Vascular Pattern Formation in Plants

Aspartic proteases gene family: Identification and expression profiles during stem vascular development in tobacco

Aspartic proteases (APs) constitute a large family in plants and are widely involved in diverse biological processes, like chloroplast metabolism, biotic and abiotic stress responses, and reproductive development. In this study, we focused on overall analysis of the APs genes in tobacco. Our analysis included the phylogeny and cis-elements in the cell wall-associated promoters of these genes. To characterize the expression patterns of APs genes in stem vascular development. The tissue expression analysis showed that NtAED3-like was preferentially expressed in the differentiating xylem and phloem cells of the vascular system. Based on histochemical staining analysis showed that the NtAED3-like gene was specifically expressed in stem vascular tissue, root vascular tissue, and petiole vascular tissue. The TdT-mediated dUTP nick-end labeling (TUNEL) assay illustrated a delayed progression of programmed cell death (PCD) within the xylem of the ko-ntaed3a-like mutant, relative to the wild type. The mutant ko-ntaed3a-like exhibited a phenotype of thinning stem circumference and changed in xylem structure and lignin content. In addition, the two-dimension heteronuclear single quantum coherent nuclear magnetic resonance (2D-HSQC) analysis of three milled wood lignins (MWLs) showed that the content of β-O-4 connection in ko-ntaed3a-like decreased slightly compared with wild type. In conclusion, this study provides our understanding of the regulation of vascular tissue development by the NtAED3-like gene in tobacco and provides a better basis for determining the molecular mechanism of the aspartic protease in secondary cell wall (SCW) development.

TARGET OF MONOPTEROS: key transcription factors orchestrating plant development and environmental response

Plants have an incredible ability to sustain root and vascular growth after initiation of the embryonic root and the specification of vascular tissue in early embryos. Microarray assays have revealed that a group of transcription factors, TARGET OF MONOPTEROS (TMO), are important for embryonic root initiation in Arabidopsis. Despite the discovery of their auxin responsiveness early on, their function and mode of action remained unknown for many years. The advent of genome editing has accelerated the study of TMO transcription factors, revealing novel functions for biological processes such as vascular development, root system architecture, and response to environmental cues. This review covers recent achievements in understanding the developmental function and the genetic mode of action of TMO transcription factors in Arabidopsis and other plant species. We highlight the transcriptional and post-transcriptional regulation of TMO transcription factors in relation to their function, mainly in Arabidopsis. Finally, we provide suggestions for further research and potential applications in plant genetic engineering.

Roles of Auxin in the Growth, Development, and Stress Tolerance of Horticultural Plants

Auxin, a plant hormone, regulates virtually every aspect of plant growth and development. Many current studies on auxin focus on the model plant Arabidopsis thaliana, or on field crops, such as rice and wheat. There are relatively few studies on what role auxin plays in various physiological processes of a range of horticultural plants. In this paper, recent studies on the role of auxin in horticultural plant growth, development, and stress response are reviewed to provide novel insights for horticultural researchers and cultivators to improve the quality and application of horticultural crops.

Deep origin and gradual evolution of transporting tissues: Perspectives from across the land plants

The evolution of transporting tissues was an important innovation in terrestrial plants that allowed them to adapt to almost all nonaquatic environments. These tissues consist of water-conducting cells and food-conducting cells and bridge plant-soil and plant-air interfaces over long distances. The largest group of land plants, representing about 95% of all known plant species, is associated with morphologically complex transporting tissue in plants with a range of additional traits. Therefore, this entire clade was named tracheophytes, or vascular plants. However, some nonvascular plants possess conductive tissues that closely resemble vascular tissue in their organization, structure, and function. Recent molecular studies also point to a highly conserved toolbox of molecular regulators for transporting tissues. Here, we reflect on the distinguishing features of conductive and vascular tissues and their evolutionary history. Rather than sudden emergence of complex, vascular tissues, plant transporting tissues likely evolved gradually, building on pre-existing developmental mechanisms and genetic components. Improved knowledge of the intimate structure and developmental regulation of transporting tissues across the entire taxonomic breadth of extant plant lineages, combined with more comprehensive documentation of the fossil record of transporting tissues, is required for a full understanding of the evolutionary trajectory of transporting tissues.

BdERECTA controls vasculature patterning and phloem-xylem organization in Brachypodium distachyon

BACKGROUND

The vascular system of plants consists of two main tissue types, xylem and phloem. These tissues are organized into vascular bundles that are arranged into a complex network running through the plant that is essential for the viability of land plants. Despite their obvious importance, the genes involved in the organization of vascular tissues remain poorly understood in grasses.

