Diphtheria toxin-mediated cell ablation reveals interregional communication during Arabidopsis seed development

The maternal embrace: the protection of plant embryos

All land plants-the embryophytes-produce multicellular embryos, as do other multicellular organisms, such as brown algae and animals. A unique characteristic of plant embryos is their immobile and confined nature. Their embedding in maternal tissues may offer protection from the environment, but also physically constrains development. Across the different land plants, a huge discrepancy is present between their reproductive structures whilst leading to similarly complex embryos. Therefore, we review the roles that maternal tissues play in the control of embryogenesis across land plants. These nurturing, constraining, and protective roles include both direct and indirect effects. In this review, we explore how the maternal surroundings affect embryogenesis and which chemical and mechanical barriers are in place. We regard these questions through the lens of evolution, and identify key questions for future research.

Arabidopsis ribosomal RNA processing meerling mutants exhibit suspensor-derived polyembryony due to direct reprogramming of the suspensor

Embryo development in Arabidopsis (Arabidopsis thaliana) starts off with an asymmetric division of the zygote to generate the precursors of the embryo proper and the supporting extraembryonic suspensor. The suspensor degenerates as the development of the embryo proper proceeds beyond the heart stage. Until the globular stage, the suspensor maintains embryonic potential and can form embryos in the absence of the developing embryo proper. We report a mutant called meerling-1 (mrl-1), which shows a high penetrance of suspensor-derived polyembryony due to delayed development of the embryo proper. Eventually, embryos from both apical and suspensor lineages successfully develop into normal plants and complete their life cycle. We identified the causal mutation as a genomic rearrangement altering the promoter of the Arabidopsis U3 SMALL NUCLEOLAR RNA-ASSOCIATED PROTEIN 18 (UTP18) homolog that encodes a nucleolar-localized WD40-repeat protein involved in processing 18S preribosomal RNA. Accordingly, root-specific knockout of UTP18 caused growth arrest and accumulation of unprocessed 18S pre-rRNA. We generated the mrl-2 loss-of-function mutant and observed asynchronous megagametophyte development causing embryo sac abortion. Together, our results indicate that promoter rearrangement decreased UTP18 protein abundance during early stage embryo proper development, triggering suspensor-derived embryogenesis. Our data support the existence of noncell autonomous signaling from the embryo proper to prevent direct reprogramming of the suspensor toward embryonic fate.

Endosperm cell death promoted by NAC transcription factors facilitates embryo invasion in Arabidopsis

In flowering plants, two fertilization products develop within the limited space of the seed: the embryo and the surrounding nutritive endosperm. The final size of the endosperm is modulated by the degree of embryo growth. In Arabidopsis thaliana, the endosperm expands rapidly after fertilization, but later gets invaded by the embryo that occupies most of the seed volume at maturity, surrounded by a single remaining aleurone-like endosperm layer.1,2,3,4 Embryo invasion is facilitated by the endosperm-expressed bHLH-type transcription factor ZHOUPI, which promotes weakening of endosperm cell walls.5,6 Endosperm elimination in zou mutants is delayed, and embryo growth is severely affected; the endosperm finally collapses around the dwarf embryo, causing the shriveled appearance of mature zou seeds.5,6,7 However, whether ZHOUPI facilitates mechanical endosperm destruction by the invading embryo or whether an active programmed cell death (PCD) process causes endosperm elimination has been subject to debate.2,8 Here we show that developmental PCD controlled by multiple NAC transcription factors in the embryo-adjacent endosperm promotes gradual endosperm elimination. Misexpressing the NAC transcription factor KIRA1 in the entire endosperm caused total endosperm elimination, generating aleurone-less mature seeds. Conversely, dominant and recessive higher-order NAC mutants led to delayed endosperm elimination and impaired cell corpse clearance. Promoting PCD in the zhoupi mutant partially rescued its embryo growth defects, while the endosperm in a zhoupi nac higher-order mutant persisted until seed desiccation. These data suggest that a combination of cell wall weakening and PCD jointly facilitates embryo invasion by an active auto-elimination of endosperm cells.

NSF/αSNAP2-mediated cis-SNARE complex disassembly precedes vesicle fusion in Arabidopsis cytokinesis

Eukaryotic membrane fusion requires trans-SNARE complexes bridging the gap between adjacent membranes1. Fusion between a transport vesicle and its target membrane transforms the trans- into a cis-SNARE complex. The latter interacts with the hexameric AAA+-ATPase N-ethylmaleimide-sensitive factor (NSF) and its co-factor alpha-soluble NSF attachment protein (αSNAP), forming a 20S complex2,3. ATPase activity disassembles the SNAP receptor (SNARE) complex into Qa-SNARE, which folds back onto itself, and its partners4,5. The fusion of identical membranes has a different sequence of events6. The fusion partners each have cis-SNARE complexes to be broken up by NSF and αSNAP. The Qa-SNARE monomers are then stabilized by interaction with Sec1/Munc18-type regulators (SM proteins) to form trans-SNARE complexes, as shown for the yeast vacuole7. Membrane fusion in Arabidopsis cytokinesis is formally akin to vacuolar fusion8. Membrane vesicles fuse with one another to form the partitioning membrane known as the cell plate. Cis-SNARE complexes of cytokinesis-specific Qa-SNARE KNOLLE and its SNARE partners are assembled at the endoplasmic reticulum and delivered by traffic via the Golgi/trans-Golgi network to the cell division plane9. The SM protein KEULE is required for the formation of trans-SNARE complexes between adjacent membrane vesicles10. Here we identify NSF and its adaptor αSNAP2 as necessary for the disassembly of KNOLLE cis-SNARE complexes, which is a prerequisite for KNOLLE-KEULE interaction in cytokinesis. In addition, we show that NSF is required for other trafficking pathways and interacts with the respective Q-SNAREs. The SNARE complex disassembly machinery is conserved in plants and plays a unique essential role in cytokinesis.

