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The intricate morphology of the flower is primarily established within floral meristems in which floral organs will be defined and from where the developing flower will emerge. Floral meristem development involves multiscale-level regulation, including lineage and positional mechanisms for establishing cell-type identity, and transcriptional regulation mediated by changes in the chromatin environment. However, many key aspects of floral meristem development remain to be determined, such as: 1) the exact role of cellular location in connecting transcriptional inputs to morphological outcomes, and 2) the precise interactions between transcription factors and chromatin regulators underlying the transcriptional networks that regulate the transition from cell proliferation to differentiation during floral meristem development. Here, we highlight recent studies addressing these points through newly developed spatial reconstruction techniques and highresolution transcription factor–chromatin environment interactions in the model plant Arabidopsis thaliana. Specifically, we feature studies that reconstructed 3D gene expression atlases of the floral meristem. We also discuss how the precise timing of floral meristem specification, floral organ patterning, and floral meristem termination is determined through temporally defined epigenetic dynamics for fine-tuning of gene expression. These studies offer fresh insights into the well-established principles of floral meristem development and outline the potential for further advances in this field in an age of integrated, powerful, multiscale resolution approaches. [ABSTRACT FROM AUTHOR]

The intricate morphology of the flower is primarily established within floral meristems in which floral organs will be defined and from where the developing flower will emerge. Floral meristem development involves multiscale-level regulation, including lineage and positional mechanisms for establishing cell-type identity, and transcriptional regulation mediated by changes in the chromatin environment. However, many key aspects of floral meristem development remain to be determined, such as: 1) the exact role of cellular location in connecting transcriptional inputs to morphological outcomes, and 2) the precise interactions between transcription factors and chromatin regulators underlying the transcriptional networks that regulate the transition from cell proliferation to differentiation during floral meristem development. Here, we highlight recent studies addressing these points through newly developed spatial reconstruction techniques and high-resolution transcription factor–chromatin environment interactions in the model plant Arabidopsis thaliana. Specifically, we feature studies that reconstructed 3D gene expression atlases of the floral meristem. We also discuss how the precise timing of floral meristem specification, floral organ patterning, and floral meristem termination is determined through temporally defined epigenetic dynamics for fine-tuning of gene expression. These studies offer fresh insights into the well-established principles of floral meristem development and outline the potential for further advances in this field in an age of integrated, powerful, multiscale resolution approaches.

The branching phenotype is an extremely important agronomic trait of plants, especially for horticultural crops. It is not only an important yield character of fruit trees, but also an exquisite ornamental trait of landscape trees and flowers. The branching characteristics of plants are determined by the periodic initiation and later development of meristems, especially the axillary meristem (AM) in the vegetative stage and the floral meristem (FM) in the reproductive stage, which jointly determine the above-ground plant architecture. The regulation of meristem initiation has made great progress in model plants in recent years. Meristem initiation is comprehensively regulated by a complex regulatory network composed of plant hormones and transcription factors. However, as it is an important trait, studies on meristem initiation in horticultural plants are very limited, and the mechanism of meristem initiation regulation in horticultural plants is largely unknown. This review summarizes recent research advances in axillary meristem regulation and mainly reviews the regulatory networks and mechanisms of AM and FM initiation regulated by transcription factors and hormones. Finally, considering the existing problems in meristem initiation studies and the need for branching trait improvement in horticulture plants, we prospect future studies to accelerate the genetic improvement of the branching trait in horticulture plants. [ABSTRACT FROM AUTHOR]

The spikelet is the basic unit of the grass inflorescence. In tetraploid (Triticum turgidum) and hexaploid wheat (Triticum aestivum), the spikelet is a short indeterminate branch with two proximal sterile bracts (glumes) followed by a variable number of florets, each including a bract (lemma) with an axillary flower. Varying levels of miR172 and/or its target gene Q (AP2L5) result in gradual transitions of glumes to lemmas, and vice versa. Here, we show that AP2L5 and its related paralog AP2L2 play critical and redundant roles in the specification of axillary floral meristems and lemma identity. AP2L2, also targeted by miR172, displayed similar expression profiles to AP2L5 during spikelet development. Loss-of-function mutants in both homeologs of AP2L2 (henceforth ap2l2) developed normal spikelets, but ap2l2 ap2l5 double mutants generated spikelets with multiple empty bracts before transitioning to florets. The coordinated nature of these changes suggest an early role of these genes in floret development. Moreover, the flowers of ap2l2 ap2l5 mutants showed organ defects in paleas and lodicules, including the homeotic conversion of lodicules into carpels. Mutations in the miR172 target site of AP2L2 were associated with reduced plant height, more compact spikes, promotion of lemma-like characters in glumes and smaller lodicules. Taken together, our results show that the balance in the expression of miR172 and AP2-like genes is crucial for the correct development of spikelets and florets, and that this balance has been altered during the process of wheat and barley (Hordeum vulgare) domestication. The manipulation of this regulatory module provides an opportunity to modify spikelet architecture and improve grain yield.

