Floral homeotic proteins modulate the genetic program for leaf development to suppress trichome formation in flowers

A space for time. Exploring temporal regulation of plant development across spatial scales

Plants continuously undergo change during their life cycle, experiencing dramatic phase transitions altering plant form, and regulating the assignment and progression of cell fates. The relative timing of developmental events is tightly controlled and involves integration of environmental, spatial, and relative age-related signals and actors. While plant phase transitions have been studied extensively and many of their regulators have been described, less is known about temporal regulation on a smaller, cell-level scale. Here, using examples from both plant and animal systems, we outline time-dependent changes. Looking at systemic scale changes, we discuss the timing of germination, juvenile-to-adult transition, flowering, and senescence, together with regeneration timing. Switching to temporal regulation on a cellular level, we discuss several instances from the animal field in which temporal control has been examined extensively at this scale. Then, we switch back to plants and summarize examples where plant cell-level changes are temporally regulated. As time cannot easily be separated from signaling derived from the environment and tissue context, we next discuss factors that have been implicated in controlling the timing of developmental events, reviewing temperature, photoperiod, nutrient availability, as well as tissue context and mechanical cues on the cellular scale. Afterwards, we provide an overview of mechanisms that have been shown or implicated in the temporal control of development, considering metabolism, division control, mobile signals, epigenetic regulation, and the action of transcription factors. Lastly, we look at remaining questions for the future study of developmental timing in plants and how recent technical advancement can enable these efforts.

The homeotic gene PhDEF regulates production of volatiles in petunia flowers by activating EOBI and EOBII

In petunia (Petunia × hybrida), MADS-box homeotic genes dictate floral organ identity. For instance, DEFICIENS (PhDEF), GLOBOSA1, and GLOBOSA2 (PhGLO1/2) are responsible for petal and stamen identity. However, whether homeotic genes, particularly PhDEF, have a function at the later stages of flower development remains elusive. In petunia flowers, scent production initiates at anthesis, when the flower is ready for pollination, and is triggered by activation of EMISSION OF BENZENOIDS I (EOBI) and EOBII, MYB transcriptional regulators of scent-related genes. Here, we revealed the role of PhDEF in mature flowers, showing that it activates scent production. PhDEF suppression using a transient viral system in petunia flowers led to a significant reduction in volatile emission and pool levels, and in the transcript levels of scent-related transcriptional regulators and enzymes. Promoter activity assays demonstrated that PhDEF activates EOBI, EOBII, and the phenylpropanoid biosynthesis genes L-PHENYLALANINE AMMONIA LYASE and PHENYLACETALDEHYDE SYNTHASE. Our findings underscore the importance of PhDEF in petunia flower development from initiation to maturation and in coordinating petal specification and the establishment of showy pollination-related traits.

The bHLH transcription factor gene EGL3 accounts for the natural diversity in Arabidopsis fruit trichome pattern and morphology

The number and distribution of trichomes, i.e. the trichome pattern, in different plant organs show a conspicuous inter- and intraspecific diversity across Angiosperms that is presumably involved in adaptation to numerous environmental factors. The genetic and molecular mechanisms accounting for the evolution of trichome patterns have just begun to be elucidated. In this study, we aimed to identify and characterize MALAMBRUNO 1 (MAU1), a locus affecting trichome number in the fruits of Arabidopsis (Arabidopsis thaliana) natural populations. To this end, we developed introgression lines (ILs) from the hairy fruit accession Doñana (Don-0) in the genetic background of the Ler strain with glabrous fruits. Genetic analyses of ILs showed that MAU1 affects fruit trichome patterns through synergistic epistasis with the MYB genes TRICHOMELESS1 (TCL1), GLABRA1 (GL1), and TRIPTYCHON (TRY). In addition, fine mapping and characterization of transgenic lines demonstrated that MAU1 is the bHLH transcription factor gene EGL3, for which Don-0 carries a gain-of-function semidominant allele. Gene expression analyses did not detect differences between EGL3 alleles, thus supporting that a structural missense mutation is the causal nucleotide polymorphism of Don-0. Further phylogenetic analyses of EGL3 showed that most Arabidopsis populations with hairy fruits belong to 3 haplogroups, suggesting that additional EGL3 natural alleles account for fruit trichome development. Finally, the characterization of EGL3 pleiotropy indicates that Don-0 hyperfunction also increases stem trichome branching. We conclude that EGL3 interactions in the core gene regulatory network of trichome development explain the Arabidopsis natural diversity for fruit trichome pattern and morphology.

