The cysteine pairs in CLV2 are not necessary for sensing the CLV3 peptide in shoot and root meristems

Receptor-like proteins (RLPs) are involved in both plant defense and developmental processes. Previous genetic and biochemical studies show that the leucine-rich repeat (LRR) receptor-like protein CLAVATA2 (CLV2) functions together with CLAVATA1 (CLV1) and CORYNE (CRN) in Arabidopsis to limit the stem cell number in shoot apical meristem, while in root it acts with CRN to trigger a premature differentiation of the stem cells after sensing the exogenously applied peptides of CLV3p, CLE19p or CLE40p. It has been proposed that disulfide bonds might be formed through two cysteine pairs in the extracellular LRR domains of CLV1 and CLV2 to stabilize the receptor complex. Here we tested the hypothesis by replacing these cysteines with alanines and showed that depletions of one or both of the cysteine pairs do not hamper the function of CLV2 in SAM maintenance. In vitro peptide assay also showed that removal of the cysteine pairs did not affect the perception of CLV3 peptides in roots. These observations allow us to conclude that the formation of disulfide bonds is not needed for the function of CLV2.

CLAVATA2 forms a distinct CLE-binding receptor complex regulating Arabidopsis stem cell specification

CLAVATA1 (CLV1), CLV2, CLV3, CORYNE (CRN), BAM1 and BAM2 are key regulators that function at the shoot apical meristem (SAM) of plants to promote differentiation by limiting the size of the organizing center that maintains stem cell identity in neighboring cells. Previous results have indicated that the extracellular domain of the receptor kinase CLV1 binds to the CLV3-derived CLE ligand. The biochemical role of the receptor-like protein CLV2 has remained largely unknown. Although genetic analysis suggested that CLV2, together with the membrane kinase CRN, acts in parallel with CLV1, recent studies using transient expression indicated that CLV2 and CRN from a complex with CLV1. Here, we report detection of distinct CLV2-CRN heteromultimeric and CLV1-BAM multimeric complexes in transient expression in tobacco and in Arabidopsis meristems. Weaker interactions between the two complexes were detectable in transient expression. We also find that CLV2 alone generates a membrane-localized CLE binding activity independent of CLV1. CLV2, CLV1 and the CLV1 homologs BAM1 and BAM2 all bind to the CLV3-derived CLE peptide with similar kinetics, but BAM receptors show a broader range of interactions with different CLE peptides. Finally, we show that BAM and CLV1 overexpression can compensate for the loss of CLV2 function in vivo. These results suggest two parallel ligand-binding receptor complexes controlling stem cell specification in Arabidopsis.

Salt-induced abnormalities on root tip mitotic cells of Allium cepa: prevention by inositol pretreatment

Salt-induced growth reduction of plants is a well-known phenomenon which poses major problem in crop productivity in places where vast majority of land plants are affected by salt. In this report, studies were carried out to reveal the effect of salt injury on the cell division pattern in roots and the role of myo-inositol in preventing the salt-induced ion disequilibrium on the chromosome and DNA degradation in roots. Present study revealed induction of various chromosomal abnormalities on the root tip mitotic cells of Allium cepa by treatment with different concentrations of NaCl (0-500 mM) for 24 h as also the amelioration of such effect by prior treatment of the roots with different concentration of myo-inositol (0-300 mM). Results showed that a narrow albeit definite range of extracellular myo-inositol (100-150 mM) is effective in preventing internucleosomal fragmentation which is the early response in roots under salt stress. Transgenic tobacco plants overexpressing Oryza (OsINO1) as well as Porteresia (PcINO1) cytosolic L: -myo-inositol-1-phosphate synthase coding genes can withstand and retain their chromosomal and DNA integrity in 100 mM NaCl solution and can subsequently prevent DNA fragmentation, caused by intracellular endonuclease activity at this salt concentration.

