Cell type-specific transcriptome analysis in the early Arabidopsis thaliana embryo

The effect of phytosulfokine alpha on haploid embryogenesis and gene expression of Brassica napus microspore cultures

Microspore embryogenesis (ME) is the most powerful tool for creating homozygous lines in plant breeding and molecular biology research. It is still based mainly on the reprogramming of microspores by temperature, osmotic and/or nutrient stress. New compounds are being sought that could increase the efficiency of microspore embryogenesis or even induce the formation of haploid embryos from recalcitrant genotypes. Among these, the mitogenic factor phytosulfokine alpha (PSK-α) is promising due to its broad spectrum of activity in vivo and in vitro. The aim of our study was to investigate the effect of PSK-α on haploid embryogenesis from microspores of oilseed rape (Brassica napus L., DH4079), one of the most important oil crops and a model plant for studying the molecular mechanisms controlling embryo formation. We tested different concentrations (0, 0.01, 0.1 and 1 µM) of the peptide and evaluated its effect on microspore viability and embryo regeneration after four weeks of culture. Our results showed a positive correlation between addition of PSK-α and cultured microspore viability and a positive effect also on the number of developed embryos. The analysis of transcriptomes across three time points (day 0, 2 and 4) with or without PSK-α supplementation (15 RNA libraries in total) unveiled differentially expressed genes pivotal in cell division, microspore embryogenesis, and subsequent regeneration. PCA grouped transcriptomes by RNA sampling time, with the first two principal components explaining 56.8% variability. On day 2 with PSK, 45 genes (15 up- and 30 down-regulated) were differentially expressed when PSK-α was added and their number increased to 304 by day 4 (30 up- and 274 down-regulated). PSK, PSKR, and PSI gene expression analysis revealed dynamic patterns, with PSK2 displaying the highest increase and overall expression during microspore culture at days 2 and 4. Despite some variations, only PSK1 showed significant differential expression upon PSK-α addition. Of 16 ME-related molecular markers, 3 and 15 exhibited significant differential expression in PSK-supplemented cultures at days 2 and 4, respectively. Embryo-specific markers predominantly expressed after 4 days of culture, with higher expression in medium without PSK, while on day 0, numerous sporophyte-specific markers were highly expressed.

Long Non-Coding RNAs: Discoveries, Mechanisms, and Research Strategies in Seeds

Seeds provide nutrients for the embryo and allow for dormancy in stressed environments to better adapt the plant to its environment. In addition, seeds are an essential source of food for human survival and are the basis for the formation of food production and quality. Therefore, the research on the genetic mechanism of seed development and germination will provide a theoretical basis and technical support for the improvement of crop yield and quality. Recent studies have shown that long non-coding RNAs (lncRNAs) occupy a pivotal position in seed development and germination. In this review, we describe the key processes in seed biology and examine discoveries and insights made in seed lncRNA, with emphasis on lncRNAs that regulate seed biology through multiple mechanisms. Given that thousands of lncRNAs are present in the seed transcriptome, characterization has lagged far behind identification. We provide an overview of research strategies and approaches including some exciting new techniques that may uncover the function of lncRNAs in seed. Finally, we discuss the challenges facing the field and the opening questions. All in all, we hope to provide a clear perspective on discoveries of seed lncRNA by linking discoveries, mechanisms, and technologies.

Asymmetric cell division in plant development

Asymmetric cell division (ACD) is a fundamental process that generates new cell types during development in eukaryotic species. In plant development, post-embryonic organogenesis driven by ACD is universal and more important than in animals, in which organ pattern is preset during embryogenesis. Thus, plant development provides a powerful system to study molecular mechanisms underlying ACD. During the past decade, tremendous progress has been made in our understanding of the key components and mechanisms involved in this important process in plants. Here, we present an overview of how ACD is determined and regulated in multiple biological processes in plant development and compare their conservation and specificity among different model cell systems. We also summarize the molecular roles and mechanisms of the phytohormones in the regulation of plant ACD. Finally, we conclude with the overarching paradigms and principles that govern plant ACD and consider how new technologies can be exploited to fill the knowledge gaps and make new advances in the field.