A mutation in CsGME encoding GDP-mannose 3,5-epimerase results in little and wrinkled leaf in cucumber

KEY MESSAGE

Map-based cloning revealed that a mutation in a highly conserved amino acid of the CsGME gene encoding GDP-mannose 3,5-epimerase, causes the phenotype of little and wrinkled leaves in cucumbers. Leaf size is a critical determinant of plant architecture in cucumbers, yet only a few genes associated with this trait have been mapped or cloned. Here, we identified and characterized a mutant with little and wrinkled leaves, named lwl-1. Genetic analysis revealed that the phenotype of the lwl-1 was controlled by a single recessive gene. Through map-based cloning, the lwl-1 locus was narrowed down to a 12.22-kb region exclusively containing one fully annotated gene CsGME (CsaV3_2G004170). CsGME encodes GDP-mannose 3,5-epimerase, which is involved in the synthesis of ascorbic acid (ASA) and one of the components of pectin, RG-II. Whole-length sequencing of the 12.22 kb DNA fragment revealed the presence of only a non-synonymous mutation located in the sixth exon of CsGME in lwl-1, resulting in an amino acid alteration from Pro363 to Leu363. This mutation was unique among 118 inbred lines from cucumber natural populations. CsGME expression significantly reduced in various organs of lwl-1, accompanied by a significant decrease in ASA and pectin content in leaves. Both CsGME and Csgme proteins were localized to the cytoplasm. The mutant phenotype exhibited partial recovery after the application of exogenous boric acid. Silencing CsGME in cucumber through VIGS confirmed its role as the causal gene for lwl-1. Transcriptome profiling revealed that CsGME greatly affected the expression of genes related to the cell division process and cell plate formation. This study represents the first report to characterize and clone the CsGME in cucumber, indicating its crucial role in regulating leaf size and development.

Flower Development in Arabidopsis

Like in other angiosperms, the development of flowers in Arabidopsis starts right after the floral transition, when the shoot apical meristem (SAM) stops producing leaves and makes flowers instead. On the flanks of the SAM emerge the flower meristems (FM) that will soon differentiate into the four main floral organs, sepals, petals, stamens, and pistil, stereotypically arranged in concentric whorls. Each phase of flower development-floral transition, floral bud initiation, and floral organ development-is under the control of specific gene networks. In this chapter, we describe these different phases and the gene regulatory networks involved, from the floral transition to the floral termination.

Gynoecium and fruit development in Arabidopsis

Flowering plants produce flowers and one of the most complex floral structures is the pistil or the gynoecium. All the floral organs differentiate from the floral meristem. Various reviews exist on molecular mechanisms controlling reproductive development, but most focus on a short time window and there has been no recent review on the complete developmental time frame of gynoecium and fruit formation. Here, we highlight recent discoveries, including the players, interactions and mechanisms that govern gynoecium and fruit development in Arabidopsis. We also present the currently known gene regulatory networks from gynoecium initiation until fruit maturation.

Case not closed: the mystery of the origin of the carpel

The carpel is a fascinating structure that plays a critical role in flowering plant reproduction and contributed greatly to the evolutionary success and diversification of flowering plants. The remarkable feature of the carpel is that it is a closed structure that envelopes the ovules and after fertilization develops into the fruit which protects, helps disperse, and supports seed development into a new plant. Nearly all plant-based foods are either derived from a flowering plant or are a direct product of the carpel. Given its importance it's no surprise that plant and evolutionary biologists have been trying to explain the origin of the carpel for a long time. Before carpel evolution seeds were produced on open leaf-like structures that are exposed to the environment. When the carpel evolved in the stem lineage of flowering plants, seeds became protected within its closed structure. The evolutionary transition from that open precursor to the closed carpel remains one of the greatest mysteries of plant evolution. In recent years, we have begun to complete a picture of what the first carpels might have looked like. On the other hand, there are still many gaps in our understanding of what the precursor of the carpel looked like and what changes to its developmental mechanisms allowed for this evolutionary transition. This review aims to present an overview of existing theories of carpel evolution with a particular emphasis on those that account for the structures that preceded the carpel and/or present testable developmental hypotheses. In the second part insights from the development and evolution of diverse plant organs are gathered to build a developmental hypothesis for the evolutionary transition from a hypothesized laminar open structure to the closed structure of the carpel.