A putative TRAPPII tethering factor is required for cell plate assembly during cytokinesis in Arabidopsis

*At the end of the cell cycle, the plant cell wall is deposited within a membrane compartment referred to as the cell plate. Little is known about the biogenesis of this transient membrane compartment. *We have positionally cloned and characterized a novel Arabidopsis gene, CLUB, identified by mutation. *CLUB/AtTRS130 encodes a putative TRAPPII tethering factor. club mutants are seedling-lethal and have a canonical cytokinesis-defective phenotype, characterized by the appearance of bi- or multinucleate cells with cell wall stubs, gaps and floating walls. Confocal microscopy showed that in club mutants, KNOLLE-positive vesicles formed and accumulated at the cell equator throughout cytokinesis, but failed to assemble into a cell plate. Similarly, electron micrographs showed large vesicles loosely connected as patchy, incomplete cell plates in club root tips. Neither the formation of KNOLLE-positive vesicles nor the delivery of these vesicles to the cell equator appeared to be perturbed in club mutants. Thus, the primary defect in club mutants appears to be an impairment in cell plate assembly. *As a putative tethering factor required for cell plate biogenesis, CLUB/AtTRS130 helps to define the identity of this membrane compartment and comprises an important handle on the regulation of cell plate assembly.

TDIF peptide signaling regulates vascular stem cell proliferation via the WOX4 homeobox gene in Arabidopsis

The indeterminate nature of plant growth and development depends on the stem cell system found in meristems. The Arabidopsis thaliana vascular meristem includes procambium and cambium. In these tissues, cell-cell signaling, mediated by a ligand-receptor pair made of the TDIF (for tracheary element differentiation inhibitory factor) peptide and the TDR/PXY (for TDIF RECEPTOR/ PHLOEM INTERCALATED WITH XYLEM) membrane protein kinase, promotes proliferation of procambial cells and suppresses their xylem differentiation. Here, we report that a WUSCHEL-related HOMEOBOX gene, WOX4, is a key target of the TDIF signaling pathway. WOX4 is expressed preferentially in the procambium and cambium, and its expression level was upregulated upon application of TDIF in a TDR-dependent manner. Genetic analyses showed that WOX4 is required for promoting the proliferation of procambial/cambial stem cells but not for repressing their commitment to xylem differentiation in response to the TDIF signal. Thus, at least two intracellular signaling pathways that diverge after TDIF recognition by TDR might regulate independently the behavior of vascular stem cells. Detailed observations in loss-of-function mutants revealed that TDIF-TDR-WOX4 signaling plays a crucial role in the maintenance of the vascular meristem organization during secondary growth.

Inter- and intrachromosomal asynchrony of cell division cycle events in root meristem cells of Allium cepa: possible connection with gradient of cyclin B-like proteins

Alternate treatments of Allium cepa root meristems with hydroxyurea (HU) and caffeine give rise to extremely large and highly elongated cells with atypical images of mitotic divisions, including internuclear asynchrony and an unknown type of interchromosomal asynchrony observed during metaphase-to-anaphase transition. Another type of asynchrony that cannot depend solely on the increased length of cells was observed following long-term incubation of roots with HU. This kind of treatment revealed both cell nuclei entering premature mitosis and, for the first time, an uncommon form of mitotic abnormality manifested in a gradual condensation of chromatin (spanning from interphase to prometaphase). Immunocytochemical study of polykaryotic cells using anti-beta tubulin antibodies revealed severe perturbations in the microtubular organization of preprophase bands. Quantitative immunofluorescence measurements of the control cells indicate that the level of cyclin B-like proteins reaches the maximum at the G2 to metaphase transition and then becomes reduced during later stages of mitosis. After long-term incubation with low doses of HU, the amount of cyclin B-like proteins considerably increases, and a significant number of elongated cells show gradients of these proteins spread along successive regions of the perinuclear cytoplasm. It is suggested that there may be a direct link between the effects of HU-mediated deceleration of S- and G2-phases and an enhanced concentration of cyclin B-like proteins. In consequence, the activation of cyclin B-CDK complexes gives rise to an abnormal pattern of premature mitotic chromosome condensation with biphasic nuclear structures having one part of chromatin decondensed, and the other part condensed.

