Inflorescence development in petunia: through the maze of botanical terminology

SUPERMAN genes: uncovering a new function in the development of complex inflorescences

The Arabidopsis SUPERMAN (SUP) gene and its orthologs in eudicots are crucial in regulating the number of reproductive floral organs. In Medicago truncatula, in addition to this function, a novel role in controlling meristem activity during compound inflorescence development was assigned to the SUP-ortholog (MtSUP). These findings led us to investigate whether the role of SUP genes in inflorescence development was legume-specific or could be extended to other eudicots. To assess that, we used Solanum lycopersicum as a model system with a cymose complex inflorescence and Arabidopsis thaliana as the best-known example of simple inflorescence. We conducted a detailed comparative expression analysis of SlSUP and SUP from vegetative stages to flower transition. In addition, we performed an exhaustive phenotypic characterisation of two different slsup and sup mutants during the plant life cycle. Our findings reveal that SlSUP is required for precise regulation of the meristems that control shoot and inflorescence architecture in tomato. In contrast, in Arabidopsis, SUP performs no meristematic function, but we found a role of SUP in floral transition. Our findings suggest that the functional divergence of SUP-like genes contributed to the modification of inflorescence architecture during angiosperm evolution.

JOINTLESS Maintains Inflorescence Meristem Identity in Tomato

JOINTLESS (J) was isolated in tomato (Solanum lycopersicum) from mutants lacking a flower pedicel abscission zone (AZ) and encodes a MADS-box protein of the SHORT VEGETATIVE PHASE/AGAMOUS-LIKE 24 subfamily. The loss of J function also causes the return to leaf initiation in the inflorescences, indicating a pivotal role in inflorescence meristem identity. Here, we compared jointless (j) mutants in different accessions that exhibit either an indeterminate shoot growth, producing regular sympodial segments, or a determinate shoot growth, due to the reduction of sympodial segments and causal mutation of the SELF-PRUNING (SP) gene. We observed that the inflorescence phenotype of j mutants is stronger in indeterminate (SP) accessions such as Ailsa Craig (AC), than in determinate (sp) ones, such as Heinz (Hz). Moreover, RNA-seq analysis revealed that the return to vegetative fate in j mutants is accompanied by expression of SP, which supports conversion of the inflorescence meristem to sympodial shoot meristem in j inflorescences. Other markers of vegetative meristems such as APETALA2c and branching genes such as BRANCHED 1 (BRC1a/b) were differentially expressed in the inflorescences of j(AC) mutant. We also found in the indeterminate AC accession that J represses homeotic genes of B- and C-classes and that its overexpression causes an oversized leafy calyx phenotype and has a dominant negative effect on AZ formation. A model is therefore proposed where J, by repressing shoot fate and influencing reproductive organ formation, acts as a key determinant of inflorescence meristems.

The RPT2a-MET1 axis regulates TERMINAL FLOWER1 to control inflorescence meristem indeterminacy in Arabidopsis

Plant inflorescence architecture is determined by inflorescence meristem (IM) activity and controlled by genetic mechanisms associated with environmental factors. In Arabidopsis (Arabidopsis thaliana), TERMINAL FLOWER1 (TFL1) is expressed in the IM and is required to maintain indeterminate growth, whereas LEAFY (LFY) is expressed in the floral meristems (FMs) formed at the periphery of the IM and is required to activate determinate floral development. Here, we address how Arabidopsis indeterminate inflorescence growth is determined. We show that the 26S proteasome subunit REGULATORY PARTICLE AAA-ATPASE 2a (RPT2a) is required to maintain the indeterminate inflorescence architecture in Arabidopsis. rpt2a mutants display reduced TFL1 expression levels and ectopic LFY expression in the IM and develop a determinate zigzag-shaped inflorescence. We further found that RPT2a promotes DNA METHYLTRANSFERASE1 degradation, leading to DNA hypomethylation upstream of TFL1 and high TFL1 expression levels in the wild-type IM. Overall, our work reveals that proteolytic input into the epigenetic regulation of TFL1 expression directs inflorescence architecture in Arabidopsis, adding an additional layer to stem cell regulation.

