The role of APETALA1 in petal number robustness

Temperature-responsive module of OfAP1 and OfLFY regulates floral transition and floral organ identity in Osmanthus fragrans

The MADS-box transcription factor APETELA1 (AP1) is crucially important for reproductive developmental processes. The function of AP1 and the classic LFY-AP1 interaction in woody plants are not widely known. Here, the OfAP1-a gene from the continuously flowering plant Osmanthus fragrans 'Sijigui' was characterized, and its roles in regulating flowering time, petal number robustness and floral organ identity were determined using overexpression in Arabidopsis thaliana and Nicotiana tabacum. The expression of OfAP1-a was significantly induced by low ambient temperature and was upregulated with the floral transition process. Ectopic expression OfAP1-a revealed its classic function in flowering and flower ABC models. The expression of OfAP1-a is inhibited by LEAFY (OfLFY) through direct promoter binding, as confirmed by yeast one-hybrid and dual luciferase assays. Arabidopsis plants overexpressing OfAP1-a exhibited accelerated flowering and altered floral organ identities. Moreover, OfAP1-a-overexpressing plants displayed variable petal numbers. Likewise, the overexpression of OfLFY in Arabidopsis and Nicotiana altered petal number robustness and inflorescence architecture, partially by regulating native AP1 in transformed plants. Furthermore, we performed RNA-seq analysis of transgenic Nicotiana plants. DEGs were identified by transcriptome analysis, and we found that the expression of several floral homeotic genes was altered in both OfAP1-a and OfLFY-overexpressing transgenic lines. Our results suggest that OfAP1-a may play important roles during floral transition and development in response to ambient temperature. OfAP1-a functions as a petal number modulator and may directly activate a subset of flowers to regulate floral organ formation. OfAP1-a and OfLFY mutually regulate the expression of each other and coregulate genes that might be involved in these phenotypes related to flowering. The results provide valuable data for understanding the function of the LFY-AP1 module in the reproductive process and shaping floral structures in woody plants.

Functional Divergence Analysis of AGL6 Genes in Prunus mume

The AGAMOUS-LIKE6 (AGL6) lineage is an important clade of MADS-box transcription factors that play essential roles in floral organ development. The genome of Prunus mume contains two homoeologous AGL6 genes that are replicated as gene fragments. In this study, two AGL6 homologs, PmAGL6-1 and PmAGL6-2, were cloned from P. mume and then functionally characterized. Sequence alignment and phylogenetic analyses grouped both genes into the AGL6 lineage. The expression patterns and protein-protein interaction patterns showed significant differences between the two genes. However, the ectopic expression of the two genes in Arabidopsis thaliana resulted in similar phenotypes, including the promotion of flowering, alteration of floral organ structure, participation in the formation of the floral meristem and promotion of pod bending. Therefore, gene duplication has led to some functional divergence of PmAGL6-1 and PmAGL6-2 but their functions are similar. We thus speculated that AGL6 genes play a crucial role in flower development in P. mume.

The selfing syndrome and beyond: diverse evolutionary consequences of mating system transitions in plants

The shift from outcrossing to self-fertilization (selfing) is considered one of the most prevalent evolutionary transitions in flowering plants. Selfing species tend to share similar reproductive traits in morphology and function, and such a set of traits is called the 'selfing syndrome'. Although the genetic basis of the selfing syndrome has been of great interest to evolutionary biologists, knowledge of the causative genes or mutations was limited until recently. Thanks to advances in population genomic methodologies combined with high-throughput sequencing technologies, several studies have successfully unravelled the molecular and genetic basis for evolution of the selfing syndrome in Capsella, Arabidopsis, Solanum and other genera. Here we first introduce recent research examples that have explored the loci, genes and mutations responsible for the selfing syndrome traits, such as reductions in petal size or in pollen production, that are mainly relevant to pre-pollination processes. Second, we review the relationship between the evolution of selfing and interspecific pollen transfer, highlighting the findings of post-pollination reproductive barriers at the molecular level. We then discuss the emerging view of patterns in evolution of the selfing syndrome, such as the pervasive involvement of loss-of-function mutations and the relative importance of selection versus neutral degradation. This article is part of the theme issue 'Genetic basis of adaptation and speciation: from loci to causative mutations'.

