DEFECTIVE KERNEL 1 promotes and maintains plant epidermal differentiation

CLE peptides act via the receptor-like kinase CRINKLY 4 in Physcomitrium patens gametophore development

The CLAVATA pathway plays a key role in the regulation of multicellular shoot and root meristems in flowering plants. In Arabidopsis, CLAVATA 3-like signaling peptides (CLEs) act via receptor-like kinases CLAVATA 1 and CRINKLY 4 (CR4). In the moss Physcomitrium patens, PpCLAVATA and PpCR4 were previously studied independently and shown to play conserved roles in the regulation of cell proliferation and differentiation. The plant calpain DEFECTIVE KERNEL 1 (DEK1) has been identified as another key regulator of cell division and cell fate in vascular plants and bryophytes. The functional interaction between CLAVATA, CR4, and DEK1 remains unknown. Here, we show that P. patens crinkly4 and dek1 mutants respond differently to CLE peptide treatments suggesting their distinct roles in the CLAVATA pathway. Reduced CLAVATA-mediated suppression of leafy shoot growth in Δcr4 mutants indicates that PpCR4 is involved in CLV3p perception, most likely as a receptor. The CLV3p strongly suppressed leaf vein development in Δcr4 mutants, suggesting that other receptors are involved in these processes and indicating a potential role of PpCR4 in organ sensitization to CLEs.

Regulation of developmental gatekeeping and cell fate transition by the calpain protease DEK1 in Physcomitrium patens

Calpains are cysteine proteases that control cell fate transitions whose loss of function causes severe, pleiotropic phenotypes in eukaryotes. Although mainly considered as modulatory proteases, human calpain targets are directed to the N-end rule degradation pathway. Several such targets are transcription factors, hinting at a gene-regulatory role. Here, we analyze the gene-regulatory networks of the moss Physcomitrium patens and characterize the regulons that are misregulated in mutants of the calpain DEFECTIVE KERNEL1 (DEK1). Predicted cleavage patterns of the regulatory hierarchies in five DEK1-controlled subnetworks are consistent with a pleiotropic and regulatory role during cell fate transitions targeting multiple functions. Network structure suggests DEK1-gated sequential transitions between cell fates in 2D-to-3D development. Our method combines comprehensive phenotyping, transcriptomics and data science to dissect phenotypic traits, and our model explains the protease function as a switch gatekeeping cell fate transitions potentially also beyond plant development.

DEFECTIVE KERNEL1 regulates cellulose synthesis and affects primary cell wall mechanics

The cell wall is one of the defining features of plants, controlling cell shape, regulating growth dynamics and hydraulic conductivity, as well as mediating plants interactions with both the external and internal environments. Here we report that a putative mechanosensitive Cys-protease DEFECTIVE KERNEL1 (DEK1) influences the mechanical properties of primary cell walls and regulation of cellulose synthesis. Our results indicate that DEK1 is an important regulator of cellulose synthesis in epidermal tissue of Arabidopsis thaliana cotyledons during early post-embryonic development. DEK1 is involved in regulation of cellulose synthase complexes (CSCs) by modifying their biosynthetic properties, possibly through interactions with various cellulose synthase regulatory proteins. Mechanical properties of the primary cell wall are altered in DEK1 modulated lines with DEK1 affecting both cell wall stiffness and the thickness of the cellulose microfibril bundles in epidermal cell walls of cotyledons.

Calpains in cyanobacteria and the origin of calpains

Calpains are cysteine proteases involved in many cellular processes. They are an ancient and large superfamily of enzymes responsible for the cleavage and irreversible modification of a large variety of substrates. They have been intensively studied in humans and other mammals, but information about calpains in bacteria is scarce. Calpains have not been found among Archaea to date. In this study, we have investigated the presence of calpains in selected cyanobacterial species using in silico analyses. We show that calpains defined by possessing CysPC core domain are present in cyanobacterial genera Anabaena, Aphanizomenon, Calothrix, Chamaesiphon, Fischerella, Microcystis, Scytonema and Trichormus. Based on in silico protein interaction analysis, we have predicted putative interaction partners for identified cyanobacterial calpains. The phylogenetic analysis including cyanobacterial, other bacterial and eukaryotic calpains divided bacterial and eukaryotic calpains into two separate monophyletic clusters. We propose two possible evolutionary scenarios to explain this tree topology: (1) the eukaryotic ancestor or an archaeal ancestor of eukaryotes obtained calpain gene from an unknown bacterial donor, or alternatively (2) calpain gene had been already present in the last common universal ancestor and subsequently lost by the ancestor of Archaea, but retained by the ancestor of Bacteria and by the ancestor of Eukarya. Both scenarios would require multiple independent losses of calpain genes in various bacteria and eukaryotes.