RESULTS

We studied in detail the vascular network in stems from the model grass Brachypodium distachyon (Brachypodium) and identified a large set of genes differentially expressed in vascular bundles versus parenchyma tissues. To decipher the underlying molecular mechanisms of vascularization in grasses, we conducted a forward genetic screen for abnormal vasculature. We identified a mutation that severely affected the organization of vascular tissues. This mutant displayed defects in anastomosis of the vascular network and uncommon amphivasal vascular bundles. The causal mutation is a premature stop codon in ERECTA, a LRR receptor-like serine/threonine-protein kinase. Mutations in this gene are pleiotropic indicating that it serves multiple roles during plant development. This mutant also displayed changes in cell wall composition, gene expression and hormone homeostasis.

CONCLUSION

In summary, ERECTA has a pleiotropic role in Brachypodium. We propose a major role of ERECTA in vasculature anastomosis and vascular tissue organization in Brachypodium.

OsARF11 Promotes Growth, Meristem, Seed, and Vein Formation during Rice Plant Development

The plant hormone auxin acts as a mediator providing positional instructions in a range of developmental processes. Studies in Arabidopsis thaliana L. show that auxin acts in large part via activation of Auxin Response Factors (ARFs) that in turn regulate the expression of downstream genes. The rice (Oryza sativa L.) gene OsARF11 is of interest because of its expression in developing rice organs and its high sequence similarity with MONOPTEROS/ARF5, a gene with prominent roles in A. thaliana development. We have assessed the phenotype of homozygous insertion mutants in the OsARF11 gene and found that in relation to wildtype, osarf11 seedlings produced fewer and shorter roots as well as shorter and less wide leaves. Leaves developed fewer veins and larger areoles. Mature osarf11 plants had a reduced root system, fewer branches per panicle, fewer grains per panicle and fewer filled seeds. Mutants had a reduced sensitivity to auxin-mediated callus formation and inhibition of root elongation, and phenylboronic acid (PBA)-mediated inhibition of vein formation. Taken together, our results implicate OsARF11 in auxin-mediated growth of multiple organs and leaf veins. OsARF11 also appears to play a central role in the formation of lateral root, panicle branch, and grain meristems.

Spatial transcriptional signatures define margin morphogenesis along the proximal-distal and medio-lateral axes in tomato (Solanum lycopersicum) leaves

Leaf morphogenesis involves cell division, expansion, and differentiation in the developing leaf, which take place at different rates and at different positions along the medio-lateral and proximal-distal leaf axes. The gene expression changes that control cell fate along these axes remain elusive due to difficulties in precisely isolating tissues. Here, we combined rigorous early leaf characterization, laser capture microdissection, and transcriptomic sequencing to ask how gene expression patterns regulate early leaf morphogenesis in wild-type tomato (Solanum lycopersicum) and the leaf morphogenesis mutant trifoliate. We observed transcriptional regulation of cell differentiation along the proximal-distal axis and identified molecular signatures delineating the classically defined marginal meristem/blastozone region during early leaf development. We describe the role of endoreduplication during leaf development, when and where leaf cells first achieve photosynthetic competency, and the regulation of auxin transport and signaling along the leaf axes. Knockout mutants of BLADE-ON-PETIOLE2 exhibited ectopic shoot apical meristem formation on leaves, highlighting the role of this gene in regulating margin tissue identity. We mapped gene expression signatures in specific leaf domains and evaluated the role of each domain in conferring indeterminacy and permitting blade outgrowth. Finally, we generated a global gene expression atlas of the early developing compound leaf.

Receptor-like kinase OsCR4 controls leaf morphogenesis and embryogenesis by fixing the distribution of auxin in rice

Cell differentiation is a key event in organ development; it involves auxin gradient formation, cell signaling, and transcriptional regulation. Yet, how these processes are orchestrated during leaf morphogenesis is poorly understood. Here, we demonstrate an essential role for the receptor-like kinase OsCR4 in leaf development. oscr4 loss-of-function mutants displayed short shoots and roots, with tiny, crinkly, or even dead leaves. The delayed outgrowth of the first three leaves and seminal root in oscr4 was due to defects in plumule and radicle formation during embryogenesis. The deformed epidermal, mesophyll, and vascular tissues observed in oscr4 leaves arose at the postembryo stage; the corresponding expression pattern of proOsCR4:GUS in embryos and young leaves suggests that OsCR4 functions in these tissues. Signals from the auxin reporter DR5rev:VENUS were found to be altered in oscr4 embryos and disorganized in oscr4 leaves, in which indole-3-acetic acid accumulation was further revealed by immunofluorescence. OsWOX3A, which is auxin responsive and related to leaf development, was activated extensively and ectopically in oscr4 leaves, partially accounting for the observed lack of cell differentiation. Our data suggest that OsCR4 plays a fundamental role in leaf morphogenesis and embryogenesis by fixing the distribution of auxin.

