Staff Publications

Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

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A Plausible Microtubule-Based Mechanism for Cell Division Orientation in Plant Embryogenesis
Chakrabortty, Bandan ; Willemsen, Viola ; Zeeuw, Thijs de; Liao, Che Yang ; Weijers, Dolf ; Mulder, Bela ; Scheres, Ben - \ 2018
Current Biology 28 (2018)19. - ISSN 0960-9822 - p. 3031 - 3043.e2.
arabidopsis - auxin - cell shape - computational modeling - cytokinesis - division orientation - embryogenesis - microtubules - systems biology

Oriented cell divisions are significant in plant morphogenesis because plant cells are embedded in cell walls and cannot relocate. Cell divisions follow various regular orientations, but the underlying mechanisms have not been clarified. We propose that cell-shape-dependent self-organization of cortical microtubule arrays is able to provide a mechanism for determining planes of early tissue-generating divisions and may form the basis for robust control of cell division orientation in the embryo. To show this, we simulate microtubules on actual cell surface shapes, from which we derive a minimal set of three rules for proper array orientation. The first rule captures the effects of cell shape alone on microtubule organization, the second rule describes the regulation of microtubule stability at cell edges, and the third rule includes the differential effect of auxin on local microtubule stability. These rules generate early embryonic division plane orientations and potentially offer a framework for understanding patterned cell divisions in plant morphogenesis. Chakrabortty et al. show that a computational model for dynamic self-organization of cortical microtubules on experimentally extracted cell shapes provides a plausible molecular mechanism for division plane orientation in the first four divisions of early stage A. thaliana embryos, in WT as well as two developmental mutants bodenlos and clasp.

A toolkit for studying cellular reorganization during early embryogenesis in Arabidopsis thaliana
Liao, Che Yang ; Weijers, Dolf - \ 2018
The Plant Journal 93 (2018)6. - ISSN 0960-7412 - p. 963 - 976.
Arabidopsis thaliana - cell biology - embryogenesis - imaging - oriented cell division
Considerable progress has been made in understanding the influence of physical and genetic factors on the patterns of cell division in various model systems. However, how each of these factors directs changes in subcellular structures has remained unclear. Generic machineries for the execution of cell expansion and division have been characterized, but how these are influenced by genetic regulators and physical cell properties remains an open question. To a large degree, the complexity of growing post-embryonic tissues and a lack of precise predictability have prevented the extraction of rigid correlations between subcellular structures and future orientation of cell division. The Arabidopsis embryo offers an exquisitely predictable and simple model for studying such correlations, but so far the tools and methodology for studying subcellular structures in the early embryo have been lacking. Here, we describe a set of markers to visualize a range of subcellular structures in the early Arabidopsis embryo. We have designed a series of fluorescent cellular reporters optimized for embryos, and demonstrate the effectiveness of using these ‘ACE’ reporters with simple three-dimensional imaging procedures that preserve delicate cellular structures. We describe the ontogeny of subcellular structures in the early embryo and find that central/peripheral cell polarity is established much earlier than suspected. In addition, we show that the actin and microtubule cytoskeleton has distinct topologies in the embryo. These tools and methods will allow detailed analysis of the events of cellular reorganization that underlie morphogenesis in the Arabidopsis embryo.
Mechanistic dissection of plant embryo initiation
Radoeva, T.M. - \ 2016
University. Promotor(en): Dolf Weijers, co-promotor(en): Sacco de Vries. - Wageningen : Wageningen University - ISBN 9789462578135 - 183 p.
embryogenesis - embryos - plants - auxins - genes - genomics - arabidopsis - cell suspensions - in vivo experimentation - zygotes - monozygotic twins - embryogenese - embryo's - planten - auxinen - genen - genomica - celsuspensies - in vivo experimenten - zygoten - monozygote tweelingen

Land plants can reproduce sexually by developing an embryo from a fertilized egg cell, the zygote. After fertilization, the zygote undergoes several rounds of controlled cell divisions to generate a mature embryo. However, embryo formation can also be induced in a variety of other cell types in many plant species. These non-zygotic embryos go through analogous developmental phases and are morphologically similar to the zygotic embryo. Despite its fundamental importance and enormous application potential, the mechanisms that alter cell fate from non-embryonic to embryonic are elusive. In the past decades, a variety of different model systems have been used to identify regulators of embryo induction, but it is unclear if these act in a common network. We recently found that inhibition of auxin response in the extra-embryonic suspensor cells cell-autonomously and predictably triggers a switch towards embryo identity. In my thesis I have used the suspensor-derived embryogenesis as a uniform model system to study the crucial first reprogramming step of embryo initiation process.

Through genome-wide transcriptional profiling upon local (suspensor-specific) auxin response inhibition (Chapter 2) and through testing the ability of fifteen known embryogenesis inducers to promote embryo formation in suspensor cells (Chapter 3), we suggest that suspensor to embryo transformation requires a defined set of genetic regulators. The results obtained in my thesis provide essential tools and basis for further research and are a step forward to understanding the first step of embryo initiation process and to unravel the mystery of totipotency in plants.

