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The development of flowers and floral organs is directed by intricate genetic programmes, many aspects of which appear to be shared among angiosperms. Early acting genes establish floral meristem identity in flower primordia initiated at the periphery of the inflorescence meristem. Later, floral organ primordia arise at precise positions within these floral meristems and take on one of the four distinct identities (sepals, petals, stamens and carpels). The ABCE model, supported by both molecular and genetic experiments in Arabidopsis, explains how a small number of regulatory genes (called floral homeotic genes or floral organ identity genes) act in different combinations to specify these different organ types. The floral organ identity genes encode transcription factors that form distinct higher order protein complexes in different regions of a flower primordium to control the expression of target genes responsible for organogenesis. Key Concepts Lateral organs produced by the shoot apical meristem during reproductive development acquire their identity as flowers through the action of floral meristem identity genes such as LEAFY and APETALA1. The identities of each of the four organ types of a flower (sepal, petal, stamen and carpel) is conferred by a unique combination of floral organ identity gene activities, referred to as class A, B, C and E in the ABCE model. The activities of the class A, B and C genes are restricted to particular regions within a developing flower primarily, but not exclusively, through transcriptional regulation. The MADS domain transcription factors encoded by the class A, B, C and E genes form unique tetrameric transcriptional regulatory complexes in cells of each floral whorl. The transcriptional regulatory complexes formed by the A, B, C and E proteins regulate distinct sets of genes at different stages of flower development. Many aspects of the genetic programmes conferring floral meristem identity and floral organ identity are conserved among all angiosperms. Keywords: flower development; homeotic genes; floral meristem identity genes; floral organ identity genes; Arabidopsis; ABCE model; quartet model

Nymphaeaceae are early diverging angiosperms with large flowers characterized by showy petals and stamens not clearly whorled but presenting a gradual morphological transition from the outer elements to the inner stamens. Such flower structure makes these plant species relevant for studying flower evolution. MADS-domain transcription factors are crucial components of the molecular network that controls flower development. We therefore isolated and characterized MADS-box genes from the water lilyNymphaea caerulea. RNA-seq experiments on floral buds have been performed to obtain the transcript sequences of floral organ identity MADS-box genes. Maximum Likelihood phylogenetic analyses confirmed their belonging to specific MADS-box gene subfamilies. Their expression was quantified by RT-qPCR in all floral organs at two stages of development. Protein interactions among these transcription factors were investigated by yeast-two-hybrid assays. We found especially interesting the involvement of two differentAGAMOUS-likegenes (NycAG1andNycAG2) in the water lily floral components. They were therefore functionally characterized by complementing Arabidopsisagandshp1 shp2mutants. The expression analysis of MADS-box genes across flower development inN. caeruleadescribed a complex scenario made of numerous genes in numerous floral components. Their expression profiles in some cases were in line with what was expected from the ABC model of flower development and its extensions, while in other cases presented new and interesting gene expression patterns, as for instance the involvement ofNycAGL6andNycFL. Although sharing a high level of sequence similarity, the twoAGAMOUS-likegenesNycAG1andNycAG2could have undergone subfunctionalization or neofunctionalization, as only one of them could partially restore theeuAGfunction in Arabidopsisag-3mutants. The hereby illustratedN. caeruleaMADS-box gene expression pattern might mirror the morphological transition from the outer to the inner floral organs, and the presence of transition organs such as the petaloid stamens. This study is intended to broaden knowledge on the role and evolution of floral organ identity genes and the genetic mechanisms causing biodiversity in angiosperm flowers.

The qualitative model presented in this work recovers the onset of the four fields that correspond to those of each floral organ whorl ofArabidopsisflower, suggesting a mechanism for the generation of the positional information required for the differential expression of the A, B, and C identity genes according to the ABC model for organ determination during early stages of flower development. Our model integrates a previous model for the emergence of WUS pattern in the floral meristem, and shows that this pre-pattern is a necessary but not sufficient condition for the posterior information of the four fields predicted by the ABC model. Furthermore, our model predicts that LFY diffusion along the L1 layer of cells is not a necessary condition for the patterning of the floral meristem.

The pseudanthial inflorescences of the sunflower family, Asteraceae, mimic a solitary flower but are composed of multiple flowers. Our studies in Gerbera hybrida indicate functional diversification for SEPALLATA (SEP)-like MADS box genes that often function redundantly in other core eudicots. We conducted phylogenetic and expression analysis for eight SEP-like GERBERA REGULATOR OF CAPITULUM DEVELOPMENT (GRCD) genes, including previously unstudied gene family members. Transgenic gerbera plants were used to infer gene functions. Adding to the previously identified stamen and carpel functions for GRCD1 and GRCD2, two partially redundant genes, GRCD4 and GRCD5, were found to be indispensable for petal development. Stepwise conversion of floral organs into leaves in the most severe RNA interference lines suggest redundant and additive GRCD activities in organ identity regulation. We show conserved and redundant functions for several GRCD genes in regulation of flower meristem maintenance, while functional diversification for three SEP1/2/4 clade genes in regulation of inflorescence meristem patterning was observed. GRCD genes show both specialized and pleiotropic functions contributing to organ differentiation and flower meristem fate, and uniquely, to patterning of the inflorescence meristem. Altogether, we provide an example of how plant reproductive evolution has used conserved genetic modules for regulating the elaborate inflorescence architecture in Asteraceae.

Stem cells (SCs) are undifferentiated cells of multicellular organisms that can divide, self-renew, and differentiate. Despite the differences in their properties, the general principles of the existence of SCs can be distinguished in all multicellular organisms. In plants, SCs are found in meristems, structures that ensure the continuous growth of a plant and provide material for the formation of various specialized tissues. There are numerous types of meristems: shoot apical meristem (SAM) and root apical meristem (RAM), and lateral meristems (LMs) (procambium, cambium, and pericycle), as well as the so-called irregular meristems, developing under certain conditions (callus, meristems of symbiotic nodules, spontaneous and pathogen-induced tumors, etc.). Specific mechanisms of regulation, which are based on the interaction of plant hormones and the major groups of transcription factors, were identified for each meristem. The activity of meristems is determined by two opposing processes: proliferation and self-renewal of SCs in the central part of the meristem and differentiation of specialized cells in the periphery. WOX-CLAVATA systems are a regulatory component conservative for different meristems, which ensures the consistency of the composition of the meristem, as well as the balance of SC proliferation and differentiation. In this review, we consider the similarities and differences between the principles of the organization of SC niches in plants and animals, as well as in a variety of meristems of higher plants; special attention will be paid to the role of WOX-CLAVATA systems in maintaining meristems and their interaction with other meristem regulators.

The gene encoding the transcription factor LEAFY was identified in the genome of the mycoheterotrophic plant, pinesap Monotropa hypopitys. In the transcriptomes of roots, bracts, and flowers of flowering pinesaps, the MhyLFY gene expression was absent. These data suggest the conservativeness of the LFY-dependent mechanism of flower meristem identity and flower formation in heterotrophic species with some differences associated to the specificity of development and the structure of such plants. The pinesap flowering under the control of the transcription factor MhyLFY may be initiated either in an embryonic inflorescence during spring dormancy release of adventitious root buds or in an inflorescence of a growing reproductive stem after photoperiodic induction.

