Radial Patterning Research Articles

Sonic Hedgehog Mediator Gli2 Regulates Bladder Mesenchymal Patterning

Congenital bladder anomalies are a major challenge to pediatric urologists. Understanding the mechanism of bladder development is crucial for advancing patient treatment. Current evidence suggests that Shh (R&D Systems) is an epithelial signal regulating bladder development, although the mechanism of the regulation is still unclear. We examined the regulation of bladder mesenchymal development. Mutation analysis, immunohistochemistry, immunoblot, in situ hybridization, and primary cell culture and transfection were performed. The mesenchyma proximal to the epithelium was defined as the inner zone and that distal to the epithelium was defined as the outer zone. We found that the Shh transcriptional factor Gli2 and the Shh target gene Bmp4 (R&D Systems) were expressed in the inner mesenchymal zone of the bladder, where active cell proliferation was observed. In Gli2(-/-) bladder primary mesenchymal cell cultures transfection with adenoviruses expressing DeltaNGli2, a constitutionally active form of Gli2, up-regulated Bmp4 expression and promoted cell proliferation. In the outer mesenchymal zone, where Gli2 and Bmp4 expression was not detectable, smooth muscle alpha-actin was expressed. In Gli2(-/-) embryo bladders Bmp4 expression in the inner zone was lost and ectopic smooth muscle was detected in the inner mesenchymal zone. Exogenous Bmp4 (10 ng/ml) in primary smooth muscle cell culture repressed smooth muscle differentiation and repression was partially rescued by the Bmp4 antagonist Noggin (R&D Systems) (300 ng/ml). Our data suggests that the Shh transcriptional factor Gli2 regulates radial patterning of the bladder mesenchyma.

Cloning of Pinus sylvestris SCARECROW gene and its expression pattern in the pine root system, mycorrhiza and NPA‐treated short roots

The SCARECROW (SCR) gene is central to root radial patterning. Its expression has not been investigated in conifers with morphologically different root types. Additional interest in SCR functions in the Pinus sylvestris root system comes from the effect of ectomycorrhiza formation on the short root apical structure. Here, the P. sylvestris SCR gene (PsySCR) was cloned and its expression investigated by northern blot and in situ hybridization of primary, lateral and short roots and mycorrhiza. Short root dichotomization was induced by auxin transport inhibitor (N-1-naphthylphthalamic acid (NPA)). PsySCR has conserved GRAS family protein motifs at the C-terminus and a variable N-terminus. PsySCR expression occurred in young root tissue and mycorrhiza. In root sections the PsySCR signal runs through the tip in initials for stele and root cap column and becomes upwards-restricted to endodermis in all root types. The PsySCR expression pattern suggests for the first time a regulatory role for SCR in maintaining the endodermal characteristics and radial patterning of roots with open meristem organization. The specific PsySCR localization is also an excellent marker for investigation of the dichotomization process in short roots.

Cortical Ventricular Zone Progenitors and Their Progeny Maintain Spatial Relationships and Radial Patterning during Preplate Development Indicating an Early Protomap

The graded expression of transcription factors by progenitors in the ventricular zone (VZ) confers positional or area identity that is inherited by subplate (SP) neurons and governs their expression of guidance molecules for thalamocortical axons and other properties required for cortical area specification. This mechanism would be most efficient if VZ progenitors and their SP neuronal progeny maintain neighbor relationships during the generation of the preplate (PP), the precursor of the SP. Therefore, a major goal of this study is to determine whether progenitors in the cortical VZ and their progeny maintain neighbor relationships during the genesis of the neocortical PP. We used time-lapse video microscopy to follow the movements of VZ progenitors and the radial movement of their progeny and distribution in the PP in whole-mount or slice cortical explants from embryonic rats at stages when PP neurons are generated. We show that labeled VZ cells proliferate and have a strong tendency to retain neighbor relationships within the VZ and that their neuronal progeny move superficially along a radial column to form the overlying PP; during this process, their neuronal progeny also retain neighbor relationships and thereby form the PP in spatial register with the VZ progenitors that generate them. This behavior differs from that reported at later stages of cortical development, when cortical plate (CP) neurons are generated, and considerable dispersion is evident among both cells within the VZ and neuronal progeny as they migrate from the VZ to the CP. However, our findings show that at the early stage of cortical development, when PP/SP neurons are generated, the VZ is, at a cellular level, a "protomap" of the PP/SP.