Microspore embryogenesis: reprogramming cell fate from pollen to embryo development
Hui Li, - \ 2014
University. Promotor(en): Gerco Angenent, co-promotor(en): Kim Boutilier. - Wageningen : Wageningen University - ISBN 9789462570702 - 224
stuifmeel - embryogenese - embryonale ontwikkeling - biologische ontwikkeling - plantenontwikkeling - in vitro kweek - plantenembryo's - brassica napus - pollen - embryogenesis - embryonic development - biological development - plant development - in vitro culture - plant embryos

Microspore embryogenesis is an expression of plant cell totipotency that leads to the production of haploid embryos. Besides being a widely exploited plant breeding tool, microspore embryogenesis is also a fascinating system that can be used to obtain a deeper mechanistic understanding of plant totipotency. This thesis aims to provide more insight into the process of microspore embryogenesis, from the formation of embryogenic cells to the outgrowth of differentiated embryos.

In Chapter 1 background information is provided on the various aspects of Brassica napus microspore culture and plant development that intersect with the topics that are studied in this thesis. Emphasis is placed on the basic requirements and limitations for successful microspore embryo culture, as well as on the roles of the plant hormone auxin and epigenetic regulation in the development of plant embryos, during both zygotic and in vitro embryo development.

Chapter 2 reviews the recent advances that have been made in understanding the developmental and molecular changes that take place during microspore embryogenesis in model systems. The commonly reported cellular changes associated with the establishment of embryo cell fate are summarized and evaluated. The subsequent differentiation of the embryo is also discussed, specifically, what is known about the establishment of polarity, with emphasis on the importance of exine rupture as a positional clue, and the processes that influence meristem maintenance during culture. Finally, the studies on the molecular changes during microspore embryo induction are put into context of male gametophytic development. Overall, the current perspective on microspore embryo initiation presents a landscape in which several routes can lead to the same final destination.

Stress treatments are widely applied to induce embryogenic growth in microspore culture. Chapter 3 explores the role of histone acetylation status in stress-induced microspore embryogenesis in Brassica napus. Inhibition of histone deactylases (HDACs) using the HDAC inhibitor trichostatin A (TSA), phenocopies the heat stress treatment that is normally used to induce embryogenic cell proliferation in B. napus microspore culture. Arabidopsis is recalcitrant for haploid embryogenesis, yet treatment with TSA also induced embryogenic cell divisions in this model species. Our observations suggest that the totipotency of the male gametophyte is kept in check by an HDAC-dependent mechanism and that the stress treatments used to induce haploid embryo development in culture impinge on this HDAC-dependent pathway. The repression of HDACs or HDAC-mediated pathways by stress and the accompanying changes in histone acetylation status could provide a single, common regulation point for the induction of haploid embryogenesis.

Chapter 4 builds on the knowledge developed in Chapter 3 on the role of HDAC proteins in plant totipotency. A wide variety of chemically distinct HDAC inhibitors was evaluated and additional inhibitors that enhance embryogenic cell induction and/or embryo yield were identified. One surprising observation was made during the course of this study: the initial donor microspore/pollen stage affects the quality of the embryo that is formed. In control cultures, embryos from progressively older stages of donor microspores/pollen became progressively compromised in their basal (axis region) region, characterized by a shift from normal embryos with apical (cotyledons) and basal (root) polarity to abnormal embryos with a reduced basal pole and ball-shaped embryos. These abnormal phenotypes could be partially complemented by treatment with HDAC inhibitors, which promoted growth of the basal region of the embryo. Progressive enhancement of embryo basal identity was accompanied by enhanced and broadened expression of the DR5 auxin response reporter. The embryo phenotypes observed in control and HDAC inhibitor treated microspore cultures are similar to the phenotypes induced by altered expression of the Arabidopsis TOPLESS (TPL)/HDAC19/BODENLOS (BDL) repressor complex, which acts to restrict expression of the AUXIN RESPONSE FACTOR ARF5/MONOPTEROS (MP) to the basal region of the embryo during zygotic embryo development.

To understand why most embryogenic callus failed to develop further, we examined the transcriptome of globular-shaped embryos that have started to histodifferentiate and compared it with embryogenic callus. The transcriptome analysis showed that the expression of many genes that regulate (auxin-related) embryo patterning were downregulated in embryogenic callus compared to globular stage embryos. This result may simply reflect the lack of patterning in these embryos or might indicate a role of auxin-signalling in embryogenic callus formation.