Pseudanthia in angiosperms: a review

BACKGROUND

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

SCOPE

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

CONCLUSIONS

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

The Amsterdam petunia germplasm collection: A tool in plant science

Petunia hybrida is a plant model system used by many researchers to investigate a broad range of biological questions. One of the reasons for the success of this organism as a lab model is the existence of numerous mutants, involved in a wide range of processes, and the ever-increasing size of this collection owing to a highly active and efficient transposon system. We report here on the origin of petunia-based research and describe the collection of petunia lines housed in the University of Amsterdam, where many of the existing genotypes are maintained.

Inflorescences of Cuscuta (Convolvulaceae): Diversity, evolution and relationships with breeding systems and fruit dehiscence modes

Cuscuta (dodder) includes ca. 200 species of plant obligate stem parasites with enormous ecological and economical significance. Inflorescences have been historically used in Cuscuta for species descriptions and identification keys, but no comprehensive study exists to date. The main objectives of this study were to survey the diversity and evolution of inflorescences and to uncover their possible form-function relationships. The inflorescence architecture of 132 Cuscuta taxa was analysed using herbarium specimens and eight species were grown to study their inflorescence development. Inflorescence traits were mapped into a genus phylogeny obtained from a combined analysis of nuclear ITS and plastid trnL-F sequences. To test the hypothesis that inflorescence architecture is connected to sexual reproduction, correlations between inflorescence traits (using Principal Components), sexual reproductive traits (pollen/ovule ratios, corolla length and diameter), fruit charaters (fruit length and width), and the modes of dehiscence were analyzed. Based on their development, three major types of inflorescences were observed: "Cuscuta type", a simple, monochasial scorpioid cyme; "Monogynella type", a compound monochasial scorpioid cymes with the longest primary axes having prolonged vegetative growth and giving the appearance of thyrses; and "Grammica type", a compound monochasial scorpiod cymes with up to five orders of axes. Maximum likelihood analyses suggested Monogynella as the ancestral type, while Cuscuta and Grammica were derived. Overall, the total length of axes exhibited a reduction trend throughout the genus evolution, but it was not correlated with the pedicels length. Inflorescences with similar architectures may exhibit contrasting pollen-ovule ratios. Positive significant correlations were noted between the size of the flower traits and pollen-ovule ratios. Several modes of dehiscence had statistically significant different total axes lengths, suggesting that the infructescence architecture is connected to the modes of dehiscence in Cuscuta and therefore seed dispersal.

Meristem transitions and plant architecture-learning from domestication for crop breeding

Genetic networks that regulate meristem transitions were recurrent targets of selection during crop domestication and allow fine-tuning of plant architecture for improved crop productivity.

Why pendulum symmetry is absent from the cymose partial inflorescences of Cannaceae? Insights into the essential characteristic of cincinni

In a typical cincinnus, the neighboring two flowers are generally enantiomorphic, which leads to the pendulum symmetry of the entire cyme. While in a two-flowered Cannaceae cincinnus, the flowers develop the same chirality. In this study, we observed several abnormal cincinni of Canna indica that extended longer than their normal form, which presented a second enantiomorphic flower, thus reflecting a typical pendulum symmetry. The chirality change of the second flower was strongly associated with the position of the lateral cincinnus meristem, which determines the angle size of the cincinnus zigzag shift and may serve as a key factor controlling the formation of pendulum symmetry. We propose that alternating floral chirality and the concomitant pendulum symmetry are the essential characteristics of a typical cincinnus. Accordingly, Canna flowers with the same chirality are arranged in modified cincinni.

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.

Plant Inflorescence Architecture: The Formation, Activity, and Fate of Axillary Meristems

The above-ground plant body in different plant species can have very distinct forms or architectures that arise by recurrent redeployment of a finite set of building blocks-leaves with axillary meristems, stems or branches, and flowers. The unique architectures of plant inflorescences in different plant families and species, on which this review focuses, determine the reproductive success and yield of wild and cultivated plants. Major contributors to the inflorescence architecture are the activity and developmental trajectories adopted by axillary meristems, which determine the degree of branching and the number of flowers formed. Recent advances in genetic and molecular analyses in diverse flowering plants have uncovered both common regulatory principles and unique players and/or regulatory interactions that underlie inflorescence architecture. Modulating activity of these regulators has already led to yield increases in the field. Additional insight into the underlying regulatory interactions and principles will not only uncover how their rewiring resulted in altered plant form, but will also enhance efforts at optimizing plant architecture in desirable ways in crop species.

