowers were identified in Papilionoideae, Cercidoideae, and Polygalaceae. However, the state-dependent diversification model was not favored. Lack of radiation associated with keel flowers suggests that diversification within Papilionoideae was not solely driven by pollinator-mediated selection, but instead an outcome of the synergistic effects of multiple innovations, including nitrogen fixation and chemical defense, as well as dispersal into subtropical and temperate regions.

Genomic, transcriptomic, and metabolomic analyses reveal convergent evolution of oxime biosynthesis in Darwin's orchid

Angraecum sesquipedale, also known as Darwin's orchid, possesses an exceptionally long nectar spur. Charles Darwin predicted the orchid to be pollinated by a hawkmoth with a correspondingly long proboscis, later identified as Xanthopan praedicta. In this plant-pollinator interaction, the A. sesquipedale flower emits a complex blend of scent compounds dominated by diurnally regulated oximes (R1R2C = N-OH) to attract crepuscular and nocturnal pollinators. The molecular mechanism of oxime biosynthesis remains unclear in orchids. Here, we present the chromosome-level genome of A. sesquipedale. The haploid genome size is 2.10 Gb and represents 19 pseudochromosomes. Cytochrome P450 encoding genes of the CYP79 family known to be involved in oxime biosynthesis in seed plants are not present in the A. sesquipedale genome nor the genomes of other members of the orchid family. Metabolomic analysis of the A. sesquipedale flower revealed a substantial release of oximes at dusk during the blooming stage. By integrating metabolomic and transcriptomic correlation approaches, flavin-containing monooxygenases (FMOs) encoded by six tandem-repeat genes in the A. sesquipedale genome are identified as catalyzing the formation of oximes present. Further in vitro and in vivo assays confirm the function of FMOs in the oxime biosynthesis. We designate these FMOs as orchid oxime synthases 1-6. The evolutionary aspects related to the CYP79 gene losses and neofunctionalization of FMO-catalyzed biosynthesis of oximes in Darwin's orchid provide new insights into the convergent evolution of biosynthetic pathways.

Genome-wide characterization of the MADS-box gene family in Paeonia ostii and expression analysis of genes related to floral organ development

BACKGROUND

Paeonia section Moutan DC. is a significant perennial subshrub, the ornamental value of which heavily depends on the type of flower it possesses. MADS-box transcription factors have a particular impact on the intricate process of floral organ development and differentiation. The release of the whole-genome data from Paeonia ostii now allows us to conduct a thorough investigation of the tree peony MADS-box gene family.

RESULTS

In this study, we identified 110 MADS-box genes in Paeonia ostii that were classified into 5 subgroups. Gene structure, domain and motif analyses revealed the conservation of the structure of these subgroups. Analysis of the cis-acting elements revealed that the 110 PoMADS genes contained different kinds of hormones and stress-related cis-acting elements in their promoter regions. Quantitative real-time PCR analysis was employed to validate the expression patterns of some PoMADS genes related to floral organ development. Genome collinearity analysis with Arabidopsis and grape revealed the conservation of PoMADS genes during evolution. A total of 857 SSRs were identified by analysing the genome sequences of identified genes. We additionally created protein‒protein interaction networks for PoMADS proteins and analysed proteins that could interact among PoMADSs in Arabidopsis thaliana and grape.

CONCLUSION

These findings offer fundamental insights for understanding the function of the MADS-box gene family, which can aid in the selection and breeding of tree peony varieties with high ornamental value in addition to supporting the understanding of the process of tree peony floral organogenesis.

Unveiling the molecular mechanism of sepal curvature in Dendrobium Section Spatulata through full-length transcriptome and RNA-seq analysis

Introduction

Orchids are renowned for their intricate floral structures, where sepals and petals contribute significantly to ornamental value and pollinator attraction. In Dendrobium Section Spatulata, the distinctive curvature of these floral organs enhances both aesthetic appeal and pollination efficiency. However, the molecular and cellular mechanisms underlying this trait remain poorly understood.

Methods

Morphological characteristics of five hybrids were analyzed, with a particular focus on hybrid H5, which exhibits pronounced sepal curling. Full-length transcriptomic sequencing was employed to assemble a reference transcriptome, while RNA-seq identified differentially expressed genes (DEGs) between sepals and petals. Gene ontology and pathway enrichment analyses were conducted to uncover biological processes associated with sepal curvature. Cytological microscopy was used to examine cell size and number, and quantitative real-time PCR (qRT-PCR) was performed to validate transcriptomic findings.

Results

The reference transcriptome contained 94,258 non-redundant transcripts, and RNA-seq identified 821 DEGs between sepals and petals, with 72.8% of these upregulated in sepals. Enrichment analysis revealed the significant involvement of DEGs in cytokinesis, cytoskeletal organization, and energy metabolism. Notably, myosin II filament organization was implicated in generating the mechanical forces responsible for curling, while metabolic pathways provided the energy necessary for these developmental processes. Cytological observations showed that the upper cell layers of the sepal were smaller and more numerous than the lower layers, indicating that differential cell growth contributes to sepal curvature. qRT-PCR analysis validated the differential expression of selected genes, supporting the transcriptomic findings.

Discussion

The interplay of cellular mechanics, cytoskeletal dynamics, and metabolic regulation is crucial in shaping sepal morphology. Future studies involving gene knockdown or overexpression experiments are recommended to validate the roles of specific genes in processes such as actin organization and myosin activity. Such work would provide deeper insights into the contributions of cytoskeletal dynamics and mechanical force generation to sepal morphogenesis.

The overall regulatory network and contributions of ABC(D)E model genes in yellowhorn flower development

BACKGROUND

Single flowers and double flowers, exhibiting distinct flower morphologies, developmental process and regulatory networks, have garnered significant attention recently. In yellowhorn (Xanthoceras sorbifolium), the cause of double flower variation has been elucidated, yet the differences in regulatory networks between single and double flowers remain unexplored.

RESULT

Here, we investigated transcriptional changes underlying flower development among yellowhorn single- and double-flowered-trees. Transcriptome analysis and weighted gene co-expression network analysis (WGCNA) were applied to identify key genes and reveal the characteristics of single and double flower traits. The involvement of development-specific and flower-type-specific DEGs related to hormone signaling, metabolism, growth and development was identified, and ABC(D)E model genes were found to be closely associated with floral organ development. Overexpression of yellowhorn ABC(D)E model genes in Arabidopsis demonstrated their roles in floral organ development, and the interactions between different pairs of genes were also validated by yeast two-hybrid (Y2H) experiments.

CONCLUSION

Therefore we elucidated differences between yellowhorn single and double flower development, highlighting the roles of yellowhorn ABC(D)E model genes in controlling flower structures, which not only enriches the understanding of yellowhorn flower researches, but also provides insights for studying flower development in other woody species.

Comparative case study of evolutionary insights and floral complexity in key early-diverging eudicot Ranunculales models

Famously referred to as "Darwin's abominable mystery," the rapid diversification of angiosperms over the last ~140 million years presents a fascinating enigma. This diversification is underpinned by complex genetic pathways that evolve and rewire to produce diverse and sometimes novel floral forms. Morphological innovations in flowers are shaped not only by genetics but also by evolutionary constraints and ecological dynamics. The importance of model organisms in addressing the long-standing scientific questions related to diverse floral forms cannot be overstated. In plant biology, Arabidopsis thaliana, a core eudicot, has emerged as a premier model system, with its genome being the first plant genome to be fully sequenced. Similarly, model systems derived from crop plants such as Oryza sativa (rice) and Zea mays (maize) have been invaluable, particularly for crop improvement. However, despite their substantial utility, these model systems have limitations, especially when it comes to exploring the evolution of diverse and novel floral forms. The order Ranunculales is the earliest-diverging lineage of eudicots, situated phylogenetically between core eudicots and monocots. This group is characterized by its exceptional floral diversity, showcasing a wide range of floral morphologies and adaptations that offer valuable insights into the evolutionary processes of flowering plants. Over the past two decades, the development of at least five model systems including, Aquilegia, Thalictrum, Nigella, Delphinium and Eschscholzia within the Ranunculales order has significantly advanced our understanding of floral evolution. This review highlights the conservation and divergence of floral organ identity programs observed among these models and discusses their importance in advancing research within the field. The review also delves into elaborate petal morphology observed in Aquilegia, Nigella, and Delphinium genera, and further discusses the contributions, limitations, and future research directions for Ranunculales model systems. Integrating these diverse models from the early-diverging eudicot order has enhanced our understanding of the complex evolutionary pathways that shape floral diversity in angiosperms, bridging the knowledge gaps essential for a comprehensive understanding of floral evolution.