Biosynthesis- and transport-mediated dynamic auxin distribution during seed development controls seed size in Arabidopsis

Auxin is indispensable to the fertilization-induced coordinated development of the embryo, endosperm, and seed coat. However, little attention has been given to the distribution pattern, maintenance mechanism, and function of auxin throughout the process of seed development. In the present study, we found that auxin response signals display a dynamic distribution pattern during Arabidopsis seed development. Shortly after fertilization, strong auxin response signals were observed at the funiculus, chalaza, and micropylar integument where the embryo attaches. Later, additional signals appeared at the middle layer of the inner integument (ii1') above the chalaza and the whole inner layer of the outer integument (oi1). These signals peaked when the seed was mature, then declined upon desiccation and disappeared in the dried seed. Auxin biosynthesis genes, including ASB1, TAA1, YUC1, YUC4, YUC8, and YUC9, contributed to the accumulation of auxin in the funiculus and seed coat. Auxin efflux carrier PIN3 and influx carrier AUX1 also contributed to the polar auxin distribution in the seed coat. PIN3 was expressed in the ii1 (innermost layer of the inner integument) and oi1 layers of the integument and showed polar localization. AUX1 was expressed in both layers of the outer integument and the endosperm and displayed a uniform localization. Further research demonstrated that the accumulation of auxin in the seed coat regulates seed size. Transgenic plants that specifically express the YUC8 gene in the oi2 or ii1 seed coat produced larger seeds. These results provide useful tools for cultivating high-yielding crops.

Spatial and temporal regulation of parent-of-origin allelic expression in the endosperm

Genomic imprinting promotes differential expression of parental alleles in the endosperm of flowering plants and is regulated by epigenetic modification such as DNA methylation and histone tail modifications in chromatin. After fertilization, the endosperm develops through a syncytial stage before it cellularizes and becomes a nutrient source for the growing embryo. Regional compartmentalization has been shown both in early and late endosperm development, and different transcriptional domains suggest divergent spatial and temporal regional functions. The analysis of the role of parent-of-origin allelic expression in the endosperm as a whole and the investigation of domain-specific functions have been hampered by the inaccessibility of the tissue for high-throughput transcriptome analyses and contamination from surrounding tissue. Here, we used fluorescence-activated nuclear sorting (FANS) of nuclear targeted GFP fluorescent genetic markers to capture parental-specific allelic expression from different developmental stages and specific endosperm domains. This approach allowed us to successfully identify differential genomic imprinting with temporal and spatial resolution. We used a systematic approach to report temporal regulation of imprinted genes in the endosperm, as well as region-specific imprinting in endosperm domains. Analysis of our data identified loci that are spatially differentially imprinted in one domain of the endosperm, while biparentally expressed in other domains. These findings suggest that the regulation of genomic imprinting is dynamic and challenge the canonical mechanisms for genomic imprinting.

Hydraulic flux-responsive hormone redistribution determines root branching

Plant roots exhibit plasticity in their branching patterns to forage efficiently for heterogeneously distributed resources, such as soil water. The xerobranching response represses lateral root formation when roots lose contact with water. Here, we show that xerobranching is regulated by radial movement of the phloem-derived hormone abscisic acid, which disrupts intercellular communication between inner and outer cell layers through plasmodesmata. Closure of these intercellular pores disrupts the inward movement of the hormone signal auxin, blocking lateral root branching. Once root tips regain contact with moisture, the abscisic acid response rapidly attenuates. Our study reveals how roots adapt their branching pattern to heterogeneous soil water conditions by linking changes in hydraulic flux with dynamic hormone redistribution.

Options for the generation of seedless cherry, the ultimate snacking product

This manuscript identifies cherry orthologues of genes implicated in the development of pericarpic fruit and pinpoints potential options and restrictions in the use of these targets for commercial exploitation of parthenocarpic cherry fruit. Cherry fruit contain a large stone and seed, making processing of the fruit laborious and consumption by the consumer challenging, inconvenient to eat 'on the move' and potentially dangerous for children. Availability of fruit lacking the stone and seed would be potentially transformative for the cherry industry, since such fruit would be easier to process and would increase consumer demand because of the potential reduction in costs. This review will explore the background of seedless fruit, in the context of the ambition to produce the first seedless cherry, carry out an in-depth analysis of the current literature around parthenocarpy in fruit, and discuss the available technology and potential for producing seedless cherry fruit as an 'ultimate snacking product' for the twenty-first century.

BABY BOOM regulates early embryo and endosperm development

The zygote is a totipotent structure that develops into an embryo with all of the cells needed to produce an entire plant. The BABY BOOM (BBM) transcription factor induces spontaneous asexual embryo development on plant organs when ectopically expressed. Although BBM is at the top of a transcriptional network that promotes asexual embryo development, little is known about its expression and role during zygotic embryogenesis. Here we show in Arabidopsis that BBM regulates the progression of zygotic embryo development and embryo patterning, and division and cellularization of the filial endosperm. In line with its role as a totipotency factor, ectopic BBM expression in the egg cell is also sufficient to induce haploid embryo development in Arabidopsis and dicot crops.

The BABY BOOM (BBM) AINTEGUMENTA-LIKE (AIL) AP2/ERF domain transcription factor is a major regulator of plant cell totipotency, as it induces asexual embryo formation when ectopically expressed. Surprisingly, only limited information is available on the role of BBM during zygotic embryogenesis. Here we reexamined BBM expression and function in the model plant Arabidopsis thaliana (Arabidopsis) using reporter analysis and newly developed CRISPR mutants. BBM was expressed in the embryo from the zygote stage and also in the maternal (nucellus) and filial (endosperm) seed tissues. Analysis of CRISPR mutant alleles for BBM (bbm-cr) and the redundantly acting AIL gene PLETHORA2 (PLT2) (plt2-cr) uncovered individual roles for these genes in the timing of embryo progression. We also identified redundant roles for BBM and PLT2 in endosperm proliferation and cellularization and the maintenance of zygotic embryo development. Finally, we show that ectopic BBM expression in the egg cell of Arabidopsis and the dicot crops Brassica napus and Solanum lycopersicon is sufficient to bypass the fertilization requirement for embryo development. Together these results highlight roles for BBM and PLT2 in seed development and demonstrate the utility of BBM genes for engineering asexual embryo development in dicot species.

SNAREs Regulate Vesicle Trafficking During Root Growth and Development

SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins assemble to drive the final membrane fusion step of membrane trafficking. Thus, SNAREs are essential for membrane fusion and vesicular trafficking, which are fundamental mechanisms for maintaining cellular homeostasis. In plants, SNAREs have been demonstrated to be located in different subcellular compartments and involved in a variety of fundamental processes, such as cytokinesis, cytoskeleton organization, symbiosis, and biotic and abiotic stress responses. In addition, SNAREs can also contribute to the normal growth and development of Arabidopsis. Here, we review recent progress in understanding the biological functions and signaling network of SNAREs in vesicle trafficking and the regulation of root growth and development in Arabidopsis.