Chapter 5 examines how embryo identity and patterning is established in two B. napus microspore embryo pathways, a zygotic-like pathway, characterized by suspensor and then embryo proper formation, and a pathway characterized by initially unorganized structures that lack a suspensor. We specifically asked the question: how can embryo patterning be established in the absence of an initial asymmetric division and in the absence of a suspensor, two key events in zygotic embryo development. Analysis of embryo fate (GRP) and auxin (PIN1, PIN7 and DR5) markers showed that embryo fate was established prior to cell division, and independent of subsequent division pattern. The suspensorless embryo program was marked by a transient auxin maximum, followed by establishment of the apical and basal poles at the globular stage, coincident with release of the embryo from the pollen exine. Unlike zygotic embryo development, polar auxin transport (PAT) was not required for embryo initiation or polarity establishment in this system. Suspensor embryos developed in a similar fashion as zygotic embryos, PAT was required for specification of the embryo proper from the suspensor. Haploid embryogenesis therefore follows at least two programs, a PAT-dependent program that requires embryo proper specification from the suspensor, and an alternative PAT-independent program marked by an initial auxin maximum.

In the final chapter, Chapter 6, the work presented in this thesis is put in context of the broader plant development field. The epigenetic regulation of developmental transitions that respond to stress and during pollen development are highlighted. A model is provided that histone acetylation levels mediated by HAT and HDAC regulate pollen fate.

Central Cell-Derived Peptides Regulate Early Embryo Patterning in Flowering Plants
Costa, L.M. ; Marshall, E. ; Tesfaye, M. ; Silverstein, K.A.T. ; Mori, M. ; Umetsu, Y. ; Otterbach, S.L. ; Papareddy, R. ; Dickinson, H.G. ; Boutilier, K.A. ; VandenBosch, K.A. ; Ohki, S. ; Gutierrez-Marcos, J.F. - \ 2014
Science 344 (2014)6180. - ISSN 0036-8075 - p. 168 - 172.
apical-basal axis - double fertilization - stomatal density - gene-expression - arabidopsis - embryogenesis - endosperm - communication - infection - origin
Plant embryogenesis initiates with the establishment of an apical-basal axis; however, the molecular mechanisms accompanying this early event remain unclear. Here, we show that a small cysteine-rich peptide family is required for formation of the zygotic basal cell lineage and proembryo patterning in Arabidopsis. EMBRYO SURROUNDING FACTOR 1 (ESF1) peptides accumulate before fertilization in central cell gametes and thereafter in embryo-surrounding endosperm cells. Biochemical and structural analyses revealed cleavage of ESF1 propeptides to form biologically active mature peptides. Further, these peptides act in a non–cell-autonomous manner and synergistically with the receptor-like kinase SHORT SUSPENSOR to promote suspensor elongation through the YODA mitogen-activated protein kinase pathway. Our findings demonstrate that the second female gamete and its sexually derived endosperm regulate early embryonic patterning in flowering plants.
Structural Basis for DNA Binding Specificity by the Auxin-Dependent ARF Transcription Factors
Boer, D.R. ; Freire Rios, A. ; Berg, W.A.M. van den; Saaki, T. ; Manfield, I.W. ; Kepinski, S. ; López-Vidrieo, I. ; Franco-Zorilla, J.M. ; Vries, S.C. de; Solano, R. ; Weijers, D. ; Coll, M. - \ 2014
Cell 156 (2014). - ISSN 0092-8674 - p. 577 - 589.
arabidopsis gene monopteros - response elements - vascular development - plant development - domains - family - embryo - embryogenesis - dimerization - recognition
Auxin regulates numerous plant developmental processes by controlling gene expression via a family of functionally distinct DNA-binding auxin response factors (ARFs), yet the mechanistic basis for generating specificity in auxin response is unknown. Here, we address this question by solving high-resolution crystal structures of the pivotal Arabidopsis developmental regulator ARF5/MONOPTEROS (MP), its divergent paralog ARF1, and a complex of ARF1 and a generic auxin response DNA element (AuxRE). We show that ARF DNA-binding domains also homodimerize to generate cooperative DNA binding, which is critical for in vivo ARF5/MP function. Strikingly, DNA-contacting residues are conserved between ARFs, and we discover that monomers have the same intrinsic specificity. ARF1 and ARF5 homodimers, however, differ in spacing tolerated between binding sites. Our data identify the DNA-binding domain as an ARF dimerization domain, suggest that ARF dimers bind complex sites as molecular calipers with ARF-specific spacing preference, and provide an atomic-scale mechanistic model for specificity in auxin response.
Local Auxin Sources Orient the Apical-Basal Axis in Arabidopsis Embryos
Robert, H.S. ; Grones, P. ; Stepanova, A.N. ; Robles, L.M. ; Lokerse, A.S. ; Alonso, J.M. ; Weijers, D. ; Friml, J. - \ 2013
Current Biology 23 (2013)24. - ISSN 0960-9822 - p. 2506 - 2512.
plant development - cotyledon development - seed development - pin proteins - biosynthesis - gene - polarity - embryogenesis - expression - efflux
Establishment of the embryonic axis foreshadows the main body axis of adults both in plants and in animals, but underlying mechanisms are considered distinct. Plants utilize directional, cell-to-cell transport of the growth hormone auxin [1 and 2] to generate an asymmetric auxin response that specifies the embryonic apical-basal axis [3, 4, 5 and 6]. The auxin flow directionality depends on the polarized subcellular localization of PIN-FORMED (PIN) auxin transporters [7 and 8]. It remains unknown which mechanisms and spatial cues guide cell polarization and axis orientation in early embryos. Herein, we provide conceptually novel insights into the formation of embryonic axis in Arabidopsis by identifying a crucial role of localized tryptophan-dependent auxin biosynthesis [ 9, 10, 11 and 12]. Local auxin production at the base of young embryos and the accompanying PIN7-mediated auxin flow toward the proembryo are required for the apical auxin response maximum and the specification of apical embryonic structures. Later in embryogenesis, the precisely timed onset of localized apical auxin biosynthesis mediates PIN1 polarization, basal auxin response maximum, and specification of the root pole. Thus, the tight spatiotemporal control of distinct local auxin sources provides a necessary, non-cell-autonomous trigger for the coordinated cell polarization and subsequent apical-basal axis orientation during embryogenesis and, presumably, also for other polarization events during postembryonic plant life [ 13 and 14].
Proteomic and mechanistic analysis of Auxin Response Factors in the Arabidopsis embryo
Llavata Peris, C.I. - \ 2013
University. Promotor(en): Dolf Weijers. - S.l. : s.n. - ISBN 9789461736734 - 143
arabidopsis - auxinen - plantengroeiregulatoren - reacties - eiwitexpressieanalyse - genexpressie - embryonale ontwikkeling - embryogenese - auxins - plant growth regulators - responses - proteomics - gene expression - embryonic development - embryogenesis