The role of the acyl modification, palmitoylation, in Arabidopsis stem cell regulation

Proper control of stem cell populations is key for the development of all multicellular organisms. In Arabidopsis, stem cells are located primarily in the shoot, root and floral meristems where they undergo complex regulation. The Arabidopsis shoot and root meristems are regulated by the related WUS and WOX5 pathways, respectively. Previous studies established that these pathways share the signal transduction components POLTERGEIST (POL) and PLL1. Our latest study in Plant Cell revealed key roles for acyl modifications and lipid binding in the regulation of these two type 2C protein phosphatases. Specifically, POL and PLL1 were shown to localize to the plasma membrane in a myristioylation- and palmitoylation-dependent manner, POL and PLL1 were shown to bind to membrane lipids, and POL activity was found to be stimulated in vitro by the phospholipid PI(4)P. Here, we will discuss what is currently known in Arabidopsis and other organisms about the mechanisms of palmitoylation and provide additional evidence supporting that POL and PLL1 are palmitoylated, including describing the identification of a putative Arabidopsis palmitoyl transferase as a PLL1 interactor.

The unique relationship between tsh4 and ra2 in patterning floral phytomers

Phytomers are developmental compartments that display stereotypical patterns dependent on whether they are initiated during the vegetative phase or the floral phases. Differences in appearance result from differential partitioning mechanisms responsible for allocation of cells to different components of the phytomer. The tasselsheath loci of maize control cell partitioning within the phytomer, indirectly influencing growth and development of its individual components. The tasselsheath4 (tsh4) gene accomplishes this through regulation of the ramosa2 (ra2) meristem determinacy gene, whereas tasselsheath1 (tsh1) appears to function differently.

Dosage effect of the short arm of chromosome 1 of rye on root morphology and anatomy in bread wheat

The spontaneous translocation of the short arm of chromosome 1 of rye (1RS) in bread wheat is associated with higher root biomass and grain yield. Recent studies have confirmed the presence of QTL for different root morphological traits on the 1RS arm in bread wheat. This study was conducted to address two questions in wheat root genetics. First, does the presence of the 1RS arm in bread wheat affect its root anatomy? Second, how does root morphology and anatomy of bread wheat respond to different dosages of 1RS? Near-isogenic plants with a different number (0 to 4 dosages) of 1RS translocations were studied for root morphology and anatomy. The F(1) hybrid, with single doses of the 1RS and 1AS arms, showed heterosis for root and shoot biomass. In other genotypes, with 0, 2, or 4 doses of 1RS, root biomass was incremental with the increase in the dosage of 1RS in bread wheat. This study also provided evidence of the presence of gene(s) influencing root xylem vessel number, size, and distribution in bread wheat. It was found that root vasculature follows a specific developmental pattern along the length of the tap root and 1RS dosage tends to affect the transitions differentially in different positions. This study indicated that the inherent differences in root morphology and anatomy of different 1RS lines may be advantageous compared to normal bread wheat to survive under stress conditions.

Arabidopsis RETINOBLASTOMA-RELATED is required for stem cell maintenance, cell differentiation, and lateral organ production

Several genes involved in the regulation of postembryonic organ initiation and growth have been identified. However, it remains largely unclear how developmental cues connect to the cell cycle. RETINOBLASTOMA RELATED (RBR) is a plant homolog of the tumor suppressor Retinoblastoma (pRb), which is a key regulator of the cell cycle. Using inducible RNA interference (RNAi) against Arabidopsis thaliana RBR (RBRi), we reduced RBR expression levels at different stages of plant development. Conditional reduction or loss of RBR function disrupted cell division patterns, promoted context-dependent cell proliferation, and negatively influenced establishment of cell differentiation. Several lineages of toti- and pluripotent cells, including shoot apical meristem stem cells, meristemoid mother cells, and procambial cells, failed to produce appropriately differentiated cells. Meristem activity was altered, leading to a disruption of the CLAVATA-WUSCHEL feedback loop and inhibition of lateral organ formation. Release of RBR from RNAi downregulation restored meristem activity. Gene profiling analyses soon after RBRi induction revealed that a change in RBR homeostasis is perceived as a stress, even before genes regulated by RBR-E2F become deregulated. The results establish RBR as a key cell cycle regulator required for coordination of cell division, differentiation, and cell homeostasis.