Auxin-dependent control of cytoskeleton and cell shape regulates division orientation in the Arabidopsis embryo

Premitotic control of cell division orientation is critical for plant development, as cell walls prevent extensive cell remodeling or migration. While many divisions are proliferative and add cells to existing tissues, some divisions are formative and generate new tissue layers or growth axes. Such formative divisions are often asymmetric in nature, producing daughters with different fates. We have previously shown that, in the Arabidopsis thaliana embryo, developmental asymmetry is correlated with geometric asymmetry, creating daughter cells of unequal volume. Such divisions are generated by division planes that deviate from a default “minimal surface area” rule. Inhibition of auxin response leads to reversal to this default, yet the mechanisms underlying division plane choice in the embryo have been unclear. Here, we show that auxin-dependent division plane control involves alterations in cell geometry, but not in cell polarity axis or nuclear position. Through transcriptome profiling, we find that auxin regulates genes controlling cell wall and cytoskeleton properties. We confirm the involvement of microtubule (MT)-binding proteins in embryo division control. Organization of both MT and actin cytoskeleton depends on auxin response, and genetically controlled MT or actin depolymerization in embryos leads to disruption of asymmetric divisions, including reversion to the default. Our work shows how auxin-dependent control of MT and actin cytoskeleton properties interacts with cell geometry to generate asymmetric divisions during the earliest steps in plant development.

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Auxin responses regulate directional cell expansion in Arabidopsis embryos

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Cell shape and division orientation are tightly coupled

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Transcriptome analysis identifies MT-associated IQD proteins in division control

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Cytoskeletal dynamics control division orientation

An Essential Function for Auxin in Embryo Development

Embryogenesis in seed plants is the process during which a single cell develops into a mature multicellular embryo that encloses all the modules and primary patterns necessary to build the architecture of the new plant after germination. This process involves a series of cell divisions and coordinated cell fate determinations resulting in the formation of an embryonic pattern with a shoot-root axis and cotyledon(s). The phytohormone auxin profoundly controls pattern formation during embryogenesis. Auxin functions in the embryo through its maxima/minima distribution, which acts as an instructive signal for tissue specification and organ initiation. In this review, we describe how disruptions of auxin biosynthesis, transport, and response severely affect embryo development. Also, the mechanism of auxin action in the development of the shoot-root axis and the three-tissue system is discussed with recent findings. Biological tools that can be implemented to study the auxin function during embryo development are presented, as they may be of interest to the reader.

Clonal sector analysis and cell ablation confirm a function for DORNROESCHEN-LIKE in founder cells and the vasculature in Arabidopsis

Inducible lineage analysis and cell ablation via conditional toxin expression in cells expressing the DORNRÖSCHEN-LIKE transcription factor represent an effective and complementary adjunct to conventional methods of functional gene analysis. Classical methods of functional gene analysis via mutational and expression studies possess inherent limitations, and therefore, the function of a large proportion of transcription factors remains unknown. We have employed two complementary, indirect methods to obtain functional information for the AP2/ERF transcription factor DORNRÖSCHEN-LIKE (DRNL), which is dynamically expressed in flowers and marks lateral organ founder cells. An inducible, two-component Cre-Lox system was used to express beta-glucuronidase GUS in cells expressing DRNL, to perform a sector analysis that reveals lineages of cells that transiently expressed DRNL throughout plant development. In a complementary approach, an inducible system was used to ablate cells expressing DRNL using diphtheria toxin A chain, to visualise the phenotypic consequences. These complementary analyses demonstrate that DRNL functionally marks founder cells of leaves and floral organs. Clonal sectors also included the vasculature of the leaves and petals, implicating a previously unidentified role for DRNL in provasculature development, which was confirmed in cotyledons by closer analysis of drnl mutants. Our findings demonstrate that inducible gene-specific lineage analysis and cell ablation via conditional toxin expression represent an effective and informative adjunct to conventional methods of functional gene analysis.

The short and intricate life of the suspensor

The suspensor is a short-lived tissue critical for proper embryonic development in many higher plants. While the tissue was initially thought to simply suspend the embryo in the endosperm, it has been found through decades of research that it serves multiple important purposes. The suspensor has been found to be vital for proper embryo patterning and numerous studies have been undertaken into the complex transcriptional cross-talk between the suspensor and the embryo proper. Indeed, many suspensor mutants also display abnormalities in the embryo. The suspensor's role as a nutrient conduit has been shown using ultrastructural and histochemical techniques. Biochemical approaches have found that the suspensor is a centre of early embryonic hormone production in several species. The suspensor has also been frequently used as a model for programmed cell death as it shows signs of termination almost immediately upon developing. This review covers the essential functions of the suspensor throughout its short existence from multiple disciplines including structural, genetic and biochemical perspectives.

Tissue-specific inactivation by cytosine deaminase/uracil phosphoribosyl transferase as a tool to study plant biology

Recent advances in the study of plant developmental and physiological responses have benefited from tissue-specific approaches, revealing the role of some cell types in these processes. Such approaches have relied on the inactivation of target cells using either toxic compounds or deleterious genes; however, both tissue-specific and truly inducible tools are lacking in order to precisely target a developmental window or specific growth response. We engineered the yeast fluorocytosine deaminase (FCY1) gene by creating a fusion with the bacterial uracil phosphoribosyl transferase (UPP) gene. The recombinant protein converts the precursor 5-fluorocytosine (5-FC) into 5-fluorouracyl, a drug used in the treatment of a range of cancers, which triggers DNA and RNA damage. We expressed the FCY-UPP gene construct in specific cell types using enhancer trap lines and promoters, demonstrating that this marker acts in a cell-autonomous manner. We also showed that it can inactivate slow developmental processes like lateral root formation by targeting pericycle cells. It also revealed a role for the lateral root cap and the epidermis in controlling root growth, a faster response. The 5-FC precursor acts systemically, as demonstrated by its ability to inhibit stomatal movements when supplied to the roots in combination with a guard cell-specific promoter. Finally, we demonstrate that the tissular inactivation is reversible, and can therefore be used to synchronize plant responses or to determine cell type-specific functions during different developmental stages. This tool will greatly enhance our capacity to understand the respective role of each cell type in plant physiology and development.