Reduced Expression of CbUFO Is Associated with the Phenotype of a Flower-Defective Cosmos bipinnatus

LEAFY (LFY) and UNUSUAL FLORAL ORGANS (UFO) homologous genes have been reported to play key roles in promoting the initiation of floral meristems in raceme- and cyme-type plants. Asteraceae, a large family of plants with more than 23,000 species, has a unique head-like inflorescence termed capitulum. Here, we report a floral defective plant of the garden cosmos named green head (gh), which shows homogeneous inflorescence, indistinguishable inflorescence periphery and center, and the replacement of flower meristems by indeterminate inflorescence meristems, coupled with iterative production of bract-like organs and higher order of inflorescences. A comparison of the LFY- and UFO-like genes (CbFLY and CbUFO) isolated from both the wild-type and gh cosmos revealed that CbUFO may play an important role in inflorescence differentiation into different structures and promotion of flower initiation, and the reduced expression of CbUFO in the gh cosmos could be associated with the phenotypes of the flower-defective plants. Further expression analysis indicated that CbUFO may promote the conversion of inflorescence meristem into floral meristem in early ray flower formation, but does not play a role in its later growth period.

Architecture and Florogenesis in Female Cannabis sativa Plants

The inflorescence is the main product of medical cannabis. Hundreds of specialized metabolites with potential bioactivity are produced and accumulated in the glandular trichomes that are highly abundant mainly on female inflorescences. Understanding the morphophysiological and genetic mechanisms governing flower and inflorescence development is therefore of high scientific and practical importance. However, in-depth investigations of cannabis florogenesis are limited. Cannabis producers and researchers consider long photoperiod to be "non-inductive" or "vegetative," but under these growth conditions, the development of solitary flowers and bracts in shoot internodes clearly indicates that the plant cannot be defined as vegetative or non-inductive in the classical sense. Most probably, induction of solitary flowers is age-dependent and controlled by internal signals, but not by photoperiod. Short photoperiod induces intense branching, which results in the development of a compound raceme. Each inflorescence consists of condensed branchlets with the same phytomer structure as that of the larger phytomers developed under long day. Each phytomer consists of reduced leaves, bracts, one or two solitary flowers, and an axillary shoot (or inflorescence). Therefore, the effect of short photoperiod on cannabis florogenesis is not flower induction, but rather a dramatic change in shoot apex architecture to form a compound racemose inflorescence structure. An understanding of the morphophysiological characteristics of cannabis inflorescence will lay the foundation for biotechnological and physiological applications to modify architecture and to maximize plant productivity and uniformity in medical Cannabis.

Evolution and genetic control of the floral ground plan

Contents Summary 70 I. Introduction 70 II. What is the floral ground plan? 71 III. Diversity and evolution of the floral ground plan 72 IV. Genetic mechanisms 77 V. What's next? 82 Acknowledgements 83 References 83 SUMMARY: The floral ground plan is a map of where and when floral organ primordia arise. New results combining the defined phylogeny of flowering plants with extensive character mapping have predicted that the angiosperm ancestor had whorls rather than spirals of floral organs in large numbers, and was bisexual. More confidently, the monocot ancestor likely had three organs in each whorl, whereas the rosid and asterid ancestor (Pentapetalae) had five, with the perianth now divided into sepals and petals. Genetic mechanisms underlying the establishment of the floral ground plan are being deduced using model species, the rosid Arabidopsis, the asterid Antirrhinum, and in grasses such as rice. In this review, evolutionary and genetic conclusions are drawn together, especially considering how known genes may control individual processes in the development and evolution of ground plans. These components include organ phyllotaxis, boundary formation, organ identity, merism (the number or organs per whorl), variation in the form of primordia, organ fusion, intercalary growth, floral symmetry, determinacy and, finally, cases where the distinction between flowers and inflorescences is blurred. It seems likely that new pathways of ground plan evolution, and new signalling mechanisms, will soon be uncovered by integrating morphological and genetic approaches.