Plant meristem cells divide and differentiate in a spatially and temporally regulated manner, ultimately giving rise to organs. In this study, we isolated the Arabidopsis jing he sheng 1 (jhs1) mutant, which exhibited retarded growth, an abnormal pattern of meristem cell division and differentiation, and morphological defects such as fasciation, an irregular arrangement of siliques, and short roots. We identified JHS1 as a homolog of human and yeast DNA Replication Helicase/Nuclease2, which is known to be involved in DNA replication and damage repair. JHS1 is strongly expressed in the meristem of Arabidopsis. The jhs1 mutant was sensitive to DNA damage stress and had an increased DNA damage response, including increased expression of genes involved in DNA damage repair and cell cycle regulation, and a higher frequency of homologous recombination. In the meristem of the mutant plants, cell cycle progression was delayed at the G2 or late S phase and genes essential for meristem maintenance were misregulated. These results suggest that JHS1 plays an important role in DNA replication and damage repair, meristem maintenance, and development in plants.

Pepper flowers are hermaphroditic; the plant's male sterility trait is characterized by its inability to produce pollen grains. In the ABC model of flower development, B-function genes play roles in petal and stamen development in the angiosperm. In this study, a B-class gene designated as PAP3 (GenBank accession no. HM104635) was isolated in pepper. The gene encoded 226 amino acids and shared high similarity with the MADS-box protein family, with a conservative MADS domain and semiconservative K domain. Furthermore, the expression of PAP3 was abundant only in petals and anthers but not in leaves. A functional study employing virus-induced gene silencing (VIGS) showed that knockdown of PAP3 led to the shriveling of pollen grains and male sterility; however, it did not affect petal development. These results suggest an essential role for PAP3 in the development of the pepper stamen and in contributing to the variation of floral traits.

During reproductive development, rice plants develop unique flower organs which determine the final grain yield. OsMADS1, one of SEPALLATA-like MADS-box genes, has been unraveled to play critical roles in rice floral organ identity specification and floral meristem determinacy. However, the molecular mechanisms underlying interactions of OsMADS1 with other floral homeotic genes in regulating flower development remains largely elusive. In this work, we studied the genetic interactions of OsMADS1 with B-, C-, and D-class genes along with physical interactions among their proteins. We show that the physical and genetic interactions between OsMADS1 and OsMADS3 are essential for floral meristem activity maintenance and organ identity specification; while OsMADS1 physically and genetically interacts with OsMADS58 in regulating floral meristem determinacy and suppressing spikelet meristem reversion. We provided important genetic evidence to support the neofunctionalization of two rice C-class genes (OsMADS3 and OsMADS58) during flower development. Gene expression profiling and quantitative RT-PCR analyses further revealed that OsMADS1 affects the expression of many genes involved in floral identity and hormone signaling, and chromatin immunoprecipitation (ChIP)–PCR assay further demonstrated that OsMADS17 is a direct target gene of OsMADS1. Taken together, these results reveal that OsMADS1 has diversified regulatory functions in specifying rice floral organ and meristem identity, probably through its genetic and physical interactions with different floral homeotic regulators.

Crocus sativus, a monocot triploid species belonging to the Iridaceae family, is cultivated for its red stigmatic lobes of the carpel that constitute saffron. Flower development has been extensively studied in different plants. Different floral developmental pathways have been deciphered in many plants. In Crocus sativus, flower is the most important part and understanding the pathway underlying the flower development can pave the way for new avenues to improve its productivity and quality. The combination of class A genes (including APETALA1; CsAP1 and APETALA2; CsAP2), class B genes (including APETALA3; CsAP3 and PISTILLATA; CsPI) and class C genes (including AGAMOUS; CsAG) that are active in each whorl, determines the identity of the organs that will later develop in that whorl. CsAP3 is a class B homeotic gene which promotes petal and stamen formation and has a very important role in flower development. It also activates other genes playing pivotal role in flower development. It has been earlier reported that CsAP3 gene has direct role in activation of CsNAP gene which promotes senescence in plants. Present work was focused on study of relative gene expression changes of CsAP3 and CsNAP gene during different stages of flower development. CsAP3 gene expression was found maximum during late-preanthesis stages of stigma development. Expression increases from stage 5 to stage 6 of flower development and then reduces again from stage 6 to stage 7. CsNAP gene had moderate expression during stage 3 to stage 4 transition and its expression increased abruptly from stage 6 to stage 7 of flower development. There is no direct concordance in the expression of CsAP3 and CsNAP gene expression in saffron. We may conclude that some other factor(s) may be responsible for initiation of CsNAP expression and CsAP3 gene may directly/indirectly be involved in regulating the factors responsible for CsNAP activation.

The shoot apical meristem contains a pool of undifferentiated stem cells and controls initiation of all aerial plant organs. In maize (Zea mays), leaves are formed throughout vegetative development; on transition to floral development, the shoot meristem forms the tassel. Due to the regulated balance between stem cell maintenance and organogenesis, the structure and morphology of the shoot meristem are constrained during vegetative development. Previous work identified loci controlling meristem architecture in a recombinant inbred line population. The study presented here expanded on this by investigating shoot apical meristem morphology across a diverse set of maize inbred lines. Crosses of these lines to common parents showed varying phenotypic expression in the F1, with some form of heterosis occasionally observed. An investigation of meristematic growth throughout vegetative development in diverse lines linked the timing of reproductive transition to flowering time. Phenotypic correlations of meristem morphology with adult plant traits showed an association between the meristem and flowering time, leaf shape, and yield traits, revealing links between the control and architecture of undifferentiated and differentiated plant organs. Finally, quantitative trait loci mapping was utilized to map the genetic architecture of these meristem traits in two divergent populations. Control of meristem architecture was mainly population-specific, with 15 total unique loci mapped across the two populations with only one locus identified in both populations.

Auxin and cytokinin regulate different critical processes involved in plant growth and environmental feedbacks. These plant hormones act either synergistically or antagonistically to control the organisation, formation and maintenance of meristem. Meristem cells can be divided to generate new tissues and organs at the locations of plant postembryonic development. The aboveground plant organs are created by the shoot apical meristem (SAM). It has been proposed that the phytohormone, cytokinin, plays a positive role in the shoot meristem function, promotes cell expansion and promotes an increasing size of the meristem in Arabidopsis, whereas it has the reverse effects in the root apical meristem (RAM). Over the last few decades, it has been believed that the apically derived auxin suppresses the shoot branching by inactivating the axillary buds. However, it has recently become clear that the mechanism of action of auxinis indirect and multifaceted. In higher plants, the regulatory mechanisms of the SAM formation and organ separation are mostly unknown. This study reviews the effects and functions of cytokinin and auxin at the shoot apical meristem. This study also highlights the merger of the transcription factor activity with the actions of cytokinin/auxin and their complex interactions with the shoot meristem in rice.