Analysis of Chromatin Accessibility, Histone Modifications, and Transcriptional States in Specific Cell Types Using Flow Cytometry

The past two decades in biomedical research have experienced an explosion of cell type-specific and single-cell studies, especially concerning the concomitant dissection of regulatory and transcriptional landscapes of those under investigation. Additionally, leveraging next-generation sequencing (NGS) platforms efforts have been undertaken to evaluate the effects of chromatin accessibility, histone modifications, or even transcription factor binding sites. We have shown that Fluorescence-Activated Nuclear Sorting (FANS) is an effective means to characterize the transcriptomes of nuclei from different tissues. In light of our own technical and experimental developments, we extend this effort to combine FACS/FANS with Assay for Transposase-Accessible Chromatin using sequencing (ATAC-seq), Chromatin Immunoprecipitation sequencing (ChIP-seq), and RNA sequencing (RNA-seq) for profiling individual cell types according to their chromatin and transcriptional states.

Independent parental contributions initiate zygote polarization in Arabidopsis thaliana

Embryogenesis of flowering plants is initiated by polarization of the zygote, a prerequisite for correct axis formation in the embryo. The daughter cells of the asymmetric zygote division form the pro-embryo and the mostly extra-embryonic suspensor.¹ The suspensor plays a pivotal role in nutrient and hormone transport and rapid growth of the embryo.^2,3 Zygote polarization is controlled by a MITOGEN-ACTIVATING PROTEIN (MAP) kinase signaling pathway including the MAPKK kinase (MAP3K) YODA (YDA)⁴ and the upstream membrane-associated proteins BRASINOSTEROID SIGNALING KINASE 1 (BSK1) and BSK2.^5,6 Furthermore, suspensor development is controlled by cysteine-rich peptides of the EMBRYO SURROUNDING FACTOR 1 (ESF1) family.⁷ While they act genetically upstream of YDA, the corresponding receptor to perceive these potential ligands is unknown. In other developmental processes, such as stomata development, YDA activity is controlled by receptor kinases of the ERECTA family (ERf).^8-12 While the receptor kinases upstream of BSK1/2 in the embryo have so far not been identified,¹ YDA is in part activated by the sperm cell-derived BSK family member SHORT SUSPENSOR (SSP) that represents a naturally occurring, constitutively active variant of BSK1.^5,13 It has been speculated that SSP might be a paternal component of a parental tug-of-war controlling resource allocation toward the embryo.^2,13 Here, we show that in addition to SSP, the receptor kinase ERECTA plays a crucial role in zygote polarization as a maternally contributed part of the embryonic YDA pathway. We conclude that two independent parental contributions initiate zygote polarization and control embryo development.

Gene expression variation in Arabidopsis embryos at single-nucleus resolution

Soon after fertilization of egg and sperm, plant genomes become transcriptionally activated and drive a series of coordinated cell divisions to form the basic body plan during embryogenesis. Early embryonic cells rapidly diversify from each other, and investigation of the corresponding gene expression dynamics can help elucidate underlying cellular differentiation programs. However, current plant embryonic transcriptome datasets either lack cell-specific information or have RNA contamination from surrounding non-embryonic tissues. We have coupled fluorescence-activated nuclei sorting together with single-nucleus mRNA-sequencing to construct a gene expression atlas of Arabidopsis thaliana early embryos at single-cell resolution. In addition to characterizing cell-specific transcriptomes, we found evidence that distinct epigenetic and transcriptional regulatory mechanisms operate across emerging embryonic cell types. These datasets and analyses, as well as the approach we devised, are expected to facilitate the discovery of molecular mechanisms underlying pattern formation in plant embryos. This article has an associated 'The people behind the papers' interview.

Advances and Opportunities in Single-Cell Transcriptomics for Plant Research

Single-cell approaches are quickly changing our view on biological systems by increasing the spatiotemporal resolution of our analyses to the level of the individual cell. The field of plant biology has fully embraced single-cell transcriptomics and is rapidly expanding the portfolio of available technologies and applications. In this review, we give an overview of the main advances in plant single-cell transcriptomics over the past few years and provide the reader with an accessible guideline covering all steps, from sample preparation to data analysis. We end by offering a glimpse of how these technologies will shape and accelerate plant-specific research in the near future.