Depletion of cellular brassinolide decreases embryo production and disrupts the architecture of the apical meristems in Brassica napus microspore-derived embryos

Exogenous applications of brassinolide (BL) increased the number and quality of microspore-derived embryos (MDEs) whereas treatments with brassinazole (BrZ), a BL biosynthetic inhibitor, had the opposite effect. At the optimal concentration (4x10(-6) M) BrZ decreased both embryo yield and conversion to less than half the value of control embryos. Metabolic studies revealed that BL levels had profound effects on glutathione and ascorbate metabolism by altering the amounts of their reduced forms (ASC and GSH) and oxidized forms [dehydroascorbate (DHA), ascorbate free radicals (AFRs), and GSSG]. Applications of BL switched the glutathione and ascorbate pools towards the oxidized forms, thereby lowering the ASC/ASC+DHA+AFR and GSH/GSH+GSSG ratios. These changes were ascribed to the ability of BL to increase the activity of ascorbate peroxidase (APX) and decrease that of glutathione reductase (GR). This trend was reversed in a BL-depleted environment, effected by BrZ applications. These metabolic alterations were associated with changes in embryo structure and performance. BL-treated MDEs developed zygotic-like shoot apical meristems (SAMs) whereas embryos treated with BrZ developed abnormal meristems. In the presence of BrZ, embryos either lacked a visible SAM, or formed SAMs in which the meristematic cells showed signs of differentiation, such as vacuolation and storage product accumulation. These abnormalities were accompanied by the lack or misexpression of three meristem marker genes isolated from Brassica napus (denoted as BnSTM, BnCLV1, and BnZLL-1) homologous to the Arabidopsis SHOOTMERISTEMLESS (STM), CLAVATA 1 (CLV1), and ZWILLE (ZLL). The expression of BnSTM and BnCLV1 increased after a few days in cultures in embryos treated with BL whereas an opposite tendency was observed with applications of BrZ. Compared with control embryos where these two genes exhibited abnormal localization patterns, BnSTM and BnCLV1 always localized throughout the subapical domains of BL-treated embryos in a zygotic-like fashion. Expression of both genes was often lost in the SAM of BrZ-treated embryos. The results suggest that maintenance of cellular BL levels is required to modulate the ascorbate and glutathione redox status during embryogenesis to ensure proper development of the embryos and formation of functional apical meristems.

Regulation of shape and patterning in plant development

Plant and animal development depend on both biochemical and biophysical responses. In certain contexts biochemical networks and gradients seem to be sufficient to explain patterning. However the translation of such patterns into shape changes also involves mechanical properties, which, in plants, largely depend on the characteristics of the structural elements, in particular the external matrix or cell wall. More generally, there is a number of emerging links between gene regulatory networks, biochemical gradients, and physical forces, involving multiple feedback loops. It is likely that combining mechanical signals and biochemical gradients could confer more robustness to plant development.

Blockage of mitosis in maize root tips using colchicine-alternatives

Mitosis in plants can be blocked by colchicine which has the capacity to bind microtubule subunits. In maize (Zea mays L.) breeding, it is frequently being used for doubling chromosome numbers of haploids for producing homozygous doubled haploids. However, colchicine is highly toxic for mammals and impacts negatively on the environment. Therefore, it was interesting to find substitutes like chemical compounds and/or physical methods which would be capable of doubling chromosome numbers in maize. For this purpose, a screening system was set up using root tips of maize. Herbicides like amiprophos methyl, oryzalin, and pronamide were identified to be effective in doubling chromosome sets of maize. Additionally, the toxicity of these compounds was lower than that of colchicine and treated seedlings recovered and grew. Therefore, they could be applied in reduced concentrations showing results comparable to colchicine.

An endoplasmic reticulum response pathway mediates programmed cell death of root tip induced by water stress in Arabidopsis

Drought induces root death in plants; however, the nature and characteristics of root cell death and its underlying mechanisms are poorly understood. Here, we provide a systematic analysis of cell death in the primary root tips in Arabidopsis during water stress. Root tip cell death occurs when high water deficit is reached. The dying cells were first detected in the apical meristem of the primary roots and underwent active programmed cell death (PCD). Transmission electron microscopic analysis shows that the cells undergoing induced death had unambiguous morphological features of autophagic cell death, including an increase in vacuole size, degradation of organelles, and collapse of the tonoplast and the plasma membrane. The results suggest that autophagic PCD occurs as a response to severe water deficit. Significant accumulation of reactive oxygen species (ROS) was detected in the stressed root tips. Expression of BAX inhibitor-1 (AtBI1) was increased in response to water stress, and atbi1-1 displayed accelerated cell death, indicating that AtBI1 and the endoplasmic reticulum (ER) stress response pathway both modulate water stress-induced PCD. These findings form the basis for further investigations into the mechanisms underlying the PCD and its role in developmental plasticity of root system architecture and subsequent adaptation to water stress.