Global Analysis of a Key Developmental Pathway in Plants

Stem cells are most often associated with mammalian development, but plants have them too. With a modular architecture that allows ongoing replacement of new stems, leaves, roots, and flowers, plants escape the debilitating effects of aging that other multicellular organisms endure. The raw materials for regeneration come from reserves of stem cells—sequestered in two meristems, one for shoots and another for roots—that adult plants draw on throughout their lives. The root meristem consists of the quiescent center (QC)—a group of four cells that maintain neighboring cells as stem cells—and the undifferentiated “initial cells” that give rise to the concentrically organized cylindrical root tissues: the epidermis, ground tissue (made up of the cortex and endodermis), and stele (pericycle and vascular cylinder). Initial cell divisions are asymmetrical, resulting in one renewed initial cell and a daughter cell that differentiates. A key regulator of root development, the transcription factor SHORT-ROOT (SHR) acts in the QC and endodermis to regulate stem-cell specification and radial patterning. The genetic workhorse of plant biology Arabidopsis thaliana has shed considerable light on the molecular pathways controlling these processes, yet many details remain obscure. In a new study, Mitch Levesque, Teva Vernoux, Philip Benfey, and colleagues focused on SHR to better understand its role in root development and identify those genes that are directly controlled by it. Before their study, only one of SHR's gene targets, SCARECROW (SCR), had been identified. The researchers also demonstrate the value of applying meta-analysis—a standard statistical approach used in many other fields to integrate and interpret the results of multiple independent studies—to the analysis of transcriptional networks in development. To identify the targets of a transcription factor, researchers typically alter their activity and then analyze genome-wide transcription with microarray analysis—an approach that proves cumbersome in multicellular organisms, where genes are often expressed in different cells at different times. Meta-analysis can overcome this problem, Levesque et al. argue, because it can detect subtle patterns in larger datasets that might be overlooked in smaller ones. Using this approach, the researchers identified eight direct SHR transcriptional targets, including SCR, as well as a long list of indirect targets involved in cell signaling and hormonal responses. They also revealed a new function for the transcription factor. First, they constructed a form of SHR that allowed them to exert precise control over its temporal and spatial expression by administering a synthetic hormone called dexamethasone (Dex). Plants lacking SHR ( shr-2 mutants) have much shorter roots and defective radial patterning. But shr-2 mutants bred to express the SHR construct had normal root growth, which the researchers attribute to restored stem-cell activity. By crossing this strain with another engineered to express a fluorescent protein upon SCR transcription, the researchers could predict when SHR targets were expressed after Dex treatment. Then, adding a compound (called cycloheximide) expected to block expression of genes that act further downstream in a pathway, they demonstrated that SHR directly targets SCR. To identify other direct SHR targets, the researchers altered SHR activity in their transgenic shr-2:SHR mutants using three different experimental treatments, then collected transcriptional profiles from the root tips of five-day-old plants. The meta-analysis of the three microarray datasets identified eight candidate targets—four of which can bind to SHR in plant cells. Which is not to say that SHR can't bind to the other four genes, only that these assays did not provide that evidence. Future studies can resolve this question. All eight candidate targets were expressed to some degree in cell types expressing SHR. Three target genes were expressed in stele tissues, the root's vascular tissue. Levesque et al. found that the stele was narrower and had fewer initial cells in shr-2 mutants, demonstrating a new role for SHR in stele differentiation and development. The researchers go on to show that hundreds of indirect target genes are either activated or repressed in response to SHR activity and that many of these genes are involved in cell or hormonal signaling pathways. They plan to investigate the functional significance of these genes in future experiments. Overall, these results suggest that SHR directly activates SCR, and in doing so influences QC specification and asymmetric cell division. It does not work alone, however, since simply adding SCR to shr-2 mutants did not correct defects associated with these processes. SHR operates in at least five regions, Levesque et al. conclude: the QC, early and late endodermis, and early and late stele. SHR controls root development, they propose, by coordinating overlapping transcription, signaling, and hormonal pathways. The product of these interactions determines how SHR influences stem-cell niche specification, radial patterning, and stele development. Functional analysis of the different targets will help the researchers test the validity of their model.