De novo activated transcription of inserted foreign coding sequences is inheritable in the plant genome

The manner in which inserted foreign coding sequences become transcriptionally activated and fixed in the plant genome is poorly understood. To examine such processes of gene evolution, we performed an artificial evolutionary experiment in Arabidopsis thaliana. As a model of gene-birth events, we introduced a promoterless coding sequence of the firefly luciferase (LUC) gene and established 386 T2-generation transgenic lines. Among them, we determined the individual LUC insertion loci in 76 lines and found that one-third of them were transcribed de novo even in the intergenic or inherently unexpressed regions. In the transcribed lines, transcription-related chromatin marks were detected across the newly activated transcribed regions. These results agreed with our previous findings in A. thaliana cultured cells under a similar experimental scheme. A comparison of the results of the T2-plant and cultured cell experiments revealed that the de novo-activated transcription concomitant with local chromatin remodelling was inheritable. During one-generation inheritance, it seems likely that the transcription activities of the LUC inserts trapped by the endogenous genes/transcripts became stronger, while those of de novo transcription in the intergenic/untranscribed regions became weaker. These findings may offer a clue for the elucidation of the mechanism by which inserted foreign coding sequences become transcriptionally activated and fixed in the plant genome.

Fluorescence activated cell sorting-A selective tool for plant cell isolation and analysis

Instrumentation for flow cytometry and sorting is designed around the assumption that samples are single-cell suspensions. However, with few exceptions, higher plants comprise complex multicellular tissues and organs, in which the individual cells are held together by shared cell walls. Single-cell suspensions can be obtained through digestion of the cells walls and release of the so-called protoplasts (plants without their cell wall). Here we describe best practices for protoplast preparation, and for analysis through flow cytometry and cell sorting. Finally, the numerous downstream applications involving sorted protoplasts are discussed.

Tissue-specific transcriptome profiling of the Arabidopsis inflorescence stem reveals local cellular signatures

Genome-wide gene expression maps with a high spatial resolution have substantially accelerated plant molecular science. However, the number of characterized tissues and growth stages is still small due to the limited accessibility of most tissues for protoplast isolation. Here, we provide gene expression profiles of the mature inflorescence stem of Arabidopsis thaliana covering a comprehensive set of distinct tissues. By combining fluorescence-activated nucleus sorting and laser-capture microdissection with next-generation RNA sequencing, we characterized the transcriptomes of xylem vessels, fibers, the proximal and distal cambium, phloem, phloem cap, pith, starch sheath, and epidermis cells. Our analyses classified more than 15,000 genes as being differentially expressed among different stem tissues and revealed known and novel tissue-specific cellular signatures. By determining overrepresented transcription factor binding regions in the promoters of differentially expressed genes, we identified candidate tissue-specific transcriptional regulators. Our datasets predict the expression profiles of an exceptional number of genes and allow hypotheses to be generated about the spatial organization of physiological processes. Moreover, we demonstrate that information about gene expression in a broad range of mature plant tissues can be established at high spatial resolution by nuclear mRNA profiling. Tissue-specific gene expression values can be accessed online at https://arabidopsis-stem.cos.uni-heidelberg.de/.

Expression of a plastid-localized sugar transporter in the suspensor is critical to embryogenesis

Plant growth and development rely on sugar transport between source and sink cells and between different organelles. The plastid-localized sugar transporter GLUCOSE-6-PHOSPHATE TRANSLOCATER1 (GPT1) is an essential gene in Arabidopsis (Arabidopsis thaliana). Using a partially rescued gpt1 mutant and cell-specific RNAi suppression of GPT1, we demonstrated that GPT1 is essential to the function of the embryo suspensor and the development of the embryo. GPT1 showed a dynamic expression/accumulation pattern during embryogenesis. Inhibition of GPT1 accumulation via RNAi using a suspensor-specific promoter resulted in embryos and seedlings with defects similar to auxin mutants. Loss of function of GPT1 in the suspensor also led to abnormal/ectopic cell division in the lower part of the suspensor, which gave rise to an ectopic embryo, resulting in twin embryos in some seeds. Furthermore, loss of function of GPT1 resulted in vacuolar localization of PIN-FORMED1 (PIN1) and altered DR5 auxin activity. Proper localization of PIN1 on the plasma membrane is essential to polar auxin transport and distribution, a key determinant of pattern formation during embryogenesis. Our findings suggest that the function of GPT1 in the embryo suspensor is linked to sugar and/or hormone distribution between the embryo proper and the maternal tissues, and is important for maintenance of suspensor identity and function during embryogenesis.

Do Plasmodesmata Play a Prominent Role in Regulation of Auxin-Dependent Genes at Early Stages of Embryogenesis?