Transcriptome profiling reveals key genes in regulation of the tepal trichome development in Lilium pumilum D.C

A number of potential genes and pathways involved in tepal trichome development were identified in a natural lily mutant by transcriptome analysis and were confirmed with trichome and trichomeless species. Trichome is a specialized structure found on the surface of the plant with an important function in survival against abiotic and biotic stress. It is also an important economic trait in crop breeding. Extensive research has investigated the foliar trichome in model plants (Arabidopsis and tomato). However, the developmental mechanism of tepal trichome remains elusive. Lilium pumilum is an edible ornamental bulb and a good breeding parent possessing cold and salt-alkali resistance. Here, we found a natural mutant of Lilium pumilum grown on a highland whose tepals are covered by trichomes. Our data indicate that trichomes of the mutant are multicellular and branchless. Notably, stomata are also developed on the tepal of the mutant as well, suggesting there may be a correlation between trichome and stomata regulation. Furthermore, we isolated 27 differentially expressed genes (DEGs) by comparing the transcriptome profiling between the natural mutant and the wild type. These 27 genes belong to 4 groups: epidermal cell cycle and division, trichome morphogenesis, stress response, and transcription factors. Quantitative real-time PCR in Lilium pumilum (natural mutant and the wild type) and other lily species (Lilium leichtlinii var. maximowiczii/trichome; Lilium davidii var. willmottiae/, trichomeless) confirmed the validation of RNA-seq data and identified several trichome-related genes.

AtENO2 functions in the development of male gametophytes in Arabidopsis thaliana

Pollen fertility is an important factor affecting the seed setting rate and seed yield of plants. The Arabidopsis thaliana enolase gene ENO2 (AtENO2) can affect the pollen morphology, germination, and pollen tube growth. AtENO2 encodes two proteins AtENO2 and AtMBP-1. To examine the effect of AtENO2 protein on pollen development, the 2nd ATG of the AtENO2 coding sequence for AtMBP-1 was mutated by site-directed mutagenesis, and transgenic plants expressing only AtENO2 but not AtMBP-1 were obtained. Phenotypic analysis indicated that AtENO2 was essential in the pollen development. The mechanisms of AtENO2 on pollen development were analyzed. AtENO2 can affect development of the pollen intine, and the mechanism may be that AtENO2 regulated the methyl esterification of pectin in pollen intine through ARF3 and AtPMEI-pi. The -734 ∼ -573 sequence of AtENO2 promoter is the main transcriptional regulatory region of AtENO2 affecting pollen development. The functional cis-acting element may be GTGANTG10(GTGA), and the trans-acting factors may be KAN, AS2 and ARF3/ETT. Moreover, the deletion of AtENO2 can cause significant difference in the expression of multiple genes related to pollen exine development. These results are useful for further studying the function of AtENO2 and exploring the mechanism of plant pollen development.

Coordination of biradial-to-radial symmetry and tissue polarity by HD-ZIP II proteins

Symmetry establishment is a critical process in the development of multicellular organs and requires careful coordination of polarity axes while cells actively divide within tissues. Formation of the apical style in the Arabidopsis gynoecium involves a bilateral-to-radial symmetry transition, a stepwise process underpinned by the dynamic distribution of the plant morphogen auxin. Here we show that SPATULA (SPT) and the HECATE (HEC) bHLH proteins mediate the final step in the style radialisation process and synergistically control the expression of adaxial-identity genes, HOMEOBOX ARABIDOPSIS THALIANA 3 (HAT3) and ARABIDOPSIS THALIANA HOMEOBOX 4 (ATHB4). HAT3/ATHB4 module drives radialisation of the apical style by promoting basal-to-apical auxin flow and via a negative feedback mechanism that finetune auxin distribution through repression of SPT expression and cytokinin sensitivity. Thus, this work reveals the molecular basis of axes-coordination and hormonal cross-talk during the sequential steps of symmetry transition in the Arabidopsis style.

The apical style in Arabidopsis is formed following a bilateral-to-radial symmetry transition in the gynoecium. Here the authors show that the final step in style radialization is coordinated by the adaxial regulators HAT3 and ATHB4, which are induced by the SPT and HEC transcription factors.