Cytokinin promotes growth cessation in the Arabidopsis root

The Arabidopsis root offers good opportunities to investigate how regulated cellular growth shapes different tissues and organs, a key question in developmental biology. Along the root's longitudinal axis, cells sequentially occupy different developmental states. Proliferative meristematic cells give rise to differentiating cells, which rapidly elongate in the elongation zone, then mature and stop growing in the differentiation zone. The phytohormone cytokinin contributes to this zonation by positioning the boundary between the meristem and the elongation zone, called the transition zone. However, the cellular growth profile underlying root zonation is not well understood, and the cellular mechanisms that mediate growth cessation remain unclear. By using time-lapse imaging, genetics, and computational analysis, we analyze the effect of cytokinin on root zonation and cellular growth. We found that cytokinin promotes growth cessation in the distal (shootward) elongation zone in conjunction with accelerating the transition from elongation to differentiation. We estimated cell-wall stiffness by using osmotic treatment experiments and found that cytokinin-mediated growth cessation is associated with cell-wall stiffening and requires the action of an auxin influx carrier, AUX1. Our measurement of growth and cell-wall mechanical properties at a cellular resolution reveal mechanisms via which cytokinin influences cell behavior to shape tissue patterns.

Quantitative live imaging of floral organ initiation and floral meristem termination in Aquilegia

In-depth investigation of any developmental process in plants requires knowledge of both the underpinning molecular networks and how they directly determine patterns of cell division and expansion over time. Floral meristems (FMs) produce floral organs, after which they undergo floral meristem termination (FMT); precise control of organ initiation and FMT is crucial to the reproductive success of any flowering plant. Using live confocal imaging, we characterized developmental dynamics during floral organ primordia initiation and FMT in Aquilegia coerulea (Ranunculaceae). Our results uncover distinct patterns of primordium initiation between stamens and staminodes compared with carpels, and provide insight into the process of FMT, which is discernable based on cell division dynamics that precede carpel initiation. To our knowledge, this is the first quantitative live imaging of meristem development in a system with numerous whorls of floral organs, as well as an apocarpous gynoecium. This study provides crucial information for our understanding of how the spatial-temporal regulation of floral meristem behavior is achieved in both evolutionary and developmental contexts.

This article has an associated ‘The people behind the papers’ interview.

Fifteen compelling open questions in plant cell biology

As scientists, we are at least as excited about the open questions-the things we do not know-as the discoveries. Here, we asked 15 experts to describe the most compelling open questions in plant cell biology. These are their questions: How are organelle identity, domains, and boundaries maintained under the continuous flux of vesicle trafficking and membrane remodeling? Is the plant cortical microtubule cytoskeleton a mechanosensory apparatus? How are the cellular pathways of cell wall synthesis, assembly, modification, and integrity sensing linked in plants? Why do plasmodesmata open and close? Is there retrograde signaling from vacuoles to the nucleus? How do root cells accommodate fungal endosymbionts? What is the role of cell edges in plant morphogenesis? How is the cell division site determined? What are the emergent effects of polyploidy on the biology of the cell, and how are any such "rules" conditioned by cell type? Can mechanical forces trigger new cell fates in plants? How does a single differentiated somatic cell reprogram and gain pluripotency? How does polarity develop de-novo in isolated plant cells? What is the spectrum of cellular functions for membraneless organelles and intrinsically disordered proteins? How do plants deal with internal noise? How does order emerge in cells and propagate to organs and organisms from complex dynamical processes? We hope you find the discussions of these questions thought provoking and inspiring.