ROS in plant development

Reactive oxygen species (ROS) are now recognized as important regulators of plant developmental programs and recent work on tip-growing systems has revealed a central role for the NADPH oxidases in generating such developmentally important ROS. Tip-growing cells have also shown that the functions of cytosolic ROS, acting as regulators of activities such as ion channel gating, are closely linked to those of ROS produced to the apoplast, where they act to modulate cell wall properties. Thus, coordination of ROS production and their activities between compartments is emerging as an important theme in our understanding of how growth and developmental programs are integrated.

How a plant builds leaves

A leaf develops from a few cells that grow, divide, and differentiate to form a complex organ that is precisely positioned relative to its neighbors. How cells communicate to achieve such coordinated growth and development is the focus of this review. We discuss (1) how the stem cells within the shoot meristem gain competence to form organs, (2) what determines the positioning and initiation of new organs, and (3) how the new organ attains its characteristic shape and polarity. Special emphasis is given to the recent integration of mathematics and physics in the study of leaf development.

Automated tracking of stem cell lineages of Arabidopsis shoot apex using local graph matching

Shoot apical meristems (SAMs) of higher plants harbor stem-cell niches. The cells of the stem-cell niche are organized into spatial domains of distinct function and cell behaviors. A coordinated interplay between cell growth dynamics and changes in gene expression is critical to ensure stem-cell homeostasis and organ differentiation. Exploring the causal relationships between cell growth patterns and gene expression dynamics requires quantitative methods to analyze cell behaviors from time-lapse imagery. Although technical breakthroughs in live-imaging methods have revealed spatio-temporal dynamics of SAM-cell growth patterns, robust computational methods for cell segmentation and automated tracking of cells have not been developed. Here we present a local graph matching-based method for automated-tracking of cells and cell divisions of SAMs of Arabidopsis thaliana. The cells of the SAM are tightly clustered in space which poses a unique challenge in computing spatio-temporal correspondences of cells. The local graph-matching principle efficiently exploits the geometric structure and topology of the relative positions of cells in obtaining spatio-temporal correspondences. The tracker integrates information across multiple slices in which a cell may be properly imaged, thus providing robustness to cell tracking in noisy live-imaging datasets. By relying on the local geometry and topology, the method is able to track cells in areas of high curvature such as regions of primordial outgrowth. The cell tracker not only computes the correspondences of cells across spatio-temporal scale, but it also detects cell division events, and identifies daughter cells upon divisions, thus allowing automated estimation of cell lineages from images captured over a period of 72 h. The method presented here should enable quantitative analysis of cell growth patterns and thus facilitating the development of in silico models for SAM growth.

Novel insights from live-imaging in shoot meristem development

Microscopic imaging of fluorescent reporters for key meristem regulators in live tissues is emerging as a powerful technique, enabling researchers to observe dynamic spatial and temporal distribution of hormonal and developmental regulators in living cells. Aided by time-lapse microphotography, new types of imaging acquisition and analysis software, and computational modeling, we are gaining significant insights into shoot apical meristem (SAM) behavior and function. This review is focused on summarizing recent advances in the understanding of SAM organization, development, and behavior derived from live-imaging techniques. This includes the revelation of mechanical forces in microtubule-controlled anisotropic growth, the role of the CLV-WUS network in the specification of peripheral zone and central zone cells, the multiple feedback loops involving cytokinin in controlling WUS expression, auxin dynamics in determining the position of new primordia, and, finally, sequence of regulatory events leading to de novo assembly of shoots from callus in culture. Future studies toward formulating "digital SAM" that incorporates multi-dimensional data ranging from images of SAM morphogenesis to a genome-scale expression map of SAM will greatly enhance our ability to understand, predict, and manipulate SAM, containing the stem cells that give rise to all above ground parts of a plant.