Locules are the seed‐bearing structure of fruits. Multiple locules are associated with increased fruit size and seed set, and therefore, control of locule number is an important agronomic trait. Locule number is controlled in part by the CLAVATA‐WUSCHEL pathway. Disruption of either the CLAVATA1 receptor‐like kinase or its ligand CLAVATA3 can cause larger floral meristems and an increased number of locules. In an EMS mutagenized population of Brassica rapa, we identified a mutant allele that raises the number of locules from four to a range of from six to eight. Linkage mapping and genetic analysis support that the mutant phenotype is due to a missense mutation in a CLAVATA 1 (CLV1) homolog. In addition to increased locule number, additional internal gynoecia are formed in brclv1 individuals, suggesting a failure to terminate floral meristem development, which results in decreased seed production. [ABSTRACT FROM AUTHOR]

Abstract Locules are the seed‐bearing structure of fruits. Multiple locules are associated with increased fruit size and seed set, and therefore, control of locule number is an important agronomic trait. Locule number is controlled in part by the CLAVATA‐WUSCHEL pathway. Disruption of either the CLAVATA1 receptor‐like kinase or its ligand CLAVATA3 can cause larger floral meristems and an increased number of locules. In an EMS mutagenized population of Brassica rapa, we identified a mutant allele that raises the number of locules from four to a range of from six to eight. Linkage mapping and genetic analysis support that the mutant phenotype is due to a missense mutation in a CLAVATA 1 (CLV1) homolog. In addition to increased locule number, additional internal gynoecia are formed in brclv1 individuals, suggesting a failure to terminate floral meristem development, which results in decreased seed production.

Accelerating the differentiation of floral meristem (FM) from shoot apical meristems (SAM) which determines the conversion from vegetative to reproductive growth is of great significance for the production of rapeseed (Brassica napus L.). In this research, the mechanisms of different nitrogen (N) application rates (low N, N1; normal N, N2; and high N, N3) on different FM development stages triggering the regulation of FM differentiation genes through the auxin biosynthetic and signal transduction were investigated. We found that the stage of FM differentiation, which was identified through a stereomicroscope and scanning electron microscope, came 4 and 7 days earlier under high N rate than under normal and low N levels, with the seed yield increased by 11.1 and 22.6%, respectively. Analysis of the auxin and its derivatives contents showed that the main biosynthesis way of auxin was the indole acetaldehyde oxime (IAOx) pathway, with 3-Indole acetonitrile dramatically accumulated during FM differentiation. At the same time, an obvious decrease of IAA contents at each FM differentiation stage was detected, and then gradually rose. Results of the expression of genes involved in auxin biosynthesis, auxin signaling transduction, and FM identification under five FM differentiation stages and three nitrogen application rates showed that genes involved in auxin biosynthesis were regulated before the FM differentiation stage, while the regulation of FM identity genes appeared mainly at the middle and later periods of the five stages, and the regulation level of genes varied under different N rates. Taken together, a high nitrogen rate could accelerate the initiation of FM differentiation, and auxin involved a lot in this regulation. [ABSTRACT FROM AUTHOR]

The branching phenotype is an extremely important agronomic trait of plants, especially for horticultural crops. It is not only an important yield character of fruit trees, but also an exquisite ornamental trait of landscape trees and flowers. The branching characteristics of plants are determined by the periodic initiation and later development of meristems, especially the axillary meristem (AM) in the vegetative stage and the floral meristem (FM) in the reproductive stage, which jointly determine the above-ground plant architecture. The regulation of meristem initiation has made great progress in model plants in recent years. Meristem initiation is comprehensively regulated by a complex regulatory network composed of plant hormones and transcription factors. However, as it is an important trait, studies on meristem initiation in horticultural plants are very limited, and the mechanism of meristem initiation regulation in horticultural plants is largely unknown. This review summarizes recent research advances in axillary meristem regulation and mainly reviews the regulatory networks and mechanisms of AM and FM initiation regulated by transcription factors and hormones. Finally, considering the existing problems in meristem initiation studies and the need for branching trait improvement in horticulture plants, we prospect future studies to accelerate the genetic improvement of the branching trait in horticulture plants.

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

Floral organs are properly developed on the basis of timed floral meristem (FM) termination in Arabidopsis. In this process, two known regulatory pathways are involved. The WUSCHEL (WUS)-CLAVATA3 (CLV3) feedback loop is vital for the spatial establishment and maintenance of the FM, while AGAMOUS (AG)-WUS transcriptional cascades temporally repress FM. At stage 6 of flower development, a C2H2-type zinc finger repressor that is a target of AG, KNUCKLES (KNU), directly represses the stem cell identity gene WUS in the organizing center for FM termination. However, how the robust FM activity is fully quenched within a limited time frame to secure carpel development is not fully understood. Here, we demonstrate that KNU directly binds to the CLV1 locus and the cis-regulatory element on CLV3 promoter and represses their expression during FM determinacy control. Furthermore, KNU physically interacts with WUS, and this interaction inhibits WUS from sustaining CLV3 in the central zone. The KNU-WUS interaction also interrupts the formation of WUS homodimers and WUS-HAIRYMERISTEM 1 heterodimers, both of which are required for FM maintenance. Overall, our findings describe a regulatory framework in which KNU plays a position-specific multifunctional role for the tightly controlled FM determinacy. [ABSTRACT FROM AUTHOR]

In the model plant Arabidopsis thaliana, the zinc-finger transcription factor KNUCKLES (KNU) plays an important role in the termination of floral meristem activity, a process that is crucial for preventing the overgrowth of flowers. The KNU gene is activated in floral meristems by the floral organ identity factor AGAMOUS (AG), and it has been shown that both AG and KNU act in floral meristem control by directly repressing the stem cell regulator WUSCHEL (WUS), which leads to a loss of stem cell activity. When we re-examined the expression pattern of KNU in floral meristems, we found that KNU is expressed throughout the center of floral meristems, which includes, but is considerably broader than the WUS expression domain. We therefore hypothesized that KNU may have additional functions in the control of floral meristem activity. To test this, we employed a gene perturbation approach and knocked down KNU activity at different times and in different domains of the floral meristem. In these experiments we found that early expression in the stem cell domain, which is characterized by the expression of the key meristem regulatory gene CLAVATA3 (CLV3), is crucial for the establishment of KNU expression. The results of additional genetic and molecular analyses suggest that KNU represses floral meristem activity to a large extent by acting on CLV3. Thus, KNU might need to suppress the expression of several meristem regulators to terminate floral meristem activity efficiently.