Cell identity specification in plants: lessons from flower development

Multicellular organisms display a fascinating complexity of cellular identities and patterns of diversification. The concept of 'cell type' aims to describe and categorize this complexity. In this review, we discuss the traditional concept of cell types and highlight the impact of single-cell technologies and spatial omics on the understanding of cellular differentiation in plants. We summarize and compare position-based and lineage-based mechanisms of cell identity specification using flower development as a model system. More than understanding ontogenetic origins of differentiated cells, an important question in plant science is to understand their position- and developmental stage-specific heterogeneity. Combinatorial action and crosstalk of external and internal signals is the key to cellular heterogeneity, often converging on transcription factors that orchestrate gene expression programs.

TCP transcription factors suppress cotyledon trichomes by impeding a cell differentiation-regulating complex

Trichomes are specialized epidermal cells that act as barriers against biotic and abiotic stresses. Although the formation of trichomes on hairy organs is well studied, the molecular mechanisms of trichome inhibition on smooth organs are still largely unknown. Here, we demonstrate that the CINCINNATA (CIN)-like TEOSINTE BRANCHED1/CYCLOIDEA/PCF (TCP) transcription factors inhibit the formation of trichomes on cotyledons in Arabidopsis (Arabidopsis thaliana). The tcp2/3/4/5/10/13/17 septuple mutant produces cotyledons with ectopic trichomes on the adaxial sides. The expression patterns of TCP genes are developmentally regulated during cotyledon development. TCP proteins directly interact with GLABRA3 (GL3), a key component of the MYB transcription factor/basic helix-loop-helix domain protein/WD40-repeat proteins (MYB-bHLH-WD40, MBW) complex essential for trichome formation, to interfere with the transactivation activity of the MBW complex in cotyledons. TCPs also disrupt the MBW complex-R3 MYB negative feedback loop by directly promoting the expression of R3 MYB genes, which enhance the repression of the MBW complex. Our findings reveal a molecular framework in which TCPs suppress trichome formation on adaxial sides of cotyledons by repressing the activity of the MBW complex at the protein level and the transcripts of R3 MYB genes at the transcriptional level.

The Relationship between AGAMOUS and Cytokinin Signaling in the Establishment of Carpeloid Features

Gynoecium development is dependent on gene regulation and hormonal pathway interactions. The phytohormones auxin and cytokinin are involved in many developmental programs, where cytokinin is normally important for cell division and meristem activity, while auxin induces cell differentiation and organ initiation in the shoot. The MADS-box transcription factor AGAMOUS (AG) is important for the development of the reproductive structures of the flower. Here, we focus on the relationship between AG and cytokinin in Arabidopsis thaliana, and use the weak ag-12 and the strong ag-1 allele. We found that cytokinin induces carpeloid features in an AG-dependent manner and the expression of the transcription factors CRC, SHP2, and SPT that are involved in carpel development. AG is important for gynoecium development, and contributes to regulating, or else directly regulates CRC, SHP2, and SPT. All four genes respond to either reduced or induced cytokinin signaling and have the potential to be regulated by cytokinin via the type-B ARR proteins. We generated a model of a gene regulatory network, where cytokinin signaling is mainly upstream and in parallel with AG activity.

Transcriptome analysis reveals rice MADS13 as an important repressor of the carpel development pathway in ovules

In angiosperms, floral homeotic genes encoding MADS-domain transcription factors regulate the development of floral organs. Specifically, members of the SEPALLATA (SEP) and AGAMOUS (AG) subfamilies form higher-order protein complexes to control floral meristem determinacy and to specify the identity of female reproductive organs. In rice, the AG subfamily gene OsMADS13 is intimately involved in the determination of ovule identity, since knock-out mutant plants develop carpel-like structures in place of ovules, resulting in female sterility. Little is known about the regulatory pathways at the base of rice gynoecium development. To investigate molecular mechanisms acting downstream of OsMADS13, we obtained transcriptomes of immature inflorescences from wild-type and Osmads13 mutant plants. Among a total of 476 differentially expressed genes (DEGs), a substantial overlap with DEGs from the SEP-family Osmads1 mutant was found, suggesting that OsMADS1 and OsMADS13 may act on a common set of target genes. Expression studies and preliminary analyses of two up-regulated genes encoding Zinc-finger transcription factors indicated that our dataset represents a valuable resource for the identification of both OsMADS13 target genes and novel players in rice ovule development. Taken together, our study suggests that OsMADS13 is an important repressor of the carpel pathway during ovule development.

