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Leaves form veins whose patterns vary from a single vein running the length of the leaf to networks of staggering complexity where huge numbers of veins connect to other veins at both ends. For the longest time, vein formation was thought to be controlled only by the polar, cell-to-cell transport of the plant hormone auxin; recent evidence suggests that is not so. Instead, it turns out that vein patterning features are best accounted for by a combination of polar auxin transport, facilitated auxin diffusion through plasmodesma intercellular channels, and auxin signal transduction—though the latter's precise contribution remains unclear. Equally unclear remain the sites of auxin production during leaf development, on which that vein patterning mechanism ought to depend. Finally, whether that vein patterning mechanism can account for the variety of vein arrangements found in nature remains unknown. Addressing those questions will be the exciting challenge of future research. [ABSTRACT FROM AUTHOR]

Over centuries of plant morphological research, biologists have enthusiastically explored how distinct vascular arrangements have diversified. These investigations have focused on the evolution of steles and secondary growth and examined the diversity of vascular tissues (xylem and phloem), including atypical developmental pathways generated through modifications to the typical development of ancestral ontogenies. A shared vernacular has evolved for communicating on the diversity of alternative ontogenies in seed plants. Botanists have traditionally used the term 'anomalous secondary growth' which was later renamed to 'cambial variants' by late Dr. Sherwin Carlquist (1988). However, the term 'cambial variants' can be vague in meaning since it is applied for developmental pathways that do not necessarily originate from cambial activity. Here, we review the 'cambial variants' concept and propose the term 'vascular variants' as a more inclusive overarching framework to interpret alternative vascular ontogenies in plants. In this framework, vascular variants are defined by their developmental origin (instead of anatomical patterns), allowing the classification of alternative vascular ontogenies into three categories: (i) procambial variants , (ii) cambial variants and (iii) ectopic cambia. Each category includes several anatomical patterns. Vascular variants, which represent broader developmental based groups, can be applied to both extant and fossil plants, and thereby offer a more adequate term from an evolutionary perspective. An overview of the developmental diversity and phylogenetic distribution of vascular variants across selected seed plants is provided. Finally, this viewpoint discusses the evolutionary implications of vascular variants. [ABSTRACT FROM AUTHOR]

The vascular cambium is an extensive and permanent secondary meristem with wood cells products of periclinal divisions commonly contributed to two directions and arranged in radial files of trees. Cambium activity is the origin of timber production. Taxodium ascendens Brongn is an exotic species in China, and its apical meristem and cambial activity are still elusive, resulting in a lack of understanding about its wood formation and improvement. We thus addressed this knowledge gap by studying Cambium activity. For studying, twigs from five 30-year-old healthy trees were collected between February-2017 and March-2018. Anatomy deciphered its apical meristem with a Cryptomeria–Abies type. The procambium appeared after leaf primordium and initially presented five lobes as observed transversely from a one-year-old shoot. The procambium under the apical differentiated into protophloem first and then protoxylem toward the inside. It means that protoxylem differentiated later than protophloem did. After dormancy, the vascular cambium began to be active, starting in early April 2017, which was later than shoot differentiation. On 25 July 2017, the cambial zone had 9–10 immature xylem cell layers. Both initiation and cessation of the xylem preceded that of the phloem. Until 10 October 2017, few immature elements were found, indicating the translation of cells from activity to dormancy. On 15 November 2017, the cambium contained 3–4 cells in radial rows, which demonstrated the dormancy of the cambium until next spring. Furthermore, immature xylem elements increased as cell layers in the cambium zone and cell fission increased. The growth pattern of T. ascendens revealed that cambial activity is highly seasonal and dependent on changes in abiotic conditions. Thus, the wood formation in the species will be significantly altered in a changing climatic pattern. These enhance our understanding of tree growth science, wood formation, wood structure, wood properties variation and wood improvement in tree breeding. [ABSTRACT FROM AUTHOR]

Background:The production of a new lateral root from parental root primary tissues has been investigated extensively, and the most important regulatory mechanisms are now well known. A first regulatory mechanism is based on the synthesis of small peptides which interact ectopically with membrane receptors to elicit a modulation of transcription factor target genes. A second mechanism involves a complex cross-talk between plant hormones. It is known that lateral roots are formed even in parental root portions characterized by the presence of secondary tissues, but there is not yet agreement about the putative tissue source providing the cells competent to become founder cells of a new root primordium. Scope:We suggest models of possible regulatory mechanisms for inducing specific root vascular cambium (VC) stem cells to abandon their activity in the production of xylem and phloem elements and to start instead the construction of a new lateral root primordium. Considering the ontogenic nature of the VC, the models which we suggest are the result of a comparative review of mechanisms known to control the activity of stem cells in the root apical meristem, procambium and VC. Stem cells in the root meristems can inherit various competences to play different roles, and their fate could be decided in response to cross-talk between endogenous and exogenous signals. Conclusions:We have found a high degree of relatedness among the regulatory mechanisms controlling the various root meristems. This fact suggests that competence to form new lateral roots can be inherited by some stem cells of the VC lineage. This kind of competence could be represented by a sensitivity of specific stem cells to factors such as those presented in our models.

The vascular cambium is an extensive and permanent secondary meristem with wood cells products of periclinal divisions commonly contributed to two directions and arranged in radial files of trees. Cambium activity is the origin of timber production. Taxodium ascendens Brongn is an exotic species in China, and its apical meristem and cambial activity are still elusive, resulting in a lack of understanding about its wood formation and improvement. We thus addressed this knowledge gap by studying Cambium activity. For studying, twigs from five 30-year-old healthy trees were collected between February-2017 and March-2018. Anatomy deciphered its apical meristem with a Cryptomeria–Abies type. The procambium appeared after leaf primordium and initially presented five lobes as observed transversely from a one-year-old shoot. The procambium under the apical differentiated into protophloem first and then protoxylem toward the inside. It means that protoxylem differentiated later than protophloem did. After dormancy, the vascular cambium began to be active, starting in early April 2017, which was later than shoot differentiation. On 25 July 2017, the cambial zone had 9–10 immature xylem cell layers. Both initiation and cessation of the xylem preceded that of the phloem. Until 10 October 2017, few immature elements were found, indicating the translation of cells from activity to dormancy. On 15 November 2017, the cambium contained 3–4 cells in radial rows, which demonstrated the dormancy of the cambium until next spring. Furthermore, immature xylem elements increased as cell layers in the cambium zone and cell fission increased. The growth pattern of T. ascendens revealed that cambial activity is highly seasonal and dependent on changes in abiotic conditions. Thus, the wood formation in the species will be significantly altered in a changing climatic pattern. These enhance our understanding of tree growth science, wood formation, wood structure, wood properties variation and wood improvement in tree breeding.