In the model plant Arabidopsis thaliana , the zinc-finger transcription factor KNUCKLES (KNU) plays an important role in the termination of floral meristem activity, a process that is crucial for preventing the overgrowth of flowers. The KNU gene is activated in floral meristems by the floral organ identity factor AGAMOUS (AG), and it has been shown that both AG and KNU act in floral meristem control by directly repressing the stem cell regulator WUSCHEL (WUS), which leads to a loss of stem cell activity. When we re-examined the expression pattern of KNU in floral meristems, we found that KNU is expressed throughout the center of floral meristems, which includes, but is considerably broader than the WUS expression domain. We therefore hypothesized that KNU may have additional functions in the control of floral meristem activity. To test this, we employed a gene perturbation approach and knocked down KNU activity at different times and in different domains of the floral meristem. In these experiments we found that early expression in the stem cell domain, which is characterized by the expression of the key meristem regulatory gene CLAVATA3 (CLV3), is crucial for the establishment of KNU expression. The results of additional genetic and molecular analyses suggest that KNU represses floral meristem activity to a large extent by acting on CLV3. Thus, KNU might need to suppress the expression of several meristem regulators to terminate floral meristem activity efficiently. [ABSTRACT FROM AUTHOR]

For an extensive period of time apical meristem (SAM) has been considered as a mysterious organ, due to its small, hidden and dynamic structure. Confocal imaging, combined with fluorescent reporters, enables researchers to unveil the mechanisms underlying cellular activities, such as gene expression, cell division, growth patterns and cell-cell communications. Recently, a series of protocols were developed for confocal imaging of inflorescence meristem (IM) and floral meristem (FM). However, the requirement of high configuration, such as the need of a water-dipping lens without coverslip and the specialized turrets associated with fixed-stage microscopes, impedes the wide adoption of these methods. We exploited an improved object slide and matching method aiming to decrease the configuration requirement. Following this protocol, various dry microscope lenses can be selected with flexibility for building 3D images of IM and FM. • Confocal imaging is a powerful technique for understanding meristem development processes. • High configuration requirement impedes the popularity of classical confocal imaging methods. • A simple method can decrease the configuration requirement without reducing the image quality. • Performance of the method breaks the shackles of water-dipping lens to acquire larger visual field. [ABSTRACT FROM AUTHOR]

A dramatic evolution of fruit size has accompanied the domestication and improvement of fruit-bearing crop species. In tomato (Solanum lycopersicum), naturally occurring cis-regulatory mutations in the genes of the CLAVATA-WUSCHEL signaling pathway have led to a significant increase in fruit size generating enlarged meristems that lead to flowers with extra organs and bigger fruits. In this work, by combining mapping-by-sequencing and CRISPR/ Cas9 genome editing methods, we isolated EXCESSIVE NUMBER OF FLORAL ORGANS (ENO), an AP2/ERF transcription factor which regulates floral meristem activity. Thus, the ENO gene mutation gives rise to plants that yield larger multilocular fruits due to an increased size of the floral meristem. Genetic analyses indicate that eno exhibits synergistic effects with mutations at the LOCULE NUMBER (encoding SlWUS) and FASCIATED (encoding SlCLV3) loci, two central players in the evolution of fruit size in the domestication of cultivated tomatoes. Our findings reveal that an eno mutation causes a substantial expansion of SlWUS expression domains in a flower-specific manner. In vitro binding results show that ENO is able to interact with the GGC-box cis-regulatory element within the SlWUS promoter region, suggesting that ENO directly regulates SlWUS expression domains to maintain floral stem-cell homeostasis. Furthermore, the study of natural allelic variation of the ENO locus proved that a cis-regulatory mutation in the promoter of ENO had been targeted by positive selection during the domestication process, setting up the background for significant increases in fruit locule number and fruit size in modern tomatoes. [ABSTRACT FROM AUTHOR]

The appropriate timing of the termination of floral meristem activity (FM determinacy) determines the number of floral organs. In Arabidopsis, two transcription factors, CRABS CRAW (CRC) and SUPERMAN (SUP), play key roles in FM determinacy. CRC belongs to the YABBY transcription factor family, whose members contain a zinc finger and a helix-loop-helix domain. The crc mutation causes the formation of unfused carpels and leads to an increase in carpel number in sensitized backgrounds. The SUP gene encodes a C2H2-type zinc-finger protein, and sup mutants produce extra carpels and stamens. However, the genetic interaction between CRC and SUP is not fully understood. Here, we show that these two transcription factors regulate multiple common downstream genes during stamen development. The crc sup double mutant had significantly more stamens and carpels than the parental lines and an enlarged floral meristem. Transcriptome data have implicated several cytokinin- and auxin-related genes as well as stress- and metabolic-related genes to function downstream of CRC and SUP during stamen development. The regulation of common downstream genes of CRC and SUP might contribute to the initiation of an appropriate number of stamens and to subsequent growth and development.