Auxin is a phytohormone that is crucial for many aspects of plant development. The processes in which this hormone has been implicated span from embryo development to flower transition, defense, tropic responses, and many other processes during plant life. A key question in auxin biology is how this molecule is able to elicit such diverse responses. Auxin regulates the transcriptional activation or repression of genes through the AUXIN RESPONSE FACTOR (ARF) family of transcription factors. In my studies I focus in the ARF transcription factors as a likely source of variation in output specificity. We consider three levels at which ARFs differ. First, ARFs differ in their ability to interact with different Aux/IAA (antagonistic family of transcription factors), or to form homo- or heterodimers. Second, ARFs assemble into different protein complexes, transcription factors interact with other transcriptional regulators or other proteins to form transcription complexes. These, when different, may contribute to different functions of ARF complexes. Thirdly, ARFs bind to and regulate different target genes. My work offers a plausible explanation how specific auxin responses are generated and through which genes the developmental responses to auxin are generated.

POPCORN Functions in the Auxin Pathway to Regulate Embryonic Body Plan and Meristem Organization in Arabidopsis
Xiang, D.Q. ; Yang, H. ; Venglat, P. ; Cao, Y.G. ; Wen, R. ; Ren, M.Z. ; Stone, S. ; Wang, E. ; Wang, H. ; Xiao, W. ; Weijers, D. ; Berleth, T. ; Laux, T. ; Selvaraj, G. ; Datla, R. - \ 2011
The Plant Cell 23 (2011)12. - ISSN 1040-4651 - p. 4348 - 4367.
stem-cell niche - cup-shaped-cotyledon - shoot meristem - wild-type - gene - embryogenesis - wuschel - fate - monopteros - thaliana
The shoot and root apical meristems (SAM and RAM) formed during embryogenesis are crucial for postembryonic plant development. We report the identification of POPCORN (PCN), a gene required for embryo development and meristem organization in Arabidopsis thaliana. Map-based cloning revealed that PCN encodes a WD-40 protein expressed both during embryo development and postembryonically in the SAM and RAM. The two pcn alleles identified in this study are temperature sensitive, showing defective embryo development when grown at 22 degrees C that is rescued when grown at 29 degrees C. In pcn mutants, meristem-specific expression of WUSCHEL (WUS), CLAVATA3, and WUSCHEL-RELATED HOMEOBOX5 is not maintained; SHOOTMERISTEMLESS, BODENLOS (BDL) and MONOPTEROS (MP) are misexpressed. Several findings link PCN to auxin signaling and meristem function: ectopic expression of DR5(rev):green fluorescent protein (GFP), pBDL:BDL-GFP, and pMP:MP-beta-glucuronidase in the meristem; altered polarity and expression of pPIN1:PIN1-GFP in the apical domain of the developing embryo; and resistance to auxin in the pcn mutants. The bdl mutation rescued embryo lethality of pcn, suggesting that improper auxin response is involved in pcn defects. Furthermore, WUS, PINFORMED1, PINOID, and TOPLESS are dosage sensitive in pcn, suggesting functional interaction. Together, our results suggest that PCN functions in the auxin pathway, integrating auxin signaling in the organization and maintenance of the SAM and RAM.
Adaptive capacity of rearing hens : effects of early life conditions
Walstra, I. - \ 2011
University. Promotor(en): Bas Kemp, co-promotor(en): Henry van den Brand; Jan ten Napel. - [S.l.] : S.n. - ISBN 9789461731265 - 147
hennen - opfoktechnieken - broeden - uitbroeden - embryogenese - experimentele infectie - warmtestress - immuniteitsreactie - immunologie - adaptatiefysiologie - hens - rearing techniques - incubation - hatching - embryogenesis - experimental infection - heat stress - immune response - immunology - adaptation physiology