Stomach Development Is Dependent on Fibroblast Growth Factor 10/Fibroblast Growth Factor Receptor 2b–Mediated Signaling

Fibroblast growth factors (Fgfs) and their receptors (Fgfrs) are important intercellular signaling molecules that are essential to mammalian embryonic development. The signaling pathways between endoderm-derived gastric epithelium and the surrounding mesenchyme are largely unknown; however, the developmental expression profile of the IIIb isoform of Fgfr2 (Fgfr2b) and its main ligand, Fgf10, suggest that they may be strong candidates. Mice lacking either component (Fgfr2b-/- or Fgf10-/-) were examined to determine the role of Fgfr2b-mediated signaling during gastric organogenesis. Stomachs from embryonic day 13.5-18.5 Fgfr2b-/-, Fgf10-/-, and wild-type littermates were collected and analyzed by conventional histology, immunohistochemistry, in situ hybridization, and electron microscopy. Fgfr2b-/- and Fgf10-/- fetuses had stomachs smaller than wild-type, consisting of relatively proportionate forestomach but disproportionately reduced glandular stomach, the mucosa of which has low cytoarchitectural complexity with a spiral arrangement of large mucosal folds. During mid to late fetal stages (embryonic day 15.5-18.5), epithelial differentiation to mucous and chief cell lineages was rudimentary, with no expression of several early cytodifferentiation markers including GATA4, GATA6, and H+/K+-adenosine triphosphatase and abnormal expression of members of the hedgehog family of signaling molecules. Fgfr2b and Fgf10 are part of a signaling network with Sonic hedgehog and Indian hedgehog that are essential to anterior-posterior and radial patterning in gastric development.

Going in circles: conserved mechanisms control radial patterning in the urinary and digestive tracts

Radial patterning in the urinary tract and gut depends on reciprocal signaling between epithelial cells, which form mucosa, and mesenchyme, which forms smooth muscle and connective tissue. These interactions depend on sonic hedgehog (Shh), which is secreted by epithelial cells and induces expression of bone morphogenetic protein 4 (Bmp4), a signaling molecule required for differentiation of smooth muscle progenitors. Patterning of the specialized mucosa lining the anterior-posterior (A-P) axis may be controlled independently by regionally expressed mesenchymal transcription factors. A study by Airik et al. in this issue of the JCI reveals that T-box 18 (Tbx18), a transcription factor selectively expressed in ureteral mesenchyme, regulates smooth muscle differentiation by maintaining Shh1 responsiveness in mesenchymal progenitors (see the related article beginning on page 663). Deletion of Tbx18 resulted in defective urothelial differentiation at the level of the ureter, suggesting that Tbx18 acts via mesenchyme as an important regulator of A-P patterning in the urinary tract.

Intercellular movement of transcription factors

Intercellular communication by the direct trafficking of transcription factors has been reported in plant developmental events such as root radial or epidermal cell patterning and shoot organogenesis. Investigations of this novel communication system have just begun and have highlighted the structural requirements for and mechanisms of transcription factor movement. Early studies suggest that plants employ both targeted (selective) and non-targeted (non-selective) intercellular movement of transcription factors. Factors that affect the intercellular movement of transcription factors through plasmodesmata have been explored.