Characterising the mechanics of cell-cell adhesion in plants

Cell-cell adhesion is a fundamental feature of multicellular organisms. To ensure multicellular integrity, adhesion needs to be tightly controlled and maintained. In plants, cell-cell adhesion remains poorly understood. Here, we argue that to be able to understand how cell-cell adhesion works in plants, we need to understand and quantitatively measure the mechanics behind it. We first introduce cell-cell adhesion in the context of multicellularity, briefly explain the notions of adhesion strength, work and energy and present the current knowledge concerning the mechanisms of cell-cell adhesion in plants. Because still relatively little is known in plants, we then turn to animals, but also algae, bacteria, yeast and fungi, and examine how adhesion works and how it can be quantitatively measured in these systems. From this, we explore how the mechanics of cell adhesion could be quantitatively characterised in plants, opening future perspectives for understanding plant multicellularity.

With an Ear Up against the Wall: An Update on Mechanoperception in Arabidopsis

Cells interpret mechanical signals and adjust their physiology or development appropriately. In plants, the interface with the outside world is the cell wall, a structure that forms a continuum with the plasma membrane and the cytoskeleton. Mechanical stress from cell wall damage or deformation is interpreted to elicit compensatory responses, hormone signalling, or immune responses. Our understanding of how this is achieved is still evolving; however, we can refer to examples from animals and yeast where more of the details have been worked out. Here, we provide an update on this changing story with a focus on candidate mechanosensitive channels and plasma membrane-localized receptors.

Physcomitrium patens: A Single Model to Study Oriented Cell Divisions in 1D to 3D Patterning

Development in multicellular organisms relies on cell proliferation and specialization. In plants, both these processes critically depend on the spatial organization of cells within a tissue. Owing to an absence of significant cellular migration, the relative position of plant cells is virtually made permanent at the moment of division. Therefore, in numerous plant developmental contexts, the (divergent) developmental trajectories of daughter cells are dependent on division plane positioning in the parental cell. Prior to and throughout division, specific cellular processes inform, establish and execute division plane control. For studying these facets of division plane control, the moss Physcomitrium (Physcomitrella) patens has emerged as a suitable model system. Developmental progression in this organism starts out simple and transitions towards a body plan with a three-dimensional structure. The transition is accompanied by a series of divisions where cell fate transitions and division plane positioning go hand in hand. These divisions are experimentally highly tractable and accessible. In this review, we will highlight recently uncovered mechanisms, including polarity protein complexes and cytoskeletal structures, and transcriptional regulators, that are required for 1D to 3D body plan formation.

Prefoldins contribute to maintaining the levels of the spliceosome LSM2-8 complex through Hsp90 in Arabidopsis

Although originally identified as the components of the complex aiding the cytosolic chaperonin CCT in the folding of actins and tubulins in the cytosol, prefoldins (PFDs) are emerging as novel regulators influencing gene expression in the nucleus. Work conducted mainly in yeast and animals showed that PFDs act as transcriptional regulators and participate in the nuclear proteostasis. To investigate new functions of PFDs, we performed a co-expression analysis in Arabidopsis thaliana. Results revealed co-expression between PFD and the Sm-like (LSM) genes, which encode the LSM2–8 spliceosome core complex, in this model organism. Here, we show that PFDs interact with and are required to maintain adequate levels of the LSM2–8 complex. Our data indicate that levels of the LSM8 protein, which defines and confers the functional specificity of the complex, are reduced in pfd mutants and in response to the Hsp90 inhibitor geldanamycin. We provide biochemical evidence showing that LSM8 is a client of Hsp90 and that PFD4 mediates the interaction between both proteins. Consistent with our results and with the role of the LSM2–8 complex in splicing through the stabilization of the U6 snRNA, pfd mutants showed reduced levels of this snRNA and altered pre-mRNA splicing patterns.