Several histone variants are posttranslationally phosphorylated. Little is known about phosphorylation of the centromere-specific histone 3 (CENH3) variant in plants. We show that CENH3 of Arabidopsis thaliana is phosphorylated in vitro by Aurora3, predominantly at serine 65. Interaction of Aurora3 and CENH3 was found by immunoprecipitation (IP) in A. thaliana and by bimolecular fluorescence complementation. Western blotting with an anti-CENH3 pS65 antibody showed that CENH3 pS65 is more abundant in flower buds than elsewhere in the plant. Substitution of serine 65 by either alanine or aspartic acid resulted in a range of phenotypic abnormalities, especially in reproductive tissues. We conclude that Aurora3 phosphorylates CENH3 at S65 and that this post-translational modification is required for the proper development of the floral meristem.

Floral meristems are dynamic systems that generate floral organ primordia at their flanks and, in most species, terminate while giving rise to the gynoecium primordia. However, we find species with floral meristems that generate additional ring meristems repeatedly throughout angiosperm history. Ring meristems produce only stamen primordia, resulting in polystemous flowers (having stamen numbers more than double that of petals or sepals), and act independently of the floral meristem activity. Most of our knowledge on floral meristem regulation is derived from molecular genetic studies of Arabidopsis thaliana, a species with a fixed number of floral organs and, as such of only limited value for understanding ring meristem function, regulation, and ecological value. This review provides an overview of the main molecular players regulating floral meristem activity in A. thaliana and summarizes our knowledge of ring primordia morphology and occurrence in dicots. Our work provides a first step toward understanding the significance and molecular genetics of ring meristem regulation and evolution.

Several histone variants are posttranslationally phosphorylated. Little is known about phosphorylation of the centromere-specific histone 3 (CENH3) variant in plants. We show that CENH3 of Arabidopsis thaliana is phosphorylated in vitro by Aurora3, predominantly at serine 65. Interaction of Aurora3 and CENH3 was found by immunoprecipitation (IP) in A. thaliana and by bimolecular fluorescence complementation. Western blotting with an anti-CENH3 pS65 antibody showed that CENH3 pS65 is more abundant in flower buds than elsewhere in the plant. Substitution of serine 65 by either alanine or aspartic acid resulted in a range of phenotypic abnormalities, especially in reproductive tissues. We conclude that Aurora3 phosphorylates CENH3 at S65 and that this post-translational modification is required for the proper development of the floral meristem. [ABSTRACT FROM AUTHOR]

Grasses have evolved complex inflorescences, where the primary unit is the specialized short branch called a spikelet. Detailed studies of the cumulative action of the genetic regulators that direct the progressive change in axillary meristem identity and their terminal differentiation are crucial to understanding the complexities of the inflorescence and the development of a determinate floret. Grass florets also pose interesting questions concerning the morphologies and functions of organs as compared to other monocots and eudicots. In this review, we summarize our current knowledge of the regulation of the transitions that occur in grass inflorescence meristems, and of the specification of floret meristems and their determinate development. We primarily use rice as a model, with appropriate comparisons to other crop models and to the extensively studied eudicot Arabidopsis. The role of MADS-domain transcription factors in floral organ patterning is well documented in many eudicots and in grasses. However, there is evidence to suggest that some of these rice floral regulators have evolved distinctive functions and that other grass species-specific factors and regulatory pathways occur – for example the LOFSEP 'E' class genes OsMADS1 and OsMAD34, and ramosa genes. A better understanding of these systems and the epigenetic regulators and hormone signaling pathways that interact with them will provide new insights into the rice inflorescence meristem and the differentiation of its floret organs, and should indicate genetic tools that can be used to control yield-related traits in both rice and other cereal crops. [ABSTRACT FROM AUTHOR]

Many developmental processes associated with fruit development occur at the floral meristem (FM). Age-regulated microRNA156 (miR156) and gibberellins (GAs) interact to control flowering time, but their interplay in subsequent stages of reproductive development is poorly understood. Here, in tomato (Solanum lycopersicum), we show that GA and miR156-targeted SQUAMOSA PROMOTER-BINDING PROTEIN-LIKE (SPL or SBP) genes interact in the tomato FM and ovary patterning. High GA responses or overexpression of miR156 (156OE), which leads to low expression levels of miR156-silenced SBP genes, resulted in enlarged FMs, ovary indeterminacy and fruits with increased locule number. Conversely, low GA responses reduced indeterminacy and locule number, and overexpression of a S. lycopersicum (Sl)SBP15 allele that is miR156 resistant (rSBP15) reduced FM size and locule number. GA responses were partially required for the defects observed in 156OE and rSBP15 fruits. Transcriptome analysis and genetic interactions revealed shared and divergent functions of miR156-targeted SlSBP genes, PROCERA/DELLA and the classical WUSCHEL/CLAVATA pathway, which has been previously associated with meristem size and determinacy. Our findings reveal that the miR156/SlSBP/GA regulatory module is deployed differently depending on developmental stage and create novel opportunities to fine-tune aspects of fruit development that have been important for tomato domestication., Competing Interests: Competing interests The authors declare no competing or financial interests., (© 2023. Published by The Company of Biologists Ltd.)