Transitions in development – an interview with Marie Monniaux

Marie Monniaux is a permanent CNRS researcher in the ‘Evo-devo of the flower’ group at the Laboratory for Plant Reproduction and Development (RDP) at the École normale supérieure (ENS) in Lyon, France. Marie uses Petunia to understand the development and evolution of the flower petal. We met Marie over Teams for a virtual chat about her career path, finding a permanent position and her ideas for the future.

Developmental stochasticity and variation in floral phyllotaxis

Floral phyllotaxis is a relatively robust phenotype; trimerous and pentamerous arrangements are widely observed in monocots and core eudicots. Conversely, it also shows variability in some angiosperm clades such as 'ANA' grade (Amborellales, Nymphaeales, and Austrobaileyales), magnoliids, and Ranunculales. Regardless of the phylogenetic relationship, however, phyllotactic pattern formation appears to be a common process. What are the causes of the variability in floral phyllotaxis and how has the variation of floral phyllotaxis contributed to floral diversity? In this review, I summarize recent progress in studies on two related fields to develop answers to these questions. First, it is known that molecular and cellular stochasticity are inevitably found in biological systems, including plant development. Organisms deal with molecular stochasticity in several ways, such as dampening noise through gene networks or maintaining function through cellular redundancy. Recent studies on molecular and cellular stochasticity suggest that stochasticity is not always detrimental to plants and that it is also essential in development. Second, studies on vegetative and inflorescence phyllotaxis have shown that plants often exhibit variability and flexibility in phenotypes. Three types of phyllotaxis variations are observed, namely, fluctuation around the mean, transition between regular patterns, and a transient irregular organ arrangement called permutation. Computer models have demonstrated that stochasticity in the phyllotactic pattern formation plays a role in pattern transitions and irregularities. Variations are also found in the number and positioning of floral organs, although it is not known whether such variations provide any functional advantages. Two ways of diversification may be involved in angiosperm floral evolution: precise regulation of organ position and identity that leads to further specialization of organs and organ redundancy that leads to flexibility in floral phyllotaxis.

Protein Farnesylation Takes Part in Arabidopsis Seed Development

Protein farnesylation is a post-translational modification regulated by the ERA1 (Enhanced Response to ABA 1) gene encoding the β-subunit of the protein farnesyltransferase in Arabidopsis. The era1 mutants have been described for over two decades and exhibit severe pleiotropic phenotypes, affecting vegetative and flower development. We further investigated the development and quality of era1 seeds. While the era1 ovary contains numerous ovules, the plant produces fewer seeds but larger and heavier, with higher protein contents and a modified fatty acid distribution. Furthermore, era1 pollen grains show lower germination rates and, at flower opening, the pistils are immature and the ovules require one additional day to complete the embryo sac. Hand pollinated flowers confirmed that pollination is a major obstacle to era1 seed phenotypes, and a near wild-type seed morphology was thus restored. Still, era1 seeds conserved peculiar storage protein contents and altered fatty acid distributions. The multiplicity of era1 phenotypes reflects the diversity of proteins targeted by the farnesyltransferase. Our work highlights the involvement of protein farnesylation in seed development and in the control of traits of agronomic interest.

Arabidopsis sepals: A model system for the emergent process of morphogenesis

During development, Arabidopsis thaliana sepal primordium cells grow, divide and interact with their neighbours, giving rise to a sepal with the correct size, shape and form. Arabidopsis sepals have proven to be a good system for elucidating the emergent processes driving morphogenesis due to their simplicity, their accessibility for imaging and manipulation, and their reproducible development. Sepals undergo a basipetal gradient of growth, with cessation of cell division, slow growth and maturation starting at the tip of the sepal and progressing to the base. In this review, I discuss five recent examples of processes during sepal morphogenesis that yield emergent properties: robust size, tapered tip shape, laminar shape, scattered giant cells and complex gene expression patterns. In each case, experiments examining the dynamics of sepal development led to the hypotheses of local rules. In each example, a computational model was used to demonstrate that these local rules are sufficient to give rise to the emergent properties of morphogenesis.