Blooming balloons: Searching for mechanisms of the inflated calyx

Studying morphological novelties offers special insights into developmental biology and evolution. The inflated calyx syndrome (ICS) is a largely unrecognized but fascinating feature of flower development, where sepals form balloon-like husks that encapsulate fruits. Despite its independent emergence in many lineages of flowering plants, the genetic and molecular mechanisms of ICS remain unknown. Early studies in the Solanaceae genus Physalis put forth key roles of MADS-box genes in ICS. However, recent work suggests these classical floral identity transcription factors were false leads. With newfound capabilities that allow rapid development of genetic systems through genomics and genome editing, Physalis has re-emerged as the most tractable model species for dissecting ICS. This review revisits current understanding of ICS and highlights how recent advancements enable a reset in the search for genetic and molecular mechanisms using unbiased, systematic approaches.

Deciphering the evolutionary development of the "Chinese lantern" within Solanaceae

MAIN CONCLUSION

The key genetic variation underlying the evo-devo of ICS in Solanaceae may be further pinpointed using an integrated strategy of forward and reverse genetics studies under the framework of phylogeny. The calyx of Physalis remains persistent throughout fruit development. Post-flowering, the fruiting calyx is inflated rapidly to encapsulate the berry, giving rise to a "Chinese lantern" structure called inflated calyx syndrome (ICS). It is unclear how this novelty arises. Over the past 2 decades, the role of MADS-box genes in the evolutionary development (evo-devo) of ICS has mainly been investigated within Solanaceae. In this review, we analyze the main achievements, challenges, and new progress. ICS acts as a source for fruit development, provides a microenvironment to protect fruit development, and assists in long-distance fruit dispersal. ICS is a typical post-floral trait, and the onset of its development is triggered by specific developmental signals that coincide with fertilization. These signals can be replaced by exogenous gibberellin and cytokinin application. MPF2-like heterotopic expression and MBP21-like loss have been proposed to be two essential evolutionary events for ICS origin, and manipulating the related MADS-box genes has been shown to affect the ICS size, sepal organ identity, and/or male fertility, but not completely disrupt ICS. Therefore, the core genes or key links in the ICS biosynthesis pathways may have undergone secondary mutations during evolution, or they have not yet been pinpointed. Recently, we have made some encouraging progress in acquiring lantern mutants in Physalis floridana. In addition to technological innovation, we propose an integrated strategy to further analyze the evo-devo mechanisms of ICS in Solanaceae using forward and reverse genetics studies under the framework of phylogeny.

HaMADS3, HaMADS7, and HaMADS8 are involved in petal prolongation and floret symmetry establishment in sunflower (Helianthus annuus L.)

The development of floral organs, crucial for the establishment of floral symmetry and morphology in higher plants, is regulated by MADS-box genes. In sunflower, the capitulum is comprised of ray and disc florets with various floral organs. In the sunflower long petal mutant (lpm), the abnormal disc (ray-like) floret possesses prolongated petals and degenerated stamens, resulting in a transformation from zygomorphic to actinomorphic symmetry. In this study, we investigated the effect of MADS-box genes on floral organs, particularly on petals, using WT and lpm plants as materials. Based on our RNA-seq data, 29 MADS-box candidate genes were identified, and their roles on floral organ development, especially in petals, were explored, by analyzing the expression levels in various tissues in WT and lpm plants through RNA-sequencing and qPCR. The results suggested that HaMADS3, HaMADS7, and HaMADS8 could regulate petal development in sunflower. High levels of HaMADS3 that relieved the inhibition of cell proliferation, together with low levels of HaMADS7 and HaMADS8, promoted petal prolongation and maintained the morphology of ray florets. In contrast, low levels of HaMADS3 and high levels of HaMADS7 and HaMADS8 repressed petal extension and maintained the morphology of disc florets. Their coordination may contribute to the differentiation of disc and ray florets in sunflower and maintain the balance between attracting pollinators and producing offspring. Meanwhile, Pearson correlation analysis between petal length and expression levels of MADS-box genes further indicated their involvement in petal prolongation. Additionally, the analysis of cis-acting elements indicated that these three MADS-box genes may regulate petal development and floral symmetry establishment by regulating the expression activity of HaCYC2c. Our findings can provide some new understanding of the molecular regulatory network of petal development and floral morphology formation, as well as the differentiation of disc and ray florets in sunflower.

Complex petal spot formation in the Beetle Daisy (Gorteria diffusa) relies on spot-specific accumulation of malonylated anthocyanin regulated by paralogous GdMYBSG6 transcription factors

Gorteria diffusa has elaborate petal spots that attract pollinators through sexual deception, but how G. diffusa controls spot development is largely unknown. Here, we investigate how pigmentation is regulated during spot formation. We determined the anthocyanin composition of G. diffusa petals and combined gene expression analysis with protein interaction assays to characterise R2R3-MYBs that likely regulate pigment production in G. diffusa petal spots. We found that cyanidin 3-glucoside pigments G. diffusa ray floret petals. Unlike other petal regions, spots contain a high proportion of malonylated anthocyanin. We identified three subgroup 6 R2R3-MYB transcription factors (GdMYBSG6-1,2,3) that likely activate the production of spot pigmentation. These genes are upregulated in developing spots and induce ectopic anthocyanin production upon heterologous expression in tobacco. Interaction assays suggest that these transcription factors regulate genes encoding three anthocyanin synthesis enzymes. We demonstrate that the elaboration of complex spots in G. diffusa begins with the accumulation of malonylated pigments at the base of ray floret petals, positively regulated by three paralogous R2R3-MYB transcription factors. Our results indicate that the functional diversification of these GdMYBSG6s involved changes in the spatial control of their transcription, and modification of the duration of GdMYBSG6 gene expression contributes towards floral variation within the species.

Comparative analysis of floral transition and floral organ formation in two contrasting species: Disocactus speciosus and D. eichlamii (Cactaceae)

KEY MESSAGE

Contrasting morphologies in Disocactus are the result of differential development of the vegetative and floral tissue where intercalary growth is involved, resulting in a complex structure, the floral axis. Species from the Cactaceae bear adaptations related with their growth in environments under hydric stress. These adaptations have translated into the reduction and modification of various structures such as leaves, stems, lateral branches, roots and the structuring of flowers in a so-called flower-shoot. While cacti flowers and fruits have a consistent structure with showy hermaphrodite or unisexual flowers that produce a fruit called cactidium, the developmental dynamics of vegetative and reproductive tissues comprising the reproductive unit have only been inferred through the analysis of pre-anthetic buds. Here we present a comparative analysis of two developmental series covering the early stages of flower formation and organ differentiation in Disocactus speciosus and Disocactus eichlamii, which have contrasting floral morphologies. We observe that within the areole, a shoot apical meristem commences to grow upward, producing lateral leaves with a spiral arrangement, rapidly transitioning to a floral meristem. The floral meristem produces tepal primordia and a staminal ring meristem from which numerous or few stamens develop in a centrifugal manner in D. speciosus and D. eichlamii, respectively. Also, the inferior ovary derives from the floral meristem flattening and an upward growth of the surrounding tissue of the underlying stem, producing the pericarpel. This structure is novel to cacti and lacks a clear anatomical delimitation with the carpel wall. Here, we present a first study that documents the early processes taking place during initial meristem determination related to pericarpel development and early floral organ formation in cacti until the establishment of mature floral organs.