The Control of Cell Expansion, Cell Division, and Vascular Development by Brassinosteroids: A Historical Perspective

Steroid hormones are important signaling molecules in plants and animals. The plant steroid hormone brassinosteroids were first isolated and characterized in the 1970s and have been studied since then for their functions in plant growth. Treatment of plants or plant cells with brassinosteroids revealed they play important roles during diverse developmental processes, including control of cell expansion, cell division, and vascular differentiation. Molecular genetic studies, primarily in Arabidopsis thaliana, but increasingly in many other plants, have identified many genes involved in brassinosteroid biosynthesis and responses. Here we review the roles of brassinosteroids in cell expansion, cell division, and vascular differentiation, comparing the early physiological studies with more recent results of the analysis of mutants in brassinosteroid biosynthesis and signaling genes. A few representative examples of other molecular pathways that share developmental roles with brassinosteroids are described, including pathways that share functional overlap or response components with the brassinosteroid pathway. We conclude by briefly discussing the origin and conservation of brassinosteroid signaling.

CsIVP functions in vasculature development and downy mildew resistance in cucumber

Domesticated crops with high yield and quality are frequently susceptible to pathogen attack, whereas enhancement of disease resistance generally compromises crop yield. The underlying mechanisms of how plant development and disease resistance are coordinately programed remain elusive. Here, we showed that the basic Helix-Loop-Helix (bHLH) transcription factor Cucumis sativus Irregular Vasculature Patterning (CsIVP) was highly expressed in cucumber vascular tissues. Knockdown of CsIVP caused severe vasculature disorganization and abnormal organ morphogenesis. CsIVP directly binds to vascular-related regulators YABBY5 (CsYAB5), BREVIPEDICELLUS (CsBP), and AUXIN/INDOLEACETIC ACIDS4 (CsAUX4) and promotes their expression. Knockdown of CsYAB5 resulted in similar phenotypes as CsIVP-RNA interference (RNAi) plants, including disturbed vascular configuration and abnormal organ morphology. Meanwhile, CsIVP-RNAi plants were more resistant to downy mildew and accumulated more salicylic acid (SA). CsIVP physically interacts with NIM1-INTERACTING1 (CsNIMIN1), a negative regulator in the SA signaling pathway. Thus, CsIVP is a novel vasculature regulator functioning in CsYAB5-mediated organ morphogenesis and SA-mediated downy mildew resistance in cucumber.

The flowering hormone florigen accelerates secondary cell wall biogenesis to harmonize vascular maturation with reproductive development

Florigen, a proteinaceous hormone, functions as a universal long-range promoter of flowering and concurrently as a generic growth-attenuating hormone across leaf and stem meristems. In flowering plants, the transition from the vegetative phase to the reproductive phase entails the orchestration of new growth coordinates and a global redistribution of resources, signals, and mechanical loads among organs. However, the ultimate cellular processes governing the adaptation of the shoot system to reproduction remain unknown. We hypothesized that if the mechanism for floral induction is universal, then the cellular metabolic mechanisms underlying the conditioning of the shoot system for reproduction would also be universal and may be best regulated by florigen itself. To understand the cellular basis for the vegetative functions of florigen, we explored the radial expansion of tomato stems. RNA-Seq and complementary genetic and histological studies revealed that florigen of endogenous, mobile, or induced origins accelerates the transcription network navigating secondary cell wall biogenesis as a unit, promoting vascular maturation and thereby adapting the shoot system to the developmental needs of the ensuing reproductive phase it had originally set into motion. We then demonstrated that a remarkably stable and broadly distributed florigen promotes MADS and MIF genes, which in turn regulate the rate of vascular maturation and radial expansion of stems irrespective of flowering or florigen level. The dual acceleration of flowering and vascular maturation by florigen provides a paradigm for coordinated regulation of independent global developmental programs.