Plant science's next top models

BACKGROUND

Model organisms are at the core of life science research. Notable examples include the mouse as a model for humans, baker's yeast for eukaryotic unicellular life and simple genetics, or the enterobacteria phage λ in virology. Plant research was an exception to this rule, with researchers relying on a variety of non-model plants until the eventual adoption of Arabidopsis thaliana as primary plant model in the 1980s. This proved to be an unprecedented success, and several secondary plant models have since been established. Currently, we are experiencing another wave of expansion in the set of plant models.

SCOPE

Since the 2000s, new model plants have been established to study numerous aspects of plant biology, such as the evolution of land plants, grasses, invasive and parasitic plant life, adaptation to environmental challenges, and the development of morphological diversity. Concurrent with the establishment of new plant models, the advent of the 'omics' era in biology has led to a resurgence of the more complex non-model plants. With this review, we introduce some of the new and fascinating plant models, outline why they are interesting subjects to study, the questions they will help to answer, and the molecular tools that have been established and are available to researchers.

CONCLUSIONS

Understanding the molecular mechanisms underlying all aspects of plant biology can only be achieved with the adoption of a comprehensive set of models, each of which allows the assessment of at least one aspect of plant life. The model plants described here represent a step forward towards our goal to explore and comprehend the diversity of plant form and function. Still, several questions remain unanswered, but the constant development of novel technologies in molecular biology and bioinformatics is already paving the way for the next generation of plant models.

Floral organ development goes live

The chance to watch floral organs develop live is not to be missed! Here, we outline reasons why quantitative, live-cell imaging is an important approach to study floral morphogenesis, and provide a basic workflow of how to get started. We highlight key advances in morphodynamics of lateral organ development, and discuss recent work that uses live confocal imaging to address the regulation of floral organ number, its robustness, and patterning mechanisms that exploit stochasticity.

Homeotic transformation from stamen to petal in Eriobotrya japonica is associated with hormone signal transduction and reduction of the transcriptional activity of EjAG

Double-flower loquat (Eriobotrya japonica) is a new germplasm with homeotic transformation of stamen into petal in whorl 3. However, little information is available on the molecular mechanism of this transformation. Herein, we analyzed the transcriptome, candidate genes and endogenous hormones to investigate the mechanisms underlying this homeotic transformation. Some transcription factors, such as MADS-box, TCP and MYB, were significantly differentially expressed. Importantly, we confirmed that one of these (DN39625_c0_g1), which encoded a C-class floral homeotic protein referred to as AGAMOUS ortholog (EjAG), was significantly downregulated. Subcellular localization of EjAG was found to be in the nucleus. Ectopic expression of EjAG rescued the development of stamens and carpels from the double-flower phenotype in an Arabidopsis ag mutant, suggesting that EjAG expression is associated with double-flower formation. Meanwhile, enrichment analyses showed that the differentially expressed genes (DEGs) were mainly involved in the metabolic pathways of hormone signal transduction. The DEGs of auxin, gibberellin A (GA) and cytokinin signaling pathways were mainly upregulated. However, the DEGs of abscisic acid (ABA) and the ethylene signaling pathway were mainly downregulated. Accordingly, the concentrations of indoleacetic acid, kinetin and GA₃ were high at the petaloid stamen stage, but the ABA concentration remained low. The identified genes and pathways provide abundant sequence resources for studying the mechanisms underlying the homeotic transformation in loquat and other Rosaceae species.

CRISPR/Cas9-Mediated Mutagenesis of RCO in Cardamine hirsuta

The small crucifer Cardamine hirsuta bears complex leaves divided into leaflets. This is in contrast to its relative, the reference plant Arabidopsis thaliana, which has simple leaves. Comparative studies between these species provide attractive opportunities to study the diversification of form. Here, we report on the implementation of the CRISPR/Cas9 genome editing methodology in C. hirsuta and with it the generation of novel alleles in the RCO gene, which was previously shown to play a major role in the diversification of form between the two species. Thus, genome editing can now be deployed in C. hirsuta, thereby increasing its versatility as a model system to study gene function and evolution.