Heterotrimeric Gα-subunit regulates flower and fruit development in CLAVATA signaling pathway in cucumber

Flowers and fruits are the reproductive organs in plants and play essential roles in natural beauty and the human diet. CLAVATA (CLV) signaling has been well characterized as regulating floral organ development by modulating shoot apical meristem (SAM) size; however, the signaling molecules downstream of the CLV pathway remain largely unknown in crops. Here, we found that functional disruption of CsCLV3 peptide and its receptor CsCLV1 both resulted in flowers with extra organs and stumpy fruits in cucumber. A heterotrimeric G protein α-subunit (CsGPA1) was shown to interact with CsCLV1. Csgpa1 mutant plants derived from gene editing displayed significantly increased floral organ numbers and shorter and wider fruits, a phenotype resembling that of Csclv mutants in cucumber. Moreover, the SAM size was enlarged and the longitudinal cell size of fruit was decreased in Csgpa1 mutants. The expression of the classical stem cell regulator WUSCHEL (WUS) was elevated in the SAM, while the expression of the fruit length stimulator CRABS CLAW (CRC) was reduced in the fruit of Csgpa1 mutants. Therefore, the Gα-subunit CsGPA1 protein interacts with CsCLV1 to inhibit floral organ numbers but promote fruit elongation, via repressing CsWUS expression and activating CsCRC transcription in cucumber. Our findings identified a new player in the CLV signaling pathway during flower and fruit development in dicots, increasing the number of target genes for precise manipulation of fruit shape during crop breeding.

CYCLOIDEA paralogs function partially redundantly to specify dorsal flower development in Mimulus lewisii

PREMISE

Duplicated genes (paralogs) are abundant in plant genomes, and their retention may influence the function of genetic programs and contribute to evolutionary novelty. How gene duplication affects genetic modules and what forces contribute to paralog retention are outstanding questions. The CYCLOIDEA(CYC)-dependent flower symmetry program is a model for understanding the evolution of gene duplication, providing multiple examples of paralog partitioning and novelty. However, a novel CYC gene lineage duplication event near the origin of higher core Lamiales (HCL) has received little attention.

METHODS

To understand the evolutionary fate of duplicated HCL CYC2 genes, we determined the effects on flower symmetry by suppressing MlCYC2A and MlCYC2B expression using RNA interference (RNAi). We determined the phenotypic effects on flower symmetry in single- and double-silenced backgrounds and coupled our functional analyses with expression surveys of MlCYC2A, MlCYC2B, and a putative downstream RADIALIS (MlRAD5) ortholog.

RESULTS

MlCYC2A and MlCYC2B jointly contribute to bilateral flower symmetry. MlCYC2B exhibits a clear dorsal flower identity function and may additionally function in carpel development. MlCYC2A functions in establishing dorsal petal shape. Further, our results suggest an MlCYC2A-MlCYC2B regulatory interaction, which may affect pathway homeostasis.

CONCLUSIONS

Our results suggest that CYC paralogs specific to higher core Lamiales may be selectively retained for their joint contribution to bilateral flower symmetry, similar to the independently derived CYC paralogs in the Lamiales model for bilateral flower symmetry research, Antirrhinum majus (snapdragon).

Cell layer-specific expression of the homeotic MADS-box transcription factor PhDEF contributes to modular petal morphogenesis in petunia

Floral homeotic MADS-box transcription factors ensure the correct morphogenesis of floral organs, which are organized in different cell layers deriving from distinct meristematic layers. How cells from these distinct layers acquire their respective identities and coordinate their growth to ensure normal floral organ morphogenesis is unresolved. Here, we studied petunia (Petunia × hybrida) petals that form a limb and tube through congenital fusion. We identified petunia mutants (periclinal chimeras) expressing the B-class MADS-box gene DEFICIENS in the petal epidermis or in the petal mesophyll, called wico and star, respectively. Strikingly, wico flowers form a strongly reduced tube while their limbs are almost normal, while star flowers form a normal tube but greatly reduced and unpigmented limbs, showing that petunia petal morphogenesis is highly modular. These mutants highlight the layer-specific roles of PhDEF during petal development. We explored the link between PhDEF and petal pigmentation, a well-characterized limb epidermal trait. The anthocyanin biosynthesis pathway was strongly downregulated in star petals, including its major regulator ANTHOCYANIN2 (AN2). We established that PhDEF directly binds to the AN2 terminator in vitro and in vivo, suggesting that PhDEF might regulate AN2 expression and therefore petal epidermis pigmentation. Altogether, we show that cell layer-specific homeotic activity in petunia petals differently impacts tube and limb development, revealing the relative importance of the different cell layers in the modular architecture of petunia petals.

Flower heterochrony and crop yield

Crop improvement has focused on enhancing yield, nutrient content, harvestability, and stress resistance using a trait-centered reductionist approach. This has downplayed the fact that plants are developmentally integrated and respond coordinately and predictably to genetic and environmental variation, with potential consequences for food production. Crop yield, including both fruit/seed production and the possibility of generating hybrid crop varieties, is highly dependent on flower morphology and sex, which, in turn, can be profoundly affected by slight shifts in the timing and rate of flower organ development (i.e., flower heterochrony). We argue that understanding the genetic and environmental bases of flower heterochrony and their effect on flower morphology and sex in cultivated plants and in their wild relatives can facilitate crop improvement.

Quantifying the complexity of plant reproductive structures reveals a history of morphological and functional integration

Vascular plant reproductive structures have undoubtedly become more complex through time, evolving highly differentiated parts that interact in specialized ways. But quantifying these patterns at broad scales is challenging because lineages produce disparate reproductive structures that are often difficult to compare and homologize. We develop a novel approach for analysing interactions within reproductive structures using networks, treating component parts as nodes and a suite of physical and functional interactions among parts as edges. We apply this approach to the plant fossil record, showing that interactions have generally increased through time and that the concentration of these interactions has shifted towards differentiated surrounding organs, resulting in more compact, functionally integrated structures. These processes are widespread across plant lineages, but their extent and timing vary with reproductive biology; in particular, seed-producing structures show them more strongly than spore or pollen-producing structures. Our results demonstrate that major reproductive innovations like the origin of seeds and angiospermy were associated with increased integration through greater interactions among parts. But they also reveal that for certain groups, particularly Mesozoic gymnosperms, millions of years elapsed between the origin of reproductive innovations and increased interactions among parts within their reproductive structures.

Organ-enriched gene expression during floral morphogenesis in wild barley

Floral morphology varies considerably between dicots and monocots. The ABCDE model explaining how floral organ development is controlled was formulated using core eudicots and applied to grass crops. Barley (Hordeum. vulgare) has unique floral morphogenesis. Wild barley (H. vulgare ssp. spontaneum), which is the immediate ancestor of cultivated barley (H. vulgare ssp. vulgare), contains a rich reservoir of genetic diversity. However, the wild barley genes involved in floral organ development are still relatively uncharacterized. In this study, we generated an organ-specific transcriptome atlas for wild barley floral organs. Genome-wide transcription profiles indicated that 22 838 protein-coding genes were expressed in at least one organ. These genes were grouped into seven clusters according to the similarities in their expression patterns. Moreover, 5619 genes exhibited organ-enriched expression, 677 of which were members of 47 transcription factor families. Gene ontology analyses suggested that the functions of the genes with organ-enriched expression influence the biological processes in floral organs. The co-expression regulatory network showed that the expression of 690 genes targeted by MADS-box proteins was highly positively correlated with the expression of ABCDE model genes during floral morphogenesis. Furthermore, the expression of 138 genes was specific to the wild barley OUH602 genome and not the Morex genome; most of these genes were highly expressed in the glume, awn, lemma, and palea. This study revealed the global gene expression patterns underlying floral morphogenesis in wild barley. On the basis of the study findings, a molecular mechanism controlling floral morphology in barley was proposed.

