The people behind the papers - Kenji Nagata and Mitsutomo Abe.

The plant epidermis is a single layer of cells that forms a crucial barrier to the outside world, but the mechanisms that control epidermal differentiation - in particular the relative importance of position and lineage - remain incompletely understood. A new paper in Development tackles this question in Arabidopsis To find out more about the story, we caught up with first author Kenji Nagata and his supervisor Mitsutomo Abe, Associate Professor at the University of Tokyo.

Induced cell fate transitions at multiple cell layers configure haustorium development in parasitic plants.

The haustorium in parasitic plants is an organ specialized for invasion and nutrient uptake from host plant tissues. Despite its importance, the developmental processes of haustoria are mostly unknown. To understand the dynamics of cell fate change and cellular lineage during haustorium development, we performed live imaging-based marker expression analysis and cell-lineage tracing during haustorium formation in the model facultative root parasite Phtheirospermum japonicum Our live-imaging analysis revealed that haustorium formation was associated with induction of simultaneous cell division in multiple cellular layers, such as epidermis, cortex and endodermis. In addition, we found that procambium-like cells, monitored by cell type-specific markers, emerged within the central region of the haustorium before xylem connection to the host plant. Our clonal analysis of cell lineages showed that cells in multiple cellular layers differentiated into procambium-like cells, whereas epidermal cells eventually transitioned into specialized cells interfacing with the host plant. Thus, our data provide a cell fate transition map during de novo haustorium organogenesis in parasitic plants.

Developmental patterning of the sub-epidermal integument cell layer in Arabidopsis seeds.

Angiosperm seed development is a paradigm of tissue cross-talk. Proper seed formation requires spatial and temporal coordination of the fertilization products - embryo and endosperm - and the surrounding seed coat maternal tissue. In early Arabidopsis seed development, all seed integuments were thought to respond homogenously to endosperm growth. Here, we show that the sub-epidermal integument cell layer has a unique developmental program. We characterized the cell patterning of the sub-epidermal integument cell layer, which initiates a previously uncharacterized extra cell layer, and identified TRANSPARENT TESTA 16 and SEEDSTICK MADS box transcription factors as master regulators of its polar development and cell architecture. Our data indicate that the differentiation of the sub-epidermal integument cell layer is insensitive to endosperm growth alone and to the repressive mechanism established by FERTILIZATION INDEPENDENT ENDOSPERM and MULTICOPY SUPPRESSOR OF IRA1 Polycomb group proteins. This work demonstrates the different responses of epidermal and sub-epidermal integument cell layers to fertilization.

MpWIP regulates air pore complex development in the liverwort Marchantia polymorpha.

The colonisation of the land by plants was accompanied by the evolution of complex tissues and multicellular structures comprising different cell types as morphological adaptations to the terrestrial environment. Here, we show that the single WIP protein in the early-diverging land plant Marchantia polymorpha L. is required for the development of the multicellular gas exchange structure: the air pore complex. This 16-cell barrel-shaped structure surrounds an opening between epidermal cells that facilitates the exchange of gases between the chamber containing the photosynthetic cells inside the plant and the air outside. MpWIP is expressed in cells of the developing air pore complex and the morphogenesis of the complex is defective in plants with reduced MpWIP function. The role of WIP proteins in the control of different multicellular structures in M. polymorpha and the flowering plant Arabidopsis thaliana suggests that these proteins controlled the development of multicellular structures in the common ancestor of land plants. We hypothesise that WIP genes were subsequently co-opted in the control of morphogenesis of novel multicellular structures that evolved during the diversification of land plants.

WUSCHEL-RELATED HOMEOBOX 2 is important for protoderm and suspensor development in the gymnosperm Norway spruce.