Deregulation of MADS-box transcription factor genes in a mutant defective in the WUSCHEL-LIKE HOMEOBOX gene EVERGREEN of Petunia hybrida

Angiosperm inflorescences develop in two fundamentally different ways. In monopodial plants, for example in Arabidopsis thaliana, the flowers are initiated as lateral appendages of a central indeterminate inflorescence meristem. In sympodial plants, flowers arise by terminal differentiation of the inflorescence meristem, while further inflorescence development proceeds from new sympodial meristems that are generated at the flank of the terminal flower. We have used the sympodial model species Petunia hybrida to investigate inflorescence development. Here, we describe a mutant, bonsai (bns), which is defective in flower formation, inflorescence branching, and control of meristem size. Detailed microscopic analysis revealed that bns meristems retain vegetative charateristics including spiral phyllotaxis. Consistent with a block in flower formation, bns mutants exhibit a deregulated expression of various MADS-box genes. Molecular analysis revealed that the bns mutant carries a transposon insertion in the previously described EVERGREEN (EVG) gene, which belongs to the WUSCHEL-LIKE HOMEOBOX (WOX) transcription factor gene family. EVG falls in the WOX9 subfamily, which has diverse developmental functions in angiosperms. The comparison of WOX9 orthologues in five model species for flowering shows that these genes play functionally divergent roles in monopodial and sympodial plants, indicating that the WOX9 regulatory node may have played an important role in the evolution of shoot architecture.

BIFURCATE FLOWER TRUSS: a novel locus controlling inflorescence branching in tomato contains a defective MAP kinase gene

A mutant line, bifurcate flower truss (bif), was recovered from a tomato genetics programme. Plants from the control line produced a mean of 0.16 branches per truss, whereas the value for bif plants was 4.1. This increase in branching was accompanied by a 3.3-fold increase in flower number and showed a significant interaction with exposure to low temperature during truss development. The control line and bif genomes were resequenced and the bif gene was mapped to a 2.01 Mbp interval on chromosome 12; all coding region polymorphisms in the interval were surveyed, and five candidate genes displaying altered protein sequences were detected. One of these genes, SlMAPK1, encoding a mitogen-activated protein (MAP) kinase, contained a leucine to stop codon mutation predicted to disrupt kinase function. SlMAPK1 is an excellent candidate for bif because knock-out mutations of an Arabidopsis orthologue MPK6 were reported to have increased flower number. An introgression browser was used to demonstrate that the origin of the bif genomic DNA at the BIF locus was Solanum galapagense and that the SlMAPK1 null mutant is a naturally occurring allele widespread only on the Galápagos Islands. This work strongly implicates SlMAPK1 as part of the network of genes controlling inflorescence branching in tomato.

Analysis of microRNA reveals cleistogamous and chasmogamous floret divergence in dimorphic plant

Cleistogenes songorica, a grass species that exhibits two spatially different type of inflorescence, chastogamy (CH), flowers localized at the top, and cleistogamy (CL) flowers embedded in leaf sheath. This study aimed at dissecting reasons underlying these distinct floral development patterns at morphological and microRNA level. Phenotyping for CH and CL was conducted and four small RNA libraries were constructed from the CH and CL flowers for high-throughput sequencing to identify the differentiated miRNAs. As results, spikelet, stigma, anther, lemma and lodicule length of CH flowers were found larger than that of CL, and so was seed setting. Also, 17 flower-related differential expression miRNAs were identified which were associated with floral organ development and morphogenesis, and the flower development. Further results showed that miR159a.1-CL3996.Contig2 pair was related to anther development, miR156a-5p-CL1954.Contig2 was linked to response to high light intensity, miR408-3p/miR408d-Unigene429 was related to pollination and Unigene429 positively regulated flower development. To our knowledge, this is the first study on differential miRNA accumulation between CH and CL flowers and our study serves as a foundation to the future elucidation of regulatory mechanisms of miRNAs in the divergent development of CL and CH flowers in a single plant.

A sister of PIN1 gene in tomato (Solanum lycopersicum) defines leaf and flower organ initiation patterns by maintaining epidermal auxin flux