The shoot apical meristem produces all of the leaves, stems and flowers of a flowering plant from a reservoir of stem cells at its growing tip. In Arabidopsis, the small polypeptide signaling molecule CLAVATA3 (CLV3), a member of the CLV3/EMBRYO SURROUNDING REGION-RELATED (CLE) gene family, is a key component of a negative feedback loop that maintains stem cell activity in shoot and floral meristems throughout development. Because in some plant species multiple CLE genes are involved in regulating shoot apical meristem activity, we tested the hypothesis that CLE genes other than CLV3 might function in stem cell homeostasis in Arabidopsis. We identified three Arabidopsis CLE genes expressed in the post-embryonic shoot apical meristem, generated loss-of-function alleles using genome editing, and analyzed the meristem phenotypes of the resulting mutant plants. We found that null mutations in CLE16, CLE17 or CLE27 affected neither vegetative nor reproductive shoot meristem activity under normal growth conditions, although CLE27 appears to slightly prolong vegetative growth. Our results indicate that the CLE16, CLE17 and CLE27 genes have largely redundant roles in the Arabidopsis shoot apical meristem and/or regulate meristem activity only under specific environmental conditions. [ABSTRACT FROM AUTHOR]

This study reports the development of a garden pea genotype ‘VRPM–901–5’ producing five flowers per peduncle at multiple flowering nodes, by using single plant selection approach from a cross ‘VL-8 × PC-531’. In addition, five other stable genetic stocks, namely VRPM-501, VRPM–502, VRPM–503, VRPM–901–3 and VRPSeL–1 producing three flowers per peduncle at multiple flowering nodes were also developed. All these unique genotypes were of either mid- or late- maturity groups. Furthermore, these multi-flowering genotypes were identified during later generations (F4 onward), which might be because of fixation of certain QTLs or recessive gene combinations. Surprisingly, a common parent PC–531, imparting multi-flowering trait in ten cross combinations was identified. Thus, the genotype PC–531 seems to harbor some recessive gene(s) or QTLs that in certain combination(s) express the multi-flowering trait. The interaction between genotype and environment showed that temperature (11–20°C) plays a key role in expression of the multi-flowering trait besides genetic background. Furthermore, the possible relationship between various multi-flowering regulatory genes such as FN, FNA, NEPTUNE, SN, DNE, HR and environmental factors was also explored, and a comprehensive model explaining the multi-flowering trait in garden pea is proposed. [ABSTRACT FROM AUTHOR]

Abstract: Proper floral patterning, including the number and position of floral organs in most plant species, is tightly controlled by the precise regulation of the persistence and size of floral meristems (FMs). In Arabidopsis, two known feedback pathways, one composed of WUSCHEL (WUS) and CLAVATA3 (CLV3) and the other composed of AGAMOUS (AG) and WUS, spatially and temporally control floral stem cells, respectively. However, mounting evidence suggests that other factors, including phytohormones, are also involved in floral meristem regulation. Here, we show that the boundary gene SUPERMAN (SUP) bridges floral organogenesis and floral meristem determinacy in another pathway that involves auxin signaling. SUP interacts with components of polycomb repressive complex 2 (PRC2) and fine‐tunes local auxin signaling by negatively regulating the expression of the auxin biosynthesis genes YUCCA1/4 (YUC1/4). In sup mutants, derepressed local YUC1/4 activity elevates auxin levels at the boundary between whorls 3 and 4, which leads to an increase in the number and the prolonged maintenance of floral stem cells, and consequently an increase in the number of reproductive organs. Our work presents a new floral meristem regulatory mechanism, in which SUP, a boundary gene, coordinates floral organogenesis and floral meristem size through fine‐tuning auxin biosynthesis. [ABSTRACT FROM AUTHOR]

In contrast to animals, plants maintain life-long post-embryonic organogenesis from specialized tissues termed meristems. Shoot meristems give rise to all aerial tissues and are precisely regulated to balance stem cell renewal and differentiation. The phytohormone auxin has a dynamic and differential distribution within shoot meristems and during shoot meristem formation. Polar auxin transport and local auxin biosynthesis lead to auxin maxima and minima to direct cell fate specification, which are critical for meristem formation, lateral organ formation, and lateral organ patterning. In recent years, feedback regulatory loops of auxin transport and signaling have emerged as major determinants of the self-organizing properties of shoot meristems. Systems biology approaches, which involve molecular genetics, live imaging, and computational modeling, have become increasingly important to unravel the function of auxin signaling in shoot meristems. [ABSTRACT FROM AUTHOR]

In Brassica napus, floral development is a decisive factor in silique formation, and it is influenced by many cultivation practices including planting date. However, the effect of planting date on floral initiation in canola is poorly understood at present. A field experiment was conducted using a split plot design, in which three planting dates (early, 15 Sep, middle, 1 Oct, and late, 15 Oct) served as main plot and five varieties differing in maturity (1358, J22, Zhongshuang 11, Zheshuang 8, and Zheyou 50) employed as subplot. The purpose of this study was to shed light on the process of floral meristem (FM) differentiation, the influence of planting date on growth period (GP) and floral initiation, and silique formation. The main stages of FM developments can be divided into four stages: first, the transition from shoot apical meristem to FM; second, flower initiation; third, gynoecium and androecium differentiation; and fourth, bud formation. Our results showed that all genotypes had increased GPs from sowing to FM differentiation as planting date was delayed while the GPs from FM differentiation to budding varied year by year except the very early variety, 1358. Based on the number of flowers present at the different reproductive stages, the flowers produced from FM differentiation to budding closely approximated the final silique even though the FM differentiated continuously after budding and peaked generally at the middle flowering stage. The ratio of siliques to maximum flower number ranged from 48% to 80%. These results suggest that (1) the period from FM differentiation to budding is vital for effective flower and silique formation although there was no significant correlation between the length of the period and effective flowers and siliques, and (2) the increased number of flowers from budding were generally ineffective. Therefore, maximizing flower numbers prior to budding will improve silique numbers, and reducing FM degeneration should also increase final silique formation. From the results of our study, we offer guidelines for planting canola varieties that differ in maturity in order to maximize effective flower numbers.