The development of secondary vascular tissue enhances the transport capacity and mechanical strength of plant bodies, while contributing a huge proportion of the world's biomass in the form of wood. Cell divisions in the cambium, which constitutes the vascular meristem, provide progenitors from which conductive xylem and phloem are derived. The cambium is a somewhat unusual stem cell population in two respects, making it an interesting subject for developmental research. Firstly, it arises post-germination, and thus represents a model for understanding stem cell initiation beyond embryogenesis. Secondly, xylem and phloem differentiate on opposing sides of cambial stem cells, making them bifacial in nature. Recent discoveries in Arabidopsis thaliana have provided insight into the molecular mechanisms that regulate the initiation, patterning, and maintenance of the cambium. In this review, the roles of intercellular signalling via mobile transcription factors, peptide–receptor modules, and phytohormones are described. Crosstalk between these regulatory pathways is becoming increasingly apparent, yet the underlying mechanisms are not fully understood. Future study of the interaction between multiple independently identified regulators, as well as the functions of their orthologues in trees, will deepen our understanding of radial growth in plants. [ABSTRACT FROM AUTHOR]

Main conclusion: Plant CLE peptides, which regulate stem cell maintenance in shoot and root meristems and in vascular bundles through LRR family receptor kinases, are novel, complex, and to some extent conserved. Over the past two decades, peptide ligands of the CLAVATA3 (CLV3) /Embryo Surrounding Region (CLE) family have been recognized as critical short- and long-distance communication signals in plants, especially for stem cell homeostasis, cell fate determination and physiological responses. Stem cells located at the shoot apical meristem (SAM), the root apical meristem (RAM) and the procambium divide and differentiate into specialized cells that form a variety of tissues such as epidermis, ground tissues, xylem and phloem. In the SAM of Arabidopsis (Arabidopsis thaliana), the CLV3 peptide restricts the number of stem cells via leucine-rich repeat (LRR)-type receptor kinases. In the RAM, root-active CLE peptides are critical negative regulators, while ROOT GROWTH FACTOR (RGF) peptides are positive regulators in stem cell maintenance. Among those root-active CLE peptides, CLE25 promotes, while CLE45 inhibits phloem differentiation. In vascular bundles, TRACHEARY ELEMENT DIFFERENTIATION INHIBITORY FACTOR (TDIF)/CLE41/CLE44 promotes procambium cell division, and prevents xylem differentiation. Orthologs of CLV3 have been identified in liverwort (Marchantia polymorpha), tomato (Solanum lycopersicum), rice (Oryza sativa), maize (Zea mays) and lotus (Lotus japonicas), suggesting that CLV3 is an evolutionarily conserved signal in stem cell maintenance. However, functional characterization of endogenous CLE peptides and corresponding receptor kinases, and the downstream signal transduction has been challenging due to their genome-wide redundancies and rapid evolution. [ABSTRACT FROM AUTHOR]

The stele concept is one of the oldest enduring concepts in plant biology. Here, I review the history of the concept and build an argument for an updated view of steles and their evolution. Studies of stelar organization have generated a widely ranging array of definitions that determine the way we classify steles and construct scenarios about the evolution of stelar architecture. Because at the organismal level biological evolution proceeds by changes in development, concepts of structure need to be grounded in development to be relevant in an evolutionary perspective. For the stele, most traditional definitions that incorporate development have viewed it as the totality of tissues that either originate from procambium – currently the prevailing view – or are bordered by a boundary layer (e.g. endodermis). Consensus between these two perspectives can be reached by recasting the stele as a structural entity of dual nature. Following a brief review of the history of the stele concept, basic terminology related to stelar organization, and traditional classifications of the steles, I revisit boundary layers from the perspective of histogenesis as a dynamic mosaic of developmental domains. I review anatomical and molecular data to explore and reaffirm the importance of boundary layers for stelar organization. Drawing on information from comparative anatomy, developmental regulation, and the fossil record, I propose a stele concept that integrates both the boundary layer and the procambial perspectives, consistent with a dual nature of the stele. This dual stele model posits that stelar architecture is determined at the apical meristem by two major cell fate specification events: a first one that specifies a provascular domain and its boundaries, and a second event that specifies a procambial domain (which will mature into conducting tissues) from cell subpopulations of the provascular domain. If the position and extent of the developmental domains defined by the two events are determined by different concentrations of the same morphogen (most likely auxin), then the distribution of this organizer factor in the shoot apical meristem, as modulated by changes in axis size and the effect of lateral organs, can explain the different stelar configurations documented among tracheophytes. This model provides working hypotheses that incorporate assumptions and generate implications that can be tested empirically. The model also offers criteria for an updated classification of steles in line with current understanding of plant development. In this classification, steles fall into two major categories determined by the configuration of boundary layers: boundary protosteles and boundary siphonosteles, each with subtypes defined by the architecture of the vascular tissues. Validation of the dual stele model and, more generally, in‐depth understanding of the regulation of stelar architecture, will necessitate targeted efforts in two areas: (i) the regulation of procambium, vascular tissue, and boundary layer specification in all extant vascular plants, considering that most of the diversity in stelar architecture is hosted by seed‐free plants, which are the least explored in terms of developmental regulation; (ii) the configuration of vascular tissues and, especially, boundary layers, in as many extinct lineages as possible. [ABSTRACT FROM AUTHOR]