Connected through the force: mechanical signals in plant development

As multicellular organisms, plants acquire characteristic shapes through a complex set of biological processes known as morphogenesis. Biochemical signalling underlies much of development, as it allows cells to acquire specific identities based on their position within tissues and organs. However, as growing physical structures, plants, and their constituent cells, also experience internal and external physical forces that can be perceived and can influence key processes such as growth, polarity, and gene expression. This process, which adds another layer of control to growth and development, has important implications for plant morphogenesis. This review provides an overview of recent research into the role of mechanical signals in plant development and aims to show how mechanical signalling can be used, in concert with biochemical signals, as a cue allowing cells and tissues to coordinate their behaviour and to add robustness to developmental processes.

Hitting the Wall-Sensing and Signaling Pathways Involved in Plant Cell Wall Remodeling in Response to Abiotic Stress

Plant cells are surrounded by highly dynamic cell walls that play important roles regulating aspects of plant development. Recent advances in visualization and measurement of cell wall properties have enabled accumulation of new data about wall architecture and biomechanics. This has resulted in greater understanding of the dynamics of cell wall deposition and remodeling. The cell wall is the first line of defense against different adverse abiotic and biotic environmental influences. Different abiotic stress conditions such as salinity, drought, and frost trigger production of Reactive Oxygen Species (ROS) which act as important signaling molecules in stress activated cellular responses. Detection of ROS by still-elusive receptors triggers numerous signaling events that result in production of different protective compounds or even cell death, but most notably in stress-induced cell wall remodeling. This is mediated by different plant hormones, of which the most studied are jasmonic acid and brassinosteroids. In this review we highlight key factors involved in sensing, signal transduction, and response(s) to abiotic stress and how these mechanisms are related to cell wall-associated stress acclimatization. ROS, plant hormones, cell wall remodeling enzymes and different wall mechanosensors act coordinately during abiotic stress, resulting in abiotic stress wall acclimatization, enabling plants to survive adverse environmental conditions.

Outside-in control - does plant cell wall integrity regulate cell cycle progression?

During recent years it has become accepted that plant cell walls are not inert objects surrounding all plant cells but are instead highly dynamic, plastic structures. They are involved in a large number of cell biological processes and contribute actively to plant growth, development and interaction with environment. Therefore, it is not surprising that cellular processes can control plant cell wall integrity (CWI) while, simultaneously, CWI can influence cellular processes. In yeast and animal cells such a bidirectional relationship also exists between the yeast/animal extracellular matrices and the cell cycle. In yeast, the CWI maintenance mechanism and a dedicated plasma membrane integrity checkpoint are mediating this relationship. Recent research has yielded insights into the mechanism controlling plant cell wall metabolism during cytokinesis. However, the knowledge regarding putative regulatory pathways controlling adaptive modifications in plant cell cycle activity in response to changes in the state of the plant cell wall are not yet identified. In this review, we summarize similarities and differences in regulatory mechanisms coordinating extracellular matrices and cell cycle activity in animal and yeast cells, discuss the available evidence supporting the existence of such a mechanism in plants and suggest that the plant CWI maintenance mechanism might also control cell cycle activity in plant cells.

Novel functions of the Arabidopsis transcription factor TCP5 in petal development and ethylene biosynthesis

The flowers of most dicotyledons have petals that, together with the sepals, initially protect the reproductive organs. Later during development petals are required to open the flower and to attract pollinators. This diverse set of functions demands tight temporal and spatial regulation of petal development. We studied the functioning of the Arabidopsis thaliana TCP5-like transcription factors (TFs) in petals. Overexpression of TCP5 in petal epidermal cells results in smaller petals, whereas tcp5 tcp13 tcp17 triple knockout lines have wider petals with an increased surface area. Comprehensive expression studies revealed effects of TCP5-like TFs on the expression of genes related to the cell cycle, growth regulation and organ growth. Additionally, the ethylene biosynthesis genes 1-amino-cyclopropane-1-carboxylate (ACC) synthase 2 (ACS2) and ACC oxidase 2 (ACO2) and several ETHYLENE RESPONSE FACTORS (ERFs) are found to be differentially expressed in TCP5 mutant and overexpression lines. Chromatin immunoprecipitation-quantitative PCR showed direct binding of TCP5 to the ACS2 locus in vivo. Ethylene is known to influence cell elongation, and the petal phenotype of the tcp5 tcp13 tcp17 mutant could be complemented by treatment of the plants with an ethylene pathway inhibitor. Taken together, this reveals a novel role for TCP5-like TFs in the regulation of ethylene-mediated petal development and growth.