In Arabidopsis, floral stem cells are maintained only at the initial stages of flower development, and they are terminated at a specific time to ensure proper development of the reproductive organs. Floral stem cell termination is a dynamic and multi-step process involving many transcription factors, chromatin remodeling factors and signaling pathways. In this review, we discuss the mechanisms involved in floral stem cell maintenance and termination, highlighting the interplay between transcriptional regulation and epigenetic machinery in the control of specific floral developmental genes. In addition, we discuss additional factors involved in floral stem cell regulation, with the goal of untangling the complexity of the floral stem cell regulatory network.

The floral meristem (FM) is self-maintaining at the early stages of flower development, but it is terminated when a fixed number of floral organs are produced. The FLORAL ORGAN NUMBER4 (FON4; also known as FON2) gene, an ortholog of Arabidopsis CLAVATA3 (CLV3), is required for regulating FM size and determinacy in rice. However, its interactions with floral homeotic genes remain unknown. Here, we report the genetic interactions between FON4 and floral homeotic genes OsMADS15 (an A-class gene), OsMADS16 (also called SUPERWOMAN1, SPW1, a B-class gene), OsMADS3 and OsMADS58 (C-class genes), OsMADS13 (a D-class gene), and OsMADS1 (an E-class gene) during flower development. We observed an additive phenotype in the fon4 double mutant with the OsMADS15 mutant allele dep (degenerative palea). The effect on the organ number of whorl 2 was enhanced in fon4 spw1. Double mutant combinations of fon4 with osmads3, osmads58, osmads13, and osmads1 displayed enhanced defects in FM determinacy and identity, respectively, indicating that FON4 and these genes synergistically control FM activity. In addition, the expression patterns of all the genes besides OsMADS13 had no obvious change in the fon4 mutant. This work reveals how the meristem maintenance gene FON4 genetically interacts with C, D, and E floral homeotic genes in specifying FM activity in monocot rice. [ABSTRACT FROM AUTHOR]

Reproductive development in plants is controlled by complex and intricate gene-regulatory networks of transcription factors. These networks integrate the information from endogenous, hormonal and environmental regulatory pathways. Many of the key players have been identified in Arabidopsis and other flowering plant species, and their interactions and molecular modes of action are being elucidated. An emerging theme is that there is extensive crosstalk between different pathways, which can be accomplished at the molecular level by modulation of transcription factor activity or of their downstream targets. In this review, we aim to summarize current knowledge on transcription factors and epigenetic regulators that control basic developmental programs during inflorescence and flower morphogenesis in the model plant Arabidopsis thaliana . This article is part of a Special Issue entitled: Plant Gene Regulatory Mechanisms and Networks, edited by Dr. Erich Grotewold and Dr. Nathan Springer. [ABSTRACT FROM AUTHOR]

Cytokinins and gibberellins (GAs) play antagonistic roles in regulating reproductive meristem activity. Cytokinins have positive effects on meristem activity and maintenance. During inflorescence meristem development, cytokinin biosynthesis is activated via a KNOX-mediated pathway. Increased cytokinin activity leads to higher grain number, whereas GAs negatively affect meristem activity. The GA biosynthesis genes GA20oxs are negatively regulated by KNOX proteins. KNOX proteins function as modulators, balancing cytokinin and GA activity in the meristem. However, little is known about the crosstalk among cytokinin and GA regulators together with KNOX proteins and how KNOX-mediated dynamic balancing of hormonal activity functions. Through map-based cloning of QTLs, we cloned a GA biosynthesis gene, Grain Number per Panicle1 (GNP1), which encodes rice GA20ox1. The grain number and yield of NIL-GNP1TQ were significantly higher than those of isogenic control (Lemont). Sequence variations in its promoter region increased the levels of GNP1 transcripts, which were enriched in the apical regions of inflorescence meristems in NIL-GNP1TQ. We propose that cytokinin activity increased due to a KNOX-mediated transcriptional feedback loop resulting from the higher GNP1 transcript levels, in turn leading to increased expression of the GA catabolism genes GA2oxs and reduced GA1 and GA3 accumulation. This rebalancing process increased cytokinin activity, thereby increasing grain number and grain yield in rice. These findings uncover important, novel roles of GAs in rice florescence meristem development and provide new insights into the crosstalk between cytokinin and GA underlying development process. [ABSTRACT FROM AUTHOR]

Morphogenesis in plants is directly linked to the mechanical elements of growing tissues, namely cell wall and inner cell pressure. Studies of these structural elements are now often performed using indentation methods such as atomic force microscopy. In these methods, a probe applies a force to the tissue surface at a subcellular scale and its displacement is monitored, yielding force-displacement curves that reflect tissue mechanics. However, the interpretation of these curves is challenging as they may depend not only on the cell probed, but also on neighboring cells, or even on the whole tissue. Here, we build a realistic three-dimensional model of the indentation of a flower bud using SOFA (Simulation Open Framework Architecture), in order to provide a framework for the analysis of force-displacement curves obtained experimentally. We find that the shape of indentation curves mostly depends on the ratio between cell pressure and wall modulus. Hysteresis in force-displacement curves can be accounted for by a viscoelastic behavior of the cell wall. We consider differences in elastic modulus between cell layers and we show that, according to the location of indentation and to the size of the probe, force-displacement curves are sensitive with different weights to the mechanical components of the two most external cell layers. Our results confirm most of the interpretations of previous experiments and provide a guide to future experimental work. [ABSTRACT FROM AUTHOR]