Flower morphology as a predictor of pollination mode in a biotic to abiotic pollination continuum

BACKGROUND AND AIMS

Wind pollination has evolved repeatedly in flowering plants, yet the identification of a wind pollination syndrome as a set of integrated floral traits can be elusive. Thalictrum (Ranunculaceae) comprises temperate perennial herbs that have transitioned repeatedly from insect to wind pollination while also exhibiting mixed pollination, providing an ideal system to test for evolutionary correlation between floral morphology and pollination mode in a biotic to abiotic continuum. Moreover, the lack of floral organ fusion across this genus allows testing for specialization to pollination vectors in the absence of this feature.

METHODS

We expanded phylogenetic sampling in the genus from a previous study using six chloroplast loci, which allowed us to test whether species cluster into distinct pollination syndromes based on floral morphology. We then used multivariate analyses on floral traits followed by ancestral state reconstruction of the emerging flower morphotypes and determined whether these traits are evolutionarily correlated under a Bayesian framework with Brownian motion.

KEY RESULTS

Floral traits fell into five distinct clusters, which were reduced to three after considering phylogenetic relatedness and were largely consistent with flower morphotypes and associated pollination vectors. Multivariate evolutionary analyses found a positive correlation between the lengths of floral reproductive structures (styles, stigmas, filaments and anthers). Shorter reproductive structures tracked insect-pollinated species and clades in the phylogeny, whereas longer structures tracked wind-pollinated ones, consistent with selective pressures exerted by biotic vs. abiotic pollination vectors, respectively.

CONCLUSIONS

Although detectable suites of integrated floral traits across Thalictrum were correlated with wind or insect pollination at the extremes of the morphospace distribution, a presumed intermediate, mixed pollination mode morphospace was also detected. Thus, our data broadly support the existence of detectable flower morphotypes from convergent evolution underlying the evolution of pollination mode in Thalictrum, presumably via different paths from an ancestral mixed pollination state.

Development of Flowers Buds and Mixed Buds in the Dichasial Inflorescence of Geranium koreanum Kom. (Geraniaceae)

Flower bud differentiation is of great significance for understanding plant evolution and ecological adaptability. The development of flower buds and mixed buds in the dichasial inflorescence of Geranium koreanum was described in this paper. The morphogenesis, surface structure, and organ morphology at different growth stages of G. koreanum buds were examined in detail using scanning electron microscope and stereo microscope. The development of mixed buds started from the flattened apical meristem. The stipule and leaf primordia arose first. Subsequently, the hemispherical meristem was divided into two hemispheres, forming a terminal bud and floral bud primordia, followed by lateral bud differentiation. The formation of the terminal and lateral buds of G. koreanum was sequential and their differentiation positions were also different. The floral bud primordia would develop into two flower units and four bracts. The primordia of a flower bud first formed the sepal primordia, then the stamen and petal primordia, and finally the pistil primordia. Compared to the stamen primordia, the growth of the petal primordia was slower. Finally, all organs, especially the petals and pistil, grew rapidly. When the pistil and petals exceeded the stamens and the petals changed color, the flower bud was ready to bloom.

A BLADE-ON-PETIOLE orthologue regulates corolla differentiation in the proximal region in Torenia fournieri

The three-dimensional shape of a flower is integrated by morphogenesis along different axes. Differentiation along the petal proximodistal axis is tightly linked to the specification of pollinators; however, it is still unclear how a petal patterns this axis. The corolla of Torenia fournieri exhibits strong differentiation along the proximodistal axis, and we previously found a proximal regulator, TfALOG3, controlling corolla neck differentiation. Here, we report another gene, TfBOP2, which is predominantly expressed in the proximal region of the corolla. TfBOP2 mutants have shorter proximal corolla tubes and longer distal lobe, demonstrating its function as a proximal regulator. Arabidopsis BOPs mutant shows similar defects, favouring a shared role of BOPs homologues. Genetic analysis demonstrates the interaction between TfBOP2 and TfALOG3, and we further found that TfALOG3 physically interacts with TfBOP2 and can recruit TfBOP2 to the nuclear region. Our study favours a hypothetical shared BOP-ALOG complex that is recruited to regulate corolla differentiation in the proximal region axis of T. fournieri.

Combined genome-wide association studies and expression quantitative trait locus analysis uncovers a genetic regulatory network of floral organ number in a tree peony (Paeonia suffruticosa Andrews) breeding population

Great progress has been made in our understanding of floral organ identity determination and its regulatory network in many species; however, the quantitative genetic basis of floral organ number variation is far less well understood for species-specific traits from the perspective of population variation. Here, using a tree peony (Paeonia suffruticosa Andrews, Paeoniaceae) cultivar population as a model, the phenotypic polymorphism and genetic variation based on genome-wide association studies (GWAS) and expression quantitative trait locus (eQTL) analysis were analyzed. Based on 24 phenotypic traits of 271 representative cultivars, the transcript profiles of 119 cultivars were obtained, which indicated abundant genetic variation in tree peony. In total, 86 GWAS-related cis-eQTLs and 3188 trans-eQTL gene pairs were found to be associated with the numbers of petals, stamens, and carpels. In addition, 19 floral organ number-related hub genes with 121 cis-eQTLs were obtained by weighted gene co-expression network analysis, among which five hub genes belonging to the ABCE genes of the MADS-box family and their spatial-temporal co-expression and regulatory network were constructed. These results not only help our understanding of the genetic basis of floral organ number variation during domestication, but also pave the way to studying the quantitative genetics and evolution of flower organ number and their regulatory network within populations.

Pollen production per flower increases with floral display size across animal-pollinated flowering plants

PREMISE

The number of open flowers on a plant (i.e., floral display size) can influence plant fitness by increasing pollinator attraction. However, diminishing marginal fitness returns with increasing floral display are expected as pollinators tend to visit more flowers per plant consecutively. An extended flower visitation sequence increases the fraction of ovules disabled by self-pollination (ovule discounting) and reduces the fraction of a plant's own pollen that is exported to sire seeds in other plants (pollen discounting). Hermaphroditic species with a genetic system that prevents self-fertilization (self-incompatibility) would avoid ovule discounting and its fitness cost, whereas species without such a genetically based barrier would not. Contrarily, pollen discounting would be an unavoidable consequence of a large floral display irrespective of selfing barriers. Nevertheless, the increasing fitness costs of ovule and pollen discounting could be offset by respectively increasing ovule and pollen production per flower.

METHODS

We compiled data on floral display size and pollen and ovule production per flower for 1241 animal-pollinated, hermaphroditic angiosperm species, including data on the compatibility system for 779 species. We used phylogenetic general linear mixed models to assess the relations of pollen and ovule production to floral display size.

RESULTS

Our findings provide evidence of increasing pollen production, but not of ovule production, with increasing display size irrespective of compatibility system and even after accounting for potentially confounding effects like flower size and growth form.

CONCLUSIONS

Our comparative study supports the pollen-discount expectation of an adaptive link between per-flower pollen production and floral display across animal-pollinated angiosperms.