The spatiotemporal localization of the plant hormone auxin acts as a positional cue during early leaf and flower organogenesis. One of the main contributors to auxin localization is the auxin efflux carrier PIN-FORMED1 (PIN1). Phylogenetic analysis has revealed that PIN1 genes are split into two sister clades; PIN1 and the relatively uncharacterized Sister-Of-PIN1 (SoPIN1). In this paper we identify entire-2 as a loss-of-function SlSoPIN1a (Solyc10g078370) mutant in Solanum lycopersicum. The entire-2 plants are unable to specify proper leaf initiation leading to a frequent switch from the wild type spiral phyllotactic pattern to distichous and decussate patterns. Leaves in entire-2 are large and less complex and the leaflets display spatial deformities in lamina expansion, vascular development, and margin specification. During sympodial growth in entire-2 the specification of organ position and identity is greatly affected resulting in variable branching patterns on the main sympodial and inflorescence axes. To understand how SlSoPIN1a functions in establishing proper auxin maxima we used the auxin signaling reporter DR5: Venus to visualize differences in auxin localization between entire-2 and wild type. DR5: Venus visualization shows a widening of auxin localization which spreads to subepidermal tissue layers during early leaf and flower organogenesis, showing that SoPIN1 functions to focus auxin signaling to the epidermal layer. The striking spatial deformities observed in entire-2 help provide a mechanistic framework for explaining the function of the SoPIN1 clade in S.lycopersicum.

Paralogous SQUAMOSA PROMOTER BINDING PROTEIN-LIKE (SPL) genes differentially regulate leaf initiation and reproductive phase change in petunia

MAIN CONCLUSION

Duplicated petunia clade-VI SPL genes differentially promote the timing of inflorescence and flower development, and leaf initiation rate. The timing of plant reproduction relative to favorable environmental conditions is a critical component of plant fitness, and is often associated with variation in plant architecture and habit. Recent studies have shown that overexpression of the microRNA miR156 in distantly related annual species results in plants with perennial characteristics, including late flowering, weak apical dominance, and abundant leaf production. These phenotypes are largely mediated through the negative regulation of a subset of genes belonging to the SQUAMOSA PROMOTER BINDING PROTEIN-LIKE (SPL) family of transcription factors. In order to determine how and to what extent paralogous SPL genes have partitioned their roles in plant growth and development, we functionally characterized petunia clade-VI SPL genes under different environmental conditions. Our results demonstrate that PhSBP1and PhSBP2 differentially promote discrete stages of the reproductive transition, and that PhSBP1, and possibly PhCNR, accelerates leaf initiation rate. In contrast to the closest homologs in annual Arabidopsis thaliana and Mimulus guttatus, PhSBP1 and PhSBP2 transcription is not mediated by the gibberellic acid pathway, but is positively correlated with photoperiod and developmental age. The developmental functions of clade-VI SPL genes have, thus, evolved following both gene duplication and speciation within the core eudicots, likely through differential regulation and incomplete sub-functionalization.

Changes in cis-regulatory elements of a key floral regulator are associated with divergence of inflorescence architectures

Higher plant species diverged extensively with regard to the moment (flowering time) and position (inflorescence architecture) at which flowers are formed. This seems largely caused by variation in the expression patterns of conserved genes that specify floral meristem identity (FMI), rather than changes in the encoded proteins. Here, we report a functional comparison of the promoters of homologous FMI genes from Arabidopsis, petunia, tomato and Antirrhinum. Analysis of promoter-reporter constructs in petunia and Arabidopsis, as well as complementation experiments, showed that the divergent expression of leafy (LFY) and the petunia homolog aberrant leaf and flower (ALF) results from alterations in the upstream regulatory network rather than cis-regulatory changes. The divergent expression of unusual floral organs (UFO) from Arabidopsis, and the petunia homolog double top (DOT), however, is caused by the loss or gain of cis-regulatory promoter elements, which respond to trans-acting factors that are expressed in similar patterns in both species. Introduction of pUFO:UFO causes no obvious defects in Arabidopsis, but in petunia it causes the precocious and ectopic formation of flowers. This provides an example of how a change in a cis-regulatory region can account for a change in the plant body plan.

Ethyl methane sulfonate induced mutations in M2 generation and physiological variations in M1 generation of peppers (Capsicum annuum L.)

