Flower development: open questions and future directions

Reflections on the ABC model of flower development

The formulation of the ABC model by a handful of pioneer plant developmental geneticists was a seminal event in the quest to answer a seemingly simple question: how are flowers formed? Fast forward 30 years and this elegant model has generated a vibrant and diverse community, capturing the imagination of developmental and evolutionary biologists, structuralists, biochemists and molecular biologists alike. Together they have managed to solve many floral mysteries, uncovering the regulatory processes that generate the characteristic spatio-temporal expression patterns of floral homeotic genes, elucidating some of the mechanisms allowing ABC genes to specify distinct organ identities, revealing how evolution tinkers with the ABC to generate morphological diversity, and even shining a light on the origins of the floral gene regulatory network itself. Here we retrace the history of the ABC model, from its genesis to its current form, highlighting specific milestones along the way before drawing attention to some of the unsolved riddles still hidden in the floral alphabet.

Morphogenesis at the shoot meristem

Shoot apical meristems are populations of stem cells which initiate the aerial parts of higher plants. Work during the last decades has revealed a complex network of molecular regulators, which control both meristem maintenance and the production of different types of organs. The behavior of this network in time and space is defined by the local interactions between regulators and also involves hormonal regulation. In particular, auxin and cytokinin are intimately implicated in the coordination of gene expression patterns. To control growth patterns at the shoot meristem the individual components of the network influence directions and rates of cell growth. This requires interference with the mechanical properties of the cells. How this complex multiscale process, characterized by multiple feedbacks, is controlled remains largely an open question. Fortunately, genetics, live imaging, computational modelling and a number of other recently developed tools offer interesting albeit challenging perspectives.

Brassinosteroid signals cooperate with katanin-mediated microtubule severing to control stamen filament elongation

Proper stamen filament elongation is essential for pollination and plant reproduction. Plant hormones are extensively involved in every stage of stamen development; however, the cellular mechanisms by which phytohormone signals couple with microtubule dynamics to control filament elongation remain unclear. Here, we screened a series of Arabidopsis thaliana mutants showing different microtubule defects and revealed that only those unable to sever microtubules, lue1 and ktn80.1234, displayed differential floral organ elongation with less elongated stamen filaments. Prompted by short stamen filaments and severe decrease in KTN1 and KTN80s expression in qui-2 lacking five BZR1-family transcription factors (BFTFs), we investigated the crosstalk between microtubule severing and brassinosteroid (BR) signaling. The BFTFs transcriptionally activate katanin-encoding genes, and the microtubule-severing frequency was severely reduced in qui-2. Taken together, our findings reveal how BRs can regulate cytoskeletal dynamics to coordinate the proper development of reproductive organs.

Circadian autonomy and rhythmic precision of the Arabidopsis female reproductive organ

The plant circadian clock regulates essential biological processes including flowering time or petal movement. However, little is known about how the clock functions in flowers. Here, we identified the circadian components and transcriptional networks contributing to the generation of rhythms in pistils, the female reproductive organ. When detached from the rest of the flower, pistils sustain highly precise rhythms, indicating organ-specific circadian autonomy. Analyses of clock mutants and chromatin immunoprecipitation assays showed distinct expression patterns and specific regulatory functions for clock activators and repressors in pistils. Genetic interaction studies also suggested a hierarchy of the repressing activities that provide robustness and precision to the pistil clock. Globally, the circadian function in pistils primarily governs responses to environmental stimuli and photosynthesis and controls pistil growth and seed weight and production. Understanding the circadian intricacies in reproductive organs may prove useful for optimizing plant reproduction and productivity.

Spatio-temporal transcriptome profiling and subgenome analysis in Brassica napus

Brassica napus is an important oil crop and an allotetraploid species. However, the detailed analysis of gene function and homoeologous gene expression in all tissues at different developmental stages was not explored. In this study, we performed a global transcriptome analysis of 24 vegetative and reproductive tissues at six developmental stages (totally 111 tissues). These samples were clustered into eight groups. The gene functions of silique pericarp were similar to roots, stems and leaves. In particular, glucosinolate metabolic process was associated with root and silique pericarp. Genes involved in protein phosphorylation were often associated with stamen, anther and the early developmental stage of seeds. Transcription factor (TF) genes were more specific than structural genes. A total of 17 100 genes that were preferentially expressed in one tissue (tissue-preferred genes, TPGs), including 889 TFs (5.2%), were identified in the 24 tissues. Some TPGs were identified as hub genes in the co-expression network analysis, and some TPGs in different tissues were involved in different hormone pathways. About 67.0% of the homoeologs showed balanced expression, whereas biased expression of homoeologs was associated with structural divergence. In addition, the spatiotemporal expression of homoeologs was related to the presence of transposable elements (TEs) and regulatory elements (REs); more TEs and fewer REs in the promoters resulted in divergent expression in different tissues. This study provides a valuable transcriptional map for understanding the growth and development of B. napus, for identifying important genes for future crop improvement, and for exploring gene expression patterns in the B. napus.