Correct developmental timing is essential for plant fitness and reproductive success. Two important transitions in shoot development—the juvenile-to-adult vegetative transition and the vegetative-to-reproductive transition—are mediated by a group of genes targeted by miR156, SQUAMOSA PROMOTER BINDING PROTEIN (SBP) genes. To determine the developmental functions of these genes in Arabidopsis thaliana, we characterized their expression patterns, and their gain-of-function and loss-of-function phenotypes. Our results reveal that SBP-LIKE (SPL) genes in Arabidopsis can be divided into three functionally distinct groups: 1) SPL2, SPL9, SPL10, SPL11, SPL13 and SPL15 contribute to both the juvenile-to-adult vegetative transition and the vegetative-to-reproductive transition, with SPL9, SP13 and SPL15 being more important for these processes than SPL2, SPL10 and SPL11; 2) SPL3, SPL4 and SPL5 do not play a major role in vegetative phase change or floral induction, but promote the floral meristem identity transition; 3) SPL6 does not have a major function in shoot morphogenesis, but may be important for certain physiological processes. We also found that miR156-regulated SPL genes repress adventitious root development, providing an explanation for the observation that the capacity for adventitious root production declines as the shoot ages. miR156 is expressed at very high levels in young seedlings, and declines in abundance as the shoot develops. It completely blocks the expression of its SPL targets in the first two leaves of the rosette, and represses these genes to different degrees at later stages of development, primarily by promoting their translational repression. These results provide a framework for future studies of this multifunctional family of transcription factors, and offer new insights into the role of miR156 in Arabidopsis development. [ABSTRACT FROM AUTHOR]

In flowering plants, the arrangement of flowers on a stem becomes an inflorescence, and a huge variety of inflorescence architecture occurs in nature. Inflorescence architecture also affects crop yield. In simple inflorescences, flowers form on a main stem; by contrast, in compound inflorescences, flowers form on branched stems and the branching pattern defines the architecture of the inflorescence. In this review, we highlight recent findings on the regulation of inflorescence architecture by cytokinin plant hormones. Results in rice and Arabidopsis show that although these two species have distinct inflorescence architectures, cytokinins have a common effect on inflorescence branching. Based on these studies, we discuss how cytokinins regulate distinct types of inflorescence architecture through their effect on meristem activities.

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. [ABSTRACT FROM AUTHOR]

In Arabidopsis, floral stem cells are maintained only at the initial stages of flower development, and they are terminated at a specific time to ensure proper development of the reproductive organs. Floral stem cell termination is a dynamic and multi-step process involving many transcription factors, chromatin remodeling factors and signaling pathways. In this review, we discuss the mechanisms involved in floral stem cell maintenance and termination, highlighting the interplay between transcriptional regulation and epigenetic machinery in the control of specific floral developmental genes. In addition, we discuss additional factors involved in floral stem cell regulation, with the goal of untangling the complexity of the floral stem cell regulatory network. [ABSTRACT FROM AUTHOR]

Polycomb Repressive Complex 2 ( PRC2) represses the transcriptional activity of target genes through trimethylation of lysine 27 of histone H3. The functions of plant PRC2 have been chiefly described in Arabidopsis, but specific functions in other plant species, especially cereals, are still largely unknown. Here we characterize mutants in the rice EMF2B gene, an ortholog of the Arabidopsis EMBRYONIC FLOWER2 ( EMF2) gene. Loss of EMF2B in rice results in complete sterility, and mutant flowers have severe floral organ defects and indeterminacy that resemble loss-of-function mutants in E-function floral organ specification genes. Transcriptome analysis identified the E-function genes Os MADS1, Os MADS6 and Os MADS34 as differentially expressed in the emf2b mutant compared with wild type. Os MADS1 and Os MADS6, known to be required for meristem determinacy in rice, have reduced expression in the emf2b mutant, whereas Os MADS34 which interacts genetically with Os MADS1 was ectopically expressed. Chromatin immunoprecipitation for H3K27me3 followed by quantitative (q) RT- PCR showed that all three genes are presumptive targets of PRC2 in the meristem. Therefore, in rice, and possibly other cereals, PRC2 appears to play a major role in floral meristem determinacy through modulation of the expression of E-function genes. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Due to the lack of cell migration, plant organogenesis relies on coordinated cell proliferation, cell growth, and differentiation. A flower possesses a complex structure, with sepals and petals constituting the perianth, and stamens and pistils where male and female gametophytes differentiate. While advances have been made in our understanding of gene regulatory networks controlling flower development, relatively little is known of how cell–cell coordination influences floral organ specification. The Arabidopsis ERECTA (ER)-family receptor kinases, ER, ER-LIKE1 (ERL1), and ERL2, regulate inflorescence architecture, organ shape, and epidermal stomatal patterning. Here it is reported that ER-family genes together regulate floral meristem organization and floral organ identity. The stem cell marker CLAVATA3 exhibits misplaced expression in the floral meristems of the er erl1 erl2 mutant. Strikingly, homeotic conversion of sepals to carpels was observed in er erl1 erl2 flowers. Consistently, ectopic expression of AGAMOUS, which determines carpel identity, was detected in er erl1 erl2 flower primordia. Among the known downstream components of ER-family receptor kinases in stomatal patterning, YODA (YDA) is also required for proper floral patterning. YDA and the ER-family show complex, synergistic genetic interactions: er erl1 erl2 yda quadruple mutant plants become extremely small, callus-like masses. While a constitutively active YDA fully rescues stomatal clustering in er erl1 erl2, it only partially rescues er erl1 erl2 flower defects. The study suggests that ER-family signalling is crucial for ensuring proper expression domains of floral meristem and floral organ identity determinants, and further implies the existence of a non-canonical downstream pathway. [ABSTRACT FROM AUTHOR]