Suspensor-derived somatic embryogenesis in Arabidopsis

In many flowering plants, asymmetric division of the zygote generates apical and basal cells with different fates. In Arabidopsis thaliana, the apical cell generates the embryo while the basal cell divides anticlinally, leading to a suspensor of 6-9 cells that remain extra-embryonic and eventually senesce. In some genetic backgrounds, or upon ablation of the embryo, suspensor cells can undergo periclinal cell divisions and eventually form a second, twin embryo. Likewise, embryogenesis can be induced from somatic cells by various genes, but the relation to suspensor-derived embryos is unclear. Here, we addressed the nature of the suspensor to embryo fate transformation, and its genetic triggers. We expressed most known embryogenesis-inducing genes specifically in suspensor cells. We next analyzed morphology and fate marker expression in embryos in which suspensor division were activated by different triggers to address the developmental paths towards reprogramming. Our results show that reprogramming of Arabidopsis suspensor cells towards embryonic identity is a specific cellular response that is triggered by defined regulators, follows a conserved developmental trajectory and shares similarity to the process of somatic embryogenesis from post-embryonic tissues.

Development of a Gateway-compatible two-component expression vector system for plants

Genetic transformation of plants offers the possibility of functional characterization of individual genes and the improvement of plant traits. Development of novel transformation vectors is essential to improve plant genetic transformation technologies for various applications. Here, we present the development of a Gateway-compatible two-component expression vector system for Agrobacterium-mediated plant transformation. The expression system contains two independent plasmid vector sets, the activator vector and the reporter vector, based on the concept of the GAL4/UAS trans-activation system. The activator vector expresses a modified GAL4 protein (GAL4-VP16) under the control of specific promoter. The GAL4-VP16 protein targets the UAS in the reporter vector and subsequently activates reporter gene expression. Both the activator and reporter vectors contain the Gateway recombination cassette, which can be rapidly and efficiently replaced by any specific promoter and reporter gene of interest, to facilitate gene cloning procedures. The efficiency of the activator-reporter expression system has been assessed using agroinfiltration mediated transient expression assay in Nicotiana benthamiana and stable transgenic expression in Arabidopsis thaliana. The reporter genes were highly expressed with precise tissue-specific and subcellular localization. This Gateway-compatible two-component expression vector system will be a useful tool for advancing plant gene engineering.

Targeted cell ablation-based insights into wound healing and restorative patterning

Plants as sessile organisms are constantly under attack by herbivores, rough environmental situations, or mechanical pressure. These challenges often lead to the induction of wounds or destruction of already specified and developed tissues. Additionally, wounding makes plants vulnerable to invasion by pathogens, which is why wound signalling often triggers specific defence responses. To stay competitive or, eventually, survive under these circumstances, plants need to regenerate efficiently, which in rigid, tissue migration-incompatible plant tissues requires post-embryonic patterning and organogenesis. Now, several studies used laser-assisted single cell ablation in the Arabidopsis root tip as a minimal wounding proxy. Here, we discuss their findings and put them into context of a broader spectrum of wound signalling, pathogen responses and tissue as well as organ regeneration.

AtNSE1 and AtNSE3 are required for embryo pattern formation and maintenance of cell viability during Arabidopsis embryogenesis

Embryogenesis is an essential process during seed development in higher plants. It has previously been shown that mutation of the Arabidopsis non-SMC element genes AtNSE1 or AtNSE3 leads to early embryo abortion, and their proteins can interact with each other directly. However, the crucial regions of these proteins in this interaction and how the proteins are cytologically involved in Arabidopsis embryo development are unknown. In this study, we found that the C-terminal including the Ring-like motif of AtNSE1 can interact with the N-terminal of AtNSE3, and only the Ring-like motif is essential for binding with three α motifs of AtNSE2 (homologous to AtMMS21). Using genetic assays and by analysing molecular markers of cell fate decisions (STM, WOX5, and WOX8) in mutant nse1 and nse3 embryos, we found that AtNSE1 and AtNSE3 work non-redundantly in early embryo development, and that differentiation of the apical meristem and the hypophysis fails in the mutants, which have disrupted auxin transportation and responses. However, the upper cells of the suspensor in the mutants seem to have proper embryo cell identity. Cytological examination showed that cell death occurred from the early embryo stage, and that vacuolar programmed cell death and necrosis in the nse1 and nse3 mutant embryos led to ovule abortion. Thus, AtNSE1 and AtNSE3 are essential for maintaining cell viability and growth during early embryogenesis. Our results improve our understanding of the functions of SMC5/6 complex in early embryogenesis in Arabidopsis.

A SOSEKI-based coordinate system interprets global polarity cues in Arabidopsis

Multicellular development requires coordinated cell polarization relative to body axes, and translation to oriented cell division^1-3. In plants, it is unknown how cell polarities are connected to organismal axes and translated to division. Here, we identify Arabidopsis SOSEKI proteins that integrate apical-basal and radial organismal axes to localize to polar cell edges. Localization does not depend on tissue context, requires cell wall integrity and is defined by a transferrable, protein-specific motif. A Domain of Unknown Function in SOSEKI proteins resembles the DIX oligomerization domain in the animal Dishevelled polarity regulator. The DIX-like domain self-interacts and is required for edge localization and for influencing division orientation, together with a second domain that defines the polar membrane domain. Our work shows that SOSEKI proteins locally interpret global polarity cues and can influence cell division orientation. Furthermore, this work reveals that, despite fundamental differences, cell polarity mechanisms in plants and animals converge on a similar protein domain.

Molecular identification of a root apical cell-specific and stress-responsive enhancer from an Arabidopsis enhancer trap line

BACKGROUND

Plant root apex is the major part to direct the root growth and development by responding to various signals/cues from internal and soil environments. To study and understand root system biology particularly at a molecular and cellular level, an Arabidopsis T-DNA insertional enhancer trap line J3411 expressing reporters (GFP) only in the root tip was adopted in this study to isolate a DNA fragment.

RESULTS

Using nested PCR, DNA sequencing and sequence homology search, the T-DNA insertion site(s) and its flanking genes were characterised in J3411 line. Subsequently, a 2000 bp plant DNA-fragment (E_rtip1) upstream of the insert position of the coding T-DNA was in silico analysed, revealing certain putative promoter/enhancer cis-regulatory elements. Cloning and transformation of this DNA fragment and its truncated segments tagged with or without 35S minimal promoter (35Smini), all of which were fused with a GFP or GUS reporter, allowed to detect GFP and GUS expression mediated only by E_rtip1 + 35mini (P_{Ertip1+35Smini}) specifically in the Arabidopsis root tip region. The P_{Ertip1+35Smini} activity was further tested to be strong and stable under many different growth conditions but suppressed by cold, salt, alkaline pH and higher ammonium and phosphorus.

CONCLUSION

This work describes a promising strategy to isolate a tissue-/cell-specific enhancer sequence from the enhancer trap lines, which are publically available. The reported synthetic promoter i.e. P_{Ertip1+35Smini} may provide a valuable and potent molecular-tool for comprehensive investigation of a gene function related to root growth and development as well as molecular engineering of root-architectural formation aiming to improve plant growth.