The paper described a new mutation that causes the development of multiple meristematic foci as part of shoot apical meristem, which can give rise to new stem axes or cause stem fasciation. The wus-1 mutation represses development of additional apical meristem in fas5 mutant, indicating to the sequential action of the genes in the formation of the shoot apical meristem and FAS5 gene participation in spatial restriction of the WUS gene expression. This function gene FAS5 performs independently of other negative regulators of WUS gene--namely genes CLV, as demonstrated by additive phenotype of double mutants fas5 clv2-1 and fas5 clv3-2. Besides the effect on the development of the shoot apical meristem fas5 mutation causes a change in the shape and number of leaves, accelerates the plant transition to the reproductive stage and leads to the development of cell neoplasms on the stem (buds, stigmatic tissues and ovule-like structures). The mutation also causes changes in apical meristems and leaf cell morphology indicating the activation in cells of DNA endoreduplication. Pleiotropic effect of the fas5 mutation on different stages of ontogeny and different organs suggests that the FAS5 gene plays a complex regulatory role at all stages of the A. thaliana shoot development, and affects many direct or indirect target genes.

Identification of new genes involved in the control of flower initiation and development, is an important problem of the plant developmental genetics. Central approach to solve it is the study of mutants with changes in these characters. The effect of pleiotropic mutation fasciata5 on the transition to the reproductive stage and flower development was studied. By analyzing double mutants we identified interactions of FASCIATA5 gene with LEAFY, APETALA1 and APETALA2, which control the floral meristem identity. The results indicate an important role of gene FASCIATA5 in upregulation of these genes.

Flower development is one of the major developmental processes that governs seed setting in angiosperms. However, little is known about the molecular mechanisms underlying flower development in legumes. Employing RNA-seq for various stages of flower development and few vegetative tissues in chickpea, we identified differentially expressed genes in flower tissues/stages in comparison to vegetative tissues, which are related to various biological processes and molecular functions during flower development. Here, we provide details of experimental methods, RNA-seq data (available at Gene Expression Omnibus database under GSE42679) and analysis pipeline published by Singh and colleagues in the Plant Biotechnology Journal (Singh et al., 2013), along with additional analysis for discovery of genes involved in shoot apical meristem (SAM) development. Our data provide a resource for exploring the complex molecular mechanisms underlying SAM and flower development and identification of gene targets for functional and applied genomics in legumes.

Plant meristems carry pools of continuously active stem cells, whose activity is controlled by developmental and environmental signals. After stem cell division, daughter cells that exit the stem cell domain acquire transit amplifying cell identity before they are incorporated into organs and differentiate. In this study, we used an integrated approach to elucidate the role of HECATE (HEC) genes in regulating developmental trajectories of shoot stem cells in Arabidopsis thaliana. Our work reveals that HEC function stabilizes cell fate in distinct zones of the shoot meristem thereby controlling the spatio-temporal dynamics of stem cell differentiation. Importantly, this activity is concomitant with the local modulation of cellular responses to cytokinin and auxin, two key phytohormones regulating cell behaviour. Mechanistically, we show that HEC factors transcriptionally control and physically interact with MONOPTEROS (MP), a key regulator of auxin signalling, and modulate the autocatalytic stabilization of auxin signalling output., eLife digest Unlike animals, plants continuously generate new organs that make up their body. At the core of this amazing capacity lie tissues called meristems, which are found at the growing tips of all plants. Meristems contain dividing stem cells. The daughters of these stem cells pass through nearby regions called transition domains. Over time, they change – or differentiate – to go on to become part of tissues like leaves, roots, stems, shoots, flowers or fruits. Stem cell differentiation has a direct impact on a plant’s architecture and eventually its reproductive success. For crops, these factors determine yield. This means that understanding this aspect of plant development is central to basic and applied plant biology. Many factors required for shoot meristem activity have been identified, with a focus so far on the processes that control the identity of the cells produced. Now, Gaillochet et al. have asked which genes are responsible for controlling when stem cells in meristems differentiate. The analysis focused on the meristem that makes all the above ground parts of model plant Arabidopsis thaliana – the shoot apical meristem. Gaillochet et al. found that HECATE genes (or HEC for short) control the timing of stem cell differentiation by regulating the balance between the activities of two plant hormones: cytokinin and auxin. These genes promote cytokinin signals at the centre of the meristem, and dampen auxin response at the edges. This acts to slow down cell differentiation in two key transition domains of the shoot meristem. These new findings provide a molecular framework that now can be further investigated in crop plants to try to improve their yield. The findings also lay the foundation for studies of animals that may define common principles shared among stem cell systems in organisms that diverged over a billion years ago.

DELLA proteins associate with transcription factors to control plant growth in response to gibberellin 1 . Semi-dwarf DELLA mutants with improved harvest index and decreased lodging greatly improved global food security during the 'green revolution' in the 1960-1970s 2 . However, DELLA mutants are pleiotropic and the developmental basis for their effects on plant architecture remains poorly understood. Here, we show that DELLA proteins have genetically separable roles in controlling stem growth and the size of the inflorescence meristem, where flowers initiate. Quantitative three-dimensional image analysis, combined with a genome-wide screen for DELLA-bound loci in the inflorescence tip, revealed that DELLAs limit meristem size in Arabidopsis by directly upregulating the cell-cycle inhibitor KRP2 in the underlying rib meristem, without affecting the canonical WUSCHEL-CLAVATA meristem size regulators 3 . Mutation of KRP2 in a DELLA semi-dwarf background restored meristem size, but not stem growth, and accelerated flower production. In barley, secondary mutations in the DELLA gain-of-function mutant Sln1d 4 also uncoupled meristem and inflorescence size from plant height. Our work reveals an unexpected and conserved role for DELLA genes in controlling shoot meristem function and suggests how dissection of pleiotropic DELLA functions could unlock further yield gains in semi-dwarf mutants.

International audience; In Arabidopsis, floral meristems appear on the flanks of the inflorescence meristem. Their stereotypic development, ultimately producing the four whorls of floral organs, is essentially controlled by a network coordinating growth and cell-fate determination. This network integrates hormonal signals, transcriptional regulators, and mechanical constraints. Mechanisms regulating floral meristem formation have been studied at many different scales, from protein structure to tissue modeling. In this paper, we review recent findings related to the emergence of the floral meristem and floral fate determination and examine how this field has been impacted by recent technological developments.

The wild sunflower (Helianthus annuus) plants develop a highly branched form with numerous small flowering heads. The origin of a no branched sunflower, producing a single large head, has been a key event in the domestication process of this species. The interaction between hormonal factors and several genes organizes the initiation and outgrowth of axillary meristems (AMs). From sunflower, we have isolated two genes putatively involved in this process, LATERAL SUPPRESSOR (LS)-LIKE (Ha-LSL) and REGULATOR OF AXILLARY MERISTEM FORMATION (ROX)-LIKE (Ha-ROXL), encoding for a GRAS and a bHLH transcription factor (TF), respectively. Typical amino acid residues and phylogenetic analyses suggest that Ha-LSL and Ha-ROXL are the orthologs of the branching regulator LS and ROX/LAX1, involved in the growth habit of both dicot and monocot species. qRT-PCR analyses revealed a high accumulation of Ha-LSL transcripts in roots, vegetative shoots, and inflorescence shoots. By contrast, in internodal stems and young leaves, a lower amount of Ha-LSL transcripts was observed. A comparison of transcription patterns between Ha-LSL and Ha-ROXL revealed some analogies but also remarkable differences; in fact, the gene Ha-ROXL displayed a low expression level in all organs analyzed. In situ hybridization (ISH) analysis showed that Ha-ROXL transcription was strongly restricted to a small domain within the boundary zone separating the shoot apical meristem (SAM) and the leaf primordia and in restricted regions of the inflorescence meristem, beforehand the separation of floral bracts from disc flower primordia. These results suggested that Ha-ROXL may be involved to establish a cell niche for the initiation of AMs as well as flower primordia. The accumulation of Ha-LSL transcripts was not restricted to the boundary zones in vegetative and inflorescence shoots, but the mRNA activity was expanded in other cellular domains of primary shoot apical meristem as well as AMs. In addition, Ha-LSL transcript accumulation was also detected in leaves and floral primordia at early stages of development. These results were corroborated by qRT-PCR analyses that evidenced high levels of Ha-LSL transcripts in very young leaves and disc flowers, suggesting a role of Ha-LSL for the early outgrowth of lateral primordia.