BACKGROUND: Distinct expression domains of WUSCHEL-RELATED HOMEOBOX (WOX) gene family members are involved in patterning and morphogenesis of the early embryo in Arabidopsis. However, the role of WOX genes in other taxa, including gymnosperms, remains elusive. Here, we use somatic embryos and reverse genetics for studying expression and function of PaWOX2, the corresponding homolog of AtWOX2 in the gymnosperm Picea abies (Pa; Norway spruce). RESULTS: The mRNA level of PaWOX2 was transiently up-regulated during early and late embryogeny. PaWOX2 mRNA in early and early late embryos was detected both in the embryonal mass and in the upper part of the suspensor. Down-regulation of PaWOX2 during development of early embryos resulted in aberrant early embryos, which failed to form a proper protoderm. Cells on the surface layer of the embryonal mass became vacuolated, and new embryogenic tissue differentiated from the embryonal mass. In addition, the aberrant early embryos lacked a distinct border between the embryonal mass, and the suspensor and the length of the suspensor cells was reduced. Down-regulation of PaWOX2 in the beginning of embryo development, before late embryos were formed, caused a significant decrease in the yield of mature embryos. On the contrary, down-regulation of PaWOX2 after late embryos were formed had no effect on further embryo development and maturation. CONCLUSIONS: Our data suggest an evolutionarily conserved function of WOX2 in protoderm formation early during embryo development among seed plants. In addition, PaWOX2 might exert a unique function in suspensor expansion in gymnosperms.

Epidermal identity is maintained by cell-cell communication via a universally active feedback loop in Arabidopsis thaliana.

The transcription factors ARABIDOPSIS THALIANA MERISTEM L1 (ATML1) and PROTODERMAL FACTOR2 (PDF2) are indispensable for epidermal cell-fate specification in Arabidopsis embryos. However, the mechanisms of regulation of these genes, particularly their relationship with cell-cell signalling pathways, although the subject of considerable speculation, remain unclear. Here we demonstrate that the receptor kinase ARABIDOPSIS CRINKLY4 (ACR4) positively affects the expression of ATML1 and PDF2 in seedlings. In contrast, ATML1- and PDF2-containing complexes directly and negatively affect both their own expression and that of ACR4. By modelling the resulting feedback loop, we demonstrate a network structure that is capable of maintaining robust epidermal cell identity post-germination. We show that a second seed-specific signalling pathway involving the subtilase ABNORMAL LEAFSHAPE1 (ALE1) and the receptor kinases GASSHO1 (GSO1) and GASSHO2 (GSO2) acts in parallel to the epidermal loop to control embryonic surface formation via an ATML1/PDF2-independent pathway. Genetic interactions between components of this linear pathway and the epidermal loop suggest that an intact embryo surface is necessary for initiation and/or stabilization of the epidermal loop, specifically during early embryogenesis.

A chromatin link that couples cell division to root epidermis patterning in Arabidopsis.

Cell proliferation and cell fate decisions are strictly coupled processes during plant embryogenesis and organogenesis. In the Arabidopsis thaliana root epidermis, expression of the homeobox GLABRA2 (GL2) gene determines hair/non-hair cell fate. This requires signalling of positional information from the underlying cortical layer, complex transcriptional regulation and a change in chromatin accessibility. However, the molecular connections among these factors and with cell division are not known. Here we have identified a GL2-expression modulator, GEM, as an interactor of CDT1, a DNA replication protein. GEM also interacts with TTG1 (TRANSPARENT TESTA GLABRA1), a WD40-repeat protein involved in GL2-dependent cell fate decision, and modulates both cell division and GL2 expression. Here we show that GEM participates in the maintenance of the repressor histone H3K9 methylation status of root patterning genes, providing a link between cell division, fate and differentiation during Arabidopsis root development.

Emergence and patterning of the five cell types of the Zea mays anther locule.