Understanding Supported Reactions in Spherical Compartments: A General Algorithm To Model and Determine Rate Constants, Diffusion Coefficients, and Spatial Product Distributions

A general algorithm allowing the numerical modeling of the time and space dependence of product formation in spherical reaction volumes is described. The algorithm is described by the complete set of mass balance equations. On the basis of these equations, the effects of the diffusion coefficient, reaction rate, bead size, reagent excess, and packing density of the resin beads on the overall reaction rates are determined for second-order reactions. Experimental data of reaction progress are employed to calculate reaction rates and diffusion coefficients in polymer-supported reactions. In addition, the conditions for shell-like product formation are determined, and various strategies for the radial patterning of resin beads are compared. The effect of diffusion on polymer-supported enzyme-catalyzed reactions of the Michaelis-Menten type is treated, as well. Finally, the effects of typical nonideal solid-phase phenomena, namely, the inhomogeneity of rate constants and the concentration dependence of diffusion coefficients, on overall rates are discussed.

The xylem and phloem transcriptomes from secondary tissues of the Arabidopsis root-hypocotyl.

The growth of secondary xylem and phloem depends on the division of cells in the vascular cambium and results in an increase in the diameter of the root and stem. Very little is known about the genetic mechanisms that control cambial activity and the differentiation of secondary xylem and phloem cell types. To begin to identify new genes required for vascular cell differentiation and function, we performed genome-wide expression profiling of xylem and phloem-cambium isolated from the root-hypocotyl of Arabidopsis (Arabidopsis thaliana). Gene expression in the remaining nonvascular tissue was also profiled. From these transcript profiles, we assembled three sets of genes with expression significantly biased toward xylem, phloem-cambium, or nonvascular tissue. We also assembled three two-tissue sets of genes with expression significantly biased toward xylem/phloem-cambium, xylem/nonvascular, or phloem-cambium/nonvascular tissues. Localizations predicted by transcript profiles were supported by results from promoter-reporter and reverse transcription-polymerase chain reaction experiments with nine xylem- or phloem-cambium-biased genes. An analysis of the members of the phloem-cambium gene set suggested that some genes involved in regulating primary meristems are also regulators of the cambium. Secondary phloem was implicated in the synthesis of auxin, glucosinolates, cytokinin, and gibberellic acid. Transcript profiles also supported the importance of class III HD ZIP and KANADI transcription factors as regulators of radial patterning during secondary growth, and identified several members of the G2-like, NAC, AP2, MADS, and MYB transcription factor families that may play roles as regulators of xylem or phloem cell differentiation and activity.

COUP-TFIIis essential for radial and anteroposterior patterning of the stomach

COUP-TFII, an orphan member of the steroid receptor superfamily, has been implicated in mesenchymal-epithelial interaction during organogenesis. The generation of a lacZ knock-in allele in the COUP-TFII locus in mice allows us to use X-gal staining to follow the expression of COUP-TFII in the developing stomach. We found COUP-TFII is expressed in the mesenchyme and the epithelium of the developing stomach. Conditional ablation of floxed COUP-TFII by Nkx3-2Cre recombinase in the gastric mesenchyme results in dysmorphogenesis of the developing stomach manifested by major patterning defects in posteriorization and radial patterning. The epithelial outgrowth, the expansion of the circular smooth muscle layer and enteric neurons as well as the posteriorization of the stomach resemble phenotypes exhibited by inhibition of hedgehog signaling pathways. Using organ cultures and cyclopamine treatment, we showed downregulation of COUP-TFII level in the stomach, suggesting COUP-TFII as a target of hedgehog signaling in the stomach. Our results are consistent with a functional link between hedgehog proteins and COUP-TFII, factors that are vital for epithelial-mesenchymal interactions.