A Robust Auxin Response Network Controls Embryo and Suspensor Development through a Basic Helix Loop Helix Transcriptional Module

Land plants reproduce sexually by developing an embryo from a fertilized egg cell. However, embryos can also be formed from other cell types in many plant species. Thus, a key question is how embryo identity in plants is controlled, and how this process is modified during nonzygotic embryogenesis. The Arabidopsis (Arabidopsis thaliana) zygote divides to produce an embryonic lineage and an extra-embryonic suspensor. Yet, normally quiescent suspensor cells can develop a second embryo when the initial embryo is damaged, or when response to the signaling molecule auxin is locally blocked. Here we used auxin-dependent suspensor embryogenesis as a model to determine transcriptome changes during embryonic reprogramming. We found that reprogramming is complex and accompanied by large transcriptomic changes before anatomical changes. This analysis revealed a strong enrichment for genes encoding components of auxin homeostasis and response among misregulated genes. Strikingly, deregulation among multiple auxin-related gene families converged upon the re-establishment of cellular auxin levels or response. This finding points to a remarkable degree of feedback regulation to create resilience in the auxin response during embryo development. Starting from the transcriptome of auxin-deregulated embryos, we identified an auxin-dependent basic Helix Loop Helix transcription factor network that mediates the activity of this hormone in suppressing embryo development from the suspensor.

Three Auxin Response Factors Promote Hypocotyl Elongation

The hormone auxin regulates growth largely by affecting gene expression. By studying Arabidopsis (Arabidopsis thaliana) mutants deficient in AUXIN RESPONSE FACTORS (ARFs), we have identified three ARF proteins that are required for auxin-responsive hypocotyl elongation. Plants deficient in these factors have reduced responses to environmental conditions that increase auxin levels, including far-red-enriched light and high temperature. Despite having decreased auxin responses, the ARF-deficient plants responded to brassinosteroid and gibberellin, indicating that different hormones can act partially independently. Aux/IAA proteins, encoded by IAA genes, interact with ARF proteins to repress auxin response. Silencing expression of multiple IAA genes increased hypocotyl elongation, suggesting that Aux/IAA proteins modulate ARF activity in hypocotyls in a potential negative feedback loop.

A Plausible Microtubule-Based Mechanism for Cell Division Orientation in Plant Embryogenesis

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.

A Comprehensive Toolkit for Inducible, Cell Type-Specific Gene Expression in Arabidopsis

Understanding the context-specific role of gene function is a key objective of modern biology. To this end, we generated a resource for inducible cell type-specific transactivation in Arabidopsis (Arabidopsis thaliana) based on the well-established combination of the chimeric GR-LhG4 transcription factor and the synthetic pOp promoter. Harnessing the flexibility of the GreenGate cloning system, we produced a comprehensive set of transgenic lines termed GR-LhG4 driver lines targeting most tissues in the Arabidopsis shoot and root with a strong focus on the indeterminate meristems. When we combined these transgenic lines with effectors under the control of the pOp promoter, we observed tight temporal and spatial control of gene expression. In particular, inducible expression in F1 plants obtained from crosses of driver and effector lines allows for rapid assessment of the cell type-specific impact of an effector with high temporal resolution. Thus, our comprehensive and flexible method is suitable for overcoming the limitations of ubiquitous genetic approaches, the outputs of which often are difficult to interpret due to the widespread existence of compensatory mechanisms and the integration of diverging effects in different cell types.

Auxin: a molecular trigger of seed development

This review by Figueiredo and Köhler describes the molecular mechanisms driving seed development. They review the role of the hormone auxin for the initial development of the three seed structures and as a trigger of fertilization-independent seed development.

The evolution of seeds defines a remarkable landmark in the history of land plants. A developing seed contains three genetically distinct structures: the embryo, the nourishing tissue, and the seed coat. While fertilization is necessary to initiate seed development in most plant species, apomicts have evolved mechanisms allowing seed formation independently of fertilization. Despite their socio–economical relevance, the molecular mechanisms driving seed development have only recently begun to be understood. Here we review the current knowledge on the role of the hormone auxin for the initial development of the three seed structures and as a trigger of fertilization-independent seed development.

KIRA1 and ORESARA1 terminate flower receptivity by promoting cell death in the stigma of Arabidopsis

Flowers have a species-specific functional life span that determines the time window in which pollination, fertilization and seed set can occur. The stigma tissue plays a key role in flower receptivity by intercepting pollen and initiating pollen tube growth toward the ovary. In this article, we show that a developmentally controlled cell death programme terminates the functional life span of stigma cells in Arabidopsis. We identified the leaf senescence regulator ORESARA1 (also known as ANAC092) and the previously uncharacterized KIRA1 (also known as ANAC074) as partially redundant transcription factors that modulate stigma longevity by controlling the expression of programmed cell death-associated genes. KIRA1 expression is sufficient to induce cell death and terminate floral receptivity, whereas lack of both KIRA1 and ORESARA1 substantially increases stigma life span. Surprisingly, the extension of stigma longevity is accompanied by only a moderate extension of flower receptivity, suggesting that additional processes participate in the control of the flower's receptive life span.

Concerted Action of Evolutionarily Ancient and Novel SNARE Complexes in Flowering-Plant Cytokinesis

Membrane vesicles delivered to the cell-division plane fuse with one another to form the partitioning membrane during plant cytokinesis, starting in the cell center. In Arabidopsis, this requires SNARE complexes involving the cytokinesis-specific Qa-SNARE KNOLLE. However, cytokinesis still occurs in knolle mutant embryos, suggesting contributions from KNOLLE-independent SNARE complexes. Here we show that Qa-SNARE SYP132, having counterparts in lower plants, functionally overlaps with the flowering plant-specific KNOLLE. SYP132 mutation causes cytokinesis defects, knolle syp132 double mutants consist of only one or a few multi-nucleate cells, and SYP132 has the same SNARE partners as KNOLLE. SYP132 and KNOLLE also have non-overlapping functions in secretion and in cellularization of the embryo-nourishing endosperm resulting from double fertilization unique to flowering plants. Evolutionarily ancient non-specialized SNARE complexes originating in algae were thus amended by the appearance of cytokinesis-specific SNARE complexes, meeting the high demand for membrane-fusion capacity during endosperm cellularization in angiosperms.