During very early stages of flower development in Arabidopsis thaliana, a series of key decisions are taken. Indeed, the position and the basic patterning of new flowers are determined in less than 4 days. Given that the scientific literature provides hard evidence for the function of only 10% of A. thaliana genes, we hypothesized that although many essential genes have already been identified, many poorly characterized genes are likely to be involved in floral patterning. In the current study, we use high-throughput sequencing to describe the transcriptome of the native inflorescence meristem, the floral meristem and the new flower immediately after the start of organ differentiation. We provide evidence that our experimental system is reliable and less affected by experimental artefacts than a widely used floral induction system. Furthermore, we show how these data can be used to identify candidate genes for functional studies, and to generate hypotheses of functional redundancies and regulatory interactions.

The shoot apical meristem contains a pool of undifferentiated stem cells and generates all above-ground organs of the plant. During vegetative growth, cells differentiate from the meristem to initiate leaves while the pool of meristematic cells is preserved; this balance is determined in part by genetic regulatory mechanisms. To assess vegetative meristem growth and genetic control in Zea mays, we investigated its morphology at multiple time points and identified three stages of growth. We measured meristem height, width, plastochron internode length, and associated traits from 86 individuals of the intermated B73 × Mo17 recombinant inbred line population. For meristem height-related traits, the parents exhibited markedly different phenotypes, with B73 being very tall, Mo17 short, and the population distributed between. In the outer cell layer, differences appeared to be related to number of cells rather than cell size. In contrast, B73 and Mo17 were similar in meristem width traits and plastochron internode length, with transgressive segregation in the population. Multiple loci (6−9 for each trait) were mapped, indicating meristem architecture is controlled by many regions; none of these coincided with previously described mutants impacting meristem development. Major loci for height and width explaining 16% and 19% of the variation were identified on chromosomes 5 and 8, respectively. Significant loci for related traits frequently coincided, whereas those for unrelated traits did not overlap. With the use of three near-isogenic lines, a locus explaining 16% of the parental variation in meristem height was validated. Published expression data were leveraged to identify candidate genes in significant regions.

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.

One of the most remarkable manifestations of plant evolution is the diversity for floral branching systems. These “inflorescences” arise from stem cell populations in shoot meristems that mature gradually to reproductive states in response to environmental and endogenous signals. The morphology of the shoot meristem maturation process is conserved across distantly related plants, raising the question of how diverse inflorescence architectures arise from seemingly common maturation programs. In tomato and related nightshades (Solanaceae), inflorescences range from solitary flowers to highly branched structures bearing hundreds of flowers. Since reproductive barriers between even closely related Solanaceae have precluded a genetic dissection, we captured and compared meristem maturation transcriptomes from five domesticated and wild species reflecting the evolutionary continuum of inflorescence complexity. We find these divergent species share hundreds of dynamically expressed genes, enriched for transcription factors. Meristem stages are defined by distinct molecular states and point to modified maturation schedules underlying architectural variation. These modified schedules are marked by a peak of transcriptome expression divergence during the reproductive transition, driven by heterochronic shifts of dynamic genes, including transcriptional regulators with known roles in flowering. Thus, evolutionary diversity in Solanaceae inflorescence complexity is determined by subtle modifications of transcriptional programs during a critical transitional window of meristem maturation, which we propose underlies similar cases of plant architectural variation. More broadly, our findings parallel the recently described transcriptome “inverse hourglass” model for animal embryogenesis, suggesting both plant and animal morphological variation is guided by a mid-development period of transcriptome divergence.

Background Although the pattern of lateral organ formation from apical meristems establishes species-specific plant architecture, the positional information that confers cell fate to cells as they transit to the meristem flanks where they differentiate, remains largely unknown. We have combined fluorescence-activated cell sorting and RNA-seq to characterise the cell-type-specific transcriptome at the earliest developmental time-point of lateral organ formation using DORNRÖSCHEN-LIKE::GFP to mark founder-cell populations at the periphery of the inflorescence meristem (IM) in apetala1 cauliflower double mutants, which overproliferate IMs. Results Within the lateral organ founder-cell population at the inflorescence meristem, floral primordium identity genes are upregulated and stem-cell identity markers are downregulated. Additional differentially expressed transcripts are involved in polarity generation and boundary formation, and in epigenetic and post-translational changes. However, only subtle transcriptional reprogramming within the global auxin network was observed. Conclusions The transcriptional network of differentially expressed genes supports the hypothesis that lateral organ founder-cell specification involves the creation of polarity from the centre to the periphery of the IM and the establishment of a boundary from surrounding cells, consistent with bract initiation. However, contrary to the established paradigm that sites of auxin response maxima pre-pattern lateral organ initiation in the IM, auxin response might play a minor role in the earliest stages of lateral floral initiation. Electronic supplementary material The online version of this article (doi:10.1186/s12864-016-3189-x) contains supplementary material, which is available to authorized users.

In the life cycle of higher plants, it is the fate of meristem cells that determines the pattern of growth and development, and therefore plant morphotype and fertility. Floral transition, the turning point from vegetative growth to reproductive development, is achieved via genetically programmed sequential changes in meristem fate from vegetative to inflorescence, and to floral, leading to flower formation and eventual seed production. The transition is rarely reversible once initiated. In this communication, we report that a bacterial infection can derail the genetically programmed fate of meristem cells, thereby drastically altering the growth pattern of the host plant. We identified four characteristic symptoms in tomato plants infected with a cell wall-less bacterium, phytoplasma. The symptoms are a manifestation of the pathogen-induced alterations of growth pattern, whereas each symptom corresponds to a distinct phase in the derailment of shoot apical meristem fate. The phases include premature floral meristem termination, suppressed floral meristem initiation, delayed conversion of vegetative meristem to inflorescence meristem, and repetitive initiation and outgrowth of lateral vegetative meristems. We further found that the pathogen-induced alterations of growth pattern were correlated with transcriptional reprogramming of key meristem switching genes. Our findings open an avenue toward understanding pathological alterations in patterns of plant growth and development, thus aiding identification of molecular targets for disease control and symptom alleviation. The findings also provide insights for understanding stem cell pluripotency and raise a tantalizing possibility for using phytoplasma as a tool to dissect the course of normal plant development and to modify plant morphogenesis by manipulating meristem fate.