Rice transcription factor MADS32 regulates floral patterning through interactions with multiple floral homeotic genes

Floral patterning is regulated by intricate networks of floral identity genes. The peculiar MADS32 subfamily genes, absent in eudicots but prevalent in monocots, control floral organ identity. However, how the MADS32 family genes interact with other floral homeotic genes during flower development is mostly unknown. We show here that the rice homeotic transcription factor OsMADS32 regulates floral patterning by interacting synergistically with E class protein OsMADS6 in a dosage-dependent manner. Furthermore, our results indicate important roles for OsMADS32 in defining stamen, pistil, and ovule development through physical and genetic interactions with OsMADS1, OsMADS58, and OsMADS13, and in specifying floral meristem identity with OsMADS6, OsMADS3, and OsMADS58, respectively. Our findings suggest that OsMADS32 is an important factor for floral meristem identity maintenance and that it integrates the action of other MADS-box homeotic proteins to sustain floral organ specification and development in rice. Given that OsMADS32 is an orphan gene and absent in eudicots, our data substantially expand our understanding of flower development in plants.

Auxin and Flower Development: A Blossoming Field

The establishment of the species-specific floral organ body plan involves many coordinated spatiotemporal processes, which include the perception of positional information that specifies floral meristem and floral organ founder cells, coordinated organ outgrowth coupled with the generation and maintenance of inter-organ and inter-whorl boundaries, and the termination of meristem activity. Auxin is integrated within the gene regulatory networks that control these processes and plays instructive roles at the level of tissue-specific biosynthesis and polar transport to generate local maxima, perception, and signaling. Key features of auxin function in several floral contexts include cell nonautonomy, interaction with cytokinin gradients, and the central role of MONOPTEROS and ETTIN to regulate canonical and noncanonical auxin response pathways, respectively. Arabidopsis flowers are not representative of the enormous angiosperm floral diversity; therefore, comparative studies are required to understand how auxin underlies these developmental differences. It will be of great interest to compare the conservation of auxin pathways among flowering plants and to discuss the evolutionary role of auxin in floral development.

Ephedra as a gymnosperm evo-devo model lineage

Established model systems in the flowering plants have greatly advanced our understanding of plant developmental biology, facilitating in turn its investigation across diverse land plants. The reliance on a limited number of model organisms, however, constitutes a barrier for future progress in evolutionary developmental biology (evo-devo). In particular, a more thorough understanding of seed plant character evolution and of its genetic and developmental basis has been hampered in part by a lack of gymnosperm model systems, since most are trees with decades-long generation times. Guided by the premise that future model organisms should be selected based on their character diversity, rather than simply phylogenetic "position," we highlight biological questions of potential interest that can be addressed via comparative studies in Ephedra (Gnetales). In addition to having relatively small genomes and shorter generation times in comparison to most other gymnosperms, Ephedra are amenable to investigations on the evolution of the key reproductive seed plant innovations of pollination and seed dispersal, as well as on polyploidy, and adaptation to extreme environments.

Histone Demethylases ELF6 and JMJ13 Antagonistically Regulate Self-Fertility in Arabidopsis

The chromatin modification H3K27me3 is involved in almost every developmental stage in Arabidopsis. Much remains unknown about the dynamic regulation of this histone modification in flower development and control of self-fertility. Here we demonstrate that the H3K27me3-specific demethylases ELF6 and JMJ13 antagonistically regulate carpel and stamen growth and thus modulate self-fertility. Transcriptome and epigenome data are used to identify potential targets of ELF6 and JMJ13 responsible for these physiological functions. We find that ELF6 relieves expansin genes of epigenetic silencing to promote cell elongation in the carpel, enhancing carpel growth and therefore encouraging out-crossing. On the other hand, JMJ13 activates genes of the jasmonic acid regulatory network alongside the auxin responsive SAUR26, to inhibit carpel growth, enhance stamen growth, and overall promote self-pollination. Our evidence provides novel mechanisms of self-fertility regulation in A. thaliana demonstrating how chromatin modifying enzymes govern the equilibrium between flower self-pollination and out-crossing.