The traditional strategy to deal with pathogens in the layer industry is based on monitoring and control methods, primarily aimed at minimizing the risk of infection with the pathogen. The aim of this thesis was to investigate whether the adaptive capacity of layers could be influenced by early life conditions as they may occur in layer practice, as an alternative strategy for improving layer health and disease resistance. The first study investigated whether suboptimal versus optimal incubation, hatch and early rearing conditions could influence the adaptive capacity during infectious challenges with Eimeria and Infectious Bronchitis (IB). The second study investigated effects of prenatal high temperature manipulation on postnatal temperature preference and adaptive response of layers to heat stress. The third study investigated effects of suboptimal and optimal incubation temperature on the adaptive response to Eimeria under normal circumstances or following exposure to a high (35oC) environmental temperature. The fourth study investigated effects of feed provision immediately after hatch (early feeding) and suppression of gram negative intestinal bacteria (by use of the antibiotic Colistin) for 21 d post hatch on microbial composition of the intestines, layer development and response to a mix challenge with lipopolysaccharide (LPS) and humane serum albumin (HuSA). Finally, effects of early feeding and Colistin treatment on organ weights and response to an infectious challenge with Eimeria were investigated. Results demonstrated that optimized incubation, hatch and rearing resulted in a better adaptive response to Eimeria and IB, as was shown by a higher feed intake and reduced weight loss. Optimal incubation as a single early life condition also had a positive influence on the adaptive response of layers toEimeria, as demonstrated by tendencies to higher feed intake and BW gain, less duodenal lesions and a lower oocyst production. Early feeding resulted in higher body and organ weights, a changed microbiota composition in the intestines, and a changed response to E. acervulina and LPS/HuSA. Colistin treatment resulted in a changed microbiota composition of the intestines and a changed response to E. acervulina and LPS/HuSA. These results confirmed the hypothesis that early life conditions can be used to influence the adaptive capacity to infectious challenges. In conclusion, improving the adaptive capacity with the use of particular early life conditions may be the first step towards an alternative method to maintain or improve layer health and disease resistance.

The role of auxin in cell specification during arabidopsis embryogenesis
Lokerse, A.S. - \ 2011
University. Promotor(en): Sacco de Vries, co-promotor(en): Dolf Weijers. - [S.l.] : S.n. - ISBN 9789461731104 - 191
arabidopsis - embryogenese - auxinen - celdifferentiatie - genexpressie - plantenembryo's - transcriptiefactoren - embryogenesis - auxins - cell differentiation - gene expression - plant embryos - transcription factors

Auxin is a structurally simple molecule, yet it elicits many different responses in plants. In Chapter 1 we have reviewed how specificity in the output of auxin signaling could be generated by distinct regulation and the unique properties of the members of the Aux/IAA and ARF transcription factor families.

In Chapter 2 we further investigated the generation of specificity in auxin responses by generating a set of sensitive transcriptional reporter lines for all Arabidopsis ARFs. This facilitated a comprehensive identification of the ARF complement within a cell/tissue of interest. Our analysis of ARF expression in the root meristem revealed both ubiquitous and specific ARF expression patterns and ARF subsets that distinguished the actively dividing cells from those undergoing elongation. Moreover, a striking correlation between cell type and ARF expression patterns was revealed in the early Arabidopsis embryo, where each cell type expressed a unique ARF complement.

In Chapter 3 we characterized a novel cell-autonomous auxin response is required for hypophysis specification and root meristem initiation, and identify Aux/IAA and ARF transcription factors that mediate this response. We show that auxin response components in the proembryo and the suspensor are intrinsically different, and their regulated, lineage-specific expression creates a prepattern enabling different developmental auxin responses. Surprisingly, we find that, in addition to mediating hypophysis specification, auxin response also acts to maintain suspensor cell identity. We show that auxin controlled maintenance of suspensor cell identity includes repression of the embryonic program. This finding gave us an experimental system in which to investigate suspensor cell identity and embryonic transformation.

In Chapter 4 the targeted and specific inhibition of auxin response in the suspensor was coupled to new embryo dissection techniques and a microarray based approach was used to generate a unique dataset which was subsequently mined for cell identity regulators. Unexpectedly, inhibition of auxin response induced the misregulation of thousands of genes, prior to gross morphological changes, revealing a high degree of transcriptional plasticity in these cells. This complicated the identification of regulators. Moreover, the dataset also included secondary/indirect changes in embryo expressed genes, which were inevitable given the connectivity and developmental connectedness between the embryo and suspensor.

One of the most striking findings from analysis of the dataset generated in Chapter 4 was the convergent regulation of members of many gene families involved in all facets of auxin homeostasis, as investigated in Chapter 5. It appears that transient auxin response inhibition is sensed as an auxin minimum and in general auxin homeostasis genes were activated or repressed in such a way that would increase cellular auxin levels (and response).