Foxd1-dependent signals control cellularity in the renal capsule, a structure required for normal renal development

Development of the metanephric kidney involves the establishment of discrete zones of induction and differentiation that are crucial to the future radial patterning of the organ. Genetic deletion of the forkhead transcription factor, Foxd1, results in striking renal abnormalities, including the loss of these discrete zones and pelvic fused kidneys. We have investigated the molecular and cellular basis of the kidney phenotypes displayed by Foxd1-null embryos and report here that they are likely to be caused by a failure in the correct formation of the renal capsule. Unlike the single layer of Foxd1-positive stroma that comprises the normal renal capsule, the mutant capsule contains heterogeneous layers of cells, including Bmp4-expressing cells, which induce ectopic phospho-Smad1 signaling in nephron progenitors. This missignaling disrupts their early patterning, which, in turn, causes mispatterning of the ureteric tree, while delaying and disorganizing nephrogenesis. In addition, the defects in capsule formation prevent the kidneys from detaching from the body wall, thus explaining their fusion and pelvic location. For the first time, functions have been ascribed to the renal capsule that include delineation of the organ and acting as a barrier to inappropriate exogenous signals, while providing a source of endogenous signals that are crucial to the establishment of the correct zones of induction and differentiation.

Mosaic analyses using marked activation and deletion clones dissect Arabidopsis SCARECROW action in asymmetric cell division.

In the Arabidopsis root meristem, ground tissue stem cell daughters perform an asymmetric division to form endodermis and cortex. The putative transcription factors SCARECROW (SCR) and SHORTROOT (SHR) regulate this radial patterning event, and the mixed cell fate in scr mutants suggests an atypical role of the SCR gene in asymmetric cell division. Here we use a newly developed site-specific gene activation/deletion system in which induced clones are positively marked with green fluorescent protein (GFP). Using this system, we show that SCR acts cell-autonomously to control asymmetric cell division within the ground tissue. We provide evidence that SCR gene expression is under autoregulatory control, that SCR limits SHR movement, and that transient SCR action is sufficient to separate endodermis and cortex fates by asymmetric cell division.

A broad competence to respond to SHORT ROOT revealed by tissue-specific ectopic expression.

In plants, cell fate specification depends primarily on position rather than lineage. Recent results indicate that positional information can be transmitted through intercellular trafficking of transcription factors. The SHORT ROOT (SHR) gene, a member of the GRAS family of putative transcription factors, is involved in root radial patterning in Arabidopsis. Correct radial patterning depends on the positional information transmitted through limited SHR intercellular movement and translated into cell division and specification by competent target cells. To investigate the regulation of SHR movement and the competence to respond to it, we drove expression of a translational fusion SHR::GFP using four different tissue-specific promoters. In a wild-type background, SHR::GFP was not able to move from either phloem companion cells or epidermal cells, both of which have been shown to support movement of other proteins, suggesting a requirement for tissue-specific factors for SHR movement. When expressed from its native promoter in plants with multiple endodermal layers, SHR::GFP was not able to move beyond the first endodermal layer, indicating that movement is not limited by a mechanism that recognizes boundaries between cell types. Surprisingly, movement of SHR::GFP was observed when ectopic expression from an epidermal promoter was placed in a scarecrow (scr) mutant background, revealing a possible role for SCR in limiting movement. Analysis of the competence to respond to SHR-mediated cell specification activity indicated that it was broadly distributed in the epidermal lineage, while competence to respond to the cell division activity of SHR appeared limited to the initials and involved induction of SCR. The spatial distribution of competence to respond to SHR highlights the importance of tightly regulated movement in generating the root radial pattern.

An auxin-responsive SCARECROW-like transcriptional activator interacts with histone deacetylase

Members of the plant-specific GRAS family of putative transcription factors are involved in various aspects of plant development. SCARECROW (SCR) is a member of this protein family and plays a significant role in the radial patterning of both roots and shoots. However, little is known about the regulation of SCR expression and its mode of action in plants. Here, we report on the isolation and characterization of a Brassica napus SCARECROW-like protein, BnSCL1, isolated by selecting for proteins that interact with the Arabidopsis histone deacetylase AtHDA19 in a yeast two-hybrid screen. BnSCL1 contains domains conserved in the GRAS family of proteins, interacts with AtHDA19 through a VHIID domain, and exerts transcription activation of reporter genes . BnSCL1 is expressed predominantly in the roots, where its expression is regulated by auxin, as it also is in shoots and mature leaves. These results indicate that BnSCL1 is a member of the GRAS family, and suggest that its mode of action in plant auxin response may involve interaction with HDA19.