Identification of ZHOUPI Orthologs in Rice Involved in Endosperm Development and Cuticle Formation

The endosperm occupies most of the available space within mature rice seeds, contains abundant nutrients, and directly influences both the quality and quantity of rice production. Initial reports noted that AtZHOUPI (AtZOU) coordinates endosperm breakdown and the concomitant separation of the embryo from this structure in Arabidopsis. The results of this study show that rice genomes contain two most closely related homologs of AtZOU, OsZOU-1 and OsZOU-2; of these, OsZOU-1 expression is limited to within the endosperm where it can be detected throughout this structure 5 days after pollination (DAP). Its expression gradually decreases from seven DAP to nine DAP. The second of the two most closely related homologs, OsZOU-2, is highly expressed in leaves and stem, but is not detected in developing seeds. Heterologous expression of OsZOU-1 and OsZOU-2 in Atzou-4 mutants also revealed that OsZOU-1 partially complements the seed phenotypes of these individuals, while its counterpart, OsZOU-2, was unable to recover these phenotypes. The over-expression of OsZOU-1 severely disrupts both seed development and plant growth in transgenic rice lines, as plants in which this gene has been knocked down failed in the separation of endosperm from embryo and cuticle formation during seed development. The results of this study therefore suggest that OsZOU-1 is orthologous to the AtZOU, and regulates both endosperm development and cuticle formation in rice.

Early plant embryogenesis-dark ages or dark matter?

In nearly all flowering plants, the basic body plan is laid down during embryogenesis. In Arabidopsis, the crucial cell types are established extremely early as reflected in the stereotypic sequence of oriented cell divisions in the developing young embryo. Research into early embryogenesis was especially focused on the role of the infamous tryptophan derivative auxin in establishing embryo polarity and generating the main body axis. However, it is becoming obvious that the mere link to auxin does not provide any mechanistic understanding of early embryo patterning. Taking recent research into account, we discuss mechanisms underlying early embryonic patterning from an evolutionary perspective.

Auxin production in the endosperm drives seed coat development in Arabidopsis

In flowering plants, seed development is initiated by the fusion of the maternal egg and central cells with two paternal sperm cells, leading to the formation of embryo and endosperm, respectively. The fertilization products are surrounded by the maternally derived seed coat, whose development prior to fertilization is blocked by epigenetic regulators belonging to the Polycomb Group (PcG) protein family. Here we show that fertilization of the central cell results in the production of auxin and most likely its export to the maternal tissues, which drives seed coat development by removing PcG function. We furthermore show that mutants for the MADS-box transcription factor AGL62 have an impaired transport of auxin from the endosperm to the integuments, which results in seed abortion. We propose that AGL62 regulates auxin transport from the endosperm to the integuments, leading to the removal of the PcG block on seed coat development.

DOI:http://dx.doi.org/10.7554/eLife.20542.001

The seeds of rice, wheat and other flowering plants store a variety of nutrients, largely in the form of sugars, proteins and oils. These stored reserves provide the main source of calories for humans and livestock all over the world, so they are of major social and economic importance.

Seed development is an intricate process. It begins after male sperm cells fuse with female gametes inside the flower. This leads to the formation of the embryo, which will develop into a new plant, and a structure called the endosperm, which nourishes the growing embryo. A protective seed coat surrounds the embryo and endosperm, which develops from certain parts of the parent flower. In order for the seed to develop successfully, these three components have to communicate so they can coordinate their growth.

Auxin is a key plant hormone that is needed for plants to grow and develop properly and is necessary for the endosperm to form. Previous research has shown that the endosperm is also required to trigger the formation of the seed coat, but the signal that triggers this process has not yet been identified. Figueiredo et al. now address this question in a small flowering plant called Arabidopsis thaliana.

The experiments show that the endosperm produces auxin, which acts as a molecular signal for the seed coat to start forming. Exposing unfertilized flowers to auxin caused a seed coat to form even though the endosperm was absent. This suggests that this hormone alone is sufficient to trigger the formation of the seed coat without any other signals. Further analysis revealed that a protein called AGL62 regulates the movement of auxin to the parts of the flower that give rise to the seed coat. In the absence of AGL62, the hormone remains trapped in the endosperm and the seed coat fails to develop. The next step following on from this work is to understand how auxin moves from the endosperm to the parts of the flower that form the seed coat.

DOI:http://dx.doi.org/10.7554/eLife.20542.002

MultiSite Gateway-Compatible Cell Type-Specific Gene-Inducible System for Plants

A powerful method to study gene function is expression or overexpression in an inducible, cell type-specific system followed by observation of consequent phenotypic changes and visualization of linked reporters in the target tissue. Multiple inducible gene overexpression systems have been developed for plants, but very few of these combine plant selection markers, control of expression domains, access to multiple promoters and protein fusion reporters, chemical induction, and high-throughput cloning capabilities. Here, we introduce a MultiSite Gateway-compatible inducible system for Arabidopsis (Arabidopsis thaliana) plants that provides the capability to generate such constructs in a single cloning step. The system is based on the tightly controlled, estrogen-inducible XVE system. We demonstrate that the transformants generated with this system exhibit the expected cell type-specific expression, similar to what is observed with constitutively expressed native promoters. With this new system, cloning of inducible constructs is no longer limited to a few special cases but can be used as a standard approach when gene function is studied. In addition, we present a set of entry clones consisting of histochemical and fluorescent reporter variants designed for gene and promoter expression studies.

Auxin production couples endosperm development to fertilization

In flowering plants, seed development is preceded by a double fertilization event, whereby two male sperm cells fuse with two female gametes: the egg and central cells. The fertilized egg cell will form the embryo, and the fertilized central cell will give rise to the triploid endosperm, whose function is to nourish and support the embryo. Even though the endosperm has an unparalleled role for human nutrition, the molecular bases for its development are yet to be understood. Our results reveal that increasing auxin levels after fertilization drive the replication of the central cell in Arabidopsis thaliana. Auxin is sufficient to trigger central cell division and is necessary for correct endosperm development, a process dependent on the MADS-box transcription factor AGL62 (AGAMOUS-LIKE 62). We propose that the epigenetic regulators of the Polycomb group (PcG) family block central cell division before fertilization by repressing the expression of auxin biosynthesis genes in the female gametophyte.