The TCP4 transcription factor regulates trichome cell differentiation by directly activating GLABROUS INFLORESCENCE STEMS in Arabidopsis thaliana

Trichomes are the first cell type to be differentiated during the morphogenesis of leaf epidermis and serve as an ideal model to study cellular differentiation. Many genes involved in the patterning and differentiation of trichome cells have been studied over the past decades, and the majority of these genes encode transcription factors that specifically regulate epidermal cell development. However, the upstream regulators of these genes that link early leaf morphogenesis with cell type differentiation are less studied. The TCP proteins are the plant-specific transcription factors involved in regulating diverse aspects of plant development including lateral organ morphogenesis by modulating cell proliferation and differentiation. Here, we show that the miR319-regulated class II TCP proteins, notably TCP4, suppress trichome branching in Arabidopsis leaves and inflorescence stem by direct transcriptional activation of GLABROUS INFLORESCENCE STEMS (GIS), a known negative regulator of trichome branching. The trichome branch number is increased in plants with reduced TCP activity and decreased in the gain-of-function lines of TCP4. Biochemical analyses show that TCP4 binds to the upstream regulatory region of GIS and activates its expression. Detailed genetic analyses show that GIS and TCP4 work in same pathway and GIS function is required for TCP4-mediated regulation of trichome differentiation. Taken together, these results identify a role for the class II TCP genes in trichome differentiation, thus providing a connection between organ morphogenesis and cellular differentiation.

Arabidopsis DEFECTIVE KERNEL1 regulates cell wall composition and axial growth in the inflorescence stem

Axial growth in plant stems requires a fine balance between elongation and stem mechanical reinforcement to ensure mechanical stability. Strength is provided by the plant cell wall, the deposition of which must be coordinated with cell expansion and elongation to ensure that integrity is maintained during growth. Coordination of these processes is critical and yet poorly understood. The plant-specific calpain, DEFECTIVE KERNEL1 (DEK1), plays a key role in growth coordination in leaves, yet its role in regulating stem growth has not been addressed. Using plants overexpressing the active CALPAIN domain of DEK1 (CALPAIN OE) and a DEK1 knockdown line (amiRNA-DEK1), we undertook morphological, biochemical, biophysical, and microscopic analyses of mature inflorescence stems. We identify a novel role for DEK1 in the maintenance of cell wall integrity and coordination of growth during inflorescence stem development. CALPAIN OE plants are significantly reduced in stature and have short, thickened stems, while amiRNA-DEK1 lines have weakened stems that are unable to stand upright. Microscopic analyses of the stems identify changes in cell size, shape and number, and differences in both primary and secondary cell wall thickness and composition. Taken together, our results suggest that DEK1 influences primary wall growth by indirectly regulating cellulose and pectin deposition. In addition, we observe changes in secondary cell walls that may compensate for altered primary cell wall composition. We propose that DEK1 activity is required for the coordination of stem strengthening with elongation during axial growth.

The roles of the cuticle in plant development: organ adhesions and beyond

Cuticles, which are composed of a variety of aliphatic molecules, impregnate epidermal cell walls forming diffusion barriers that cover almost all the aerial surfaces in higher plants. In addition to revealing important roles for cuticles in protecting plants against water loss and other environmental stresses and aggressions, mutants with permeable cuticles show major defects in plant development, such as abnormal organ formation as well as altered seed germination and viability. However, understanding the mechanistic basis for these developmental defects represents a significant challenge due to the pleiotropic nature of phenotypes and the altered physiological status/viability of some mutant backgrounds. Here we discuss both the basis of developmental phenotypes associated with defects in cuticle function and mechanisms underlying developmental processes that implicate cuticle modification. Developmental abnormalities in cuticle mutants originate at early developmental time points, when cuticle composition and properties are very difficult to measure. Nonetheless, we aim to extract principles from existing data in order to pinpoint the key cuticle components and properties required for normal plant development. Based on our analysis, we will highlight several major questions that need to be addressed and technical hurdles that need to be overcome in order to advance our current understanding of the developmental importance of plant cuticles.

A mechanosensitive Ca2+ channel activity is dependent on the developmental regulator DEK1

Responses of cells to mechanical stress are thought to be critical in coordinating growth and development. Consistent with this idea, mechanically activated channels play important roles in animal development. For example, the PIEZO1 channel controls cell division and epithelial-layer integrity and is necessary for vascular development in mammals. In plants, the actual contribution of mechanoperception to development remains questionable because very few putative mechanosensors have been identified and the phenotypes of the corresponding mutants are rather mild. Here, we show that the Arabidopsis Defective Kernel 1 (DEK1) protein, which is essential for development beyond early embryogenesis, is associated with a mechanically activated Ca2+ current in planta, suggesting that perception of mechanical stress plays a critical role in plant development.