Phylogenetic Analysis of Spliceosome SF3a2 in Different Plant Species

The formation of mature mRNA requires cutting introns and splicing exons. The occurrence of splicing involves the participation of the spliceosome. Common spliceosomes mainly include five snRNPs: U1, U2, U4/U6, and U5. SF3a2, an essential component of spliceosome U2 snRNP, participates in splicing a series of genes. There is no definition of SF3a2 in plants. The paper elaborated on SF3a2s from a series of plants through protein sequence similarity. We constructed the evolutionary relationship of SF3a2s in plants. Moreover, we analyzed the similarities and differences in gene structure, protein structure, the cis-element of the promoter, and expression pattern; we predicted their interacting proteins and constructed their collinearity. We have preliminarily analyzed SF3a2s in plants and clarified the evolutionary relationship between different species; these studies can better serve for in-depth research on the members of the spliceosome in plants.

The origin and evolution of carpels and fruits from an evo-devo perspective

The flower is an evolutionary innovation in angiosperms that drives the evolution of biodiversity. The carpel is integral to a flower and develops into fruits after fertilization, while the perianth, consisting of the calyx and corolla, is decorative to facilitate pollination and protect the internal organs, including the carpels and stamens. Therefore, the nature of flower origin is carpel and stamen origin, which represents one of the greatest and fundamental unresolved issues in plant evolutionary biology. Here, we briefly summarize the main progress and key genes identified for understanding floral development, focusing on the origin and development of the carpels. Floral ABC models have played pioneering roles in elucidating flower development, but remain insufficient for resolving flower and carpel origin. The genetic basis for carpel origin and subsequent diversification leading to fruit diversity also remains elusive. Based on current research progress and technological advances, simplified floral models and integrative evolutionary-developmental (evo-devo) strategies are proposed for elucidating the genetics of carpel origin and fruit evolution. Stepwise birth of a few master regulatory genes and subsequent functional diversification might play a pivotal role in these evolutionary processes. Among the identified transcription factors, AGAMOUS (AG) and CRABS CLAW (CRC) may be the two core regulatory genes for carpel origin as they determine carpel organ identity, determinacy, and functionality. Therefore, a comparative identification of their protein-protein interactions and downstream target genes between flowering and non-flowering plants from an evo-devo perspective may be primary projects for elucidating carpel origin and development.

Geranium sylvaticum increases pollination probability by sexually dimorphic flowers

Sexual dimorphism is expressed as different morphologies between the sexes of a species. Dimorphism is pronounced in gynodioecious populations which consist of female and hermaphrodite individuals. The small size of female flowers in gynodioecious species is often explained by resource re-allocation to seed production instead of large flowers. However, pollinator attraction is critical to female fitness, and factors other than resource savings are needed to explain the small size of female flowers. We hypothesized that the floral size dimorphism in the perennial gynodioecious Geranium sylvaticum (L.) is adaptive in terms of pollination. To test this "pollination hypothesis," we video recorded the small female and large hermaphrodite G. sylvaticum flowers. We parameterized floral visitor behavior when visiting a flower and calculated pollination probabilities by a floral visitor as the probability of touching anther and stigma with the same body part. Pollination probability differed in terms of flower sex and pollinator species. Bumblebees had the highest pollination probability. The small female flowers were more likely to receive pollen via several pollinator groups than the large hermaphrodite flowers. The pollen display of hermaphrodites matched poorly with the stigma display of hermaphrodites, but well with that of females. Although the small size of female flowers is commonly explained by resource re-allocation, we show that sexual dimorphism in flower size may increase the main reproductive functions of the females and hermaphrodites. Dimorphism increases pollination probability in females and fathering probability of the hermaphrodites likely driving G. sylvaticum populations towards dioecy.

Genomic basis of the giga-chromosomes and giga-genome of tree peony Paeonia ostii

Tree peony (Paeonia ostii) is an economically important ornamental plant native to China. It is also notable for its seed oil, which is abundant in unsaturated fatty acids such as α-linolenic acid (ALA). Here, we report chromosome-level genome assembly (12.28 Gb) of P. ostii. In contrast to monocots with giant genomes, tree peony does not appear to have undergone lineage-specific whole-genome duplication. Instead, explosive LTR expansion in the intergenic regions within a short period (~ two million years) may have contributed to the formation of its giga-genome. In addition, expansion of five types of histone encoding genes may have helped maintain the giga-chromosomes. Further, we conduct genome-wide association studies (GWAS) on 448 accessions and show expansion and high expression of several genes in the key nodes of fatty acid biosynthetic pathway, including SAD, FAD2 and FAD3, may function in high level of ALAs synthesis in tree peony seeds. Moreover, by comparing with cultivated tree peony (P. suffruticosa), we show that ectopic expression of class A gene AP1 and reduced expression of class C gene AG may contribute to the formation of petaloid stamens. Genomic resources reported in this study will be valuable for studying chromosome/genome evolution and tree peony breeding.

Gene identification and tissue expression analysis inform the floral organization and color in the basal angiosperm Magnolia polytepala (Magnoliaceae)

In Magnolia polytepala, the formation of floral organization and color was attributed to tissue-dependent differential expression levels of MADS-box genes and anthocyanin biosynthetic genes. In angiosperms, the diversity of floral morphology and organization suggests its value in exploring plant evolution. Magnolia polytepala, an endemic basal angiosperm species in China, possesses three green sepal-like tepals in the outermost whorl and pink petal-like tepals in the inner three whorls, forming unique floral morphology and organization. However, we know little about its underlying molecular regulatory mechanism. Here, we first reported the full-length transcriptome of M. polytepala using PacBio sequencing. A total of 16 MADS-box transcripts were obtained from the transcriptome data, including floral homeotic genes (e.g., MpAPETALA3) and other non-floral homeotic genes (MpAGL6, etc.). Phylogenetic analysis and spatial expression pattern reflected their putative biological function as their homologues in Arabidopsis. In addition, nine structural genes involved in anthocyanin biosynthesis pathway had been screened out, and tepal color difference was significantly associated with their tissue-dependent differential expression levels. This study provides a relatively comprehensive investigation of the MADS-box family and anthocyanin biosynthetic genes in M. polytepala, and will facilitate our understanding of the regulatory mechanism underlying floral organization and color in basal angiosperms.

Temperature-mediated flower size plasticity in Arabidopsis

Organisms can rapidly mitigate the effects of environmental changes by changing their phenotypes, known as phenotypic plasticity. Yet, little is known about the temperature-mediated plasticity of traits that are directly linked to plant fitness such as flower size. We discovered substantial genetic variation in flower size plasticity to temperature both among selfing Arabidopsis thaliana and outcrossing A. arenosa individuals collected from a natural growth habitat. Genetic analysis using a panel of 290 A. thaliana accession and mutant lines revealed that MADS AFFECTING FLOWERING (MAF) 2-5 gene cluster, previously shown to regulate temperature-mediated flowering time, was associated to the flower size plasticity to temperature. Furthermore, our findings pointed that the control of plasticity differs from control of the trait itself. Altogether, our study advances the understanding of genetic and molecular factors underlying plasticity on fundamental fitness traits, such as flower size, in response to future climate scenarios.