In contrast to animals, which complete organogenesis early in their development, plants continuously produce organs, and structures throughout their entire lifecycle. Plants achieve the continuous growth of organs through the initiation and maintenance of meristems that populate the plant body. Plants contain two apical meristems, one at the shoot and one root, to produce the lateral organs of the shoot and the cell files of the root, respectively. Additional meristems within the plant produce branches while others produce the cell types within the vasculature system. Throughout development, plants must balance producing organs and maintaining their meristems, which requires tightly controlled regulations. This review focuses on the various plant meristems, how cells within these meristems maintain their identity, and particularly the molecular players that regulate stem cell maintenance. In addition, we summarize cell types which share molecular features with meristems, but do not follow the same rules regarding maintenance, including pericycle and rachis founder cells. Together, these populations of cells contribute to the entire organogenesis of plants. [ABSTRACT FROM AUTHOR]

Aragoa is a shrubby genus endemic to páramo in the northern Andes representing the sister group to Plantago and Limosella. Stem and leaf structure of Aragoa corrugatifolia were studied to clarify the evolutionary pathways and ecological significance of their anatomical traits. Aragoa and Plantago share a non-fascicular primary vascular system, rayless wood and secondary phloem, and anomocytic stomata. Aragoa is distinctive from most Plantaginaceae in the presence of cortical aerenchyma and of helical thickenings in vessels. Its procambium emerges in the primary meristem ring as a continuous cylinder. The view on the ring meristem and procambial strands as developmental stages in the formation of a primary vascular system is not relevant for Aragoa, and probably for other Plantaginaceae. The raylessness is synapomorphic for the crown clade of Plantaginaceae comprising Aragoa, Littorella, Plantago, Veronica, Picrorhiza, Wulfenia, and Veronicastrum. The loss of rays is thought to be predetermined by procambium rather than by the vascular cambium. The extremely narrow vessels with helical thickenings are presumably adaptive to hydric and thermic conditions of páramo. Cortical aerenchyma is thought to be a response to the local hypoxia caused by the water retained by ericoid leaves. Trichomes on juvenile leaves are expected to be the traits of considerable taxonomic importance.

In plants, cells do not migrate. Tissues are frequently arranged in concentric rings; thus, expansion of inner layers is coordinated with cell division and/or expansion of cells in outer layers. In Arabidopsis stems, receptor kinases, PXY and ER, genetically interact to coordinate vascular proliferation and organisation via inter-tissue signalling. The contribution of PXY and ER paralogues to stem patterning is not known, nor is their function understood in hypocotyls, which undergo considerable radial expansion. Here, we show that removal of all PXY and ER gene-family members results in profound cell division and organisation defects. In hypocotyls, these plants failed to transition to true radial growth. Gene expression analysis suggested that PXY and ER cross- and inter-family transcriptional regulation occurs, but it differs between stem and hypocotyl. Thus, PXYand ER signalling interact to coordinate development in a distinct manner in different organs. We anticipate that such specialised local regulatory relationships, where tissue growth is controlled via signals moving across tissue layers, may coordinate tissue layer expansion throughout the plant body. [ABSTRACT FROM AUTHOR]

Summary: The post‐embryonic growth of plants requires the activities of apical meristems and lateral meristems. In the meristems, self‐proliferation and differentiation of stem cells is tightly modulated by plant hormone signaling networks and specific transcription factors. Despite extensive studies on stem cell maintenance in plants, the mechanism by which stem cells are initially established is largely unknown. Vascular stem cells consisting of procambial/cambial cells give rise to xylem and phloem cells. In this study, we analyzed the establishment of procambial cells using the in vitro culture system VISUAL, in which mesophyll cells rapidly differentiate into xylem tracheary elements and phloem sieve elements via procambial cells. We found that procambial cell formation in VISUAL is initiated by light, which can be replaced by application of gibberellin (GA). Gibberellin was able to promote procambial cell formation through degradation of DELLA, whereas light did not elevate the endogenous GA content. Indeed, light in combination with bikinin reduced the accumulation of DELLA protein in VISUAL. Consistently, overexpression of a constitutively active DELLA protein repressed vascular cell differentiation even under light. These combined results suggest that DELLA signaling suppresses procambial cell formation during vascular development in VISUAL. [ABSTRACT FROM AUTHOR]

Background: Multicellular organisms rely on the transmission of information between cells to coordinate various biological processes during growth and development. Plants, like animals, utilize small peptide ligands as signaling molecules to transmit information between cells. These polypeptides typically act as extracellular messengers that are perceived by membrane-bound receptors, which then transduce the signal into the recipient cell to modify downstream gene transcription. The CLAVATA3/EMBRYO SURROUNDING REGION-RELATED (CLE) proteins represent one of the largest and best understood families of small polypeptides in plants. Members of the CLE family play critical roles in mediating cell fate decisions during plant development, particularly within the unique meristem structures that contain stem cell reservoirs acting as sources of cells for continuous organ formation. Objective: Here we review the roles of CLE family members in regulating the activity of the shoot apical meristems that generate the aerial parts of the plants, and of the vascular meristems that produce the sugar- and water-conducting tissues. Methods: A systematic literature search was performed using the Google Scholar and PubMed search engines. The keywords 'CLE', 'CLV3', 'TDIF', 'meristem', and 'plant stem cells' were used as search terms. The 95 retrieved articles, dating from 1992, were organized by topic and their key findings incorporated into the text. Results: We summarize our current understanding of how the CLE peptide CLV3 orchestrates the activity of shoot apical meristems, describing its expression, processing and movement, as well as its intracellular signal transduction pathways, key target genes and downstream gene regulatory networks. We also discuss the roles of CLE peptide signaling in the vascular meristems to promote procambial cell proliferation and suppress xylem differentiation. Conclusions: Signaling pathways mediated by CLE peptides are critical for stem cell maintenance and differentiation in shoot apical and vascular meristems in plants, exposing CLE genes as potential targets for increasing yield and biomass production. While large numbers of CLE genes are being discovered in plants, only a few have been functionally characterized. We anticipate that future research will continue to elucidate the roles of the CLE family in plant development, and their potential impacts on agriculture and commerce. [ABSTRACT FROM AUTHOR]

Based on mutant phenotypes the MONOPTEROS (MP)/Auxin Response Factor 5 (ARF5) gene acts in several developmental processes including leaf vein development. Since overlapping functions among ARF genes are common, we assessed the related ARF 3-8 and 19 genes for potential overlap in expression during vein development using in-situ hybridization. Like MP/ARF5, ARF3 was expressed in preprocambial and procambial cells. ARF7 was also expressed in procambial cells, close to and during vein differentiation. ARF19 was expressed in differentiating vessel elements. To assess if genes with vein expression have overlapping functions, double mutants were generated. While arf3, 5 and 7 mutants formed leaves normally, double mutant combinations of mp/arf5 with arf3 or arf7 resulted in a breakdown of leaf formation. Instead, novel structures not present in any of the single mutants formed. The results implicate ARF3 and ARF7 in rosette leaf formation and suggest that their functions overlap and act in parallel with MP/ARF5 in this process. The observed vascular expression patterns suggest unique functions (ARF7 and 19) and potentially overlapping functions (ARF3 and 5) in vein development. Since arf3 arf5 double mutants do not form leaves, assessment of their potential combined action in vein development will require the use of conditional mutants.