Plasmodesmata form intercellular channels which ensure the transport of various molecules during embryogenesis and postembryonic growth. However, high permeability of plasmodesmata may interfere with the establishment of auxin maxima, which are required for cellular patterning and the development of distinct tissues. Therefore, diffusion through plasmodesmata is not always desirable and the symplastic continuum must be broken up to induce or accomplish some developmental processes. Many data show the role of auxin maxima in the regulation of auxin-responsive genes and the establishment of various cellular patterns. However, still little is known whether and how these maxima are formed in the embryo proper before 16-cell stage, that is, when there is still a nonpolar distribution of auxin efflux carriers. In this work, we focused on auxin-dependent regulation of plasmodesmata function, which may provide rapid and transient changes of their permeability, and thus take part in the regulation of gene expression.

Genome-wide analysis of spatiotemporal expression patterns during rice leaf development

BACKGROUND

Rice leaves consist of three distinct regions along a proximal-distal axis, namely the leaf blade, sheath, and blade-sheath boundary region. Each region has a unique morphology and function, but the genetic programs underlying the development of each region are poorly understood. To fully elucidate rice leaf development and discover genes with unique functions in rice and grasses, it is crucial to explore genome-wide transcriptional profiles during the development of the three regions.

RESULTS

In this study, we performed microarray analysis to profile the spatial and temporal patterns of gene expression in the rice leaf using dissected parts of leaves sampled in broad developmental stages. The dynamics in each region revealed that the transcriptomes changed dramatically throughout the progress of tissue differentiation, and those of the leaf blade and sheath differed greatly at the mature stage. Cluster analysis of expression patterns among leaf parts revealed groups of genes that may be involved in specific biological processes related to rice leaf development. Moreover, we found novel genes potentially involved in rice leaf development using a combination of transcriptome data and in situ hybridization, and analyzed their spatial expression patterns at high resolution. We successfully identified multiple genes that exhibit localized expression in tissues characteristic of rice or grass leaves.

CONCLUSIONS

Although the genetic mechanisms of leaf development have been elucidated in several eudicots, direct application of that information to rice and grasses is not appropriate due to the morphological and developmental differences between them. Our analysis provides not only insights into the development of rice leaves but also expression profiles that serve as a valuable resource for gene discovery. The genes and gene clusters identified in this study may facilitate future research on the unique developmental mechanisms of rice leaves.

Transcriptomic Profiling of the Arabidopsis Embryonic Epidermis Using FANS in Combination with RNAseq

The fundamental mechanisms of cell identity and tissue establishment are important already from the very beginning of a plant's life and reiterate later during development. In order to unravel and understand the underlying mechanisms to generate differences that in turn lead to cell or tissue types, plant cells have to be separated and their transcriptional setup analyzed. We have previously demonstrated that fluorescence-activated nuclear sorting (FANS) is a powerful tool to generate nuclear transcriptomic profiles of the most inaccessible embryonic tissues. In this protocol, we extend this effort to combine FANS with next generation RNA sequencing (RNA-seq) to achieve early embryonic transcriptomes of Arabidopsis epidermis precursor tissue (protoderm) and the inner tissue counterpart.

Developmental and genomic architecture of plant embryogenesis: from model plant to crops

Embryonic development represents an important reproductive phase of sexually reproducing plant species. The fusion of egg and sperm produces the plant zygote, a totipotent cell that, through cell division and cell identity specification in early embryogenesis, establishes the major cell lineages and tissues of the adult plant. The subsequent morphogenesis phase produces the full-sized embryo, while the late embryogenesis maturation process prepares the seed for dormancy and subsequent germination, ensuring continuation of the plant life cycle. In this review on embryogenesis, we compare the model eudicot Arabidopsis thaliana with monocot crops, focusing on genome activation, paternal and maternal regulation of early zygote development, and key organizers of patterning, such as auxin and WOX transcription factors. While the early stages of embryo development are apparently conserved among plant species, embryo maturation programs have diversified between eudicots and monocots. This diversification in crop species reflects the likely effects of domestication on seed quality traits that are determined during embryo maturation, and also assures seed germination in different environmental conditions. This review describes the most important features of embryonic development in plants, and the scope and applications of genomics in plant embryo studies.