One fundamental difference between plants and animals is the existence of a germ-line in animals and its absence in plants. In flowering plants, the sexual organs (stamens and carpels) are composed almost entirely of somatic cells, a small subset of which switch to meiosis; however, the mechanism of meiotic cell fate acquisition is a long-standing botanical mystery. In the maize (Zea mays) anther microsporangium, the somatic tissues consist of four concentric cell layers that surround and support reproductive cells as they progress through meiosis and pollen maturation. Male sterility, defined as the absence of viable pollen, is a common phenotype in flowering plants, and many male sterile mutants have defects in somatic and reproductive cell fate acquisition. However, without a robust model of anther cell fate acquisition based on careful observation of wild-type anther ontogeny, interpretation of cell fate mutants is limited. To address this, the pattern of cell proliferation, expansion, and differentiation was tracked in three dimensions over 30 days of wild-type (W23) anther development, using anthers stained with propidium iodide (PI) and/or 5-ethynyl-2'-deoxyuridine (EdU) (S-phase label) and imaged by confocal microscopy. The pervading lineage model of anther development claims that new cell layers are generated by coordinated, oriented cell divisions in transient precursor cell types. In reconstructing anther cell division patterns, however, we can only confirm this for the origin of the middle layer (ml) and tapetum, while young anther development appears more complex. We find that each anther cell type undergoes a burst of cell division after specification with a characteristic pattern of both cell expansion and division. Comparisons between two inbreds lines and between ab- and adaxial anther florets indicated near identity: anther development is highly canalized and synchronized. Three classical models of plant organ development are tested and ruled out; however, local clustering of developmental events was identified for several processes, including the first evidence for a direct relationship between the development of ml and tapetal cells. We speculate that small groups of ml and tapetum cells function as a developmental unit dedicated to the development of a single pollen grain.

AhNRAMP1 iron transporter is involved in iron acquisition in peanut.

Peanut/maize intercropping is a sustainable and effective agroecosystem to alleviate iron-deficiency chlorosis. Using suppression subtractive hybridization from the roots of intercropped and monocropped peanut which show different iron nutrition levels, a peanut gene, AhNRAMP1, which belongs to divalent metal transporters of the natural resistance-associated macrophage protein (NRAMP) gene family was isolated. Yeast complementation assays suggested that AhNRAMP1 encodes a functional iron transporter. Moreover, the mRNA level of AhNRAMP1 was obviously induced by iron deficiency in both roots and leaves. Transient expression, laser microdissection, and in situ hybridization analyses revealed that AhNRAMP1 was mainly localized on the plasma membrane of the epidermis of peanut roots. Induced expression of AhNRAMP1 in tobacco conferred enhanced tolerance to iron deprivation. These results suggest that the AhNRAMP1 is possibly involved in iron acquisition in peanut plants.

[Structural and cytochemical aspects of Brassica rapa embryogenesis under conditions of clinorotation].

The results of study of embryo development in B. rapa plants, as well as the rate and the character of nutrient substances accumulation in their cells under slow horizontal clinorotation and laboratory control are presented. Significant similarity of the peculiarities of embryo differentiation and character of nutrient substance accumulation in both variants was shown. The cases of different deviations during embryo differentiation, and rate and quantity of reserve nutrient substances in their cells are revealed under clinorotation compared to the laboratory control.

GASSHO1 and GASSHO2 encoding a putative leucine-rich repeat transmembrane-type receptor kinase are essential for the normal development of the epidermal surface in Arabidopsis embryos.

Receptor-like kinases (RLKs) containing leucine-rich repeats (LRRs) act as both signal receptor and signal transducer in ligand-mediated communication between cells. It is believed that many LRR-RLKs are present in the Arabidopsis genome, but the functions of most are unknown. We recently identified Bnms4D-82, an expressed sequence tag (EST) in Brassica napus that encodes an LRR-RLK and is expressed at an early stage of its microspore embryogenesis. To elucidate the function of this gene we used GASSHO1 (GSO1) and GSO2, two Arabidopsis genes with a high degree of homology with Bnms4D-82. The products of transcripts of GSO1 and GSO2 accumulate in parts of the embryo and in seedlings, but not in true leaves. Plants that lacked both GSO1 and GSO2 exhibited pleiotropy, including abnormal bending of embryos, ectopic adhesion between cotyledons, a highly permeable epidermal structure, and an abnormal pattern of distribution of stomata on cotyledons in embryos and seedlings. However, plants homozygous for either gso1-1 or gso2-1 had no visible abnormality. These results suggest that GASSHO genes are essential for the formation of a normal epidermal surface during embryogenesis.

