Revisiting floral fusion: the evolution and molecular basis of a developmental innovation

Convergent evolutionary patterns of heterostyly across angiosperms support the pollination-precision hypothesis

Since the insights by Charles Darwin, heterostyly, a floral polymorphism with morphs bearing stigmas and anthers at reciprocal heights, has become a model system for the study of natural selection. Based on his archetypal heterostylous flower, including regular symmetry, few stamens and a tube, Darwin hypothesised that heterostyly evolved to promote outcrossing through efficient pollen transfer between morphs involving different areas of a pollinator's body, thus proposing his seminal pollination-precision hypothesis. Here we update the number of heterostylous and other style-length polymorphic taxa to 247 genera belonging to 34 families, notably expanding known cases by 20%. Using phylogenetic and comparative analyses across the angiosperms, we show numerous independent origins of style-length polymorphism associated with actinomorphic, tubular flowers with a low number of sex organs, stamens fused to the corolla, and pollination by long-tongued insects. These associations provide support for the Darwinian pollination-precision hypothesis as a basis for convergent evolution of heterostyly across angiosperms.

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.

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.

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.

A phylogenomic analysis of Lonicera and its bearing on the evolution of organ fusion

PREMISE

The ~140 species of Lonicera are characterized by variously fused leaves, bracteoles, and ovaries, making it a model system for studying the evolution and development of organ fusion. However, previous phylogenetic analyses, based mainly on chloroplast DNA markers, have yielded uncertain and conflicting results. A well-supported phylogeny of Lonicera will allow us to trace the evolutionary history of organ fusion.

METHODS

We inferred the phylogeny of Lonicera using restriction site-associated DNA sequencing (RADSeq), sampling all major clades and 18 of the 23 subsections. This provided the basis for inferring the evolution of five fusion-related traits.

RESULTS

RADSeq data yielded a well-resolved and well-supported phylogeny. The two traditionally recognized subgenera (Periclymenum and Chamaecerasus), three of the four sections (Isoxylosteum, Coeloxylosteum, and Nintooa), and half of the subsections sampled were recovered as monophyletic. However, the large and heterogeneous section Isika was strongly supported as paraphyletic. Nintooa, a clade of ~22 mostly vine-forming species, including L. japonica, was recovered in a novel position, raising the possibility of cytonuclear discordance. We document the parallel evolution of fused leaves, bracteoles, and ovaries, with rare reversals. Most strikingly, complete cupules, in which four fused bracteoles completely enclose two unfused ovaries, arose at least three times. Surprisingly, these appear to have evolved directly from ancestors with free bracteoles instead of partial cupules.

CONCLUSIONS

We provide the most comprehensive and well-supported phylogeny of Lonicera to date. Our inference of multiple evolutionary shifts in organ fusion provides a solid foundation for in depth developmental and functional analyses.

The NAC transcription factors play core roles in flowering and ripening fundamental to fruit yield and quality

Fruits are derived from flowers and play an important role in human food, nutrition, and health. In general, flowers determine the crop yield, and ripening affects the fruit quality. Although transcription factors (TFs) only account for a small part of plant transcriptomes, they control the global gene expression and regulation. The plant-specific NAC (NAM, ATAF, and CUC) TFs constitute a large family evolving concurrently with the transition of both aquatic-to-terrestrial plants and vegetative-to-reproductive growth. Thus, NACs play an important role in fruit yield and quality by determining shoot apical meristem (SAM) inflorescence and controlling ripening. The present review focuses on the various properties of NACs together with their function and regulation in flower formation and fruit ripening. Hitherto, we have a better understanding of the molecular mechanisms of NACs in ripening through abscisic acid (ABA) and ethylene (ETH), but how NACs regulate the expression of the inflorescence formation-related genes is largely unknown. In the future, we should focus on the analysis of NAC redundancy and identify the pivotal regulators of flowering and ripening. NACs are potentially vital manipulation targets for improving fruit quantity and quality.