Radial Patterning of Arabidopsis Shoots by Class III HD-ZIP and KANADI Genes

Background: Shoots of all land plants have a radial pattern that can be considered to have an adaxial (central)-abaxial (peripheral) polarity. In Arabidopsis, gain-of-function alleles of PHAVOLUTA and PHABULOSA, members of the class III HD-ZIP gene family, result in adaxialization of lateral organs. Conversely, loss-of-function alleles of the KANADI genes cause an adaxialization of lateral organs. Thus, the class III HD-ZIP and KANADI genes comprise a genetic system that patterns abaxial-adaxial polarity in lateral organs produced from the apical meristem.Results: We show that gain-of-function alleles of REVOLUTA, another member of the class III HD-ZIP gene family, are characterized by adaxialized lateral organs and alterations in the radial patterning of vascular bundles in the stem. The gain-of-function phenotype can be obtained by changing only the REVOLUTA mRNA sequence and without changing the protein sequence; this finding indicates that this phenotype is likely mediated through an interference with microRNA binding. Loss of KANADI activity results in similar alterations in vascular patterning as compared to REVOLUTA gain-of-function alleles. Simultaneous loss-of-function of PHABULOSA, PHAVOLUTA, and REVOLUTA abaxializes cotyledons, abolishes the formation of the primary apical meristem, and in severe cases, eliminates bilateral symmetry; these phenotypes implicate these three genes in radial patterning of both embryonic and postembryonic growth.Conclusions: Based on complementary vascular and leaf phenotypes of class III HD-ZIP and KANADI mutants, we propose that a common genetic program dependent upon miRNAs governs adaxial-abaxial patterning of leaves and radial patterning of stems in the angiosperm shoot. This finding implies that a common patterning mechanism is shared between apical and vascular meristems.

Characterization and genetic analysis of a lettuce (Lactuca sativa L.) mutant, weary, that exhibits reduced gravitropic response in hypocotyls and inflorescence stems.

A lettuce (Lactuca sativa L.) mutant that exhibits a procumbent growth habit was identified and characterized. In two wild type (WT) genetic backgrounds, segregation patterns revealed that the mutant phenotype was controlled by a recessive allele at a single locus, which was designated weary. Hypocotyls and inflorescence stems of plants homozygous for the weary allele exhibited reduced gravitropic responses compared with WT plants, but roots exhibited normal gravitropism. Microscopic analysis revealed differences in the radial distribution of amyloplasts in hypocotyl and inflorescence stem cells of weary and WT plants. Amyloplasts occurred in a single layer of endodermal cells in WT hypocotyls and inflorescence stems. By contrast, amyloplasts were observed in several layers of cortical cells in weary hypocotyls, and weary inflorescence stem cells lacked amyloplasts entirely. These results are consistent with the proposed role of sedimenting amyloplasts in shoot gravitropism of higher plants. The phenotype associated with the weary mutant is similar to that described for the Arabidopsis mutant sgr1/scr, which is defective in radial patterning and gravitropism.

SCARECROW is involved in positioning the stem cell niche in the Arabidopsis root meristem.

Stem cells self-renew and produce daughter cells that differentiate. How stem cells are specified and maintained is a central question in developmental biology. Plant stem cells occupy a small region or niche in larger zones of mitotic activity called meristems. Here we provide molecular evidence that in the Arabidopsis root meristem, the stem cell population depends on a central group of cells, the quiescent center (QC), which positions the stem cell niche. We show that the putative transcription factor SCARECROW (SCR), first identified by its role in radial patterning, is required cell-autonomously for distal specification of the QC, which in turn regulates stem cell fate of immediately surrounding cells.