Cell wall biomechanics: a tractable challenge in manipulating plant cell walls 'fit for purpose'!

The complexity and recalcitrance of plant cell walls has contributed to the success of plants colonising land. Conversely, these attributes have also impeded progress in understanding the roles of walls in controlling and directing developmental processes during plant growth and also in unlocking their potential for biotechnological innovation. Recent technological advances have enabled the probing of how primary wall structures and molecular interactions of polysaccharides define their biomechanical (and hence functional) properties. The outputs have led to a new paradigm that places greater emphasis on understanding how the wall, as a biomechanical construct and cell surface sensor, modulates both plant growth and material properties. Armed with this knowledge, we are gaining the capacity to design walls 'fit for (biotechnological) purpose'!

Developing a 'thick skin': a paradoxical role for mechanical tension in maintaining epidermal integrity?

Plant aerial epidermal tissues, like animal epithelia, act as load-bearing layers and hence play pivotal roles in development. The presence of tension in the epidermis has morphogenetic implications for organ shapes but it also constantly threatens the integrity of this tissue. Here, we explore the multi-scale relationship between tension and cell adhesion in the plant epidermis, and we examine how tensile stress perception may act as a regulatory input to preserve epidermal tissue integrity and thus normal morphogenesis. From this, we identify parallels between plant epidermal and animal epithelial tissues and highlight a list of unexplored questions for future research.

Regulation of cell wall genes in response to DEFECTIVE KERNEL1 (DEK1)-induced cell wall changes

Defective Kernel1 (DEK1) is a plant-specific calpain involved in epidermis specification and maintenance. DEK1 regulation of the epidermal cell wall is proposed to be key to ensure tissue integrity and coordinated growth. Changes in the expression of DEK1 are correlated with changes in the expression of cell wall-related genes. For example, we have found that Lipid transfer protein 3 (LTP3), EXPANSIN 11 (EXP11), and an AP2 transcription factor (AP2TF) are misexpressed in plants with constitutively altered levels of DEK1 activity. RT-qPCR studies show that LTP3 and AP2TF may respond to a DEK1-generated signal whereas EXP11 is not altered immediately after dexamethasone induction of CALPAIN suggesting it is not in the direct signaling pathway downstream of DEK1. Our data suggest these genes are regulated by a feedback mechanism in response to DEK1-induced changes in the cell wall, and contribute to the phenotypes seen in plants with altered DEK1 expression.

Signaling in Early Maize Kernel Development

Developing the next plant generation within the seed requires the coordination of complex programs driving pattern formation, growth, and differentiation of the three main seed compartments: the embryo (future plant), the endosperm (storage compartment), representing the two filial tissues, and the surrounding maternal tissues. This review focuses on the signaling pathways and molecular players involved in early maize kernel development. In the 2 weeks following pollination, functional tissues are shaped from single cells, readying the kernel for filling with storage compounds. Although the overall picture of the signaling pathways regulating embryo and endosperm development remains fragmentary, several types of molecular actors, such as hormones, sugars, or peptides, have been shown to be involved in particular aspects of these developmental processes. These molecular actors are likely to be components of signaling pathways that lead to transcriptional programming mediated by transcriptional factors. Through the integrated action of these components, multiple types of information received by cells or tissues lead to the correct differentiation and patterning of kernel compartments. In this review, recent advances regarding the four types of molecular actors (hormones, sugars, peptides/receptors, and transcription factors) involved in early maize development are presented.

Proteomic Characterization of Differential Abundant Proteins Accumulated between Lower and Upper Epidermises of Fleshy Scales in Onion (Allium cepa L.) Bulbs