Classical histology describes the histological organization in Zea mays as having a “closed organization” that differs from Arabidopsis with the development of xylem conforming to predictable rules. We speculated that root apical meristem organization in a wild subspecies of Z. mays (a teosinte) would differ from a domestic sweetcorn cultivar (‘Honey Bantam’). Careful comparison could contribute to understanding how evolutionary processes and the domestication of maize have affected root development. Root tips of seedlings were prepared and sectioned for light microscopy. Most sections were treated with RNase before staining to increase contrast between the walls and cytoplasm. Longitudinal and serial transverse sections were analyzed using computer imaging to determine the position and timing of key xylem developmental events. Metaxylem development in mexicana teosinte differed from sweetcorn only in that the numbers of late-maturing metaxylem vessels in the latter are typically two-fold greater and the number of cells in the transverse section of procambium were greater in the latter, but parenchymatous cell sizes were not statistically different. Promeristems of both were nearly identical in size and organization, but did not operate quite as previously described. Mitotic activity was rare in the quiescent centers, but occasionally a synchronized pulse of mitoses was observed there. Our reinterpretation of histogen theory and procambium development should be useful for future detailed studies of regulation of development, and perhaps its evolution, in this species.

The change of pectin epitopes during procambium-cambium continuum development was investigated by immunolocalization in poplar. The monoclonal antibody JIM5 labels homogalacturonan (HGA) with a low degree of esterification, and the monoclonal antibody JIM7 labels HGA with a high degree of methyl-esterification. Arabinan, rather than galactan, and HGA with low degree of esterification were located in the cell walls of procambial, while HGA with a low degree of esterification was located in the tangential walls, and galactan was located in both the tangential and radial walls of procambial, yet nearly no arabinan was located in the tangential walls of the cambial cells. The changes in pectin distribution took place when periclinal divisions appeared within a procambial trace. The distribution difference of pectin epitopes was also present in procambium-cambium derivatives. The arabinan existed in all cell walls of primary xylem, but was absent from the tangential walls of secondary xylem cells. The galactan existed only in mature primary phloem. Furthermore, 19 pectin methylesterases (PMEs) genes were identified by RNA sequencing, six genes presented highly differentially and were supposed to be involved in the cell wall esterification process. The results provide direct evidence of the dynamic changes of pectin epitopes during the development of the procambium-cambium continuum in poplar. [ABSTRACT FROM AUTHOR]

To understand the molecular mechanisms of rice aerial organ development, we identified a mutant gene that caused a significant decrease in the width of aerial organs, termed ABNORMAL VASCULAR BUNDLES ( AVB)., Histological analysis showed that the slender aerial organs were caused by cell number reduction. In avb, the number of vascular bundles in aerial organs was reduced, whereas the area of the vascular bundles was increased., Ploidy analysis and the in situ expression patterns of histone H4 confirmed that cell proliferation was impaired during lateral primordia development, whereas procambium cells showed a greater ability to undergo cell division in avb. RNA sequencing ( RNA-seq) showed that the development process was affected in avb. Map-based cloning and genetic complementation demonstrated that AVB encodes a land plant conserved protein with unknown functions., Our research shows that AVB is involved in the maintenance of the normal cell division pattern in lateral primordia development and that the AVB gene is required for procambium establishment following auxin signaling. [ABSTRACT FROM AUTHOR]

Vascular tissue, comprising xylem and phloem, is responsible for the transport of water and nutrients throughout the plant body. Such tissue is continually produced from stable populations of stem cells, specifically the procambium during primary growth and the cambium during secondary growth. As the majority of plant biomass is produced by the cambium, there is an obvious demand for an understanding of the genetic mechanisms that control the rate of vascular cell division. Moreover, wood is an industrially important product of the cambium, and research is beginning to uncover similar mechanisms in trees such as poplar. This review focuses upon recent work that has identified the major molecular pathways that regulate procambial and cambial activity. [ABSTRACT FROM AUTHOR]

Over centuries of plant morphological research, biologists have enthusiastically explored how distinct vascular arrangements have diversified. These investigations have focused on the evolution of steles and secondary growth and examined the diversity of vascular tissues (xylem and phloem), including atypical developmental pathways generated through modifications to the typical development of ancestral ontogenies. A shared vernacular has evolved for communicating on the diversity of alternative ontogenies in seed plants. Botanists have traditionally used the term 'anomalous secondary growth' which was later renamed to 'cambial variants' by late Dr. Sherwin Carlquist (1988). However, the term 'cambial variants' can be vague in meaning since it is applied for developmental pathways that do not necessarily originate from cambial activity. Here, we review the 'cambial variants' concept and propose the term 'vascular variants' as a more inclusive overarching framework to interpret alternative vascular ontogenies in plants. In this framework, vascular variants are defined by their developmental origin (instead of anatomical patterns), allowing the classification of alternative vascular ontogenies into three categories: (i) procambial variants , (ii) cambial variants and (iii) ectopic cambia . Each category includes several anatomical patterns. Vascular variants, which represent broader developmental based groups, can be applied to both extant and fossil plants, and thereby offer a more adequate term from an evolutionary perspective. An overview of the developmental diversity and phylogenetic distribution of vascular variants across selected seed plants is provided. Finally, this viewpoint discusses the evolutionary implications of vascular variants., Competing Interests: The author declare no conflicts of interest., (© The Author(s) 2023. Published by Oxford University Press on behalf of the Annals of Botany Company.)