This study was conducted to enhance genetic variability in peppers (Capsicum annuum, cv B12) using ethyl methanesulphonate (EMS). Exposure to an EMS concentration of 0.6%, v/v for 12 h was used to mutagenize 2000 seeds for the first generation (M1). It was observed that the growth behaviors including plant height, flowering date, and number of seeds per first fruit were different in the M1 generation than in wild type (WT) plants. In addition one phenotypic mutation (leaf shape and plant architecture) was observed during the M1 generation. During the seedling stage in the M2 generation, the observed changes were in the form of slow growth or chlorophyll defect (e.g., albino, pale green, and yellow seedlings). At maturity, there were three kinds of phenotypic mutations observed in three different families of the mutant population. The first observed change was a plant with yellow leaf color, and the leaves of this mutant plant contained 62.19% less chlorophyll a and 64.06% less chlorophyll b as compared to the wild-type. The second mutation resulted in one dwarf plant with a very short stature (6 cm), compact internodes and the leaves and stem were rough and thick. The third type of mutation occurred in four plants and resulted in the leaves of these plants being very thick and longer than those of WT plants. Furthermore, anatomical observations of the leaf blade section of this mutant plant type contained more xylem and collenchyma tissue in the leaf midrib of the mutant plant than WT. In addition, its leaf blade contained thicker palisade and spongy tissue than the WT.

Capsicum annuum S (CaS) promotes reproductive transition and is required for flower formation in pepper (Capsicum annuum)

The genetic control of the transition to flowering has mainly been studied in model species, while few data are available in crop species such as pepper (Capsicum spp.). To elucidate the genetic control of the transition to flowering in pepper, mutants that lack flowers were isolated and characterized. Genetic mapping and sequencing allowed the identification of the gene disrupted in the mutants. Double mutants and expression analyses were used to characterize the relationships between the mutated gene and other genes controlling the transition to flowering and flower differentiation. The mutants were characterized by a delay in the initiation of sympodial growth, a delay in the termination of sympodial meristems and complete inhibition of flower formation. Capsicum annuum S (CaS), the pepper (Capsicum annuum) ortholog of tomato (Solanum lycopersicum) COMPOUND INFLORESCENCE and petunia (Petunia hybrida) EVERGREEN, was found to govern the mutant phenotype. CaS is required for the activity of the flower meristem identity gene Ca-ANANTHA and does not affect the expression of CaLEAFY. CaS is epistatic over other genes controlling the transition to flowering with respect to flower formation. Comparative homologous mutants in the Solanaceae indicate that CaS has uniquely evolved to have a critical role in flower formation, while its role in meristem maturation is conserved in pepper, tomato and petunia.

Inflorescence development in tomato: gene functions within a zigzag model

Tomato is a major crop plant and several mutants have been selected for breeding but also for isolating important genes that regulate flowering and sympodial growth. Besides, current research in developmental biology aims at revealing mechanisms that account for diversity in inflorescence architectures. We therefore found timely to review the current knowledge of the genetic control of flowering in tomato and to integrate the emerging network into modeling attempts. We developed a kinetic model of the tomato inflorescence development where each meristem was represented by its "vegetativeness" (V), reflecting its maturation state toward flower initiation. The model followed simple rules: maturation proceeded continuously at the same rate in every meristem (dV); floral transition and floral commitment occurred at threshold levels of V; lateral meristems were initiated with a gain of V (ΔV) relative to the V level of the meristem from which they derived. This last rule created a link between successive meristems and gave to the model its zigzag shape. We next exploited the model to explore the diversity of morphotypes that could be generated by varying dV and ΔV and matched them with existing mutant phenotypes. This approach, focused on the development of the primary inflorescence, allowed us to elaborate on the genetic regulation of the kinetic model of inflorescence development. We propose that the lateral inflorescence meristem fate in tomato is more similar to an immature flower meristem than to the inflorescence meristem of Arabidopsis. In the last part of our paper, we extend our thought to spatial regulators that should be integrated in a next step for unraveling the relationships between the different meristems that participate to sympodial growth.

Meristem maturation and inflorescence architecture--lessons from the Solanaceae

Plant apical meristems (AMs) grow continuously by delicately balancing cells leaving at the periphery to form lateral organs with slowly dividing central domain cells that replenish reservoirs of pluripotent cells. This balance can be modified by signals originating from within and outside the meristem, and their integration results in a gradual maturation process that often culminates with the meristem differentiating into a flower. Accompanying this 'meristem maturation' are changes in spacing and size of lateral organs and in rates at which lateral meristems are released from apical dominance. Modulation of distinct meristem maturation parameters through environmental and genetic changes underlies the remarkable diversity of shoot architectures. Here, we discuss recent studies relating the dynamics of meristem maturation with organization of floral branching systems--inflorescences--in the nightshades. From this context, we suggest general principles on how factors coordinating meristem maturation impact shoot organization more broadly.