Molecular mechanisms and evolutionary history of phytomelatonin in flowering

Flowering is a critical stage in plant life history, which is coordinated by environmental signals and endogenous cues. Phytomelatonin is a widely distributed indoleamine present in all living organisms and plays pleiotropic roles in plant growth and development. Recent evidence has established that phytomelatonin could modulate flowering in many species, probably in a concentration-dependent manner. Phytomelatonin seems to associate with floral meristem identification and floral organ formation, and the fluctuation of phytomelatonin might be important for flowering. Regarding the underlying mechanisms, phytomelatonin interacts with the central components of floral gene regulatory networks directly or indirectly, including the MADS-box gene family, phytohormones, and reactive oxygen species (ROS). From an evolutionary point of view, the actions of phytomelatonin in flowering probably evolved during the period of the diversification of flowering plants and could be regarded as a functional extension of its primary activities. The presumed evolutionary history of phytomelatonin-modulated flowering is proposed, presented in the chronological order of the appearance of phytomelatonin and core flowering regulators, namely DELLA proteins, ROS, and phytohormones. Further efforts are needed to address some intriguing aspects, such as the exploration of the association between phytomelatonin and photoperiodic flowering, phytomelatonin-related floral MADS-box genes, the crosstalk between phytomelatonin and phytohormones, as well as its potential applications in agriculture.

Mutualist- and antagonist-mediated selection contribute to trait diversification of flowers

Flowers are generally short-lived, and they all face a multidimensional challenge because they have to attract mutualists, compel them to vector pollen with minimal investment in rewards, and repel floral enemies during this short time window. Their displays are under complex selection, either consistent or conflicting, to maximize reproductive fitness under heterogeneous environments. The phenological or morphological mismatches between flowers and visitors will influence interspecific competition, resource access, mating success and, ultimately, population and community dynamics. To better understand the effects of the plant visitors on floral traits, it is necessary to determine the functional significance of specific floral traits for the visitors; how plants respond to both mutualists and antagonists through adaptive changes; and to evaluate the net fitness effects of biological mutualisms and antagonism on plants. In this review, we bring together insights from fields as diverse as floral biology, insect behavioral responses, and evolutionary biology to explain the processes and patterns of floral diversity evolution. Then, we discuss the ecological significance of plant responses to mutualists and antagonists from a community perspective, and propose a set of research questions that can guide the research field to integrate studies of plant defense and reproduction.

Integrative Analysis of miRNAs and Their Targets Involved in Ray Floret Growth in Gerbera hybrida

MicroRNAs (miRNAs) are involved in regulating many aspects of plant growth and development at the post-transcriptional level. Gerbera (Gerbera hybrida) is an important ornamental crop. However, the role of miRNAs in the growth and development of gerbera is still unclear. In this study, we used high-throughput sequencing to analyze the expression profiles of miRNAs in ray floret during inflorescence opening. A total of 164 miRNAs were obtained, comprising 24 conserved miRNAs and 140 novel miRNAs. Ten conserved and 15 novel miRNAs were differentially expressed during ray floret growth, and 607 differentially expressed target genes of these differentially expressed miRNAs were identified using psRNATarget. We performed a comprehensive analysis of the expression profiles of the miRNAs and their targets. The changes in expression of five miRNAs (ghy-miR156, ghy-miR164, ghy-miRn24, ghy-miRn75 and ghy-miRn133) were inversely correlated with the changes in expression of their eight target genes. The miRNA cleavage sites in candidate target gene mRNAs were determined using 5′-RLM-RACE. Several miRNA-mRNA pairs were predicted to regulate ray floret growth and anthocyanin biosynthesis. In conclusion, the results of small RNA sequencing provide valuable information to reveal the mechanisms of miRNA-mediated ray floret growth and anthocyanin accumulation in gerbera.

Convergent evolution of a blood-red nectar pigment in vertebrate-pollinated flowers

Beyond sugars, many types of nectar solutes play important ecological roles; however, the molecular basis for the diversity of nectar composition across species is less explored. One rare trait among flowering plants is the production of colored nectar, which may function to attract and guide prospective pollinators. Our findings indicate convergent evolution of a red-colored nectar has occurred across two distantly related plant species. Behavioral data show that the red pigment attracts diurnal geckos, the likely pollinator of one of these plants. These findings join a growing list of examples of distinct biochemical and molecular mechanisms underlying evolutionary convergence and provide a fascinating system for testing how interactions across species drive the evolution of novel pigments in an understudied context.

Nearly 90% of flowering plants depend on animals for reproduction. One of the main rewards plants offer to pollinators for visitation is nectar. Nesocodon mauritianus (Campanulaceae) produces a blood-red nectar that has been proposed to serve as a visual attractant for pollinator visitation. Here, we show that the nectar’s red color is derived from a previously undescribed alkaloid termed nesocodin. The first nectar produced is acidic and pale yellow in color, but slowly becomes alkaline before taking on its characteristic red color. Three enzymes secreted into the nectar are either necessary or sufficient for pigment production, including a carbonic anhydrase that increases nectar pH, an aryl-alcohol oxidase that produces a pigment precursor, and a ferritin-like catalase that protects the pigment from degradation by hydrogen peroxide. Our findings demonstrate how these three enzymatic activities allow for the condensation of sinapaldehyde and proline to form a pigment with a stable imine bond. We subsequently verified that synthetic nesocodin is indeed attractive to Phelsuma geckos, the most likely pollinators of Nesocodon. We also identify nesocodin in the red nectar of the distantly related and hummingbird-visited Jaltomata herrerae and provide molecular evidence for convergent evolution of this trait. This work cumulatively identifies a convergently evolved trait in two vertebrate-pollinated species, suggesting that the red pigment is selectively favored and that only a limited number of compounds are likely to underlie this type of adaptation.

Abscisic acid mediates the reduction of petunia flower size at elevated temperatures due to reduced cell division

Elevated temperatures suppress cell division in developing petunia buds leading to smaller flowers, mediated by ABA. Flower size is one of the most important showy traits in determining pollinator attraction, and a central factor determining the quality of floricultural products. Whereas the adverse effects of elevated temperatures on showy traits have been described in detail, its underlining mechanisms is poorly understood. Here, we investigated the physiological mechanism responsible for the reduction of flower size in petunia under elevated temperatures. We found that the early stages of flower-bud development were most sensitive to elevated temperatures, resulting in a drastic reduction of flower diameter that was almost independent of flower load. We demonstrated that the temperature-mediated flower size reduction occurred due to a shorter growth period, and a lower rate of corolla cell division. Consistently, local application of cytokinin, a phytohormone that promotes cell division, resulted in recovery of flower dimensions when grown under elevated temperatures. Hormone analysis of temperature-inhibited flower buds revealed no significant changes in levels of cytokinin, and a specific increase of abscisic acid (ABA) levels, known to inhibit cell division. Moreover, local application of ABA on flower buds caused a reduction of flower dimensions as a result of lower levels of cell division, suggesting that ABA mediates the reduction of flower size at elevated temperatures. Taken together, our results shed light on the mechanism by which elevated temperatures decrease petunia flower size, and show that temperature-mediated reduction of flower size can be alleviated by increasing the cytokinin/ABA ratio.

Humans Share More Preferences for Floral Phenotypes With Pollinators Than With Pests

Studies on the selection of floral traits usually consider pollinators and sometimes herbivores. However, humans also exert selection on floral traits of ornamental plants. We compared the preferences of bumblebees (Bombus terrestris), thrips (Frankliniella occidentalis), and humans for flowers of snapdragon. From a cross of two species, Antirrhinum majus and Antirrhinum linkianum, we selected four Recombinant Inbred Lines (RILs). We characterised scent emission from whole flowers and stamens, pollen content and viability, trichome density, floral shape, size and colour of floral parts. We tested the preferences of bumblebees, thrips, and humans for whole flowers, floral scent bouquets, stamen scent, and individual scent compounds. Humans and bumblebees showed preferences for parental species, whereas thrips preferred RILs. Colour and floral scent, in combination with other floral traits, seem relevant phenotypes for all organisms. Remarkably, visual traits override scent cues for bumblebees, although, scent is an important trait when bumblebees cannot see the flowers, and methyl benzoate was identified as a key attractant for them. The evolutionary trajectory of flowers is the result of multiple floral traits interacting with different organisms with different habits and modes of interaction.