A significant proportion of terrestrial biomass is constituted of xylem cells that make up woody plant tissue. Xylem is required for water transport, and is present in the vascular tissue with a second conductive tissue, phloem, required primarily for nutrient transport. Both xylem and phloem are derived from cell divisions in vascular meristems known as the cambium and procambium. One major component that influences several aspects of plant vascular development, including cell division in the vascular meristem, vascular organization and differentiation of vascular cell types, is a signalling module characterized by a peptide ligand called TRACHEARY ELEMENT DIFFERENTIATION INHIBITORY FACTOR (TDIF) and its cognate receptor, PHLOEM INTERCALATED WITH XYLEM (PXY). In this review, we explore the literature that describes signalling components, phytohormones and transcription factors that interact with these two central factors, to control the varying outputs required in vascular tissues for normal organization and elaboration of plant vascular tissue. [ABSTRACT FROM AUTHOR]

The formation of meristemoids and the ontogeny of the shoot apical meristem (SAM) were studied using cultured zygotic embryos of Arabidopsis thaliana (L.) Heynh. LER ecotype. In the callus induction treatment, the procambial cells within the cotyledons of the embryo explants proliferated and gave rise to callus tissues. At the end of the treatment, a band of small cytoplasmic-rich cells derived from the procambium was produced and located at the outer surface of the callus. Upon transfer to the shoot induction medium (SIM) in the absence of abscisic acid (ABA), the cytoplasmic cells differentiated mainly into vascular elements and vacuolated parenchyma cells. This pattern of development negatively affected the explants' ability to produce meristemoids and SAMs. Contrary to the control, the inclusion of ABA in the SIM resulted first in starch synthesis and accumulation in the surface cytoplasmic cells. This was followed by the formation of cytoplasmic cells among the starch-rich cells; further proliferation of the cytoplasmic cells resulted in the formation of meristemoids. The formation of tracheary elements was suppressed in the ABA-containing SIM. Upon transferring to the shoot development medium, which lacked plant growth regulators, some meristemoids differentiated into apical meristem cells. These cells had distinct nuclei and nucleoli, with little starch present. Additional cell divisions increased the size of the future SAM. Shoot buds with distinct SAMs were clearly delineated with the appearance of leaf primordia. [ABSTRACT FROM AUTHOR]

Here, we describe the complex interactions that occur between HD–ZIP III transcription factors, auxin signaling, and thermospermine biosynthesis genes during vascular development. Through this work, we present a model of positive and negative feedback between auxin and HD–ZIP gene expression that tunes the rate of xylem differentiation.The role of auxin as main regulator of vascular differentiation is well established, and a direct correlation between the rate of xylem differentiation and the amount of auxin reaching the (pro)cambial cells has been proposed. It has been suggested that thermospermine produced by ACAULIS5 (ACL5) and BUSHY AND DWARF2 (BUD2) is one of the factors downstream to auxin contributing to the regulation of this process in Arabidopsis. Here, we provide an in-depth characterization of the mechanism through which ACL5 modulates xylem differentiation. We show that an increased level of ACL5 slows down xylem differentiation by negatively affecting the expression of homeodomain-leucine zipper (HD–ZIP) III and key auxin signaling genes. This mechanism involves the positive regulation of thermospermine biosynthesis by the HD–ZIP III protein ARABIDOPSIS THALIANA HOMEOBOX8 tightly controlling the expression of ACL5 and BUD2. In addition, we show that the HD–ZIP III protein REVOLUTA contributes to the increased leaf vascularization and long hypocotyl phenotype of acl5 likely by a direct regulation of auxin signaling genes such as LIKE AUXIN RESISTANT2 (LAX2) and LAX3. We propose that proper formation and differentiation of xylem depend on a balance between positive and negative feedback loops operating through HD–ZIP III genes. [ABSTRACT FROM PUBLISHER]

Plant vasculatures are complex tissues consisting of (pro)cambium, phloem, and xylem. The (pro)cambium serves as vascular stem cells that produce all vascular cells. The Arabidopsis ERECTA (ER) receptor kinase is known to regulate the architecture of inflorescence stems. It was recently reported that the er mutation enhances a vascular phenotype induced by a mutation of TDR/PXY, which plays a significant role in procambial proliferation, suggesting that ER participates in vascular development. However, detailed molecular mechanisms of the ER-dependent vascular regulation are largely unknown. Here, this work found that ER and its paralogue, ER-LIKE1, were redundantly involved in procambial development of inflorescence stems. Interestingly, their activity in the phloem was sufficient for vascular regulation. Furthermore, two endodermis-derived peptide hormones, EPFL4 and EPFL6, were redundantly involved in such regulation. It has been previously reported that EPFL4 and EPFL6 act as ligands of phloem-expressed ER for stem elongation. Therefore, these findings indicate that cell–cell communication between the endodermis and the phloem plays an important role in procambial development as well as stem elongation. Interestingly, similar EPFL–ER modules control two distinct developmental events by slightly changing their components: the EPFL4/6–ER module for stem elongation and the EPFL4/6–ER/ERL1 module for vascular development. [ABSTRACT FROM AUTHOR]

In plants, the cambium and procambium are meristems from which vascular tissue is derived. In contrast to most plant cells, stem cells within these tissues are thin and extremely long. They are particularly unusual as they divide down their long axis in a highly ordered manner, parallel to the tangential axis of the stem. CLAVATA3-LIKE/ESR-RELATED 41 (CLE41) and PHLOEM INTERCALATED WITH XYLEM (PXY) are a multifunctional ligand-receptor pair that regulate vascular cell division, vascular organisation and xylem differentiation in vascular tissue. A transcription factor gene, WUSCHEL HOMEOBOX RELATED 4 (WOX4) has been shown to act downstream of PXY. Here we show that WOX4 acts redundantly with WOX14 in the regulation of vascular cell division, but that these genes have no function in regulating vascular organisation. Furthermore, we identify an interaction between PXY and the receptor kinase ERECTA (ER) that affects the organisation of the vascular tissue but not the rate of cell division, suggesting that cell division and vascular organisation are genetically separable. Our observations also support a model whereby tissue organisation and cell division are integrated via PXY and ER signalling, which together coordinate development of different cell types that are essential for normal stem formation. [ABSTRACT FROM AUTHOR]