A functional imaging study of germinating oilseed rape seed

Germination, the process whereby a dry, quiescent seed springs to life, has been a focus of plant biologist for many years, yet the early events following water uptake, during which metabolism of the embryo is restarted, remain enigmatic. Here, the nature of the cues required for this restarting in oilseed rape (Brassica napus) seed has been investigated. A holistic in vivo approach was designed to display the link between the entry and allocation of water, metabolic events and structural changes occurring during germination. For this, we combined functional magnetic resonance imaging with Fourier transform infrared microscopy, fluorescence-based respiration mapping, computer-aided seed modeling and biochemical tools. We uncovered an endospermal lipid gap, which channels water to the radicle tip, from whence it is distributed via embryonic vasculature toward cotyledon tissues. The resumption of respiration is initiated first in the endosperm, only later spreading to the embryo. Sugar metabolism and lipid utilization are linked to the spatiotemporal sequence of tissue rehydration. Together, this imaging study provides insights into the spatial aspects of key events in oilseed rape seeds leading to germination. It demonstrates how seed architecture predetermines the pattern of water intake, which sets the stage for the orchestrated restart of life.

Wound signaling of regenerative cell reprogramming

Plants are sessile organisms that must deal with various threats resulting in tissue damage, such as herbivore feeding, and physical wounding by wind, snow or crushing by animals. During wound healing, phytohormone crosstalk orchestrates cellular regeneration through the establishment of tissue-specific asymmetries. In turn, hormone-regulated transcription factors and their downstream targets coordinate cellular responses, including dedifferentiation, cell cycle reactivation and vascular regeneration. By comparing different examples of wound-induced tissue regeneration in the model plant Arabidopsis thaliana, a number of key regulators of developmental plasticity of plant cells have been identified. We present the relevance of these findings and of the dynamic establishment of differential auxin gradients for cell reprogramming after wounding.

Plant Phosphoglycerolipids: The Gatekeepers of Vascular Cell Differentiation

In higher plants, the plant vascular system has evolved as an inter-organ communication network essential to deliver a wide range of signaling factors among distantly separated organs. To become conductive elements, phloem and xylem cells undergo a drastic differentiation program that involves the degradation of the majority of their organelles. While the molecular mechanisms regulating such complex process remain poorly understood, it is nowadays clear that phosphoglycerolipids display a pivotal role in the regulation of vascular tissue formation. In animal cells, this class of lipids is known to mediate acute responses as signal transducers and also act as constitutive signals that help defining organelle identity. Their rapid turnover, asymmetrical distribution across subcellular compartments as well as their ability to rearrange cytoskeleton fibers make phosphoglycerolipids excellent candidates to regulate complex morphogenetic processes such as vascular differentiation. Therefore, in this review we aim to summarize, emphasize and connect our current understanding about the involvement of phosphoglycerolipids in phloem and xylem differentiation.

Control of vein network topology by auxin transport

Background

Tissue networks such as the vascular networks of plant and animal organs transport signals and nutrients in most multicellular organisms. The transport function of tissue networks depends on topological features such as the number of networks’ components and the components’ connectedness; yet what controls tissue network topology is largely unknown, partly because of the difficulties in quantifying the effects of genes on tissue network topology. We address this problem for the vein networks of plant leaves by introducing biologically motivated descriptors of vein network topology; we combine these descriptors with cellular imaging and molecular genetic analysis; and we apply this combination of approaches to leaves of Arabidopsis thaliana that lack function of, overexpress or misexpress combinations of four PIN-FORMED (PIN) genes—PIN1, PIN5, PIN6, and PIN8—which encode transporters of the plant signal auxin and are known to control vein network geometry.

Results

We find that PIN1 inhibits vein formation and connection, and that PIN6 acts redundantly to PIN1 in these processes; however, the functions of PIN6 in vein formation are nonhomologous to those of PIN1, while the functions of PIN6 in vein connection are homologous to those of PIN1. We further find that PIN8 provides functions redundant and homologous to those of PIN6 in PIN1-dependent inhibition of vein formation, but that PIN8 has no functions in PIN1/PIN6-dependent inhibition of vein connection. Finally, we find that PIN5 promotes vein formation; that all the vein-formation-promoting functions of PIN5 are redundantly inhibited by PIN6 and PIN8; and that these functions of PIN5, PIN6, and PIN8 are independent of PIN1.

Conclusions

Our results suggest that PIN-mediated auxin transport controls the formation of veins and their connection into networks.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-015-0208-3) contains supplementary material, which is available to authorized users.