New roles of NO TRANSMITTING TRACT and SEEDSTICK during medial domain development in Arabidopsis fruits

The gynoecium, the female reproductive part of the flower, is key for plant sexual reproduction. During its development, inner tissues such as the septum and the transmitting tract tissue, important for pollen germination and guidance, are formed. In Arabidopsis, several transcription factors are known to be involved in the development of these tissues. One of them is NO TRANSMITTING TRACT (NTT), essential for transmitting tract formation. We found that the NTT protein can interact with several gynoecium-related transcription factors, including several MADS-box proteins, such as SEEDSTICK (STK), known to specify ovule identity. Evidence suggests that NTT and STK control enzyme and transporter-encoding genes involved in cell wall polysaccharide and lipid distribution in gynoecial medial domain cells. The results indicate that the simultaneous loss of NTT and STK activity affects polysaccharide and lipid deposition and septum fusion, and delays entry of septum cells to their normal degradation program. Furthermore, we identified KAWAK, a direct target of NTT and STK, which is required for the correct formation of fruits in Arabidopsis These findings position NTT and STK as important factors in determining reproductive competence.

Doubled Haploid 'CUDH2107' as a Reference for Bulb Onion (Allium cepa L.) Research: Development of a Transcriptome Catalogue and Identification of Transcripts Associated with Male Fertility

Researchers working on model plants have derived great benefit from developing genomic and genetic resources using ‘reference’ genotypes. Onion has a large and highly heterozygous genome making the sharing of germplasm and analysis of sequencing data complicated. To simplify the discovery and analysis of genes underlying important onion traits, we are promoting the use of the homozygous double haploid line ‘CUDH2107’ by the onion research community. In the present investigation, we performed transcriptome sequencing on vegetative and reproductive tissues of CUDH2107 to develop a multi-organ reference transcriptome catalogue. A total of 396 million 100 base pair paired reads was assembled using the Trinity pipeline, resulting in 271,665 transcript contigs. This dataset was analysed for gene ontology and transcripts were classified on the basis of putative biological processes, molecular function and cellular localization. Significant differences were observed in transcript expression profiles between different tissues. To demonstrate the utility of our CUDH2107 transcriptome catalogue for understanding the genetic and molecular basis of various traits, we identified orthologues of rice genes involved in male fertility and flower development. These genes provide an excellent starting point for studying the molecular regulation, and the engineering of reproductive traits.

The Rosa chinensis cv. Viridiflora Phyllody Phenotype Is Associated with Misexpression of Flower Organ Identity Genes

Phyllody is a flower abnormality in which leaf-like structures replace flower organs in all whorls. Here, we investigated the origin and the molecular mechanism of phyllody phenotype in Rosa chinensis cv. Viridiflora, an ancient naturally occurring Chinese mutant cultivar. Reciprocal grafting experiments and microscopy analyses, demonstrated that the phyllody phenotype in Viridiflora is not associated with phytoplasmas infection. Transcriptome comparisons by the mean of RNA-Seq identified 672 up-regulated and 666 down-regulated genes in Viridiflora compared to its closely related genotype R. chinensis cv. Old Blush. A fraction of these genes are putative homologs of genes known to be involved in flower initiation and development. We show that in flower whorl 2 of Viridiflora, a down-regulation of the floral organ identity genes RcPISTILLATA (RcPI), RcAPETALA3 (RcAP3) and RcSEPALLATA3 (RcSEP3), together with an up-regulation of the putative homolog of the gene SUPPRESSOR of OVEREXPRESSION of CONSTANS1 (RcSOC1) are likely at the origin of the loss of petal identity and leaf-like structures formation. In whorl 3 of Viridiflora, ectopic expression of RcAPETALA2 (RcAP2) along with the down regulation of RcPI, RcAP3, and RcSEP3 is associated with loss of stamens identity and leaf-like structures formation. In whorl 4, the ectopic expression of RcAP2 associated with a down-regulation of RcSEP3 and of the C-class gene RcAGAMOUS correlate with loss of pistil identity. The latter also suggested the antagonist effect between the A and C class genes in the rose. Together, these data suggest that modified expression of the ABCE flower organ identity genes is associated with the phyllody phenotype in the rose Viridiflora and that these genes are important for normal flower organs development.