Blurring the Boundaries between a Branch and a Flower: Potential Developmental Venues in CACTACEAE

Flowers are defined as short shoots that carry reproductive organs. In Cactaceae, this term acquires another meaning, since the flower is interpreted as a branch with a perianth at the tip, with all reproductive organs embedded within the branch, thus giving way to a structure that has been called a "flower shoot". These organs have long attracted the attention of botanists and cactologists; however, the understanding of the morphogenetic processes during the development of these structures is far from clear. In this review, we present and discuss some classic flower concepts used to define floral structures in Cactaceae in the context of current advances in flower developmental genetics and evolution. Finally, we propose several hypotheses to explain the origin of these floral shoot structures in cacti, and we suggest future research approaches and methods that could be used to fill the gaps in our knowledge regarding the ontogenetic origin of the "flower" in the cactus family.

Spatial patterning of scent in petunia corolla is discriminated by bees and involves the ABCG1 transporter

Floral guides are patterned cues that direct the pollinator to the plant reproductive organs. The spatial distribution of showy visual and olfactory traits allows efficient plant-pollinator interactions. Data on the mechanisms underlying floral volatile patterns or their interactions with pollinators are lacking. Here we characterize the spatial emission patterns of volatiles from the corolla of the model plant Petunia × hybrida and reveal the ability of honeybees to distinguish these patterns. Along the adaxial epidermis, in correlation with cell density, the petal base adjacent to reproductive organs emitted significantly higher levels of volatiles than the distal petal rim. Volatile emission could also be differentiated between the two epidermal surfaces: emission from the adaxial side was significantly higher than that from the abaxial side. Similar emission patterns were also observed in other petunias, Dianthus caryophyllus (carnation) and Argyranthemum frutescens (Marguerite daisy). Analyses of transcripts involved in volatile production/emission revealed lower levels of the plasma-membrane transporter ABCG1 in the abaxial versus adaxial epidermis. Transient overexpression of ABCG1 enhanced emission from the abaxial epidermis to the level of the adaxial epidermis, suggesting its involvement in spatial emission patterns in the epidermal layers. Proboscis extension response experiments showed that differences in emission levels along the adaxial epidermis, that is, petal base versus rim, detected by GC-MS are also discernible by honeybees.

Floral infrared emissivity estimates using simple tools

BACKGROUND

Floral temperature has important consequences for plant biology, and accurate temperature measurements are therefore important to plant research. Thermography, also referred to as thermal imaging, is beginning to be used more frequently to measure and visualize floral temperature. Accurate thermographic measurements require information about the object's emissivity (its capacity to emit thermal radiation with temperature), to obtain accurate temperature readings. However, there are currently no published estimates of floral emissivity available. This is most likely to be due to flowers being unsuitable for the most common protocols for emissivity estimation. Instead, researchers have used emissivity estimates collected on vegetative plant tissue when conducting floral thermography, assuming these tissues to have the same emissivity. As floral tissue differs from vegetative tissue, it is unclear how appropriate and accurate these vegetative tissue emissivity estimates are when they are applied to floral tissue.

RESULTS

We collect floral emissivity estimates using two protocols, using a thermocouple and a water bath, providing a guide for making estimates of floral emissivity that can be carried out without needing specialist equipment (apart from the thermal camera). Both protocols involve measuring the thermal infrared radiation from flowers of a known temperature, providing the required information for emissivity estimation. Floral temperature is known within these protocols using either a thermocouple, or by heating the flowers within a water bath. Emissivity estimates indicate floral emissivity is high, near 1, at least across petals. While the two protocols generally indicated the same trends, the water bath protocol gave more realistic and less variable estimates. While some variation with flower species and location on the flower is observed in emissivity estimates, these are generally small or can be explained as resulting from artefacts of these protocols, relating to thermocouple or water surface contact quality.

CONCLUSIONS

Floral emissivity appears to be high, and seems quite consistent across most flowers and between species, at least across petals. A value near 1, for example 0.98, is recommended for accurate thermographic measurements of floral temperature. This suggests that the similarly high values based on vegetation emissivity estimates used by previous researchers were appropriate.

Petal Cellular Identities

Petals are typified by their conical epidermal cells that play a predominant role for the attraction and interaction with pollinators. However, cell identities in the petal can be very diverse, with different cell types in subdomains of the petal, in different cell layers, and depending on their adaxial-abaxial or proximo-distal position in the petal. In this mini-review, we give an overview of the main cell types that can be found in the petal and describe some of their functions. We review what is known about the genetic basis for the establishment of these cellular identities and their possible relation with petal identity and polarity specifiers expressed earlier during petal development, in an attempt to bridge the gap between organ identity and cell identity in the petal.

Ray flower initiation in the Helianthus radula inflorescence is influenced by a functional allele of the HrCYC2c gene

The radiate pseudanthium, with actinomorphic disk flowers surrounded by showy marginal zygomorphic ray flowers, is the most common inflorescence in the Helianthus genus. In Helianthus radula, ray flower primordia are normally absent at the dorsal domain of the inner phyllaries (discoid heads) while the occurrence of radiate inflorescences is uncommon. In Helianthus spp., flower symmetry and inflorescence architecture are mainly controlled by CYCLOIDEA (CYC)-like genes but the putative role of these genes in the development of discoid inflorescences has not been investigate. Three CYC genes of H. radula with a role in ray flower identity (HrCYC2c, HrCYC2d, and HrCYC2e) were isolated. The phylogenetic analysis placed these genes within the CYC2 subclade. We identified two different alleles for the HrCYC2c gene. A mutant allele, designed HrCYC2c-m, shows a thymine to adenine transversion, which generates a TGA stop codon after a translation of 14 amino acids. We established homozygous dominant (HrCYC2c/HrCYC2c) and recessive (HrCYC2c-m/HrCYC2c-m) plants for this nonsense mutation. Inflorescences of both HrCYC2c/HrCYC2c and HrCYC2c/HrCYC2c-m plants initiated ray flowers, despite at low frequency. By contrast, plants homozygous for the mutant allele (HrCYC2c-m/HrCYC2c-m) failed at all to develop ray flowers. The results support, for the first time, a role of the HrCYC2c gene on the initiation of ray flower primordia. However, also in the two dominant phenotypes, discoid heads are the prevalent architecture suggesting that this gene is required but not sufficient to initiate ray flowers in pseudanthia. Other unknown major genes are most likely required in the shift from discoid to radiate inflorescence.

The genetic control of leaf and petal allometric variations in Arabidopsis thaliana

BACKGROUND

Organ shape and size covariation (allometry) factors are essential concepts for the study of evolution and development. Although ample research has been conducted on organ shape and size, little research has considered the correlated variation of these two traits and quantitatively measured the variation in a common framework. The genetic basis of allometry variation in a single organ or among different organs is also relatively unknown.

RESULTS

A principal component analysis (PCA) of organ landmarks and outlines was conducted and used to quantitatively capture shape and size variation in leaves and petals of multiparent advanced generation intercross (MAGIC) populations of Arabidopsis thaliana. The PCA indicated that size variation was a major component of allometry variation and revealed negatively correlated changes in leaf and petal size. After quantitative trait loci (QTL) mapping, five QTLs for the fourth leaf, 11 QTLs for the seventh leaf, and 12 QTLs for petal size and shape were identified. These QTLs were not identical to those previously identified, with the exception of the ER locus. The allometry model was also used to measure the leaf and petal allometry covariation to investigate the evolution and genetic coordination between homologous organs. In total, 12 QTLs were identified in association with the fourth leaf and petal allometry covariation, and eight QTLs were identified to be associated with the seventh leaf and petal allometry covariation. In these QTL confidence regions, there were important genes associated with cell proliferation and expansion with alleles unique to the maximal effects accession. In addition, the QTLs associated with life-history traits, such as days to bolting, stem length, and rosette leaf number, which were highly coordinated with climate change and local adaption, were QTL mapped and showed an overlap with leaf and petal allometry, which explained the genetic basis for their correlation.