Nectary development in Cleome violacea

Nectaries are a promising frontier for plant evo-devo research, and are particularly fascinating given their diversity in form, position, and secretion methods across angiosperms. Emerging model systems permit investigations of the molecular basis for nectary development and nectar secretion across a range of taxa, which addresses fundamental questions about underlying parallelisms and convergence. Herein, we explore nectary development and nectar secretion in the emerging model taxa, Cleome violacea (Cleomaceae), which exhibits a prominent adaxial nectary. First, we characterized nectary anatomy and quantified nectar secretion to establish a foundation for quantitative and functional gene experiments. Next, we leveraged RNA-seq to establish gene expression profiles of nectaries across three key stages of development: pre-anthesis, anthesis, and post-fertilization. We then performed functional studies on five genes that were putatively involved in nectary and nectar formation: CvCRABSCLAW (CvCRC), CvAGAMOUS (CvAG), CvSHATTERPROOF (CvSHP), CvSWEET9, and a highly expressed but uncharacterized transcript. These experiments revealed a high degree of functional convergence to homologues from other core Eudicots, especially Arabidopsis. CvCRC, redundantly with CvAG and CvSHP, are required for nectary initiation. Concordantly, CvSWEET9 is essential for nectar formation and secretion, which indicates that the process is eccrine based in C. violacea. While demonstration of conservation is informative to our understanding of nectary evolution, questions remain. For example, it is unknown which genes are downstream of the developmental initiators CvCRC, CvAG, and CvSHP, or what role the TCP gene family plays in nectary initiation in this family. Further to this, we have initiated a characterization of associations between nectaries, yeast, and bacteria, but more research is required beyond establishing their presence. Cleome violacea is an excellent model for continued research into nectary development because of its conspicuous nectaries, short generation time, and close taxonomic distance to Arabidopsis.

Evolutionary history of the grass gynoecium

The grass family (Poaceae) includes cereal crops that provide a key food source for the human population. The food industry uses the starch deposited in the cereal grain, which develops directly from the gynoecium. Morphological interpretation of the grass gynoecium remains controversial. We re-examine earlier hypotheses and studies of morphology and development in the context of more recent analyses of grass phylogenetics and developmental genetics. Taken in isolation, data on gynoecium development in bistigmatic grasses do not contradict its interpretation as a solitary ascidiate carpel. Nevertheless, in the context of other data, this interpretation is untenable. Broad comparative analysis in a modern phylogenetic context clearly demonstrates that the grass gynoecium is pseudomonomerous. A bistigmatic grass gynoecium has two sterile carpels, each producing a stigma, and a fertile carpel that lacks a stigma. To date, studies of grass developmental genetics and developmental morphology have failed to fully demonstrate the composite nature of the grass gynoecium be-cause its complex evolutionary history is hidden by extreme organ integration. It is problematic to interpret the gynoecium of grasses in terms of normal angiosperm gynoecium typology. Even the concept of a carpel becomes misleading in grasses; instead, we recommend the term pistil for descriptive purposes.

Refined Interpretation of the Pistillate Flower in Ceratophyllum Sheds Fresh Light on Gynoecium Evolution in Angiosperms

Molecular phylogenetic analyses have revealed a superclade of mesangiosperms with five extant lineages: monocots, eudicots, magnoliids, Ceratophyllum and Chloranthaceae. Both Ceratophyllum and Chloranthaceae are ancient lineages with a long fossil record; their precise placement within mesangiosperms is uncertain. Morphological studies have suggested that they form a clade together with some Cretaceous fossils, including Canrightia, Montsechia and Pseudoasterophyllites. Apart from Canrightia, members of this clade share unilocular gynoecia commonly interpreted as monomerous with ascidiate carpels. Alternatively, the gynoecium of Ceratophyllum has also been interpreted as syncarpous with a single fertile carpel (pseudomonomerous). We investigate patterns of morphological, anatomical and developmental variation in gynoecia of three Ceratophyllum species to explore the controversial interpretation of its gynoecium as either monomerous or pseudomonomerous. We use an angiosperm-wide morphological data set and contrasting tree topologies to estimate the ancestral gynoecium type in both Ceratophyllum and mesangiosperms. Gynoecia of all three Ceratophyllum species possess a small (sometimes vestigial) glandular appendage on the abaxial side and an occasionally bifurcating apex. The ovary is usually unilocular with two procambium strands, but sometimes bilocular and/or with three strands in C. demersum. None of the possible phylogenetic placements strongly suggest apocarpy in the stem lineage of Ceratophyllum. Rescoring Ceratophyllum as having two united carpels affects broader-scale reconstructions of the ancestral gynoecium in mesangiosperms. Our interpretation of the glandular appendage as a tepal or staminode homologue makes the Ceratophyllum ovary inferior, thus resembling (semi)inferior ovaries of most Chloranthaceae and potentially related fossils Canrightia and Zlatkocarpus. The entire structure of the flower of Ceratophyllum suggests strong reduction following a long and complex evolutionary history. The widely accepted notion that apocarpy is ancestral in mesangiosperms (and angiosperms) lacks robust support, regardless of which modes of carpel fusion are considered. Our study highlights the crucial importance of incorporating fossils into large-scale analyses to understand character evolution.