Finally, many bHLH superfamily members were misregulated upon the inhibition of suspensor auxin response and subsequently found to have specific expression patterns in the embryo, the focus of in Chapter 6. Several bHLHs were shown to lose their lineage specific expression patterns upon inhibition of auxin response in the suspensor, validating further research to place these factors into the auxin response pathways controlling cell identity in the embryo.

A mutually inhibitory interaction between auxin and cytokinin specifies vascular pattern in roots.
Bishopp, A. ; Help, H. ; El-Showk, S. ; Weijers, D. ; Scheres, B.J.G. ; Friml, J. ; Benkova, E. ; Pekka Mahonen, A. ; Helariutta, Y. - \ 2011
Current Biology 21 (2011)11. - ISSN 0960-9822 - p. 917 - 926.
cup-shaped-cotyledon - stem-cell niche - class iiihd-zip - arabidopsis root - meristem activity - hormonal-control - gene family - embryo - efflux - embryogenesis
Background Whereas the majority of animals develop toward a predetermined body plan, plants show iterative growth and continually produce new organs and structures from actively dividing meristems. This raises an intriguing question: How are these newly developed organs patterned? In Arabidopsis embryos, radial symmetry is broken by the bisymmetric specification of the cotyledons in the apical domain. Subsequently, this bisymmetry is propagated to the root promeristem. Results Here we present a mutually inhibitory feedback loop between auxin and cytokinin that sets distinct boundaries of hormonal output. Cytokinins promote the bisymmetric distribution of the PIN-FORMED (PIN) auxin efflux proteins, which channel auxin toward a central domain. High auxin promotes transcription of the cytokinin signaling inhibitor AHP6, which closes the interaction loop. This bisymmetric auxin response domain specifies the differentiation of protoxylem in a bisymmetric pattern. In embryonic roots, cytokinin is required to translate a bisymmetric auxin response in the cotyledons to a bisymmetric vascular pattern in the root promeristem. Conclusions Our results present an interactive feedback loop between hormonal signaling and transport by which small biases in hormonal input are propagated into distinct signaling domains to specify the vascular pattern in the root meristem. It is an intriguing possibility that such a mechanism could transform radial patterns and allow continuous vascular connections between other newly emerging organs.
Green beginnings - Pattern formation during plant embryogenesis
Llavata Peris, C.I. ; Rademacher, E.H. ; Weijers, D. - \ 2010
Current Topics in Developmental Biology 91 (2010). - ISSN 0070-2153 - p. 1 - 27.
arabidopsis shoot meristem - dependent auxin gradients - root apical meristem - epidermal-cell fate - polar transport - axis formation - mapkk kinase - basal axis - gene - embryogenesis
Embryogenesis in plants transforms the zygote into a relatively simple structure, the seedling, which contains all tissues and organs that later form the mature plant body. Despite a profound diversity in cell division patterns among plant species, embryogenesis yields remarkably homologous seedling architectures. In this review, we describe the formative events during plant embryogenesis and discuss the molecular mechanisms that regulate these processes, focusing on Arabidopsis. Even though only a relatively small number of factors are known that regulate each patterning step, a picture emerges where locally acting transcription factors and intercellular signaling contribute to the specification and spatio-temporal coordination of the various cell types in the embryo. Notably, several patterning processes are controlled by the plant hormone auxin. Most regulators that were identified in Arabidopsis have orthologs in other sequenced plant genomes, and several of these are expressed in similar patterns. Therefore, it appears that robust conserved mechanisms may underlie pattern formation in plant embryos
Effect of excessive, hormonally induced intrauterine crowding in the gilt on fetal development on day 40 of pregnancy
Waaij, E.H. van der; Hazeleger, W. ; Soede, N.M. ; Laurenssen, B.F.A. ; Kemp, B. - \ 2010
Journal of Animal Science 88 (2010)8. - ISSN 0021-8812 - p. 2611 - 2619.
ovulation rate - litter size - swine - survival - number - sows - pig - fertilization - embryogenesis - responses