Physcomitrella patens: a model to investigate the role of RAC/ROP GTPase signalling in tip growth

Polarized cell expansion plays an important role in plant morphogenesis. Tip growth is a dramatic form of this process, which is widely used as a model to study its regulation by RAC/ROP GTPase signalling. During the dominant haploid phase of its life cycle, the moss Physcomitrella patens contains different types of cells that expand by tip growth. Physcomitrella is a highly attractive experimental system because its genome has been sequenced, and transgene integration by homologous recombination occurs in this plant at frequencies allowing effective gene targeting. Furthermore, together with the vascular spikemoss Selaginella moellendorffii, whose genome has also been sequenced, the non-vascular moss Physcomitrella provides an evolutionary link between green algae and angiosperms. BLAST searches established that the Physcomitrella and Selaginella genomes encode not only putative RAC/ROP GTPases, but also homologues of all known regulators of polarized RAC/ROP signalling, as well as of key effectors acting in signalling cascades downstream of RAC/ROP activity. Nucleotide sequence relationships within seven different families of Physcomitrella, Selaginella, Arabidopsis thaliana and Nicotiana tabacum (tobacco) genes with distinct functions in RAC/ROP signalling were characterized based on extensive maximum likelihood and Neighbor-Joining analyses. The results of these analyses are interpreted in the light of current knowledge concerning expression patterns and molecular functions of RAC/ROP signalling proteins in angiosperms. A key aim of this study is to facilitate the use of Physcomitrella as a model to investigate the molecular control of tip growth in plants.

Seasonal water relations of Lyginia barbata (Southern rush) in relation to root xylem development and summer dormancy of root apices

*Periods of dormancy in shallow roots allow perennial monocotyledons to establish deep root systems, but we know little about patterns of xylem maturation, water-transport capacities and associated economies in water use of growing and dormant roots. *Xylem development, anatomy, conductance and in situ cellular [K] and [Cl] were investigated in roots of field-grown Lyginia barbata (Restionaceae) in Mediterranean southwestern Australia. Parallel studies of gas exchange, culm relative water loss and soil water content were conducted. *Stomatal conductance and photosynthesis decreased during summer drought as soil profiles dried, but rates recovered when dormant roots became active with the onset of wetter conditions. Anatomical studies identified sites of close juxtaposition of phloem and xylem in dormant and growing roots. Ion data and dye tracing showed mature late metaxylem of growing roots was located >or= 100 mm from the tip, but at only <or= 10 mm for dormant roots. Dormant roots remained hydrated in dry soils (0.001-0.005 g g(-1)). *Effective regulation of growth and water-conserving/obtaining properties permits the survival of shallow roots of L. barbata during summer drought and may represent important strategies for establishing deeper perennial root systems in other monocotyledonous plants adapted to seasonally dry habitats.

MGOUN1 encodes an Arabidopsis type IB DNA topoisomerase required in stem cell regulation and to maintain developmentally regulated gene silencing

Maintenance of stem cells in the Arabidopsis thaliana shoot meristem is regulated by signals from the underlying cells of the organizing center, provided through the transcription factor WUSCHEL (WUS). Here, we report the isolation of several independent mutants of MGOUN1 (MGO1) as genetic suppressors of ectopic WUS activity and enhancers of stem cell defects in hypomorphic wus alleles. mgo1 mutants have previously been reported to result in a delayed progression of meristem cells into differentiating organ primordia (Laufs et al., 1998). Genetic analyses indicate that MGO1 functions together with WUS in stem cell maintenance at all stages of shoot and floral meristems. Synergistic interactions of mgo1 with several chromatin mutants suggest that MGO1 affects gene expression together with chromatin remodeling pathways. In addition, the expression states of developmentally regulated genes are randomly switched in mgo1 in a mitotically inheritable way, indicating that MGO1 stabilizes epigenetic states against stochastically occurring changes. Positional cloning revealed that MGO1 encodes a putative type IB topoisomerase, which in animals and yeast has been shown to be required for regulation of DNA coiling during transcription and replication. The specific developmental defects in mgo1 mutants link topoisomerase IB function in Arabidopsis to stable propagation of developmentally regulated gene expression.