A key feature of the development of a higher plant is the continuous formation of new organs from the meristems. Originally patterned during embryogenesis, the meristems must activate cell division de novo at the time of germination, in order to initiate post-embryonic development. In a mutagenesis screen aimed at finding new players in early seedling cell division control, we identified ELONGATA3 (ELO3) as a key regulator of meristem cell cycle activation in Arabidopsis. Our results show that plants carrying a hypomorphic allele of ELO3 fail to activate cell division in the meristems following germination, which leads to seedling growth arrest and lethality. Further analyses suggest that this is due to a failure in DNA replication, followed by cell cycle arrest, in the meristematic tissue. Interestingly, the meristem cell cycle arrest in elo3 mutants, but not the later leaf developmental defects that have been linked to the loss of ELO3 activities, can be relieved by the addition of metabolic sugars in the growth medium. This finding points to a new role by which carbohydrate availability promotes meristem growth. Furthermore, growth arrested elo3 mutants suffer a partial loss of shoot meristem identity, which provides further evidence that cell cycle activities can influence the control of tissue identity.

Phosphatidylserine (PS), a quantitatively minor membrane phospholipid, is involved in many biological processes besides its role in membrane structure. One PS synthesis gene, PHOSPHATIDYLSERINE SYNTHASE1 (PSS1), has been discovered to be required for microspore development in Arabidopsis thaliana L. but how PSS1 affects postembryonic development is still largely unknown. Here, we show that PSS1 is also required for inflorescence meristem and organ development in Arabidopsis. Disruption of PSS1 causes severe dwarfism, smaller lateral organs and reduced size of inflorescence meristem. Morphological and molecular studies suggest that both cell division and cell elongation are affected in the pss1-1 mutant. RNA in situ hybridization and promoter GUS analysis show that expression of both WUSCHEL (WUS) and CLAVATA3 (CLV3) depend on PSS1. Moreover, the defect in meristem maintenance is recovered and the expression of WUS and CLV3 are restored in the pss1-1 clv1-1 double mutant. Both SHOOTSTEMLESS (STM) and BREVIPEDICELLUS (BP) are upregulated, and auxin distribution is disrupted in rosette leaves of pss1-1. However, expression of BP, which is also a regulator of internode development, is lost in the pss1-1 inflorescence stem. Our data suggest that PSS1 plays essential roles in inflorescence meristem maintenance through the WUS-CLV pathway, and in leaf and internode development by differentially regulating the class I KNOX genes.

The control of organ growth by coordinating cell proliferation and differentiation is a fundamental developmental process. In plants, postembryonic root growth is sustained by the root meristem. For maintenance of root meristem size, the rate of cell differentiation must equal the rate of cell division. Cytokinin and auxin interact to affect the cell proliferation and differentiation balance and thus control root meristem size. However, the genetic and molecular mechanisms that determine root meristem size still remain largely unknown. Here, we report that da1-related protein2 (dar2) mutants produce small root meristems due to decreased cell division and early cell differentiation in the root meristem of Arabidopsis (Arabidopsis thaliana). dar2 mutants also exhibit reduced stem cell niche activity in the root meristem. DAR2 encodes a Lin-11, Isl-1, and Mec-3 domain-containing protein and shows an expression peak in the border between the transition zone and the elongation zone. Genetic analyses show that DAR2 functions downstream of cytokinin and SHORT HYPOCOTYL2 to maintain normal auxin distribution by influencing auxin transport. Further results indicate that DAR2 acts through the PLETHORA pathway to influence root stem cell niche activity and therefore control root meristem size. Collectively, our findings identify the role of DAR2 in root meristem size control and provide a novel link between several key regulators influencing root meristem size.

The shoot apical meristem is the ultimate source of the cells that constitute the entire aboveground portion of the plant body. In Arabidopsis thaliana, meristem maintenance is regulated by the negative feedback loop of WUSCHEL-CLAVATA (WUS-CLV). Although CLV-like genes, such as FLORAL ORGAN NUMBER1 (FON1) and FON2, have been shown to be involved in maintenance of the reproductive meristems in rice (Oryza sativa), current understanding of meristem maintenance remains insufficient. In this article, we demonstrate that the FON2-LIKE CLE PROTEIN1 (FCP1) and FCP2 genes encoding proteins with similar CLE domains are involved in negative regulation of meristem maintenance in the vegetative phase. In addition, we found that WUSCHEL-RELATED HOMEOBOX4 (WOX4) promotes the undifferentiated state of the meristem in rice and that WOX4 function is associated with cytokinin action. Consistent with similarities in the shoot apical meristem phenotypes caused by overexpression of FCP1 and downregulation of WOX4, expression of WOX4 was negatively regulated by FCP1 (FCP2). Thus, FCP1/2 and WOX4 are likely to be involved in maintenance of the vegetative meristem in rice.

A long-standing question in plants and animals is how spatial patterns are maintained within stem cell niches despite ongoing cell divisions. Here we address how, during shoot meristem formation in Arabidopsis thaliana, the three apical cell layers acquire stem cell identity. Using a sensitized mutant screen, we identified miR394 as a mobile signal produced by the surface cell layer (the protoderm) that confers stem cell competence to the distal meristem by repressing the F box protein LEAF CURLING RESPONSIVENESS. This repression is required to potentiate signaling from underneath the stem cells by the transcription factor WUSCHEL, maintaining stem cell pluripotency. The interaction of two opposing signaling centers provides a mechanistic framework of how stem cells are localized at the tip of the meristem. Although the constituent cells change, the surface layer provides a stable point of reference in the self-organizing meristem.

For successful molecular breeding it is important to identify targets to the gene family level, and in the specific species of interest, in this case Pisum sativum L. The cytokinins have been identified as a key breeding target due to their influence on plant architecture, and on seed size and sink activity. We focused on the cytokinin biosynthetic gene family (the IPTs) and the gene family key to the destruction of cytokinins (the CKXs), as well as other gene families potentially affected by changing cytokinin levels. These included key meristem genes (WUS and BAM1) and the transporter gene families, sucrose transporters (SUTs) and amino acid permeases (AAPs). We used reverse transcription quantitative PCR (RT-qPCR) to monitor gene expression in the vegetative meristem and in pre- and post-fertilisation young pea fruits. PsWUS expression was specific to the shoot apical meristem while PsBAM1 was highly expressed in the shoot apical meristem (SAM) but was also expressed at a low level in the young fruit. Differential expression was shown between genes and within gene families for IPT, CKX, SUT, and AAP. PsCKX7 showed strong gene family member-specific expression in the SAM, and was also expressed in young pea fruits. We suggest that PsCKX7 is a potential target for downregulation via molecular breeding or gene editing.