ZmZHOUPI, an endosperm-specific basic helix-loop-helix transcription factor involved in maize seed development

In angiosperm seeds the embryo is embedded within the endosperm, which is in turn enveloped by the seed coat, making inter-compartmental communication essential for coordinated seed growth. In this context the basic helix-loop-helix domain transcription factor AtZHOUPI (AtZOU) fulfils a key role in both the lysis of the transient endosperm and in embryo cuticle formation in Arabidopsis thaliana. In maize (Zea mays), a cereal with a persistent endosperm, a single gene, ZmZOU, falls into the same phylogenetic clade as AtZOU. Its expression is limited to the endosperm where it peaks during the filling stage. In ZmZOU-RNA interference knock-down lines embryo size is slightly reduced and the embryonic suspensor and the adjacent embryo surrounding region show retarded breakdown. Ectopic expression of ZmZOU reduces stomatal number, possibly due to inappropriate protein interactions. ZmZOU forms functional heterodimers with AtICE/AtSCREAM and the closely related maize proteins ZmICEb and ZmICEc, but its interaction is more efficient with the ZmICEa protein, which shows sequence divergence and only has close homologues in other monocotyledonous species. Consistent with the observation that these complexes can trans-activate target gene promoters from Arabidopsis, ZmZOU partially complements the Atzou-4 mutant. However, structural, trans-activation and gene expression data support the hypothesis that ZmZOU and ZmICEa may have coevolved to form a functional complex unique to monocot seeds. This divergence may explain the reduced functionality of ZmZOU in Arabidopsis, and reflect functional specificities which are unique to the monocotyledon lineage.

Direct evidence that suspensor cells have embryogenic potential that is suppressed by the embryo proper during normal embryogenesis

The suspensor is a temporary supporting structure of proembryos. It has been proposed that suspensor cells also possess embryogenic potential, which is suppressed by the embryo as an effect of the embryo-suspensor interaction. However, data to support this hypothesis are not yet available. In this report, using an in vivo living cell laser ablation technique, we show that Arabidopsis suspensor cells can develop into embryos after removing the embryo proper. The embryo proper plays a critical role in maintaining suspensor cell identity. However, this depends on the developmental stage; after the globular embryo stage, the suspensors no longer possess the potential to develop into embryos. We also reveal that hypophysis formation may be essential for embryo differentiation. Furthermore, we show that, after removing the embryo, auxin gradually accumulates in the top suspensor cell where cell division occurs to produce an embryo. Auxin redistribution likely reprograms the fate of the suspensor cell and triggers embryogenesis in suspensor cells. Thus, we provide direct evidence that the embryo suppresses the embryogenic potential of suspensor cells.

A Universal Positive-Negative Selection System for Gene Targeting in Plants Combining an Antibiotic Resistance Gene and Its Antisense RNA

Gene targeting (GT) is a useful technology for accurate genome engineering in plants. A reproducible approach based on a positive-negative selection system using hygromycin resistance and the diphtheria toxin A subunit gene as positive and negative selection markers, respectively, is now available. However, to date, this selection system has been applied exclusively in rice (Oryza sativa). To establish a universally applicable positive-negative GT system in plants, we designed a selection system using a combination of neomycin phosphotransferaseII (nptII) and an antisense nptII construct. The concomitant transcription of both sense and antisense nptII suppresses significantly the level of expression of the sense nptII gene, and transgenic calli and plants become sensitive to the antibiotic geneticin. In addition, we were able to utilize the sense nptII gene as a positive selection marker and the antisense nptII construct as a negative selection marker for knockout of the endogenous rice genes Waxy and 33-kD globulin through GT, although negative selection with this system is relatively less efficient compared with diphtheria toxin A subunit. The approach developed here, with some additional improvements, could be applied as a universal selection system for the enrichment of GT cells in several plant species.

Live-cell imaging and optical manipulation of Arabidopsis early embryogenesis

Intercellular communications are essential for cell proliferation and differentiation during plant embryogenesis. However, analysis of intercellular communications in living material in real time is difficult owing to the restricted accessibility of the embryo within the flower. We established a live-embryo imaging system to visualize cell division and cell fate specification in Arabidopsis thaliana from zygote division in real time. We generated a cell-division lineage tree for early embryogenesis in Arabidopsis. Lineage analysis showed that both the direction and time course of cell division between sister cells differed along the apical-basal or radial axes. Using the Arabidopsis kpl mutant, in which single-fertilization events are frequent, we showed that endosperm development is not required for pattern formation during early embryogenesis. Optical manipulation demonstrated that damage to the embryo initial cell induces cell fate conversion of the suspensor cell to compensate for the disrupted embryo initial cell even after cell fate is specified.

Signalling events regulating seed coat development

The evolution of seeds was a major reason for the rise of angiosperms to ecological dominance. Seeds of angiosperms are composed of three main structures: the embryo, which will give rise to the next generation; the endosperm, a nurturing tissue whose main function is to deliver nutrients from the mother plant to the embryo; and the seed coat (or testa), a tissue that is derived from the maternal integuments and which provides support and protection to the growing embryo. All three seed components need to exchange signals to ensure co-ordinated growth and development. The present review discusses the structure of the seed coat, its interaction with the endosperm, and bidirectional signalling events between endosperm and seed coat that co-ordinate growth of both tissues. Angiosperm seeds are not only of evolutionary significance, but also of major agronomic importance, demanding a thorough understanding of the events governing seed growth and development.

A roadmap to embryo identity in plants

Although plant embryogenesis is usually studied in the context of seed development, there are many alternative roads to embryo initiation. These include somatic embryogenesis in tissue culture and microspore embryogenesis, both widely used in breeding and crop propagation, but also include other modes of ectopic embryo initiation. In the past decades several genes, mostly transcription factors, were identified that can induce embryogenesis in somatic cells. Because the genetic networks in which such regulators operate to promote embryogenesis are largely unknown, a key question is how their activity relates to zygotic and alternative embryo initiation. We describe here the many roads to plant embryo initiation and discuss a framework for defining the developmental roles and mechanisms of plant embryogenesis regulators.

Physical, metabolic and developmental functions of the seed coat

The conventional understanding of the role of the seed coat is that it provides a protective layer for the developing zygote. Recent data show that the picture is more nuanced. The seed coat certainly represents a first line of defense against adverse external factors, but it also acts as channel for transmitting environmental cues to the interior of the seed. The latter function primes the seed to adjust its metabolism in response to changes in its external environment. The purpose of this review is to provide the reader with a comprehensive view of the structure and functionality of the seed coat, and to expose its hidden interaction with both the endosperm and embryo. Any breeding and/or biotechnology intervention seeking to increase seed size or modify seed features will have to consider the implications on this tripartite interaction.