Geminivirus C4 protein alters Arabidopsis development

The C4 protein of beet curly top virus [BCTV-B (US:Log:76)] induces hyperplasia in infected phloem tissue and tumorigenic growths in transgenic plants. The protein offers an excellent model for studying cell cycle control, cell differentiation, and plant development. To investigate the role of the C4 protein in plant development, transgenic Arabidopsis thaliana plants were generated in which the C4 transgene was expressed under the control of an inducible promoter. A detailed analysis of the developmental changes that occur in cotyledons and hypocotyls of seedlings expressing the C4 transgene showed extensive cell division in all tissues types examined, radically altered tissue layer organization, and the absence of a clearly defined vascular system. Induced seedlings failed to develop true leaves, lateral roots, and shoot and root apical meristems, as well as vascular tissue. Specialized epidermis structures, such as stomata and root hairs, were either absent or developmentally impaired in seedlings that expressed C4 protein. Exogenous application of brassinosteroid and abscisic acid weakly rescued the C4-induced phenotype, while induced seedlings were hypersensitive to gibberellic acid and kinetin. These results indicate that ectopic expression of the BCTV C4 protein in A. thaliana drastically alters plant development, possibly through the disruption of multiple hormonal pathways.

A dynamic model for stem cell homeostasis and patterning in Arabidopsis meristems

Plants maintain stem cells in their meristems as a source for new undifferentiated cells throughout their life. Meristems are small groups of cells that provide the microenvironment that allows stem cells to prosper. Homeostasis of a stem cell domain within a growing meristem is achieved by signalling between stem cells and surrounding cells. We have here simulated the origin and maintenance of a defined stem cell domain at the tip of Arabidopsis shoot meristems, based on the assumption that meristems are self-organizing systems. The model comprises two coupled feedback regulated genetic systems that control stem cell behaviour. Using a minimal set of spatial parameters, the mathematical model allows to predict the generation, shape and size of the stem cell domain, and the underlying organizing centre. We use the model to explore the parameter space that allows stem cell maintenance, and to simulate the consequences of mutations, gene misexpression and cell ablations.

Plant primary meristems: shared functions and regulatory mechanisms

Primary plant meristems are the shoot and root meristems that are initiated at opposite poles of the plant embryo. They contain stem cells, which remain undifferentiated, and supply new cells for growth and the formation of tissues. The maintenance of a long-lasting stem cell population in meristems is achieved by signal exchange between organizing regions and the stem cells, and also by feedback signals emanating from differentiating cells. Related peptide signals that make use of different receptor classes were found to control the stem cell populations in both meristem types by regulating evolutionarily conserved homeodomain transcription factors. The precise interplay of auxin and cytokinin signaling pathways is central to keep cells in the meristem, or direct them toward differentiation.

The AUX1 LAX family of auxin influx carriers is required for the establishment of embryonic root cell organization in Arabidopsis thaliana

BACKGROUND AND AIMS

The root meristem of the Arabidopsis thaliana mature embryo is a highly organized structure in which individual cell shape and size must be regulated in co-ordination with the surrounding cells. The objective of this study was to determine the role of the AUX1 LAX family of auxin import carriers during the establishment of the embryonic root cell pattern.

METHODS

The radicle apex of single and multiple aux1 lax mutant mature embryos was used to evaluate the effect of this gene family upon embryonic root organization and root cap size, cell number and cell size.

KEY RESULTS

It was demonstrated here that mutations within the AUX1 LAX family are associated with changes in cell pattern establishment in the embryonic quiescent centre and columella. aux1 lax mutants have a larger radicle root cap than the wild type and this is associated with a significant increase in the root-cap cell number, average cell size, or both. Extreme disorganization of the radicle apex was observed among quadruple aux1 lax1 lax2 lax3 mutant embryos, but not in single aux1 null or in lax1, lax2 and lax3 single mutants, indicating redundancy within the AUX1 LAX family.

CONCLUSIONS

It was determined that the AUX1 LAX family of auxin influx facilitators participates in the establishment of cell pattern within the apex of the embryonic root in a gene-redundant fashion. It was demonstrated that aux1 lax mutants are affected in cell proliferation and cell growth within the radicle tip. Thus AUX1 LAX auxin importers emerge as new players in morphogenetic processes involved in patterning during embryonic root formation.