TERMINAL FLOWER 1 (TFL1) is a key regulator of Arabidopsis plant architecture that responds to developmental and environmental signals to control flowering time and the fate of shoot meristems. TFL1 expression is dynamic, being found in all shoot meristems, but not in floral meristems, with the level and distribution changing throughout development. Using a variety of experimental approaches we have analysed the TFL1 promoter to elucidate its functional structure. TFL1 expression is based on distinct cis-regulatory regions, the most important being located 3' of the coding sequence. Our results indicate that TFL1 expression in the shoot apical versus lateral inflorescence meristems is controlled through distinct cis-regulatory elements, suggesting that different signals control expression in these meristem types. Moreover, we identified a cis-regulatory region necessary for TFL1 expression in the vegetative shoot and required for a wild-type flowering time, supporting that TFL1 expression in the vegetative meristem controls flowering time. Our study provides a model for the functional organisation of TFL1 cis-regulatory regions, contributing to our understanding of how developmental pathways are integrated at the genomic level of a key regulator to control plant architecture., This work was supported by a Joint Project Grant from the Royal Society [ESEP/JP 15057] to D.B. and F.M. The laboratory of F.M. was funded by grants from the Spanish Ministerio de Ciencia e Innovacion [BIO2009-10876 and CSD2007-00057], the Spanish Ministerio de Economia y Competitividad [BFU2012-38929] and from the Generalitat Valenciana [ACOMP09-083 and ACOMP2012-099]. Work in the Y.H. lab was supported by the Plant Genome Research Program from the National Science Foundation [NSF-PGRP-IOS-1339388]. P.F.-N. was supported by a fellowship from the I3P Program of Consejo Superior de Investigaciones Cientificas.

Plant meristems are responsible for the generation of all plant tissues and organs. Here we show that the transcription factor (TF) FAR-RED ELONGATED HYPOCOTYL3 (FHY3) plays an important role in both floral meristem (FM) determinacy and shoot apical meristem maintenance in Arabidopsis, in addition to its well-known multifaceted roles in plant growth and development during the vegetative stage. Through genetic analyses, we show that WUSCHEL (WUS) and CLAVATA3 (CLV3), two central players in the establishment and maintenance of meristems, are epistatic to FHY3 Using genome-wide ChIP-seq and RNA-seq data, we identify hundreds of FHY3 target genes in flowers and find that FHY3 mainly acts as a transcriptional repressor in flower development, in contrast to its transcriptional activator role in seedlings. Binding motif-enrichment analyses indicate that FHY3 may coregulate flower development with three flower-specific MADS-domain TFs and four basic helix-loop-helix TFs that are involved in photomorphogenesis. We further demonstrate that CLV3, SEPALLATA1 (SEP1), and SEP2 are FHY3 target genes. In shoot apical meristem, FHY3 directly represses CLV3, which consequently regulates WUS to maintain the stem cell pool. Intriguingly, CLV3 expression did not change significantly in fhy3 and phytochrome B mutants before and after light treatment, indicating that FHY3 and phytochrome B are involved in light-regulated meristem activity. In FM, FHY3 directly represses CLV3, but activates SEP2, to ultimately promote FM determinacy. Taken together, our results reveal insights into the mechanisms of meristem maintenance and determinacy, and illustrate how the roles of a single TF may vary in different organs and developmental stages.

Plant hormones are signalling molecules that control growth and development. Growth of the aerial parts of higher plants requires the continuous activity of the shoot apical meristem, a small mound of cells at the apex of a plant. KNOTTED1-like HOMEOBOX (KNOX) genes are involved in regulating meristem activity, however, little is known about how this regulation is mediated. Recent evidence suggests that KNOX transcription factors may control meristem development by regulating the balance of activities of multiple hormones.

Background Plant profilin genes encode core cell-wall structural proteins and are evidenced for their up-regulation under cotton domestication. Notwithstanding striking discoveries in the genetics of cell-wall organization in plants, little is explicit about the manner in which profilin-mediated molecular interplay and corresponding networks are altered, especially during cellular signalling of apical meristem determinacy and flower development. Results Here we show that the ectopic expression of GhPRF1 gene in tobacco resulted in the hyperactivation of apical meristem and early flowering phenotype with increased flower number in comparison to the control plants. Spatial expression alteration in CLV1, a key meristem-determinacy gene, is induced by the GhPRF1 overexpression in a WUS-dependent manner and mediates cell signalling to promote flowering. But no such expression alterations are recorded in the GhPRF1-RNAi lines. The GhPRF1 transduces key positive flowering regulator AP1 gene via coordinated expression of FT4, SOC1, FLC1 and FT1 genes involved in the apical-to-floral meristem signalling cascade which is consistent with our in silico profilin interaction data. Remarkably, these positive and negative flowering regulators are spatially controlled by the Actin-Related Protein (ARP) genes, specifically ARP4 and ARP6 in proximate association with profilins. This study provides a novel and systematic link between GhPRF1 gene expression and the flower primordium initiation via up-regulation of the ARP genes, and an insight into the functional characterization of GhPRF1 gene acting upstream to the flowering mechanism. Also, the transgenic plants expressing GhPRF1 gene show an increase in the plant height, internode length, leaf size and plant vigor. Conclusions Overexpression of GhPRF1 gene induced early and increased flowering in tobacco with enhanced plant vigor. During apical meristem determinacy and flower development, the GhPRF1 gene directly influences key flowering regulators through ARP-genes, indicating for its role upstream in the apical-to-floral meristem signalling cascade. Electronic supplementary material The online version of this article (doi:10.1186/s12870-016-0798-0) contains supplementary material, which is available to authorized users.

In rice, inflorescence architecture is established at early stages of reproductive development and contributes directly to grain yield potential. After induction of flowering, the complexity of branching, and therefore the number of seeds on the panicle, is determined by the activity of different meristem types and the timing of transitions between them. Although some of the genes involved in these transitions have been identified, an understanding of the network of transcriptional regulators controlling this process is lacking. To address this we used a precise laser microdissection and RNA-sequencing approach in Oryza sativa ssp. japonica cv. Nipponbare to produce quantitative data that describe the landscape of gene expression in four different meristem types: the rachis meristem, the primary branch meristem, the elongating primary branch meristem (including axillary meristems), and the spikelet meristem. A switch in expression profile between apical and axillary meristem types followed by more gradual changes during transitions in axillary meristem identity was observed, and several genes potentially involved in branching were identified. This resource will be vital for a mechanistic understanding of the link between inflorescence development and grain yield., This project was jointly supported by Fondation Agropolis through the Investissements d'avenir programme (ANR-10-LABX-0001-01) and Fondazione Cariplo under the EVOREPRICE 1201-004 project. Israr Ud Din's Ph.D. fellowship was funded by IBGE, The University of Agriculture Peshawar, KPK, Pakistan.

Shoot apical meristem activity is controlled by complex regulatory networks in which components such as transcription factors, miRNAs, small peptides, hormones, enzymes and epigenetic marks all participate. Many key genes that determine the inherent characteristics of the shoot apical meristem have been identified through genetic approaches. Recent advances in genome-wide studies generating extensive transcriptomic and DNA-binding datasets have increased our understanding of the interactions within the regulatory networks that control the activity of the meristem, identifying new regulators and uncovering connections between previously unlinked network components. In this review, we focus on recent studies that illustrate the contribution of whole genome analyses to understand meristem function., This work was supported by grants from the Spanish Ministerio de Economía y Competitividad (BFU2014-58289-P) and from the Agencia de Gestió d’Ajuts Universitaris I de Recerca (2014 SGR 1406) to JLR.

485 I. 485 II. 486 III. 491 IV. 491 V. 495 495 References 495 SUMMARY: Boundaries, established and maintained in different regions of the plant body, have diverse functions in development. One role is to separate different cell groups, for example the differentiating cells of a leaf primordium from the pluripotent cells of the apical meristem. Boundary zones are also established during compound leaf development, to separate young leaflets from each other, and in many other positions of the plant body. Recent studies have demonstrated that different boundary zones share similar properties. They are characterized by a low rate of cell divisions and specific patterns of gene expression. In addition, the levels of the plant hormones auxin and brassinosteroids are down-regulated in boundary zones, resulting in a low differentiation level of boundary cells. This feature seems to be crucial for a second important role of boundary zones, the formation of new meristems. The primary shoot meristem, as well as secondary and ectopic shoot meristems, initiate from boundary cells that exhibit competence for meristem formation.