Background: The classical (C) MIKC-type MADS-box transcription factors comprise one gene family that plays diverse roles in the flowering process ranging from floral initiation to the development of floral organs. Despite their importance in regulating developmental processes that impact crop yield, they remain largely unexplored in the major legume oilseed crop, soybean. Results: We identified 57 MIKCc-type transcription factors from soybean and determined the in silico gene expression profiles of the soybean MIKCc-type genes across different tissues. Our study implicates three MIKCc-type transcription factors as novel members of the AGAMOUS LIKE 6 (AGL6) subfamily of the MIKCC-type MADS-box genes, and we named this sister clade PsMADS3. While similar genes were identified in other legume species, poplar and grape, no such gene is represented in Arabidopsis thaliana or rice. RT-PCR analysis on these three soybean PsMADS3 genes during early floral initiation processes revealed their temporal expression similar to that of APETALA1, a gene known to function as a floral meristem identity gene. However, RNA in situ hybridisation showed that their spatial expression patterns are markedly different from those of APETALA1. Conclusion: Legume flower development system differs from that in the model plant, Arabidopsis. There is an overlap in the initiation of different floral whorls in soybean, and inflorescent meristems can revert to leaf production depending on the environmental conditions. MIKCC-type MADS-box genes have been shown to play key regulatory roles in different stages of flower development. We identified members of the PsMADS3 sub-clade in legumes that show differential spatial expression during floral initiation, indicating their potential novel roles in the floral initiation process. The results from this study will contribute to a better understanding of legume-specific floral developmental processes. [ABSTRACT FROM AUTHOR]

Abstract: The study of floral ontogeny across the entire Rubiaceae family is essential to an understanding of its floral evolution. However, studies reporting on the development of the gynoecium, as well as the formation of the carpel septa, placenta and ovules are scarce. This work, therefore aimed to assess placentation and gynosporogenesis, as well as carpel, septum, and ovule development, in Psychotria carthagenensis Jacq. (Psychotrieae) and Rudgea macrophylla Benth. (Palicoureeae). A new ovule type, Psychotria, is here recognized to accommodate the unique combination of ovule characters observed in these two species and possibly the Psychotrieae–Palicoureeae clade. In this ovule type the carpels are considered as sterile appendices, and the placenta represents a continuation of the floral meristem based on its position relative to the carpels. The ovules are erect and located at the basal portion of the locule. The nucellar epidermis appears at early developmental stages as a flat surface and, later on, as a dome-like structure. The new type is closely related to the Phyllis type but is distinguished by the erect ovules as opposed to pendulous in the Phyllis type. [Copyright &y& Elsevier]

Plant development and architecture is regulated by meristems that initiate lateral organs on their flanks. The gene regulatory networks that govern the transition of a vegetative shoot apical meristem into an inflorescence meristem (IM), together with those necessary to specify floral meristem (FM) identity have been elucidated in Arabidopsis thaliana and are highly complex and redundant. FMs are initiated in the axils of cryptic bracts and evidence suggests that FMs emerge and differentiate along an abaxial/adaxial axis, in contrast to existing models of centroradial positional information within FMs. Real-time imaging has revealed dynamic cell division and gene expression patterns associated with incipient primordia in the IM. This review, however, outlines how little is known concerning the identity of these primordia, the timing of FM specification and commitment in relation to the establishment of FM identity, and the interplay between bract and FM founder cell recruitment and development. [ABSTRACT FROM AUTHOR]

The inflorescence of Helianthus annuus L. has two types of flowers (or florets) on a single capitulum; central hermaphrodite disc florets and peripheral pistillate ray florets. In both florets, reproductive development starts with the conversion of apical meristem into floral meristem that will produce floral organ primordia. The only difference between hermaphrodite and pistillate florets in apical meristem stage is that apical meristem of the pistillate florets is not as apparent and curvaceous as apical meristem of the hermaphrodite florets. The differentiation of apical meristem into floral meristem is in the same progress in both florets. In hermaphrodite florets, flower organs; petals, stamens and carpels develop from floral meristem. Differentiation of five petal primordia takes place in the same way in both florets. Firstly filament and then anther differentiates in a stamen. Two carpel primordia appear below the stamen primordia in hermaphrodite florets. In following stages, carpel primordia are lengthened and formed inferior ovary, style, stigma respectively. In pistillate florets, flower organs; petals and carpels develop from floral meristem. They pass directly from the periant initiation to the start of carpel formation. Stamen primordia don't appear and the further development of carpel primordia stops in a short time, as a result, stigma and style do not exist in pistillate florets. However, an inferior ovary with no ovule forms. In the capitulum of hermaphrodite florets, the development takes place in a centripetal manner; it starts firstly on the outermost whorl, and it proceeds towards inner whorl. However, this is not the case in pistillate florets. [ABSTRACT FROM AUTHOR]