Flow Cytometry and Sorting in Arabidopsis

Flow cytometry and sorting represents a valuable and mature experimental platform for the analysis of cellular populations. Applications involving higher plants started to emerge around 40 years ago and are now widely employed both to provide unique information regarding basic and applied questions in the biosciences and to advance agricultural productivity in practical ways. Further development of this platform is being actively pursued, and this promises additional progress in our understanding of the interactions of cells within complex tissues and organs. Higher plants offer unique challenges in terms of flow cytometric analysis, first since their organs and tissues are, almost without exception, three-dimensional assemblies of different cell types held together by tough cell walls, and, second, because individual plant cells are generally larger than those of mammals.This chapter, which updates work last reviewed in 2014 [Galbraith DW (2014) Flow cytometry and sorting in Arabidopsis. In: Sanchez Serrano JJ, Salinas J (eds) Arabidopsis Protocols, 3rd ed. Methods in molecular biology, vol 1062. Humana Press, Totowa, pp 509-537], describes the application of techniques of flow cytometry and sorting to the model plant species Arabidopsis thaliana, in particular emphasizing (a) fluorescence labeling in vivo of specific cell types and of subcellular components, (b) analysis using both conventional cytometers and spectral analyzers, (c) fluorescence-activated sorting of protoplasts and nuclei, and (d) transcriptome analyses using sorted protoplasts and nuclei, focusing on population analyses at the level of single protoplasts and nuclei. Since this is an update, details of new experimental methods are emphasized.

The Arabidopsis embryo as a quantifiable model for studying pattern formation

Phenotypic diversity of flowering plants stems from common basic features of the plant body pattern with well-defined body axes, organs and tissue organisation. Cell division and cell specification are the two processes that underlie the formation of a body pattern. As plant cells are encased into their cellulosic walls, directional cell division through precise positioning of division plane is crucial for shaping plant morphology. Since many plant cells are pluripotent, their fate establishment is influenced by their cellular environment through cell-to-cell signaling. Recent studies show that apart from biochemical regulation, these two processes are also influenced by cell and tissue morphology and operate under mechanical control. Finding a proper model system that allows dissecting the relationship between these aspects is the key to our understanding of pattern establishment. In this review, we present the Arabidopsis embryo as a simple, yet comprehensive model of pattern formation compatible with high-throughput quantitative assays.

The integral spliceosomal component CWC15 is required for development in Arabidopsis

Efficient mRNA splicing is a prerequisite for protein biosynthesis and the eukaryotic splicing machinery is evolutionarily conserved among species of various phyla. At its catalytic core resides the activated splicing complex Bact consisting of the three small nuclear ribonucleoprotein complexes (snRNPs) U2, U5 and U6 and the so-called NineTeen complex (NTC) which is important for spliceosomal activation. CWC15 is an integral part of the NTC in humans and it is associated with the NTC in other species. Here we show the ubiquitous expression and developmental importance of the Arabidopsis ortholog of yeast CWC15. CWC15 associates with core components of the Arabidopsis NTC and its loss leads to inefficient splicing. Consistent with the central role of CWC15 in RNA splicing, cwc15 mutants are embryo lethal and additionally display strong defects in the female haploid phase. Interestingly, the haploid male gametophyte or pollen in Arabidopsis, on the other hand, can cope without functional CWC15, suggesting that developing pollen might be more tolerant to CWC15-mediated defects in splicing than either embryo or female gametophyte.

Plasmogamic Paternal Contributions to Early Zygotic Development in Flowering Plants

Flowering plant zygotes possess complete developmental potency, and the mixture of male and female genetic and cytosolic materials in the zygote is a trigger to initiate embryo development. Plasmogamy, the fusion of the gamete cytoplasms, facilitates the cellular dynamics of the zygote. In the last decade, mutant analyses, live cell imaging-based observations, and direct observations of fertilized egg cells by in vitro fusion of isolated gametes have accelerated our understanding of the post-plasmogamic events in flowering plants including cell wall formation, gamete nuclear migration and fusion, and zygotic cell elongation and asymmetric division. Especially, it has become more evident that paternal parent-of-origin effects, via sperm cytoplasm contents, not only control canonical early zygotic development, but also activate a biparental signaling pathway critical for cell fate determination after the first cell division. Here, we summarize the plasmogamic paternal contributions via the entry of sperm contents during/after fertilization in flowering plants.

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.