Meristem identity and phyllotaxis in inflorescence development

Inflorescence morphology is incredibly diverse. This diversity of form has been a fruitful source of inquiry for plant morphologists for more than a century. Work in the grasses (Poaceae), the tomato family (Solanaceae), and Arabidopsis thaliana (Brassicaceae) has led to a richer understanding of the molecular genetics underlying this diversity. The character of individual meristems, a combination of the number (determinacy) and nature (identity) of the products a meristem produces, is key in the development of plant form. A framework that describes inflorescence development in terms of shifting meristem identities has emerged and garnered empirical support in a number of model systems. We discuss this framework and highlight one important aspect of meristem identity that is often considered in isolation, phyllotaxis. Phyllotaxis refers to the arrangement of lateral organs around a central axis. The development and evolution of phyllotaxis in the inflorescence remains underexplored, but recent work analyzing early inflorescence development in the grasses identified an evolutionary shift in primary branch phyllotaxis in the Pooideae. We discuss the evidence for an intimate connection between meristem identity and phyllotaxis in both the inflorescence and vegetative shoot, and touch on what is known about the establishment of phyllotactic patterns in the meristem. Localized auxin maxima are instrumental in determining the position of lateral primordia. Upstream factors that regulate the position of these maxima remain unclear, and how phyllotactic patterns change over the course of a plant's lifetime and evolutionary time, is largely unknown. A more complete understanding of the molecular underpinnings of phyllotaxis and architectural diversity in inflorescences will require capitalizing on the extensive resources available in existing genetic systems, and developing new model systems that more fully represent the diversity of plant morphology.

What lies beyond the eye: the molecular mechanisms regulating tomato fruit weight and shape

Domestication of fruit and vegetables resulted in a huge diversity of shapes and sizes of the produce. Selections that took place over thousands of years of alleles that increased fruit weight and altered shape for specific culinary uses provide a wealth of resources to study the molecular bases of this diversity. Tomato (Solanum lycopersicum) evolved from a wild ancestor (S. pimpinellifolium) bearing small and round edible fruit. Molecular genetic studies led to the identification of two genes selected for fruit weight: FW2.2 encoding a member of the Cell Number Regulator family; and FW3.2 encoding a P450 enzyme and the ortholog of KLUH. Four genes were identified that were selected for fruit shape: SUN encoding a member of the IQD family of calmodulin-binding proteins leading to fruit elongation; OVATE encoding a member of the OVATE family proteins involved in transcriptional repression leading to fruit elongation; LC encoding most likely the ortholog of WUSCHEL controlling meristem size and locule number; FAS encoding a member in the YABBY family controlling locule number leading to flat or oxheart shape. For this article, we will provide an overview of the putative function of the known genes, when during floral and fruit development they are hypothesized to act and their potential importance in regulating morphological diversity in other fruit and vegetable crops.

Towards an ontogenetic understanding of inflorescence diversity

BACKGROUNDS AND AIMS

Conceptual and terminological conflicts in inflorescence morphology indicate a lack of understanding of the phenotypic diversity of inflorescences. In this study, an ontogeny-based inflorescence concept is presented considering different meristem types and developmental pathways. By going back to the ontogenetic origin, diversity is reduced to a limited number of types and terms.

METHODS

Species from 105 genera in 52 angiosperm families are investigated to identify their specific reproductive meristems and developmental pathways. Based on these studies, long-term experience with inflorescences and literature research, a conceptual framework for the understanding of inflorescences is presented.

KEY RESULTS

Ontogeny reveals that reproductive systems traditionally called inflorescences fall into three groups, i.e. 'flowering shoot systems' (FSS), 'inflorescences' sensu stricto and 'floral units' (FUs). Our concept is, first, based on the identification of reproductive meristem position and developmental potential. The FSS, defined as a seasonal growth unit, is used as a reference framework. As the FSS is a leafy shoot system bearing reproductive units, foliage and flowering sequence play an important role. Second, the identification of two different flower-producing meristems is essential. While 'inflorescence meristems' (IMs) share acropetal primordia production with vegetative meristems, 'floral unit meristems' (FUMs) resemble flower meristems in being indeterminate. IMs produce the basic inflorescence types, i.e. compound and simple racemes, panicles and botryoids. FUMs give rise to dense, often flower-like units (e.g. heads). They occur solitarily at the FSS or occupy flower positions in inflorescences, rendering the latter thyrses in the case of cymose branching.