Dissection of the Arabidopsis HUA-PEP gene activity reveals that ovule fate specification requires restriction of the floral A-function

Ovules are essential for sexual plant reproduction and seed formation, and are fundamental for agriculture. However, our understanding of the molecular mechanisms governing ovule development is far from complete. In Arabidopsis, ovule identity is determined by homeotic MADS-domain proteins that define the floral C- (AG) and D- (SHP1/SHP2, STK) functions. Pre-mRNA processing of these genes is critical and mediated by HUA-PEP activity, composed of genes encoding RNA-binding proteins. In strong hua-pep mutants, functional transcripts for C- and D-function genes are reduced, resulting in homeotic transformation of ovules. Thus, hua-pep mutants provide an unique sensitized background to study ovule morphogenesis when C- and D-functions are simultaneously compromised. We found that hua-pep ovules are morphologically sepaloid and show ectopic expression of the homeotic class-A gene AP1. Inactivation of AP1 or AP2 (A-function genes) in hua-pep mutants reduced homeotic conversions, rescuing ovule identity while promoting carpelloid traits in transformed ovules. Interestingly, increased AG dosage led to similar results. Our findings strongly suggest that HUA-PEP activity is required for correct C and D floral functions, which in turn prevents ectopic expression of class-A genes in ovules for their proper morphogenesis, evoking the classic A-C antagonism of the ABC model for floral organ development.

Can the French flag and reaction-diffusion models explain flower patterning? Celebrating the 50th anniversary of the French flag model

It has been 50 years since Lewis Wolpert introduced the French flag model proposing the patterning of different cell types based on threshold concentrations of a morphogen diffusing in the tissue. Sixty-seven years ago, Alan Turing introduced the idea of patterns initiating de novo from a reaction-diffusion network. Together these models have been used to explain many patterning events in animal development, so here we take a look at their applicability to flower development. First, although many plant transcription factors move through plasmodesmata from cell to cell, in the flower there is little evidence that they specify fate in a concentration-dependent manner, so they cannot yet be described as morphogens. Secondly, the reaction-diffusion model appears to be a reasonably good description of the formation of spots of pigment on petals, although additional nuances are present. Thirdly, aspects of both of these combine in a new fluctuation-based patterning system creating the scattered pattern of giant cells in Arabidopsis sepals. In the future, more precise imaging and manipulations of the dynamics of patterning networks combined with mathematical modeling will allow us to better understand how the multilayered complex and beautiful patterns of flowers emerge de novo.

Age-dependent heteroblastic development of leaf hairs in Arabidopsis

Higher plants progress through a juvenile and an adult phase of development before they enter the reproductive phase. The transition from the juvenile to the adult phase is referred to as vegetative phase change, and this is signified by the production of trichomes on the abaxial side of leaf blades in Arabidopsis; however, the molecular mechanism underlying this process is poorly understood. We identified a dominant mutation (gl1-D) in a forward genetic screen that accelerates abaxial trichome production during shoot development. This phenotype is the result of a G-to-A substitution in the 3' noncoding region in the GLABRA1(GL1) gene. We show that TOE1, an AP2-like transcription factor that acts downstream of the miR156-SPL pathway, represses GL1 expression by directly binding to this site, and that gl1-D prevents TOE1 binding. Our work reveals a molecular link between vegetative phase change and abaxial trichome production in Arabidopsis, and answers a long-standing question of how the vegetative phase change pathway regulates vegetative traits.

The Dynamic Genetic-Hormonal Regulatory Network Controlling the Trichome Development in Leaves

Plant trichomes are outgrowths developed from an epidermal pavement cells of leaves and other organs. Trichomes (also called 'hairs') play well-recognized roles in defense against insect herbivores, forming a physical barrier that obstructs insect movement and mediating chemical defenses. In addition, trichomes can act as a mechanosensory switch, transducing mechanical stimuli (e.g., insect movement) into physiological signals, helping the plant to respond to insect attacks. Hairs can also modulate plant responses to abiotic stresses, such as water loss, an excess of light and temperature, and reflect light to protect plants against UV radiation. The structure of trichomes is species-specific and this trait is generally related to their function. These outgrowths are easily analyzed and their origin represents an outstanding subject to study epidermal cell fate and patterning in plant organs. In leaves, the developmental control of the trichomatous complement has highlighted a regulatory network based on four fundamental elements: (i) genes that activate and/or modify the normal cell cycle of epidermal pavement cells (i.e., endoreduplication cycles); (ii) transcription factors that create an activator/repressor complex with a central role in determining cell fate, initiation, and differentiation of an epidermal cell in trichomes; (iii) evidence that underlines the interplay of the aforesaid complex with different classes of phytohormones; (iv) epigenetic mechanisms involved in trichome development. Here, we reviewed the role of genes in the development of trichomes, as well as the interaction between genes and hormones. Furthermore, we reported basic studies about the regulation of the cell cycle and the complexity of trichomes. Finally, this review focused on the epigenetic factors involved in the initiation and development of hairs, mainly on leaves.