Interspecific complementation-restoration of phenotype in Arabidopsis cuc2cuc3 mutant by sugarcane CUC2 gene

BACKGROUND

In plants, a critical balance between differentiation and proliferation of stem cells at the shoot apical meristem zone is essential for proper growth. The spatiotemporal regulation of some crucial genes dictates the formation of a boundary within and around budding organs. The boundary plays a pivotal role in distinguishing one tissue type from another and provides a defined shape to the organs at their developed stage. NAM/CUC subfamily of the NAC transcription factors control the boundary formation during meristematic development.

RESULTS

Here, we have identified the CUP-SHAPED COTYLEDON (CUC) genes in sugarcane and named SsCUC2 (for the orthologous gene of CUC1 and CUC2) and SsCUC3. The phylogenetic reconstruction showed that SsCUCs occupy the CUC2 and CUC3 clade together with monocots, whereas eudicot CUC2 and CUC3 settled separately in the different clade. The structural analysis of CUC genes showed that most of the CUC3 genes were accompanied by an intron gain during eudicot divergence. Besides, the study of SsCUCs expression in the RNA-seq obtained during different stages of ovule development revealed that SsCUCs express in developing young tissues, and the expression of SsCUC2 is regulated by miR164. We also demonstrate that SsCUC2 (a monocot) could complement the cuc2cuc3 mutant phenotype of Arabidopsis (eudicot).

CONCLUSIONS

This study further supports that CUC2 has diverged in CUC1 and CUC2 during the evolution of monocots and eudicots from ancestral plants. The functional analysis of CUC expression patterns during sugarcane ovule development and ectopic expression of SsCUC2 in Arabidopsis showed that SsCUC2 has a conserved role in boundary formation. Overall, these findings improve our understanding of the functions of sugarcane CUC genes. Our results reveal the crucial functional role of CUC genes in sugarcane.

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.

The interplay between ABA/ethylene and NAC TFs in tomato fruit ripening: a review

This review contains functional roles of NAC transcription factors in the transcriptional regulation of ripening in tomato fruit, describes the interplay between ABA/ethylene and NAC TFs in tomato fruit ripening. Fruit ripening is regulated by a complex network of transcription factors (TFs) and genetic regulators in response to endogenous hormones and external signals. Studying the regulation of fruit ripening has important significance for controlling fruit quality, enhancing nutritional value, improving storage conditions and extending shelf-life. Plant-specific NAC (named after no apical meristem (NAM), Arabidopsis transcription activator factor 1/2 (ATAF1/2) and Cup-shaped cotyledon (CUC2)) TFs play essential roles in plant development, ripening and stress responses. In this review, we summarize the recent progress on the regulation of NAC TFs in fruit ripening, discuss the interactions between NAC and other factors in controlling fruit development and ripening, and emphasize how NAC TFs are involved in tomato fruit ripening through the ethylene and abscisic acid (ABA) pathways. The signaling network regulating ripening is complex, and both hormones and individual TFs can affect the status or activity of other network participants, which can alter the overall ripening network regulation, including response signals and fruit ripening. Our review helps in the systematic understanding of the regulation of NAC TFs involved in fruit ripening and provides a basis for the development or establishment of complex ripening regulatory network models.

Insights into the evo-devo of plant reproduction using next-generation sequencing approaches

The development of plant model organisms has traditionally been analyzed using resource-heavy, tailored applications that are not easily transferable to distantly related non-model taxa. Thus, our understanding of plant development has been limited to a subset of traits, and evolutionary studies conducted most effectively either across very wide [e.g. Arabidopsis thaliana and Oryza sativa (rice)] or narrow (i.e. population level) phylogenetic distances. As plant biologists seek to capitalize on natural diversity for crop improvement, enhance ecosystem functioning, and better understand plant responses to climate change, high-throughput and broadly applicable forms of existing molecular biology assays are becoming an invaluable resource. Next-generation sequencing (NGS) is increasingly becoming a powerful tool in evolutionary developmental biology (evo-devo) studies, particularly through its application to understanding trait evolution at different levels of gene regulation. Here, I review some of the most common and emerging NGS-based methods, using exemplar studies in reproductive plant evo-devo to illustrate their potential.