Prenatal plumbing--vascular tissue formation in the plant embryo

The first vascular tissue precursors are specified early during embryogenesis. These precursors give rise to the multi-layered cylinder of hypocotyl and root through controlled, oriented divisions. Concomitant with its growth, the bundle is patterned into xylem and phloem tissues, and intervening procambial cells. These patterns are later maintained during post-embryonic growth and vascular cells will eventually differentiate, displaying characteristic secondary cell wall modifications. Given that the vascular system forms de novo in a simple yet predictable fashion, the embryo provides an excellent model system to study early developmental aspects of vascular tissue formation. However, the benefits of this model are only beginning to be exploited, and most knowledge about the vascular development is derived from growing post-embryonic tissues. Importantly, it is unclear how much of these established post-embryonic mechanisms can be extrapolated to tissue formation during embryogenesis. Here we review concepts established in the model plant Arabidopsis thaliana and focus on recent advances made in understanding embryonic vascular development.

Intercellular trafficking of transcription factors in the vascular tissue patterning

Throughout life cycles, plants grow in an indeterminate manner by adding new cells and organs with specialized functions. Newly emerging cells acquire their identities depending on their positions relative to the neighboring cells. Exchanging positional signals between cells is critical in this process. Recent studies showed that many transcription factors move between cells or between organs in forms of proteins and messenger RNA (mRNA). Some of these were found to be important positional signals for cell type patterning. Cell type patterning in the vascular system is no exception from this. In this review, we describe recent discoveries of mobile transcription factors that function as positional signals for vascular tissue patterning and propose how these transcription factors integrate with other forms of signals.

Message in a bottle: small signalling peptide outputs during growth and development

Classical and recently found phytohormones play an important role in plant growth and development, but plants additionally control these processes through small signalling peptides. Over 1000 potential small signalling peptide sequences are present in the Arabidopsis genome. However, to date, a mere handful of small signalling peptides have been functionally characterized and few have been linked to a receptor. Here, we assess the potential small signalling peptide outputs, namely the molecular, biochemical, and morphological changes they trigger in Arabidopsis. However, we also include some notable studies in other plant species, in order to illustrate the varied effects that can be induced by small signalling peptides. In addition, we touch on some evolutionary aspects of small signalling peptides, as studying their signalling outputs in single-cell green algae and early land plants will assist in our understanding of more complex land plants. Our overview illustrates the growing interest in the small signalling peptide research area and its importance in deepening our understanding of plant growth and development.

Role of hormones in controlling vascular differentiation and the mechanism of lateral root initiation

The vascular system in plants is induced and controlled by streams of inductive hormonal signals. Auxin produced in young leaves is the primary controlling signal in vascular differentiation. Its polar and non-polar transport pathways and major controlling mechanisms are clarified. Ethylene produced in differentiating protoxylem vessels is the signal that triggers lateral root initiation, while tumor-induced ethylene is a limiting and controlling factor of crown gall development and its vascular differentiation. Gibberellin produced in mature leaves moves non-polarly and promotes elongation, regulates cambium activity and induces long fibers. Cytokinin from the root cap moves upward to promote cambial activity and stimulate shoot growth and branching, while strigolactone from the root inhibits branching. Furthermore, the role of the hormonal signals in controlling the type of differentiating vascular elements and gradients of conduit size and density, and how they regulate plant adaptation and have shaped wood evolution are elucidated.

The plant vascular system: evolution, development and functions

The emergence of the tracheophyte-based vascular system of land plants had major impacts on the evolution of terrestrial biology, in general, through its role in facilitating the development of plants with increased stature, photosynthetic output, and ability to colonize a greatly expanded range of environmental habitats. Recently, considerable progress has been made in terms of our understanding of the developmental and physiological programs involved in the formation and function of the plant vascular system. In this review, we first examine the evolutionary events that gave rise to the tracheophytes, followed by analysis of the genetic and hormonal networks that cooperate to orchestrate vascular development in the gymnosperms and angiosperms. The two essential functions performed by the vascular system, namely the delivery of resources (water, essential mineral nutrients, sugars and amino acids) to the various plant organs and provision of mechanical support are next discussed. Here, we focus on critical questions relating to structural and physiological properties controlling the delivery of material through the xylem and phloem. Recent discoveries into the role of the vascular system as an effective long-distance communication system are next assessed in terms of the coordination of developmental, physiological and defense-related processes, at the whole-plant level. A concerted effort has been made to integrate all these new findings into a comprehensive picture of the state-of-the-art in the area of plant vascular biology. Finally, areas important for future research are highlighted in terms of their likely contribution both to basic knowledge and applications to primary industry.