BEX1/ARF1A1C is required for BFA-sensitive recycling of PIN auxin transporters and auxin-mediated development in Arabidopsis

Correct positioning of membrane proteins is an essential process in eukaryotic organisms. The plant hormone auxin is distributed through intercellular transport and triggers various cellular responses. Auxin transporters of the PIN-FORMED (PIN) family localize asymmetrically at the plasma membrane (PM) and mediate the directional transport of auxin between cells. A fungal toxin, brefeldin A (BFA), inhibits a subset of guanine nucleotide exchange factors for ADP-ribosylation factor small GTPases (ARF GEFs) including GNOM, which plays a major role in localization of PIN1 predominantly to the basal side of the PM. The Arabidopsis genome encodes 19 ARF-related putative GTPases. However, ARF components involved in PIN1 localization have been genetically poorly defined. Using a fluorescence imaging-based forward genetic approach, we identified an Arabidopsis mutant, bfa-visualized exocytic trafficking defective1 (bex1), in which PM localization of PIN1-green fluorescent protein (GFP) as well as development is hypersensitive to BFA. We found that in bex1 a member of the ARF1 gene family, ARF1A1C, was mutated. ARF1A1C localizes to the trans-Golgi network/early endosome and Golgi apparatus, acts synergistically to BEN1/MIN7 ARF GEF and is important for PIN recycling to the PM. Consistent with the developmental importance of PIN proteins, functional interference with ARF1 resulted in an impaired auxin response gradient and various developmental defects including embryonic patterning defects and growth arrest. Our results show that ARF1A1C is essential for recycling of PIN auxin transporters and for various auxin-dependent developmental processes.

Local auxin sources orient the apical-basal axis in Arabidopsis embryos

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 to generate an asymmetric auxin response that specifies the embryonic apical-basal axis. The auxin flow directionality depends on the polarized subcellular localization of PIN-FORMED (PIN) auxin transporters. 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. 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.

Embryo defective12 encodes the plastid initiation factor 3 and is essential for embryogenesis in maize

Embryo-specific mutants in maize define a unique class of genetic loci that affect embryogenesis without a significant deleterious impact on endosperm development. Here we report the characterization of an embryo specific12 (emb12) mutant in maize. Embryogenesis in the emb12 mutants is arrested at or before transition stage. The mutant embryo at an early stage exhibits abnormal cell structure with increased vacuoles and dramatically reduced internal membrane organelles. In contrast, the mutant endosperm appears normal in morphology, cell structure, starch, lipid and protein accumulation. The Emb12 locus was cloned by transposon tagging and predicts a protein with a high similarity to prokaryotic translation initiation factor 3 (IF3). EMB12-GFP fusion analysis indicates that EMB12 is localized in plastids. The RNA in situ hybridization and protein immunohistochemical analyses indicate that a high level of Emb12 expression localizes in the embryo proper at early developmental stages and in the embryo axis at later stages. Western analysis indicates that plastid protein synthesis is impaired. These results indicate that Emb12 encodes the plastid IF3 which is essential for embryogenesis but not for endosperm development in maize.

SNARE complexes of different composition jointly mediate membrane fusion in Arabidopsis cytokinesis

Membrane fusion is mediated by soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complexes. Although membrane fusion is required for separating daughter cells in eukaryotic cytokinesis, the SNARE complexes involved are not known. In plants, membrane vesicles targeted to the cell division plane fuse with one another to form the partitioning membrane, progressing from the center to the periphery of the cell. In Arabidopsis, the cytokinesis-specific Qa-SNARE KNOLLE interacts with two other Q-SNAREs, SNAP33 and novel plant-specific SNARE 11 (NPSN11), whose roles in cytokinesis are not clear. Here we show by coimmunoprecipitation that KNOLLE forms two SNARE complexes that differ in composition. One complex is modeled on the trimeric plasma membrane type of SNARE complex and includes, in addition to KNOLLE, the promiscuous Qb,c-SNARE SNAP33 and the R-SNARE vesicle-associated membrane protein (VAMP) 721,722, also involved in innate immunity. In contrast, the other KNOLLE-containing complex is tetrameric and includes Qb-SNARE NPSN11, Qc-SNARE SYP71, and VAMP721,722. Elimination of only one or the other type of KNOLLE complex by mutation, including the double mutant npsn11 syp71, causes a mild or no cytokinesis defect. In contrast, the two double mutants snap33 npsn11 and snap33 syp71 eliminate both types of KNOLLE complexes and display knolle-like cytokinesis defects. Thus the two distinct types of KNOLLE complexes appear to jointly mediate membrane fusion in Arabidopsis cytokinesis.

A protodermal miR394 signal defines a region of stem cell competence in the Arabidopsis shoot meristem

A long-standing question in plants and animals is how spatial patterns are maintained within stem cell niches despite ongoing cell divisions. Here we address how, during shoot meristem formation in Arabidopsis thaliana, the three apical cell layers acquire stem cell identity. Using a sensitized mutant screen, we identified miR394 as a mobile signal produced by the surface cell layer (the protoderm) that confers stem cell competence to the distal meristem by repressing the F box protein LEAF CURLING RESPONSIVENESS. This repression is required to potentiate signaling from underneath the stem cells by the transcription factor WUSCHEL, maintaining stem cell pluripotency. The interaction of two opposing signaling centers provides a mechanistic framework of how stem cells are localized at the tip of the meristem. Although the constituent cells change, the surface layer provides a stable point of reference in the self-organizing meristem.

Arabidopsis ETHE1 encodes a sulfur dioxygenase that is essential for embryo and endosperm development

Mutations in human (Homo sapiens) ETHYLMALONIC ENCEPHALOPATHY PROTEIN1 (ETHE1) result in the complex metabolic disease ethylmalonic encephalopathy, which is characterized in part by brain lesions, lactic acidemia, excretion of ethylmalonic acid, and ultimately death. ETHE1-like genes are found in a wide range of organisms; however, the biochemical and physiological role(s) of ETHE1 have not been examined outside the context of ethylmalonic encephalopathy. In this study we characterized Arabidopsis (Arabidopsis thaliana) ETHE1 and determined the effect of an ETHE1 loss-of-function mutation to investigate the role(s) of ETHE1 in plants. Arabidopsis ETHE1 is localized in the mitochondrion and exhibits sulfur dioxygenase activity. Seeds homozygous for a DNA insertion in ETHE1 exhibit alterations in endosperm development that are accompanied by a delay in embryo development followed by embryo arrest by early heart stage. Strong ETHE1 labeling was observed in the peripheral and chalazal endosperm of wild-type seeds prior to cellularization. Therefore, ETHE1 appears to play an essential role in regulating sulfide levels in seeds.