Plants produce new organs from a population of pluripotent cells which are located in specific tissues called meristems. One of these meristems, the shoot apical meristem (SAM), gives rise to leaves during the vegetative phase and flowers during the reproductive phase. The transition from vegetative SAM to an inflorescence meristem (IM) is a dramatic developmental switch, which has been particularly well studied in the model species Arabidopsis thaliana. This developmental switch is controlled by multiple environmental signals such as day length (or photoperiod), and it is accompanied by changes in expression of hundreds of genes. A major interest in plant biology is to identify and characterize those genes which are regulated in the stem cells of the SAM in response to the photoperiodic signals. In this sense, techniques such as RNA in situ hybridization (RNA ISH) have been very successfully employed to detect the temporal and spatial expression patterns of genes in the SAM. This method can be specifically optimized for photoperiodic-flowering studies. In this chapter, we describe improved methods to generate plant material and histological samples to be combined with RNA ISH in flowering-related studies.

SUMMARY The shoot apical meristem, a small dome-shaped structure at the shoot apex, is responsible for the initiation of all post-embryonic shoot organs. Pluripotent stem cells within the meristem replenish themselves and provide daughter cells that become incorporated into lateral organ primordia around the meristem periphery. We have identified three novel regulators of shoot apical meristem activity in Arabidopsis thaliana that encode related AIL/PLT transcription factors: AINTEGUMENTA (ANT), AINTEGUMENTA-LIKE6 (AIL6)/PLETHORA3 (PLT3) and AINTEGUMENTA-LIKE7 (AIL7)/PLETHORA7 (PLT7). Loss of these genes results in plants that initiate only a few leaves prior to termination of shoot apical meristem activity. In 7-day-old ant ail6 ail7 seedlings, we observed reduced cell division in the meristem region, differentiation of meristematic cells and altered expression of the meristem regulators WUSCHEL (WUS), CLAVATA3 (CLV3) and SHOOT MERISTEMLESS (STM). Genetic experiments suggest that these three AIL genes do not act specifically in either the WUS/CLV or STM pathway regulating meristem function. Furthermore, these studies indicate that ANT, AIL6 and AIL7 have distinct functions within the meristem rather than acting in a strictly redundant manner. Our study thus identifies three new genes whose distinct functions are together required for continuous shoot apical meristem function.

The indeterminate growth pattern displayed by shoots is mediated by the proper maintenance of the shoot meristem. Meristem maintenance is dependent upon the balance of stem cell perpetuation in the central zone (CZ) and organogenesis in the peripheral zone (PZ). Although the mechanisms that coordinate CZ and PZ function is not understood, meristem cell fate is likely achieved by the spatial interplay between gene regulatory networks and hormone signaling pathways. During shoot maturation, the identity of the shoot meristem as well as the lateral organs are transformed during the vegetative and reproductive transitions. Studies in model plant systems indicate that three amino acid extension (TALE) homeodomain proteins integrate signaling events that transform the identity of the shoot meristem and establish reproductive patterns of growth. This review will highlight the function of TALE homeodomain transcription factors that regulate shoot meristem cell fate and also function with phase specific regulators to maintain shoot meristem identity.

PpMADS1, a member of the euAP1 clade of the class A genes, was previously cloned from peach. In this study, PpMADS1 was constitutively expressed in Arabidopsis thaliana to study its function in plant development. The transgenic A. thaliana plants containing 35S::PpMADS1 showed severe phenotype variation including early flowering, conversion of inflorescence branches to solitary flowers, formation of terminal flowers, production of higher number of carpels, petals, and stamens than non-transgenic plants, and prevention of pod shatter. Significantly, the transgenic plants produced more than one silique from a single flower. The results obtained by using cDNA microarray and real-time PCR analyses in the transgenic Arabidopsis indicated that PpMADS1 might play dual roles in regulating the floral meristem development by activating or repressing different sets of genes that would determine the different fate of a floral meristem. In addition, the PpMADS1 gene promoter was further cloned, and deletion analyses were conducted by using fused GUS as a reporter gene in transgenic A. thaliana. Histochemical staining of different organs from transgenic plants revealed the region between −197 and −454 bp was specific for GUS expression in flower primordium, and the region between −454 and −678 bp was specific for GUS expression in sepals and petals. In contrast, a negative regulatory element present between −678 and −978 bp could suppress GUS expression in filament.

Post-embryonic development depends on the activity of meristems in plants, and thus control of cell fate in the meristem is crucial to plant development and its architecture. In grasses such as rice and maize, the fate of reproductive meristems changes from indeterminate meristems, such as inflorescence and branch meristems, to determinate meristems, such as the spikelet meristem. Here we analyzed a recessive mutant of rice, aberrant spikelet and panicle1 (asp1), that showed pleiotropic phenotypes such as a disorganized branching pattern, aberrant spikelet morphology, and disarrangement of phyllotaxy. Close examination revealed that regulation of meristem fate was compromised in asp1: degeneration of the inflorescence meristem was delayed, transition from the branch meristem to the spikelet meristem was accelerated, and stem cell maintenance in both the branch meristem and the spikelet meristem was compromised. The genetic program was also disturbed in terms of spikelet development. Gene isolation revealed that ASP1 encodes a transcriptional co-repressor that is related to TOPLESS (TPL) in Arabidopsis and RAMOSA ENHANCER LOCUS2 (REL2) in maize. It is likely that the pleiotropic defects are associated with de-repression of multiple genes related to meristem function in the asp1 mutant. The asp1 mutant also showed de-repression of axillary bud growth and disturbed phyllotaxy in the vegetative phase, suggesting that the function of this gene is closely associated with auxin action. Consistent with these observations and the molecular function of Arabidopsis TPL, auxin signaling was also compromised in the rice asp1 mutant. Taken together, these results indicate that ASP1 regulates various aspects of developmental processes and physiological responses as a transcriptional co-repressor in rice.

Large flower size is an important trait that influences the economic value of floricultural plants. Large flower size is conferred by two morphological changes i.e. increased petal number and individual petal size. Increased petal number is induced by enlargement of the floral meristem or homeotic conversion of stamens and carpels into petals. Genes regulating meristematic competence mediate enlargement of the floral meristem, while floral homeotic genes mediate the homeotic conversion of floral organs. Increased individual petal size is induced by increased cell size or cell number. Polyploidization of the nuclear genome increases petal cell size, while increased petal cell number is induced by enhanced expression of the genes involved in auxin signal transduction and cytokinin biosynthesis. In this review, the molecular mechanisms affecting flower size, elucidated mainly using model plants such as Arabidopsis, are summarized. Further, our research on the roles of cytokinin biosynthesis and signal transduction in increased flower size in petunia, induced by the single major gene Grandiflora, is discussed. Based on these results, a possible systematic breeding method for increasing flower size is discussed.