Auxin-associated initiation of vascular cell differentiation by LONESOME HIGHWAY

Plant vascular tissues are essential for the existence of land plants. Many studies of transcriptional regulation and cell-cell communication have revealed the process underlying the development of vascular tissues from vascular initial cells. However, the initiation of vascular cell differentiation is still a mystery. Here, we report that LONESOME HIGHWAY (LHW), which encodes a bHLH transcription factor, is expressed in pericycle-vascular mother cells at the globular embryo stage and is required for proper asymmetric cell division to generate vascular initial cells. In addition, ectopic expression of LHW elicits an ectopic auxin response. Moreover, LHW is required for the correct expression patterns of components related to auxin flow, such as PIN-FORMED 1 (PIN1), MONOPTEROS (MP) and ATHB-8, and ATHB-8 partially rescues the vascular defects of lhw. These results suggest that LHW functions as a key regulator to initiate vascular cell differentiation in association with auxin regulation.

The plant-specific dof transcription factors family: new players involved in vascular system development and functioning in Arabidopsis

In higher plants phloem and xylem are responsible for long-distance transport of water, nutrients, and signals that act systemically at short or long-distance to coordinate developmental processes. The formation of the plant vascular system is a complex process that integrates signaling events and gene regulation at transcriptional and posttranscriptional levels. Thanks to transcriptomic and proteomic analysis we start to better understand the mechanisms underlying the formation and the functioning of the vascular system. The role of the DNA-binding with one finger (Dof TFs), a group of plant-specific transcription factors, recently emerged as part of the transcriptional regulatory networks acting on the formation and functioning of the vascular tissues. More than half of the members of this TF family are expressed in the vascular system. In addition some of them have been proposed to be mobile proteins, suggesting a possible role in the control of short- or long-distance signaling as well. This review summarizes the current knowledge on Dof TFs family in Arabidopsis with a special focus on their role in vascular development and functioning.

Deletion of MP/ARF5 domains III and IV reveals a requirement for Aux/IAA regulation in Arabidopsis leaf vascular patterning

Combinatorial interactions of AUXIN RESPONSE FACTORs (ARFs) and auxin/indole acetic acid (Aux/IAA) proteins through their common domains III and IV regulate auxin responses, but insight into the functions of individual proteins is still limited. As a new tool to explore this regulatory network, we generated a gain-of-function ARF genotype by eliminating domains III and IV from the functionally well-characterized ARF MONOPTEROS(MP)/ARF5. This truncated version of MP, termed MPΔ, conferred complementing MP activity, but also displayed a number of semi-dominant traits affecting auxin signaling and organ patterning. In MPΔ, the expression levels of many auxin-inducible genes, as well as rooting properties and vascular tissue abundance, were enhanced. Lateral organs were narrow, pointed and filled with parallel veins. This effect was epistatic over the vascular hypotrophy imposed by certain Aux/IAA mutations. Further, in MPΔ leaves, failure to turn off the procambium-selecting gene PIN1 led to the early establishment of oversized central procambial domains and very limited subsequent lateral growth of the leaf lamina. We conclude that MPΔ can selectively uncouple a single ARF from regulation by Aux/IAA proteins and can be used as a genetic tool to probe auxin pathways and explore leaf development.

Signaling and gene regulatory programs in plant vascular stem cells

A key question about the development of multicellular organisms is how they precisely control the complex pattern formation during their growth. For plants to grow for many years, a tight balance between pluripotent dividing cells and cells undergoing differentiation should be maintained within stem cell populations. In this process, cell-cell communication plays a central role by creating positional information for proper cell type patterning. Cell-type specific gene regulatory networks govern differentiation of cells into particular cell types. In this review, we will provide a comprehensive overview of emerging key signaling and regulatory programs in the stem cell population that direct morphogenesis of plant vascular tissues.

Auxin, self-organisation, and the colonial nature of plants

Evolution has provided at least two particularly successful independent solutions to the problems of multicellularity - animals and higher plants. An obvious requirement for successful multicellularity is communication between different parts of the organism, both locally, for example between neighbouring cells, and over very long distances. Recent advances in understanding hormone signalling networks in plants are beginning to reveal how co-ordination of activity across the whole plant body can be achieved despite the lack of a control centre, typical of animal systems. Of particular importance in this distributed regulatory approach are the self-organising properties of the transport system for the plant hormone auxin. This review examines the integrative role of the auxin transport network in co-ordinating plant growth and development.