Growth and biomechanics of shoot organs

Plant organs arise through complex interactions between biological and physical factors that control morphogenesis. While there has been tremendous progress in the understanding of the genetics behind development, we know much less about how mechanical forces control growth in plants. In recent years, new multidisciplinary research combining genetics, live-imaging, physics, and computational modeling has begun to fill this gap by revealing the crucial role of biomechanics in the establishment of plant organs. In this review, we provide an overview of our current understanding of growth during initiation, patterning, and expansion of shoot lateral organs. We discuss how growth is controlled by physical forces, and how mechanical stresses generated during growth can control morphogenesis at the level of both cells and tissues. Understanding the mechanical basis of growth and morphogenesis in plants is in its early days, and many puzzling facts are yet to be deciphered.

Brassicaceae flowers: diversity amid uniformity

The mustard family Brassicaceae, which includes the model plant Arabidopsis thaliana, exhibits morphological stasis and significant uniformity of floral plan. Nonetheless, there is untapped diversity in almost every aspect of floral morphology in the family that lends itself to comparative study, including organ number, shape, form, and color. Studies on the genetic basis of morphological diversity, enabled by extensive genetic tools and genomic resources and the close phylogenetic distance among mustards, have revealed a mosaic of conservation and divergence in numerous floral traits. Here I review the morphological diversity of the flowers of Brassicaceae and discuss studies addressing the underlying genetic and developmental mechanisms shaping floral diversity. To put flowers in the context of the floral display, I describe diversity in inflorescence morphology and the variation that exists in the structures preceding the floral organs. Reconstructing the floral morphospace in Brassicaceae coupled with next-generation sequencing data and unbiased approaches to interrogate gene function in species throughout the mustard phylogeny offers promising ways to understand how developmental mechanisms originate and diversify.

Duplication of a domestication locus neutralized a cryptic variant that caused a breeding barrier in tomato

Genome editing technologies are being widely adopted in plant breeding¹. However, a looming challenge of engineering desirable genetic variation in diverse genotypes is poor predictability of phenotypic outcomes due to unforeseen interactions with pre-existing cryptic mutations^2-4. In tomato, breeding with a classical MADS-box gene mutation that improves harvesting by eliminating fruit stem abscission frequently results in excessive inflorescence branching, flowering and reduced fertility due to interaction with a cryptic variant that causes partial mis-splicing in a homologous gene^5-8. Here, we show that a recently evolved tandem duplication carrying the second-site variant achieves a threshold of functional transcripts to suppress branching, enabling breeders to neutralize negative epistasis on yield. By dissecting the dosage mechanisms by which this structural variant restored normal flowering and fertility, we devised strategies that use CRISPR-Cas9 genome editing to predictably improve harvesting. Our findings highlight the under-appreciated impact of epistasis in targeted trait breeding and underscore the need for a deeper characterization of cryptic variation to enable the full potential of genome editing in agriculture.

Emergence of robust patterns from local rules during plant development

The formation of spatial and temporal patterns is an essential component of organismal development. Patterns can be observed on every level from subcellular to organismal and may emerge from local rules that correspond to the interactions between molecules, cells, or tissues. The emergence of robust patterns may seem in contradiction with the prominent heterogeneity at subcellular and cellular scales, however it has become increasingly clear that heterogeneity can be instrumental for pattern formation. Here we review recent examples in plant development, involving genetic regulation, cell arrangement, growth and signal gradient. We discuss how patterns emerge from local rules, whether heterogeneity is stochastic or can be patterned, and whether stochastic noise is amplified or requires filtering for robust patterns to be achieved. We also stress the importance of modelling in investigating such questions.