SCHIZORIZA controls an asymmetric cell division and restricts epidermal identity in the Arabidopsis root.

The primary root of Arabidopsis has a simple cellular organisation. The fixed radial cell pattern results from stereotypical cell divisions that occur in the meristem. Here we describe the characterisation of schizoriza (scz), a mutant with defective radial patterning. In scz mutants, the subepidermal layer (ground tissue) develops root hairs. Root hairs normally only form on epidermal cells of wild-type plants. Moreover, extra periclinal divisions (new wall parallel to surface of the root) occur in the scz root resulting in the formation of supernumerary layers in the ground tissue. Both scarecrow (scr) and short root (shr) suppress the extra periclinal divisions characteristic of scz mutant roots. This results in the formation of a single layered ground tissue in the double mutants. Cells of this layer develop root hairs, indicating that mis-specification of the ground tissue in scz mutants is uncoupled to the cell division defect. This suggests that during the development of the ground tissue SCZ has two distinct roles: (1) it acts as a suppressor of epidermal fate in the ground tissue, and (2) it is required to repress periclinal divisions in the meristem. It may act in the same pathway as SCR and SHR.

Apical-basal pattern formation in Arabidopsis embryogenesis.

Embryogenesis transforms the fertilized egg cell into a multicellular organism. In higher animals, the mature embryo is a miniature variant of the adult animal, and whatever changes may take place during post‐embryonic development, they occur within the confines of the body organization established during embryogenesis. By contrast, higher plant embryogenesis generates a juvenile form, the seedling, which lacks most species‐specific features of the adult plant. Post‐embryonic development originates from two primary meristems, stem‐cell systems that occupy opposite ends of the main body axis. The primary shoot meristem at the top end is the source of cells for new organs, such as leaves, and secondary shoot meristems, including flowers. The primary root meristem at the bottom end produces cells for extension growth of the primary root. In addition, root branches are initiated from specific cell groups within the primary root. These primordia recapitulate radial patterning and root meristem establishment as occurs in embryogenesis (Malamy and Benfey, 1997). Thus, the considerable increase in architectural complexity during post‐embryonic development is contingent upon the basic body organization laid down during embryogenesis. Formally, the seedling body organization can be viewed as a superimposition of two patterns: an apical–basal pattern along the main axis of polarity and a radial pattern across the axis (Figure 1). Apical–basal pattern elements are, from top to bottom: shoot meristem, one or two cotyledons (embryonic leaves), hypocotyl (embryonic stem), radicle (embryonic root) and root meristem. The radial pattern consists of tissue layers that are arranged concentrically from the periphery to the centre: epidermis, cortex and endodermis (both derived from ground tissue), pericycle and vascular tissue (xylem and phloem). The origin of seedling structures can be traced back to cell groups in the young embryo, due to the highly invariant pattern of cell division in Arabidopsis early embryogenesis (Figure 1; for …

Role of PHABULOSA and PHAVOLUTA in determining radial patterning in shoots.

The upper side of the angiosperm leaf is specialized for efficient capture of sunlight whereas the lower side is specialized for gas exchange. In Arabidopsis, the establishment of polarity in the leaf probably requires the generation and perception of positional information along the radial (adaxial versus abaxial or central versus peripheral) dimension of the plant. This is because the future upper (adaxial) side of the leaf develops from cells closer to the centre of the shoot, whereas the future under (abaxial) side develops from cells located more peripherally. Here we implicate the Arabidopsis PHABULOSA and PHAVOLUTA genes in the perception of radial positional information in the leaf primordium. Dominant phabulosa (phb) and phavoluta (phv) mutations cause a dramatic transformation of abaxial leaf fates into adaxial leaf fates. They do so by altering the predicted sterol/lipid-binding domains of ATHB14 and ATHB9, proteins of previously unknown function that also contain DNA-binding motifs. This change probably renders the protein constitutively active, implicating this domain as a central regulator of protein function and the PHB and PHV proteins as receptors for an adaxializing signal.