CONCLUSIONS

This study explored the genetic basis for leaf and petal allometry and their interaction, which may provide important information for investigating the correlated variation and evolution of organ shape and size in Arabidopsis.

Parallelism in Flower Evolution and Development

Flower evolution is characterized by widespread repetition, with adaptations to pollinator environment evolving in parallel. Recent studies have expanded our understanding of the developmental basis of adaptive floral novelties—petal fusion, bilateral symmetry, heterostyly, and floral dimensions. In this article, we describe patterns of trait evolution and review developmental genetic mechanisms underlying floral novelties. We discuss the diversity of mechanisms for parallel adaptation, the evidence for constraints on these mechanisms, and how constraints help explain observed macroevolutionary patterns. We describe parallel evolution resulting from similarities at multiple hierarchical levels—genetic, developmental, morphological, functional—which indicate general principles in floral evolution, including the central role of hormone signaling. An emerging pattern is mutational bias that may contribute to rapid patterns of parallel evolution, especially if the derived trait can result from simple degenerative mutations. We argue that such mutational bias may be less likely to govern the evolution of novelties patterned by complex developmental pathways.

Differential Gene Expression with an Emphasis on Floral Organ Size Differences in Natural and Synthetic Polyploids of Nicotiana tabacum (Solanaceae)

Floral organ size, especially the size of the corolla, plays an important role in plant reproduction by facilitating pollination efficiency. Previous studies have outlined a hypothesized organ size pathway. However, the expression and function of many of the genes in the pathway have only been investigated in model diploid species; therefore, it is unknown how these genes interact in polyploid species. Although correlations between ploidy and cell size have been shown in many systems, it is unclear whether there is a difference in cell size between naturally occurring and synthetic polyploids. To address these questions comparing floral organ size and cell size across ploidy, we use natural and synthetic polyploids of Nicotiana tabacum (Solanaceae) as well as their known diploid progenitors. We employ a comparative transcriptomics approach to perform analyses of differential gene expression, focusing on candidate genes that may be involved in floral organ size, both across developmental stages and across accessions. We see differential expression of several known floral organ candidate genes including ARF2, BIG BROTHER, and GASA/GAST1. Results from linear models show that ploidy, cell width, and cell number positively influence corolla tube circumference; however, the effect of cell width varies by ploidy, and diploids have a significantly steeper slope than both natural and synthetic polyploids. These results demonstrate that polyploids have wider cells and that polyploidy significantly increases corolla tube circumference.

Reconstructing trait evolution in plant evo-devo studies

Our planet is teeming with an astounding diversity of plants. In a mere single group of closely related species, tremendous diversity can be observed in their form and function - the colour of petals in flowering plants, the shape of the fronds in ferns, and the branching pattern of the gametophyte in mosses. Diversity can also be found in subtler traits, such as the resistance to pathogens or the ability to recruit symbiotic microbes from the environment. Plant traits can also be highly conserved - at the cellular and metabolic levels, entire biosynthetic pathways are present in all plant groups, and morphological characteristics such as vascular tissues have been conserved for hundreds of millions of years. The research community that seeks to understand these traits - both the diverse and the conserved - by taking an evolutionary point-of-view on plant biology is growing. Here, we summarize a subset of the different aspects of plant evolutionary biology, provide a guide for structuring comparative biology approaches and discuss the pitfalls that (plant) researchers should avoid when embarking on such studies.

A role for the Auxin Response Factors ARF6 and ARF8 homologs in petal spur elongation and nectary maturation in Aquilegia

The petal spur of the basal eudicot Aquilegia is a key innovation associated with the adaptive radiation of the genus. Previous studies have shown that diversification of Aquilegia spur length can be predominantly attributed to variation in cell elongation. However, the genetic pathways that control the development of petal spurs are still being investigated. Here, we focus on a pair of closely related homologs of the AUXIN RESPONSE FACTOR family, AqARF6 and AqARF8, to explore their roles in Aquileiga coerulea petal spur development. Expression analyses of the two genes show that they are broadly expressed in vegetative and floral organs, but have relatively higher expression in petal spurs, particularly at later stages. Knockdown of the two AqARF6 and AqARF8 transcripts using virus-induced gene silencing resulted in largely petal-specific defects, including a significant reduction in spur length due to a decrease in cell elongation. These spurs also exhibited an absence of nectar production, which was correlated with downregulation of STYLISH homologs that have previously been shown to control nectary development. This study provides the first evidence of ARF6/8 homolog-mediated petal development outside the core eudicots. The genes appear to be specifically required for cell elongation and nectary maturation in the Aquilegia petal spur.

HEARTBREAK Controls Post-translational Modification of INDEHISCENT to Regulate Fruit Morphology in Capsella

Morphological variation is the basis of natural diversity and adaptation. For example, angiosperms (flowering plants) evolved during the Cretaceous period more than 100 mya and quickly colonized terrestrial habitats [1]. A major reason for their astonishing success was the formation of fruits, which exist in a myriad of different shapes and sizes [2]. Evolution of organ shape is fueled by variation in expression patterns of regulatory genes causing changes in anisotropic cell expansion and division patterns [3, 4, 5]. However, the molecular mechanisms that alter the polarity of growth to generate novel shapes are largely unknown. The heart-shaped fruits produced by members of the Capsella genus comprise an anatomical novelty, making it particularly well suited for studies on morphological diversification [6, 7, 8]. Here, we show that post-translational modification of regulatory proteins provides a critical step in organ-shape formation. Our data reveal that the SUMO protease, HEARTBREAK (HTB), from Capsella rubella controls the activity of the key regulator of fruit development, INDEHISCENT (CrIND in C. rubella), via de-SUMOylation. This post-translational modification initiates a transduction pathway required to ensure precisely localized auxin biosynthesis, thereby facilitating anisotropic cell expansion to ultimately form the heart-shaped Capsella fruit. Therefore, although variation in the expression of key regulatory genes is known to be a primary driver in morphological evolution, our work demonstrates how other processes—such as post-translational modification of one such regulator—affects organ morphology.

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HTB encodes a SUMO protease required for fruit shape in Capsella

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Anisotropic cell growth is suppressed in the fruit valves of the htb mutant

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HTB stabilizes CrIND through de-SUMOylation to facilitate local auxin biosynthesis

Identification of the target genes of AqAPETALA3-3 (AqAP3-3) in Aquilegia coerulea (Ranunculaceae) helps understand the molecular bases of the conserved and nonconserved features of petals

Identification and comparison of the conserved and variable downstream genes of floral organ identity regulators are critical to understanding the mechanisms underlying the commonalities and peculiarities of floral organs. Yet, because of the lack of studies in nonmodel species, a general picture of the regulatory evolution between floral organ identity genes and their targets is still lacking. Here, by conducting extensive chromatin immunoprecipitation followed by high-throughput sequencing (ChIP-seq), electrophoretic mobility shift assay and bioinformatic analyses, we identify and predict the target genes of a petal identity gene, AqAPETALA3-3 (AqAP3-3), in Aquilegia coerulea (Ranunculaceae) and compare them with those of its counterpart in Arabidopsis thaliana, AP3. In total, 7049 direct target genes are identified for AqAP3-3, of which 2394 are highly confident and 1085 are shared with AP3. Gene Ontology enrichment analyses further indicate that conserved targets are largely involved in the formation of identity-related features, whereas nonconserved targets are mostly required for the formation of species-specific features. These results not only help understand the molecular bases of the conserved and nonconserved features of petals, but also pave the way to studying the regulatory evolution between floral organ identity genes and their targets.