Abstract: Unlike most animal tissue networks, the patterns of vein networks in plant leaves are variable and plastic, suggesting distinct control mechanisms. Thus, knowledge of the gene regulatory circuits that pattern leaf vein networks could suggest new control mechanisms of tissue network formation. However, the cis-regulatory elements required for expression at early stages of vein development are largely unknown. Here we show that the Arabidopsis genes CYCLIN A2;1 (CYCA2;1) and CYCLIN A2;4 (CYCA2;4), previously shown to act redundantly in vein cell proliferation, are expressed at early stages of vein development. We show that stage-specific expression of CYCA2;1 and CYCA2;4 in vein development depends on regulatory elements containing, respectively, one and three evolutionarily conserved transcription-factor binding sites. Our data suggest that early vein expression is encoded in regulatory elements of different structures. [Copyright &y& Elsevier]

Leaf venation patterns vary considerably between species and between leaves within a species. A mechanism based on canalization of auxin transport has been suggested as the means by which plastic yet organized venation patterns are generated. This study assessed the plasticity of Arabidopsis thaliana leaf venation in response to ectopic ground or procambial cell divisions and auxin transport inhibition (ATI). Ectopic ground cell divisions resulted in vascular fragments between major veins, whereas ectopic procambial cell divisions resulted in additional, abnormal vessels along major veins, with more severely perturbed lines forming incomplete secondary and higher-order venation. These responses imply limited vascular plasticity in response to unscheduled cell divisions. Surprisingly, a combination of ectopic ground cell divisions and ATI resulted in massive vascular overgrowth. It is hypothesized that the vascular overproduction in auxin transport-inhibited wild-type leaves is limited by simultaneous differentiation of ground cells into mesophyll cells. Ectopic ground cell divisions may negate this effect by providing undifferentiated ground cells that respond to accumulated auxin by differentiation into vascular cells. [ABSTRACT FROM PUBLISHER]

Meristems are morphogenic plant tissues comprising the pool of stem cells initiating specialized tissues. The balance of stem cell proliferation and differentiation depends on antagonistic action of genes WOX encoding homeodomain-containing transcription factors and CLAVATA-like systems controlling their expression. The CLE (CLAVATA3/ENDOSPERM SURROUNDING REGION) peptides and their receptor protein kinases comprise the basis of the CLAVATA-like systems. Definite member of the family of CLE peptides (CLV3 in the shoot, CLE40 in the root, and CLE41/CLE44 in the procambium) are central regulators of primary meristem development. The role of CLE peptides in the development of secondary meristems, such as nodule meristems of legumes, is studied. CLE peptides are found also outside the plant kingdom, e.g., parasitic nematodes. The review considers CLE peptides as universal regulators of meristem development. [ABSTRACT FROM AUTHOR]

The regulation of cell specification in plants is particularly important in vascular development. The vascular system is comprised two differentiated tissue types, the xylem and phloem, which form conductive elements for the transport of water, nutrients and signaling molecules. A meristematic layer, the procambium, is located between these two differentiated cell types and divides to initiate vascular growth. We report the identification of a receptor-like kinase (RLK) that is expressed in the vasculature. Histochemical analyses of mutants in this kinase display an aberrant accumulation of highly lignified cells, typical of xylem or fiber cells, within the phloem. In addition, phloem cells are sometimes located adjacent to xylem cells in these mutants. We, therefore, named this RLK XYLEM INTERMIXED WITH PHLOEM 1 (XIP1). Analyses of longitudinal profiles of xip1 mutant stems show malformed cell files, indicating defects in oriented cell divisions or cell morphology. We propose that XIP1 prevents ectopic lignification in phloem cells and is necessary to maintain the organization of cell files or cell morphology in conductive elements. [ABSTRACT FROM AUTHOR]

Abstract: Genes expressed in vascular tissues have been identified by several strategies, usually with a focus on mature vascular cells. In this study, we explored the possibility of using two opposite types of altered tissue compositions in combination with a double-filter selection to identify genes with a high probability of vascular expression in early organ primordia. Specifically, we generated full-transcriptome microarray profiles of plants with (a) genetically strongly reduced and (b) pharmacologically vastly increased vascular tissues and identified a reproducible cohort of 158 transcripts that fulfilled the dual requirement of being underrepresented in (a) and overrepresented in (b). In order to assess the predictive value of our identification scheme for vascular gene expression, we determined the expression patterns of genes in two unbiased subsamples. First, we assessed the expression patterns of all twenty annotated transcription factor genes from the cohort of 158 genes and found that seventeen of the twenty genes were preferentially expressed in leaf vascular cells. Remarkably, fifteen of these seventeen vascular genes were clearly expressed already very early in leaf vein development. Twelve genes with published leaf expression patterns served as a second subsample to monitor the representation of vascular genes in our cohort. Of those twelve genes, eleven were preferentially expressed in leaf vascular tissues. Based on these results we propose that our compendium of 158 genes represents a sample that is highly enriched for genes expressed in vascular tissues and that our approach is particularly suited to detect genes expressed in vascular cell lineages at early stages of their inception. [Copyright &y& Elsevier]

Background and Aims Understanding the fate and dynamics of cells during callus formation is essential to understanding totipotency and the mechanisms of somatic embryogenesis. Here, the fate of leaf explant cells during the development of embryogenic callus was investigated in the model legume Medicago truncatula. Methods Callus development was examined from cultured leaf explants of the highly regenerable genotype Jemalong 2HA (2HA) and from mesophyll protoplasts of 2HA and wild-type Jemalong. Callus development was studied by histology, manipulation of the culture system, detection of early production of reactive oxygen species and visualization of SERK1 (SOMATIC EMBRYO RECEPTOR KINASE1) gene expression. Key Results Callus formation in leaf explants initiates at the cut surface and within veins of the explant. The ontogeny of callus development is dominated by the division and differentiation of cells derived from pluripotent procambial cells and from dedifferentiated mesophyll cells. Procambium-derived cells differentiated into vascular tissue and rarely formed somatic embryos, whereas dedifferentiated mesophyll cells were competent to form somatic embryos. Interestingly, explants incubated adaxial-side down had substantially less cell proliferation associated with veins yet produced similar numbers of somatic embryos to explants incubated abaxial-side down. Somatic embryos mostly formed on the explant surface originally in contact with the medium, while in protoplast microcalli, somatic embryos only fully developed once at the surface of the callus. Mesophyll protoplasts of 2HA formed embryogenic callus while Jemalong mesophyll protoplasts produced callus rich in vasculature. Conclusions The ontogeny of embryogenic callus in M. truncatula relates to explant orientation and is driven by the dynamics of pluripotent procambial cells, which proliferate and differentiate into vasculature. The ontogeny is also related to de-differentiated mesophyll cells that acquire totipotency and form the majority of embryos. This contrasts with other species where totipotent embryo-forming initials mostly originate from procambial cells. [ABSTRACT FROM PUBLISHER]