Selection for litter size may result in an increase in uterine crowding due to a faster increase in ovulation rate than in litter size. Increased ovulation rate does not result in a proportionally increased number of piglets born alive. In this study, the effect of ovulation rate on vitality characteristics of fetal-placental units at d 40 of pregnancy was investigated. For this, 43 Large White gilts were treated with hormones to induce superovulation. Average ovulation rate was 45.16 ± 13.22; average number of vital fetuses at d 40 of pregnancy was 17.09 ± 3.61 that weighed 11.26 ± 1.99 g; their placenta weighed 31.88 ± 14.79 g; and they occupied 11.69 ± 4.90 cm of the uterus. Loss in oocytes (i.e., that did not result in a vital fetus at d 40) increased with increasing ovulation rate and occurred before (early mortality; P = 0.0003) and after implantation (late mortality, i.e., traces visible at d 40; P <0.0001). With respect to the vital fetuses, increased ovulation rate resulted in decreased fetal (P = 0.0008) and placental weight (P = 0.0008) and decreased length of the area in the uterus that was occupied by the placenta (P = 0.0011). Strong correlations existed between placental and fetal weight [0.68; 95% confidence interval (CI) = 0.64 to 0.72], and placental weight and length (0.78; 95% CI = 0.74 to 0.82). Fetal-placental characteristics were weakly correlated to distance to the implantation sites of neighboring fetuses, a measure of crowdedness [-0.002 (95% CI = -0.042 to 0.038) with fetal weight to 0.16 (95% CI = 0.12 to 0.20) with placental length]. Increased ovulation rates, but more specifically increased late mortality rates, have negative effects on the remaining vital fetuses with respect to the fetal (P = 0.0085) and placental weight (P <0.0001) and length of the implantation site (P = 0.0016). The most extreme effect was on placental weight, in which a uterus with 18 cases of late mortality (P <0.0001). Furthermore, increased ovulation rates resulted in decreased within litter variation for fetal (P = 0.0018) and placental weight (P = 0.0084). At increased ovulation rates, the number of live fetuses remained similar, but placental development is impaired and the growth of the fetus is retarded compared with reduced ovulation rate, with effects likely lasting into adult life.
MONOPTEROS controls embryonic root initiation by regulating a mobile transcription factor
Schlereth, A. ; Moller, B.K. ; Liu, W. ; Kientz, M. ; Flipse, J. ; Rademacher, E.H. ; Schmid, M. ; Juergens, G. ; Weijers, D. - \ 2010
Nature 464 (2010). - ISSN 0028-0836 - p. 913 - 916.
auxin-response factors - loop-helix proteins - arabidopsis-thaliana - gene-expression - family - embryogenesis - activation - mutation - receptor - encodes
Acquisition of cell identity in plants relies strongly on positional information1, hence cell–cell communication and inductive signalling are instrumental for developmental patterning. During Arabidopsis embryogenesis, an extra-embryonic cell is specified to become the founder cell of the primary root meristem, hypophysis, in response to signals from adjacent embryonic cells2. The auxin-dependent transcription factor MONOPTEROS (MP) drives hypophysis specification by promoting transport of the hormone auxin from the embryo to the hypophysis precursor. However, auxin accumulation is not sufficient for hypophysis specification, indicating that additional MP-dependent signals are required3. Here we describe the microarray-based isolation of MP target genes that mediate signalling from embryo to hypophysis. Of three direct transcriptional target genes, TARGET OF MP 5 (TMO5) and TMO7 encode basic helix–loop–helix (bHLH) transcription factors that are expressed in the hypophysis-adjacent embryo cells, and are required and partially sufficient for MP-dependent root initiation. Importantly, the small TMO7 transcription factor moves from its site of synthesis in the embryo to the hypophysis precursor, thus representing a novel MP-dependent intercellular signal in embryonic root specification
Identification of novel auxin responses during Arabidopsis embryogenesis
Rademacher, E.H. - \ 2009
University. Promotor(en): Sacco de Vries, co-promotor(en): Dolf Weijers. - [S.l. : S.n. - ISBN 9789085854906 - 110
arabidopsis - embryogenese - plantenfysiologie - auxinen - celfysiologie - embryogenesis - plant physiology - auxins - cell physiology
Plants normally form one embryo per seed. Under special circumstances, such as death of the embryo, a second embryo can develop from a supportive structure called the suspensor. These suspensor cells therefore provide a reservoir of stem cells for the generation of secondary embryos. At the start of this project, the mechanisms that control the formation of secondary embryos were completely unclear.
By conducting a systematic screen for cellular responses to the plant hormone auxin during embryogenesis we found that auxin prevents embryo development from suspensor cells. The detailed analysis of auxin response components allowed us to identify the auxin-dependent transcription factors that mediate auxin action in the suspensor. Furthermore, we found that the control of expression of these auxin response transcription factors contributes to early embryo pattern formation. This work identified the first molecular players in the control of suspensor-embryo transformation and provides a stepping stone for elucidating the genetic networks that control embryo identity in plants.