In recent years, the role of WOX genes encoding homeodomain transcription factors in the development of the apical meristem of shoots and roots has been actively investigated. However, the role of WOX genes in the control of the cell proliferation in other meristem types is poorly studied. In our work, we have studied the role of the WOX5 gene in the development of the meristem in nitrogen-fixing nodules developing on the roots of legumes in a symbiosis with rhizobia. We have shown that the WOX5 gene is involved in the development of the nodule meristem in legumes, have quantitatively evaluated the gene’s expression at different nodule formation stages, and have studied the localization of its expression using a construct containing the WOX5 promoter fused with a reporter gene. The role of the WOX5 transcription factor in the nodule organogenesis and its possible interaction with the hormonal system in the course of the nodule development has been discussed.

An indeterminate developmental program allows plant organs to grow continuously by maintaining functional meristems over time. The molecular mechanisms involved in the maintenance of the root apical meristem are not completely understood. We have identified a new Arabidopsis thaliana mutant named moots koom 1 (mko1) that showed complete root apical meristem exhaustion of the primary root by 9 days post-germination. MKO1 is essential for maintenance of root cell proliferation. In the mutant, cell division is uncoupled from cell growth in the region corresponding to the root apical meristem. We established the sequence of cellular events that lead to meristem exhaustion in this mutant. Interestingly, the SCR and WOX5 promoters were active in the mko1 quiescent center at all developmental stages. However, during meristem exhaustion, the mutant root tip showed defects in starch accumulation in the columella and changes in auxin response pattern. Therefore, contrary to many described mutants, the determinate growth in mko1 seedlings does not appear to be a consequence of incorrect establishment or affected maintenance of the quiescent center but rather of cell proliferation defects both in stem cell niche and in the rest of the apical meristem. Our results support a model whereby the MKO1 gene plays an important role in the maintenance of the root apical meristem proliferative capacity and indeterminate root growth, which apparently acts independently of the SCR/SHR and WOX5 regulatory pathways.

AGAMOUS-LIKE6 (AGL6) genes play essential roles in flower development, but whether and how they work with floral organ identity genes remain less understood. Here, we describe interactions of the rice (Oryza sativa) AGL6 gene MADS6 with other rice floral homeotic genes in flower development. Genetic analyses revealed that MADS6 specifies the identity of the three inner whorls and floral meristem determinacy redundantly with SUPERWOMAN1/MADS16 (B-gene) or MADS3 (C-gene). MADS6 was shown to define carpel/ovule development and floral determinacy by interacting with MADS13 (D-gene) and control the palea and floral meristem identities together with the YABBY gene DROOPING LEAF. Expression analyses revealed that the transcript levels of six B-, C-, and E-class genes were reduced in mads6-1 at the early flower developmental stage, suggesting that MADS6 is a key regulator of early flower development. Moreover, MADS6 can directly bind to a putative regulatory motif on MADS58 (C-gene), and mads6-1 mads58 displayed phenotypes similar to that of mads6-1. These results suggest that MADS6 is a key player in specifying flower development via interacting with other floral homeotic genes in rice, thus providing new insights into the mechanism by which flower development is controlled.

In plants, the meristem has to maintain a separate population of pluripotent cells that serve two main tasks, i.e., self-maintenance and organ initiation, which are separated spatially in meristem. Prior to our study, WUS and WUS-like WOX genes had been reported as essential for the development of the SAM. In this study, the consequences of gain of WOX1 function are described. Here we report the identification of an Arabidopsis gain-of-function mutant wox1-D, in which the expression level of the WOX1 (WUSCHEL HOMEOBOX 1) was elevated and subtle defects in meristem development were observed. The wox1-D mutant phenotype is dwarfed and slightly bushy, with a smaller shoot apex. The wox1-D mutant also produced small and dark green leaves, and exhibited a failure in anther dehiscence and male sterility. Molecular evidences showed that the transcription of the stem cell marker gene CLV3 was down-regulated in the meristem of wox1-D but accumulated in the other regions, i.e., in the root-hypocotyl junction and at the sites for lateral root initiation. The fact that the organ size and cell size in leaves of wox1-D are smaller than those in wild type suggests that cell expansion is possibly affected in order to have partially retarded the development of lateral organs, possibly through alteration of CLV3 expression pattern in the meristem. An S-adenosylmethionine decarboxylase (SAMDC) protein, SAMDC1, was found able to interact with WOX1 by yeast two-hybrid and pull-down assays in vitro. HPLC analysis revealed a significant reduction of polyamine content in wox1-D. Our results suggest that WOX1 plays an important role in meristem development in Arabidopsis, possibly via regulation of SAMDC activity and polyamine homeostasis, and/or by regulating CLV3 expression.

VFL and VvTFL1 genes expression patterns and the effects of sucrose on the expression of VFL and VvTFL1 genes in different organs of the “Xiangfei” grapevine (Vitis vinifera L.) were investigated. VvTFL1 gene expression was detected in the meristem of the apical bud and lateral bud, but was not detected during inflorescence differentiation and flower organ development. After sucrose treatment, VvTFL1 gene expression increased in the apical bud, but decreased in the lateral bud. These results suggested that the VvTFL1 gene might be mainly involved in the apical growth process of shoots, and exogenous sucrose had an effect on the VvTFL1 gene by increasing shoot apical meristem initiation of apical buds. The VFL gene was expressed primarily during inflorescence differentiation and early flower organ development, but it gradually reduced in later flower development. After sucrose treatment, VFL gene expression increased in the inflorescence and small or middle flower, but a little change was seen in the large flower. These results suggested that the VFL gene plays important roles in the initiation of inflorescence meristems and the morphological formation of flower organs. Exogenous sucrose had an effect on VFL gene expression at the early stage of flower development.

Grass plants develop unique floral patterns that determine grain production. However, the molecular mechanism underlying the specification of floral organ identities and meristem determinacy, including the interaction among floral homeotic genes, remains largely unknown in grasses. Here, we report the interactions of rice (Oryza sativa) floral homeotic genes, OsMADS3 (a C-class gene), OsMADS13 (a D-class gene), and DROOPING LEAF (DL), in specifying floral organ identities and floral meristem determinacy. The interaction among these genes was revealed through the analysis of double mutants. osmads13-3 osmads3-4 displayed a loss of floral meristem determinacy and generated abundant carpelloid structures containing severe defective ovules in the flower center, which were not detectable in the single mutant. In addition, in situ hybridization and yeast two-hybrid analyses revealed that OsMADS13 and OsMADS3 did not regulate each other’s transcription or interact at the protein level. This indicates that OsMADS3 plays a synergistic role with OsMADS13 in both ovule development and floral meristem termination. Strikingly, osmads3-4 dl-sup6 displayed a severe loss of floral meristem determinacy and produced supernumerary whorls of lodicule-like organs at the forth whorl, suggesting that OsMADS3 and DL synergistically terminate the floral meristem. Furthermore, the defects of osmads13-3 dl-sup6 flowers appeared identical to those of dl-sup6, and the OsMADS13 expression was undetectable in dl-sup6 flowers. These observations suggest that DL and OsMADS13 may function in the same pathway specifying the identity of carpel/ovule and floral meristem. Collectively, we propose a model to illustrate the role of OsMADS3, DL, and OsMADS13 in the specification of flower organ identity and meristem determinacy in rice.