Photosynthetic activity of reproductive organs

During seed development, carbon is reallocated from maternal tissues to support germination and subsequent growth. As this pool of resources is depleted post-germination, the plant begins autotrophic growth through leaf photosynthesis. Photoassimilates derived from the leaf are used to sustain the plant and form new organs, including other vegetative leaves, stems, bracts, flowers, fruits, and seeds. In contrast to the view that reproductive tissues act only as resource sinks, many studies demonstrate that flowers, fruits, and seeds are photosynthetically active. The photosynthetic contribution to development is variable between these reproductive organs and between species. In addition, our understanding of the developmental control of photosynthetic activity in reproductive organs is vastly incomplete. A further complication is that reproductive organ photosynthesis (ROP) appears to be particularly important under suboptimal growth conditions. Therefore, the topic of ROP presents the community with a challenge to integrate the fields of photosynthesis, development, and stress responses. Here, we attempt to summarize our understanding of the contribution of ROP to development and the molecular mechanisms underlying its control.

Maize YABBY genes drooping leaf1 and drooping leaf2 regulate floret development and floral meristem determinacy

Floral morphology is shaped by factors that modulate floral meristem activity and size, and the identity, number and arrangement of the lateral organs they form. We report here that the maize CRABS CLAW co-orthologs drooping leaf1 (drl1) and drl2 are required for development of ear and tassel florets. Pistillate florets of drl1 ears are sterile with unfused carpels that fail to enclose an expanded nucellus-like structure. Staminate florets of drl1 tassels have extra stamens and fertile anthers. Natural variation and transposon alleles of drl2 enhance drl1 mutant phenotypes by reducing floral meristem (FM) determinacy. The drl paralogs are co-expressed in lateral floral primordia, but not within the FM. drl expression together with the more indeterminate mutant FMs suggest that the drl genes regulate FM activity and impose meristem determinacy non-cell-autonomously from differentiating cells in lateral floral organs. We used gene regulatory network inference, genetic interaction and expression analyses to suggest that DRL1 and ZAG1 target each other and a common set of downstream genes that function during floret development, thus defining a regulatory module that fine-tunes floret patterning and FM determinacy.

Gynoecium development: networks in Arabidopsis and beyond

Life has always found a way to preserve itself. One strategy that has been developed for this purpose is sexual reproduction. In land plants, the gynoecium is considered to be at the top of evolutionary innovation, since it has been a key factor in the success of the angiosperms. The gynoecium is composed of carpels with different tissues that need to develop and differentiate in the correct way. In order to control and guide gynoecium development, plants have adapted elements of pre-existing gene regulatory networks (GRNs) but new ones have also evolved. The GRNs can interact with internal factors (e.g. hormones and other metabolites) and external factors (e.g. mechanical signals and temperature) at different levels, giving robustness and flexibility to gynoecium development. Here, we review recent findings regarding the role of cytokinin-auxin crosstalk and the genes that connect these hormonal pathways during early gynoecium development. We also discuss some examples of internal and external factors that can modify GRNs. Finally, we make a journey through the flowering plant lineage to determine how conserved are these GRNs that regulate gynoecium and fruit development.

Cytokinin - A Developing Story

In the past decade tremendous advances have been made in understanding the biosynthesis, perception, and signaling pathways of the plant hormone cytokinin. It also became clear that interfering with any of these steps greatly impacts all on stages of growth and development. This has recently spurted renewed effort to understand how cytokinin signaling affects developmental processes. As a result, new insights on the role of cytokinin signaling and the downstream targets during, for example, shoot apical meristem, flower, female gametophyte, stomata and vascular development are being unraveled. In this review we aim to give a comprehensive overview of recent findings on how cytokinin influences growth and development in plants, and highlight areas for future research.