The SERK1 protein complexes
Karlova, R.B. - \ 2008
University. Promotor(en): Sacco de Vries, co-promotor(en): Jacques Vervoort. - S.l. : S.n. - ISBN 9789085852865 - 142
biochemische receptoren - kinasen - signaaltransductie - embryogenese - plantenfysiologie - biochemical receptors - kinases - signal transduction - embryogenesis - plant physiology
Cell fate in plant cells is highly flexible and even differentiated cells can change fate back to the totipotent state. In this study we show that plant steroid hormones are required for this cell fate change. Brassinosteroids are perceived by receptors of the BRI1 and SERK type present in the plasma membrane of plant cells. SERK1 phosphorilation status in vivo is enhanced by brassinosteroid perception and in vitro SERK1 appears to be the most active kinase compared with the other members of the family. Using a combination of biochemical, molecular and cell biological tools the work presented in this thesis shows that the BRI1 and SERK1 receptors transduce the signal from the membrane directly to the nucleus via a transcription factor, AGL15. This is a novel finding in plants where it was not shown that such short signaling transduction pathways are operational.
Early-age housing temperature affects subsequent broiler chicken performance
Baarendse, P.J.J. ; Kemp, B. ; Brand, H. van den - \ 2006
British Poultry Science 47 (2006)2. - ISSN 0007-1668 - p. 125 - 130.
gallus-domesticus - thermal manipulations - thermotolerance - ascites - thermoregulation - embryogenesis - responses - system
1. The influence of housing temperature in early life on subsequent growth and development of broiler chickens was investigated. 2. Hatchlings were exposed to an ambient temperature of 34°C (NT) or 28°C (LT) on d¿1. Both temperature regimes decreased with 1°C per day for 5¿d. At d¿6 the ambient temperature of the LT group was increased to the same ambient temperature as the NT group. At d¿29 all chickens were exposed to 10°C for 7¿d. 3. Navel temperature was lower in the LT group than in the NT group from d¿2 to 5. 4. Body weight of the chickens was higher in the NT group than in the LT group and the difference between both groups increased in time. 5. Temperature treatment during the early post-hatching period did not result in a long-term alternation in organ development, haematocrit value, energy and protein metabolism or the occurrence of ascites. 6. Although not significantly, the course of metabolism suggested that early thermal treatment had long-term effects. Before cold treatment in week 5, the NT group showed higher values for energy and protein metabolism than the LT group, but during cold exposure, the opposite was found. 7. We concluded that exposure of chickens to a moderate reduction in house temperatures during early post-hatching life seemed to have long-term negative effects on the performance of these chickens, but, on the other hand, it seemed that these chickens were better prepared to withstand cold challenge later in life.
Metabolic Responses of Chick Embryos to Short-Term Temperature Fluctuations
Lourens, A. ; Brand, H. van den; Heetkamp, M.J.W. ; Meijerhof, R. ; Kemp, B. - \ 2006
Poultry Science 85 (2006)6. - ISSN 0032-5791 - p. 1081 - 1086.
unturned eggs - incubation - embryogenesis
Two experiments were carried out to study embryonic metabolic responses to short-term temperature fluctuations in order to explore the possibilities of using embryonic metabolic responses as a tool to control the incubation process. In the first experiment, eggshell temperature (ET) in the control group was kept constant at 37.8°C, and embryos in the experimental group were exposed to varying ET within the range of 36.8 to 38.8°C using ET steps of 0.2°C and time steps of 3 h. This was repeated in 3 periods between 6.5 and 9.5 d, 10.5 and 13.5 d, and 14.5 and 17.5 d. In the studied ET range, heat production (HP) increased linearly at 4.9% per 1°C ET. In the second experiment, a standard machine temperature (MT) was used for the control group, and eggs in the experimental group were exposed to low (MT ¿ 0.3°C) or high (MT + 0.3°C) temperatures for 1 h of time at d 8, 9, and 11 to 16. When MT was decreased, CO2 production initially increased at 0.5% and decreased thereafter. When MT was increased, CO2 production initially decreased at 0.4% and increased thereafter. It was concluded that embryonic HP responded linearly with short-term ET changes in the studied ET range of 36.8 to 38.8°C. Changes in CO2 concentration due to short-term MT changes could not be explained by embryonic HP only. It can be speculated that blood flow through the chorio-allantoic membrane changes with MT, affecting heat transfer and diffusion of CO2. A second, delayed response to MT changes was in accordance with the findings in Experiment 1. Within the studied temperature range it will be difficult to use embryonic metabolic responses as a tool to control the incubation process. Because HP is linearly related to ET as in the studied temperature range, other factors such as O2 availability or CO2 release may limit embryo development at higher ET. At this moment, research on the effects of gas exchange at different temperatures on embryo development and survival is lacking.
Evaluation of crucial factors for implementing shed-microspore culture of Indonesian hot pepper (Capsicum annuum L.) cultivars
Supena, E.D.J. ; Muswita, W. ; Suharsono, S. ; Custers, J.B.M. - \ 2006
Scientia Horticulturae 107 (2006)3. - ISSN 0304-4238 - p. 226 - 232.
triticum-aestivum l. - anther culture - antibiotic timentin - chloroplast number - plant-regeneration - tissue-cultures - colchicine - cell - embryogenesis - haploids
A shed-microspore culture protocol was developed in Wageningen for producing doubled haploid plants in several genotypes of Indonesian hot pepper (Capsicum annuum L.). For transfer of technology to Indonesia, three factors were studied that appeared crucial for successful implementation in practice. First, application in the culture medium of a combination of the antibiotics timentin and rifampicin at the concentrations of 200 and 10 mg/l, respectively, prevented bacterial contamination from the donor explants. Second, in vitro application of colchicine (100 ¿M) during the first week of culture was highly effective in increasing the percentage of doubled haploid plants. Third, a comparative analysis of the ploidy level of plants regenerated from shed-microspore-derived embryos using chloroplast counts in guard cells of leaf stomata and flow cytometric measurement of leaf nuclear DNA content, revealed that the first procedure is a reliable and an easy to use method for ploidy determination with hot pepper
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