∙ In seed plants, current knowledge concerning embryonic pattern formation by polar auxin transport (PAT) and WUSCHEL-related homeobox (WOX) gene activity is primarily derived from studies on angiosperms, while less is known about these processes in gymnosperms. In view of the differences in their embryogeny, and the fact that somatic embryogenesis is used for mass propagation of conifers, a better understanding of embryo development is vital. ∙ The expression patterns of PaWOX2 and PaWOX8/ 9 were followed with quantitative reverse transcription-polymerase chain reaction (qRT-PCR) and in situ hybridization (ISH) during seed and somatic embryo development in Norway spruce (Picea abies), and in somatic embryos treated with the PAT inhibitor N-1- naphthylphthalamic acid (NPA). ∙ Both PaWOX2 and PaWOX8/ 9 were highly expressed at the early growth stages of zygotic and somatic embryos, and shared a similar expression pattern over the entire embryo. At later embryo stages, high expression of PaWOX8/ 9 became restricted to cotyledon primordia, epidermis, procambium and root apical meristem (RAM), which became most evident in NPA-treated somatic embryos, while expression of PaWOX2 was much lower. ∙ Our results suggest an ancestral role of WOX in seed plant embryo development, and strengthen the proposed connection between PAT, PIN-FORMED (PIN) and WOX in the regulation of embryo patterning in seed plants. [ABSTRACT FROM AUTHOR]

Abstract: While many regulatory mechanisms controlling the development and function of root and shoot apical meristems have been revealed, our knowledge of similar processes in lateral meristems, including the vascular cambium, is still limited. Our understanding of even the anatomy and development of lateral meristems (procambium or vascular cambium) is still relatively incomplete, let alone their genetic regulation. Research into this particular tissue type has been mostly hindered by a lack of suitable molecular markers, as well as the fact that thus far very few mutants affecting plant secondary development have been described. The development of suitable molecular markers is a high priority in order to help define the anatomy, especially the location and identity of cambial stem cells and the developmental phases and molecular regulatory mechanisms of the cambial zone. To date, most of the advances have been obtained by studying the role of the major plant hormones in vascular development. Thus far auxin, cytokinin, gibberellin and ethylene have been implicated in regulating the maintenance and activity of cambial stem cells; the most logical question in research would be how these hormones interact during the various phases of cambial development. [Copyright &y& Elsevier]

The plant vascular system provides transport and support capabilities that are essential for plant growth and development, yet the mechanisms directing the arrangement of vascular bundles within the shoot inflorescence stem remain unknown. We used computational and experimental biology to evaluate the role of auxin and brassinosteroid hormones in vascular patterning in Arabidopsis. We show that periodic auxin maxima controlled by polar transport and not overall auxin levels underlie vascular bundle spacing, whereas brassinosteroids modulate bundle number by promoting early procambial divisions. Overall, this study demonstrates that auxin polar transport coupled to brassinosteroid signaling is required to determine the radial pattern of vascular bundles in shoots. [ABSTRACT FROM AUTHOR]

Figures from independent studies of 1941 suggested a similarity in the initiation of the vascular system of the arborescent palms and the unrelated and herbaceous lily turf, Ophiopogon. In order to compare more rigorously the vascular pattern of Ophiopogon with the intensively studied vascular system of the palms, cross sections of the rhizome of Ophiopogon were serially analyzed. All procambial strands were mapped in the 1.5-mm-long tip of a rhizome of Ophiopogon containing the insertions of 18 leaves. Charting procambia effected a description of the origins of the three major traces and the two intermediate traces associated with each leaf. All traces arise by merger of originally isolated procambia. A primary thickening meristem (PTM) with a horizontal roof and sloping sides furnishes the more distal components of the traces. The most proximal segment of a major trace differentiates in isolation in the residual meristem near the first horizontal procambia generated by the roof of the PTM. The procambial segment becomes continuous with the leaf as its major trace when the proximal segment simultaneously links with the segment from the PTM at one end and grows into the leaf at the other. Before this capture by the leaf, the distal end of the horizontal segment joined procambial segments formed earlier in the older, distalPTM. Unions with these same strands in the distalPTMfurnish the distal segments of the intermediate leaf traces. The most proximal segment of an intermediate trace originates by isolated dedifferentiation from leaf parenchyma near isolated horizontal procambia that differentiate farther from the center of the roof of the PTM after those procambia that become components of the major traces. The proximal ends of major and intermediate traces near their insertions are displaced and stretched upward and centrifugally by the expansion of the parenchyma of the axis. Such distortion is less for intermediate traces since they begin later and farther from the center than major traces. Thus, major traces develop more of a centripetal component to their descent before resuming the centrifugal descent of intermediate traces. Stretching brings the proximal portions of the traces above the path of the rising, expanding PTM. As thePTMpasses over the ends of the traces near their insertions, it generates more isolated procambia that become the proximal branches of the major and intermediate leaf traces. While these characteristics of the shoot of Ophiopogon resemble aspects of vascular ontogeny in the palms, these features differ from procambial characteristics of several other monocotyledonous orders. [ABSTRACT FROM AUTHOR]