CONCLUSIONS

The ontogenetic concept differs from all existing inflorescence concepts in being based on meristems and developmental processes. It includes clear terms and allows homology statements. Transitional forms are an explicit part of the concept, illustrating the ontogenetic potential for character transformation in evolution.

CaJOINTLESS is a MADS-box gene involved in suppression of vegetative growth in all shoot meristems in pepper

In aiming to decipher the genetic control of shoot architecture in pepper (Capsicum spp.), the allelic late-flowering mutants E-252 and E-2537 were identified. These mutants exhibit multiple pleiotropic effects on the organization of the sympodial shoot. Genetic mapping and sequence analysis indicated that the mutants are disrupted at CaJOINTLESS, the orthologue of the MADS-box genes JOINTLESS and SVP in tomato and Arabidopsis, respectively. Late flowering of the primary and sympodial shoots of Cajointless indicates that the gene functions as a suppressor of vegetative growth in all shoot meristems. While CaJOINTLESS and JOINTLESS have partially conserved functions, the effect on flowering time and on sympodial development in pepper, as well as the epistasis over FASCICULATE, the homologue of the major determinant of sympodial development SELF-PRUNING, is stronger than in tomato. Furthermore, the solitary terminal flower of pepper is converted into a structure composed of flowers and leaves in the mutant lines. This conversion supports the hypothesis that the solitary flowers of pepper have a cryptic inflorescence identity that is suppressed by CaJOINTLESS. Formation of solitary flowers in wild-type pepper is suggested to result from precocious maturation of the inflorescence meristem.

Repression of floral meristem fate is crucial in shaping tomato inflorescence

Tomato is an important crop and hence there is a great interest in understanding the genetic basis of its flowering. Several genes have been identified by mutations and we constructed a set of novel double mutants to understand how these genes interact to shape the inflorescence. It was previously suggested that the branching of the tomato inflorescence depends on the gradual transition from inflorescence meristem (IM) to flower meristem (FM): the extension of this time window allows IM to branch, as seen in the compound inflorescence (s) and falsiflora (fa) mutants that are impaired in FM maturation. We report here that JOINTLESS (J), which encodes a MADS-box protein of the same clade than SHORT VEGETATIVE PHASE (SVP) and AGAMOUS LIKE 24 (AGL24) in Arabidopsis, interferes with this timing and delays FM maturation, therefore promoting IM fate. This was inferred from the fact that j mutation suppresses the high branching inflorescence phenotype of s and fa mutants and was further supported by the expression pattern of J, which is expressed more strongly in IM than in FM. Most interestingly, FA - the orthologue of the Arabidopsis LEAFY (LFY) gene - shows the complementary expression pattern and is more active in FM than in IM. Loss of J function causes premature termination of flower formation in the inflorescence and its reversion to a vegetative program. This phenotype is enhanced in the absence of systemic florigenic protein, encoded by the SINGLE FLOWER TRUSS (SFT) gene, the tomato orthologue of FLOWERING LOCUS T (FT). These results suggest that the formation of an inflorescence in tomato requires the interaction of J and a target of SFT in the meristem, for repressing FA activity and FM fate in the IM.

Open and closed inflorescences: more than simple opposites

The absence of a terminal flower in inflorescences ('open inflorescences') is currently explained by the maintenance of putative stem-cells in the central zone (CZ) of the inflorescence meristem (IM) governed by the CLAVATA-WUSCHEL regulatory loop. Disruption of this regulatory pathway, as in Arabidopsis TERMINAL FLOWER LOCUS 1 mutants, leads to terminal flower production. However, recent studies in other taxa reveal novel mechanisms of inflorescence termination; for example, the SEPALLATA-like MADS-box floral identity gene GERBERA REGULATOR OF CAPITULUM DEVELOPMENT 2 in Gerbera excludes the retention of a CZ as an ontogenetic cause for the openness of these inflorescences. Moreover, comparative histological studies show that the retention of a CZ in the IM, mostly a feature of the 'typical open families', is absent in open inflorescences of other families. Concerning these groups, new evidence suggests that spatial constraints at the IM could play a role at the time when terminal flower production (or not) is determined. This indicates that the multiple loss and re-gain of the terminal flower in angiosperms is necessarily based on more than one ontogenetic pathway.