Evolution by gene duplication of Medicago truncatula PISTILLATA-like transcription factors

PISTILLATA (PI) is a member of the B-function MADS-box gene family, which controls the identity of both petals and stamens in Arabidopsis thaliana. In Medicago truncatula (Mt), there are two PI-like paralogs, known as MtPI and MtNGL9. These genes differ in their expression patterns, but it is not known whether their functions have also diverged. Describing the evolution of certain duplicated genes, such as transcription factors, remains a challenge owing to the complex expression patterns and functional divergence between the gene copies. Here, we report a number of functional studies, including analyses of gene expression, protein-protein interactions, and reverse genetic approaches designed to demonstrate the respective contributions of each M. truncatula PI-like paralog to the B-function in this species. Also, we have integrated molecular evolution approaches to determine the mode of evolution of Mt PI-like genes after duplication. Our results demonstrate that MtPI functions as a master regulator of B-function in M. truncatula, maintaining the overall ancestral function, while MtNGL9 does not seem to have a role in this regard, suggesting that the pseudogenization could be the functional evolutionary fate for this gene. However, we provide evidence that purifying selection is the primary evolutionary force acting on this paralog, pinpointing the conservation of its biochemical function and, alternatively, the acquisition of a new role for this gene.

Gene network analysis of Arabidopsis thaliana flower development through dynamic gene perturbations

Understanding how flowers develop from undifferentiated stem cells has occupied developmental biologists for decades. Key to unraveling this process is a detailed knowledge of the global regulatory hierarchies that control developmental transitions, cell differentiation and organ growth. These hierarchies may be deduced from gene perturbation experiments, which determine the effects on gene expression after specific disruption of a regulatory gene. Here, we tested experimental strategies for gene perturbation experiments during Arabidopsis thaliana flower development. We used artificial miRNAs (amiRNAs) to disrupt the functions of key floral regulators, and expressed them under the control of various inducible promoter systems that are widely used in the plant research community. To be able to perform genome-wide experiments with stage-specific resolution using the various inducible promoter systems for gene perturbation experiments, we also generated a series of floral induction systems that allow collection of hundreds of synchronized floral buds from a single plant. Based on our results, we propose strategies for performing dynamic gene perturbation experiments in flowers, and outline how they may be combined with versions of the floral induction system to dissect the gene regulatory network underlying flower development.

Adaptation in flower form: a comparative evodevo approach

Evolutionary developmental biology (evodevo) attempts to explain how the process of organismal development evolves, utilizing a comparative approach to investigate changes in developmental pathways and processes that occur during the evolution of a given lineage. Evolutionary genetics uses a population approach to understand how organismal changes in form or function are linked to underlying genetics, focusing on changes in gene and genotype frequencies within populations and the fixation of genotypic variation into traits that define species or evoke speciation events. Microevolutionary processes, including mutation, genetic drift, natural selection and gene flow, can provide the foundation for macroevolutionary patterns observed as morphological evolution and adaptation. The temporal element linking microevolutionary processes to macroevolutionary patterns is development: an organism's genotype is converted to phenotype by ontogenetic processes. Because selection acts upon the phenotype, the connection between evolutionary genetics and developmental evolution becomes essential to understanding adaptive evolution in organismal form and function. Here, we discuss how developmental genetic studies focused on key developmental processes could be linked within a comparative framework to study the developmental genetics of adaptive evolution, providing examples from research on two key processes of plant evodevo - floral symmetry and organ fusion - and their role in the adaptation of floral form.

Combinatorial activities of SHORT VEGETATIVE PHASE and FLOWERING LOCUS C define distinct modes of flowering regulation in Arabidopsis

Background

The initiation of flowering is an important developmental transition as it marks the beginning of the reproductive phase in plants. The MADS-box transcription factors (TFs) FLOWERING LOCUS C (FLC) and SHORT VEGETATIVE PHASE (SVP) form a complex to repress the expression of genes that initiate flowering in Arabidopsis. Both TFs play a central role in the regulatory network by conferring seasonal patterns of flowering. However, their interdependence and biological relevance when acting as a complex have not been extensively studied.

Results

We characterized the effects of both TFs individually and as a complex on flowering initiation using transcriptome profiling and DNA-binding occupancy. We find four major clusters regulating transcriptional responses, and that DNA binding scenarios are highly affected by the presence of the cognate partner. Remarkably, we identify genes whose regulation depends exclusively on simultaneous action of both proteins, thus distinguishing between the specificity of the SVP:FLC complex and that of each TF acting individually. The downstream targets of the SVP:FLC complex include a higher proportion of genes regulating floral induction, whereas those bound by either TF independently are biased towards floral development. Many genes involved in gibberellin-related processes are bound by the SVP:FLC complex, suggesting that direct regulation of gibberellin metabolism by FLC and SVP contributes to their effects on flowering.

Conclusions

The regulatory codes controlled by SVP and FLC were deciphered at the genome-wide level revealing substantial flexibility based on dependent and independent DNA binding that may contribute to variation and robustness in the regulation of flowering.

Electronic supplementary material

The online version of this article (doi:10.1186/s13059-015-0597-1) contains supplementary material, which is available to authorized users.