Alpha-glucosidase I is required for cellulose biosynthesis and morphogenesis in Arabidopsis.

Novel mutations in the RSW1 and KNOPF genes were identified in a large-scale screen for mutations that affect cell expansion in early Arabidopsis embryos. Embryos from both types of mutants were radially swollen with greatly reduced levels of crystalline cellulose, the principal structural component of the cell wall. Because RSW1 was previously shown to encode a catalytic subunit of cellulose synthase, the similar morphology of knf and rsw1-2 embryos suggests that the radially swollen phenotype of knf mutants is largely due to their cellulose deficiency. Map-based cloning of the KNF gene and enzyme assays of knf embryos demonstrated that KNF encodes alpha-glucosidase I, the enzyme that catalyzes the first step in N-linked glycan processing. The strongly reduced cellulose content of knf mutants indicates that N-linked glycans are required for cellulose biosynthesis. Because cellulose synthase catalytic subunits do not appear to be N glycosylated, the N-glycan requirement apparently resides in other component(s) of the cellulose synthase machinery. Remarkably, cellular processes other than extracellular matrix biosynthesis and the formation of protein storage vacuoles appear unaffected in knf embryos. Thus in Arabidopsis cells, like yeast, N-glycan trimming is apparently required for the function of only a small subset of N-glycoproteins.

Creating a two-dimensional pattern de novo during Arabidopsis trichome and root hair initiation.

During plant epidermal differentiation, root hairs and leaf hairs (trichomes) become specified in a regular pattern. Although the underlying mechanisms appear to be different in that the position of root hairs is determined by their position with respect to the underlying cortical cells and that of the trichomes appears to be generated de novo, a common set of genes was found to operate in both systems. A complex of transcription factors appears to be involved in creating the pattern and cell-cell movement of small transcription factors is postulated to mediate cell-cell communication.

Growing up green: cellular basis of plant development.

Plant development is a biphasic process. Pattern formation during embryogenesis generates a basic body organisation, including self-maintaining stem-cell systems called meristems at opposite ends of the main axis of polarity. During post-embryonic development, the meristems produce new organs with reference to the existing body, transforming the juvenile seedling into the species-specific adult plant. Studies in Arabidopsis indicate that patterning in plants involves not only cell surface interactions but also unique modes of communication such as movement of transcription factors between cells and directional transport of the signaling molecule auxin.

TORNADO1 regulates root epidermal patterning through the WEREWOLF pathway in Arabidopsis thaliana.

Cell fate in the root epidermis of Arabidopsis thaliana is determined in a position-dependent manner. SCRAMBLED (SCM), an atypical leucine-rich repeat receptor-like kinase, mediates this positional regulation via its effect on WEREWOLF (WER) expression, and subsequently, its downstream transcription factor, GLABRA2 (GL2), which are required for nonhair cell development. Previously, TORNADO1 (TRN1), a plant-specific protein with a leucine-rich repeat ribonuclease inhibitor-like domain, was shown to be required for proper epidermal patterning in Arabidopsis roots. In this work, we analyzed the possible involvement of TRN1 in the known root epidermal gene network. We discovered that the trn1 mutant caused the ectopic expression of WER and the randomized expression of GL2 and EGL3. This suggests that TRN1 regulates the position-dependent cell fate determination by affecting WER expression in Arabidopsis root epidermis. Additionally, the distinct phenotypes of the aerial parts of the trn1-t and scm-2 mutant suggest that TRN1 and SCM might have different functions in the development of aerial parts.

Embryonic cuticle establishment: the great (apoplastic) divide.

The plant cuticle, a dynamic interface between plants and their environment, is formed by the secretion of hydrophobic lipids and waxes into the outer wall of aerial epidermal cells. Cuticle formation is such a ubiquitous feature of epidermal cells, and is of such fundamental importance for plant survival, that identifying and understanding specific developmental roles for this structure has been a major challenge for plant scientists. In recent work, we have tried to understand the functional relationships between a signaling feedback loop required for epidermal cell specification in developing plant embryos, and a seed specific signaling cascade, involving components localized both in the embryo and in the embryo surrounding endosperm, and necessary for embryo cuticle function. Analysis of the strongly synergistic genetic relationships between these 2 independent pathways, combined with mathematical simulations of the behavior of the signaling feedback loop, have allowed us to propose an important, and hitherto unsuspected, role for the embryonic cuticle as an apoplastic diffusion barrier, necessary for preventing the excessive diffusion of developmentally important signaling molecules away from developing embryo into surrounding tissues.