The onion (Allium cepa L.) is widely planted worldwide as a valuable vegetable crop. The scales of an onion bulb are a modified type of leaf. The one-layer-cell epidermis of onion scales is commonly used as a model experimental material in botany and molecular biology. The lower epidermis (LE) and upper epidermis (UE) of onion scales display obvious differences in microscopic structure, cell differentiation and pigment synthesis; however, associated proteomic differences are unclear. LE and UE can be easily sampled as single-layer-cell tissues for comparative proteomic analysis. In this study, a proteomic approach based on 2-DE and mass spectrometry (MS) was applied to compare LE and UE of fleshy scales from yellow and red onions. We identified 47 differential abundant protein spots (representing 31 unique proteins) between LE and UE in red and yellow onions. These proteins are mainly involved in pigment synthesis, stress response, and cell division. Particularly, the differentially accumulated chalcone-flavanone isomerase and flavone O-methyltransferase 1-like in LE may result in the differences in the onion scale color between red and yellow onions. Moreover, stress-related proteins abundantly accumulated in both LE and UE. In addition, the differential accumulation of UDP-arabinopyranose mutase 1-like protein and β-1,3-glucanase in the LE may be related to the different cell sizes between LE and UE of the two types of onion. The data derived from this study provides new insight into the differences in differentiation and developmental processes between onion epidermises. This study may also make a contribution to onion breeding, such as improving resistances and changing colors.

Cell adhesion in plants is under the control of putative O-fucosyltransferases

Cell-to-cell adhesion in plants is mediated by the cell wall and the presence of a pectin-rich middle lamella. However, we know very little about how the plant actually controls and maintains cell adhesion during growth and development and how it deals with the dynamic cell wall remodeling that takes place. Here we investigate the molecular mechanisms that control cell adhesion in plants. We carried out a genetic suppressor screen and a genetic analysis of cell adhesion-defective Arabidopsis thaliana mutants. We identified a genetic suppressor of a cell adhesion defect affecting a putative O-fucosyltransferase. Furthermore, we show that the state of cell adhesion is not directly linked with pectin content in the cell wall but instead is associated with altered pectin-related signaling. Our results suggest that cell adhesion is under the control of a feedback signal from the state of the pectin in the cell wall. Such a mechanism could be necessary for the control and maintenance of cell adhesion during growth and development.

Highlighted article: Pectin-related signaling, rather than absolute pectin levels, are proposed to contribute to cell adhesion during growth and development in Arabidopsis thaliana.

Calpain-Mediated Positional Information Directs Cell Wall Orientation to Sustain Plant Stem Cell Activity, Growth and Development

Eukaryotic development and stem cell control depend on the integration of cell positional sensing with cell cycle control and cell wall positioning, yet few factors that directly link these events are known. The DEFECTIVE KERNEL1 (DEK1) gene encoding the unique plant calpain protein is fundamental for development and growth, being essential to confer and maintain epidermal cell identity that allows development beyond the globular embryo stage. We show that DEK1 expression is highest in the actively dividing cells of seeds, meristems and vasculature. We further show that eliminating Arabidopsis DEK1 function leads to changes in developmental cues from the first zygotic division onward, altered microtubule patterns and misshapen cells, resulting in early embryo abortion. Expression of the embryonic marker genes WOX2, ATML1, PIN4, WUS and STM, related to axis organization, cell identity and meristem functions, is also altered in dek1 embryos. By monitoring cell layer-specific DEK1 down-regulation, we show that L1- and 35S-induced down-regulation mainly affects stem cell functions, causing severe shoot apical meristem phenotypes. These results are consistent with a requirement for DEK1 to direct layer-specific cellular activities and set downstream developmental cues. Our data suggest that DEK1 may anchor cell wall positions and control cell division and differentiation, thereby balancing the plant's requirement to maintain totipotent stem cell reservoirs while simultaneously directing growth and organ formation. A role for DEK1 in regulating microtubule-orchestrated cell wall orientation during cell division can explain its effects on embryonic development, and suggests a more general function for calpains in microtubule organization in eukaryotic cells.

Communication is key: Reducing DEK1 activity reveals a link between cell-cell contacts and epidermal cell differentiation status

Plant epidermis development requires not only the initial acquisition of tissue identity, but also the ability to differentiate specific cell types over time and to maintain these differentiated states throughout the plant life. To set-up and maintain differentiation, plants activate specific transcriptional programs. Interfering with these programs can prevent differentiation and/or force differentiated cells to lose their identity and re-enter a proliferative state. We have recently shown that the Arabidopsis Defective Kernel 1 (DEK1) protein is required both for the differentiation of epidermal cells and for the maintenance of their fully differentiated state. Defects in DEK1 activity lead to a deregulation of the expression of epidermis-specific differentiation–promoting HD-ZIP IV transcription factors. Here we propose a working model in which DEK1, by maintaining cell-cell contacts, and thus communication between neighboring cells, influences HD-ZIP IV gene expression and epidermis differentiation.