The formation of vein patterns in leaves has captivated biologists, mathematicians, and philosophers. In leaf development, files of vein-forming procambial cells emerge from within a seemingly homogeneous subepidermal tissue through the selection of anatomically inconspicuous preprocambial cells. Although the molecular details underlying the orderly formation of veins in the leaf remain elusive, gradually restricted transport paths of the plant signaling molecule auxin have long been implicated in defining sites of vein differentiation. Several recent advances converge to more precisely define the role of auxin flow at successive stages of vascular development. The picture that emerges is that of vein formation as a self-organizing, reiterative, auxin-transport-dependent process. [ABSTRACT FROM AUTHOR]

Vascular development is a central theme in plant science. However, little is known about the mechanism of vascular development in monocotyledons (compared with dicotyledons). Therefore, we investigated sequential processes of differentiation into various different vascular cells by carrying out detailed observations using serial sections of the bases of developing leaves of rice and maize. The developmental process of the longitudinal vascular bundles was divided into six stages in rice and five stages in maize. The initiation of differentiation into procambial progenitor cells forming the commissural vein arose in a circular layer cell that was adjacent to both a metaxylem vessel and one or a few phloem cells in stage V longitudinal vascular bundles. In most cases the differentiation of ground meristem cells into procambial progenitor cells extended in one direction, toward the next longitudinal vascular bundle, and subsequent periclinal divisions and further differentiation produced a vessel element, two companion cells and a sieve element to form a commissural vein. These results suggest the presence of an intercellular signal(s) that induces differentiation of the circular layer cell and the ground meristem cells into procambial progenitor cells, forming a commissural vein sequentially. [ABSTRACT FROM AUTHOR]

The origin of polystichy in the maize ear and central tassel spike continues to challenge our understanding of evolution in this important crop species. In this paper we tested the hypothesis that the change in phyllotaxy occurs in the region of husk leaf production before the transition to reproductive growth. Young ear or presumptive ear primordia were dissected to examine the transition from distichous husk leaves below the ear through spiral phyllotaxy to the polystichous arrangement of spikelet pair primordia in the young ear. Serial transverse sections were used to document the thickness of successive disks of insertion of lateral primordia and to reconstruct the path of procambial differentiation. The transition in phyllotaxy, though variable, typically occurs in the vegetative zone and is associated with periodic heterogeneity in the thickness of leaf bases and a delay in the development of waves of procambial differentiation into the base of the young ear. [ABSTRACT FROM AUTHOR]

Every procambial strand was mapped in the apical six disks of insertion comprising the shoot tip of Symplocarpus. Detection of longitudinally discontinuous procambia confirmed that the procambial leaf traces were originally isolated in this monocotyledonous shoot. The initially separate procambia of a nascent leaf trace became linked with a leaf as its leaf trace by further procambial differentiation among the procambial segments and the leaf to be served. The procambial components of the major leaf traces were formed by a succession of five waves within each of the superposed disks of insertion. Space for the additional procambia in the successive waves was made by the expansion growth of each disk. Waves 1-4 made the procambial components of the major leaf traces near the center of the disk, but wave 5 caused the lower end of a major trace to become centrifugal. The first intermediate leaf traces began after the major traces, since only waves 3-5 were involved. The mantle of the meristematic cap was the principal source of new isolated procambia during wave 5. The major and first intermediate traces comprised the inner procambial system. The last intermediate traces comprised the outer or cortical system. The last intermediate traces were made only during wave 5 outside of the meristematic cap. The upper part of a bud trace is like a last intermediate trace, but the lower part may become linked to the inner system by the meristematic cap. The patterns to the initiation of leaf traces discovered in Symplocarpus support a state of procambial initiation in monocotyledonous shoots that involves discontinuous strands. [ABSTRACT FROM AUTHOR]

Abstract: Plant-specific Dof transcription factors play various roles in plants. We show the strictly regulated activity of two Dof gene promoters during procambium formation, an early process for vascular development, in Arabidopsis thaliana (L.) Heynh. The AtDof2.4 promoter was active in procambial cells of leaf primordia, roots and embryos, whereas the AtDof5.8 promoter activity was specifically detectable in the cells of prospective veins in leaf primordia of seedlings and cotyledons of developing embryos, and the vascular tissue of developing flower buds. The AtDof5.8 promoter but not the AtDof2.4 promoter showed strong activity in advance of perceptible procambium formation. AtDof2.4 and AtDof5.8 might function in the early but different processes for vascular development. [Copyright &y& Elsevier]

The SOMATIC EMBRYOGENESIS RECEPTOR KINASE 1 (SERK1) gene is expressed in the procambium of the vascular bundles in roots, hypocotyls, and inflorescence stems. In younger parts of roots and hypocotyls, SERK1 expression was less restricted and was also observed in protoxylem cells, immature metaxylem cells and phloem companion cells. In roots, SERK1 expression was first detected in root vascular stem cells and was notably absent from the QC. In general, the SERK1 protein level as visualized by expression of a SERK1-YFP fusion protein closely followed the pattern of gene expression. In hypocotyls, prolonged application of 2,4-D resulted in extensive unorganized proliferation of SERK1 expressing cells originating from the procambium and pericycle. In roots, 2,4-D treatment results in an increase in SERK1 transcription that results in a moderate increase in the amount of SERK1-YFP fusion protein. The restricted vascular pattern of SERK1 expression in roots remains unaffected after 2,4-D treatment. [ABSTRACT FROM PUBLISHER]

Formation of leaf vascular pattern requires regulation of a number of cellular processes, including cell proliferation. To assess the role of cell proliferation during vein order formation, leaf development in genetic lines exhibiting aberrant cell proliferation patterns due to altered expression patterns of ANT and ICK1 genes was analyzed. Modification of cell proliferation patterns alters the number of higher order veins and the number of minor tertiary veins remodeled as intersecondary veins in Arabidopsis rosette leaves. Minor vein complexity, as indicated by branch point and freely ending veinlet number, is highly correlated with a decrease or increase in cell proliferation. Observations of procambial strand formation in modified cell proliferation pattern lines showed that vein pattern is specified early in leaf development and that formation of freely ending veinlets is temporally correlated with the expansion of ground meristem when cell proliferation is terminated prematurely. Taken together, our observations indicate that: (1) genes that modulate cell proliferation play a key role in regulating the meristematic competence of ground meristem cells to form procambium and vein pattern during leaf development, and (2) ANT is a crucial part of this regulation. [ABSTRACT FROM AUTHOR]