The patterning of epidermal hairs in Arabidopsis--updated.

Epidermal hairs of Arabidopsis thaliana emerge in regular spacing patterns providing excellent model systems for studies of biological pattern formation. A number of root-hair and leaf-trichome patterning mutants and tools for cell-specific and tissue-specific manipulation of patterning protein activities have been combined in cycles of experimentation and mathematical modelling. These approaches have provided insight into molecular mechanisms of epidermal patterning. During the last two years, endoreplication has, unexpectedly, been found to control cell-fate maintenance during trichome patterning. New genetic interactions between a downstream, positive transcriptional regulator and lateral inhibitors of trichome or non-root-hair fate specification have been uncovered. A lateral inhibitor and a new positive regulator have been identified as major loci affecting trichome patterning in natural Arabidopsis populations. Finally, factors that modify root-hair patterning from the underlying cell layer have been discovered.

From priming to plasticity: the changing fate of rhizodermic cells.

The fate of root epidermal cells is controlled by a complex interplay of transcriptional regulators, generating a genetically determined, position-biased arrangement of root hair cells. This pattern is altered during postembryonic development and in response to environmental signals to confer developmental plasticity that acclimates the plant to the prevailing conditions. Based on the hypothesis that events downstream of this initial mechanism can modulate the pattern installed during embryogenesis, we have developed a reaction diffusion model that reproduces the root hair patterning previously observed experimentally. Under all growth conditions, an almost equal spacing between root hair forming cells was observed both in vitro and in silico, indicating that long-range intercellular communication is crucial for the trichoblasts' decision to form a root hair. We assume that a hair growth promoter (HGP) is upregulated in root-hair-forming cells by a trichoblast-specific component. Once established, HGP production is self-enhancing. The autocatalytic regulation of HGP is antagonized by an HGP-produced hair growth inhibitor (HGI). HGI is exported from trichoblasts and diffuses to neighboring cells, where it inhibits further HGP production and promotes the non-hair cell fate. Under conditions of phosphate deficiency, we hypothesise that HGP production is increased and HGI diffusion rate is reduced, leading to a position-independent formation of extra root hairs.

The endosperm-specific ZHOUPI gene of Arabidopsis thaliana regulates endosperm breakdown and embryonic epidermal development.

During Arabidopsis seed development, the growing embryo invades and consumes the surrounding endosperm tissue. The signalling pathways that coordinate the separation of the embryo from the endosperm and the concomitant breakdown of the endosperm are poorly understood. We have identified a novel bHLH transcription factor, ZHOUPI (ZOU), which mediates these processes. ZOU is expressed exclusively in the endosperm of developing seeds. It is activated in the central cell immediately after fertilization and is initially expressed uniformly in endosperm, subsequently resolving to the embryo surrounding region (ESR). However, zou mutant embryos have defects in cuticle formation and in epidermal cell adhesion, suggesting that ZOU functions non-autonomously to regulate embryonic development. In addition, the endosperm of zou mutant seeds fails to separate from the embryo, restricting embryo expansion and resulting in the production of shrivelled collapsed seeds. zou seeds retain more endosperm than do wild-type seeds at maturity, suggesting that ZOU also controls endosperm breakdown. We identify several target genes whose expression in the ESR is regulated by ZOU. These include ABNORMAL LEAF SHAPE1, which encodes a subtilisin-like protease previously shown to have a similar role to ZOU in regulating endosperm adhesion and embryonic epidermal development. However, expression of several other ESR-specific genes is independent of ZOU. Therefore, ZOU is not a general regulator of endosperm patterning, but rather controls specific signalling pathways that coordinate embryo invasion and breakdown of surrounding endosperm tissues.