Specification and regulation of vascular tissue identity in the Arabidopsis embryo

Development of plant vascular tissues involves tissue identity specification, growth, pattern formation and cell-type differentiation. Although later developmental steps are understood in some detail, it is still largely unknown how the tissue is initially specified. We used the early Arabidopsis embryo as a simple model to study this process. Using a large collection of marker genes, we found that vascular identity was specified in the 16-cell embryo. After a transient precursor state, however, there was no persistent uniform tissue identity. Auxin is intimately connected to vascular tissue development. We found that, although an AUXIN RESPONSE FACTOR5/MONOPTEROS (ARF5/MP)-dependent auxin response was required, it was not sufficient for tissue specification. We therefore used a large-scale enhanced yeast one-hybrid assay to identify potential regulators of vascular identity. Network and functional analysis of candidate regulators suggest that vascular identity is under robust, complex control. We found that one candidate regulator, the G-class bZIP transcription factor GBF2, can modulate vascular gene expression by tuning MP output through direct interaction. Our work uncovers components of a gene regulatory network that controls the initial specification of vascular tissue identity.

The promise of single-cell genomics in plants

Single-cell genomic approaches have the potential to revolutionize the study of plant systems. Here, we highlight newly developed techniques to analyze transcriptomes at single-cell resolution. We focus on the rigorous standards necessary to generate and compare these data sets introducing analysis methods that can be applied to interpret their results. Lastly, we discuss the inherent limitations of single-cell studies and address future directions for plant single-cell genomics.

Cell lineage-specific transcriptome analysis for interpreting cell fate specification of proembryos

In Arabidopsis, a zygote undergoes asymmetrical cell division that establishes the first two distinct cell types of early proembryos, apical and basal cells. However, the genome-wide transcriptional activities that guide divergence of apical and basal cell development remain unknown. Here, we present a comprehensive transcriptome analysis of apical and basal cell lineages, uncovering distinct molecular pathways during cell lineage specification. Selective deletion of inherited transcripts and specific de novo transcription contribute to the establishment of cell lineage-specific pathways for cell fate specification. Embryo-related pathways have been specifically activated in apical cell lineage since 1-cell embryo stage, but quick transcriptome remodeling toward suspensor-specific pathways are found in basal cell lineage. Furthermore, long noncoding RNAs and alternative splicing isoforms may be involved in cell lineage specification. This work also provides a valuable lineage-specific transcriptome resource to elucidate the molecular pathways for divergence of apical and basal cell lineages at genome-wide scale.

Asymmetric division of the Arabidopsis zygote produces apical and basal cells that mainly develop into embryo and suspensor, respectively. Here, Zhou et al. show that de novo transcription and selective RNA turnover establish distinct apical and basal transcriptomes as early as the 1-cell stage of embryo development.

Laser-Assisted Microdissection of Plant Embryos for Transcriptional Profiling

Transcriptomic studies have proven powerful and effective as a tool to study the molecular underpinnings of plant development. Still, it remains challenging to disentangle cell- or tissue-specific transcriptomes in complex structures like the plant seed. In particular, the embryo of flowering plants is embedded in the endosperm, a nurturing tissue, which, in turn, is enclosed by the maternal seed coat. Here, we describe laser-assisted microdissection (LAM) to isolate highly pure embryo tissue from whole seeds. This technique is applicable to virtually any plant seed, and we illustrate the use of LAM to isolate embryos from species of the Boechera and Solanum genera. LAM is a tool that will greatly help to increase the repertoires of tissue-specific transcriptomes, including those of embryos and parts thereof, in nonmodel plants.

Big data from small tissues: extraction of high-quality RNA for RNA-sequencing from different oilseed Brassica seed tissues during seed development

BACKGROUND

Obtaining high-quality RNA for gene expression analyses from different seed tissues is challenging due to the presence of various contaminants, such as polyphenols, polysaccharides and lipids which interfere with RNA extraction methods. At present, the available protocols for extracting RNA from seeds require high amounts of tissue and are mainly focused on extracting RNA from whole seeds. However, extracting RNA at the tissue level enables more detailed studies regarding tissue specific transcriptomes during seed development.

RESULTS

Seeds from heart stage embryo to mature developmental stages of Brassica napus and B. oleracea were sampled for isolation of the embryo, endosperm and seed coat tissues. Ovules and ovary wall tissue were also collected from pre-fertilized buds. Subsequent to testing several RNA extraction methods, modifications applied to E.Z.N.A. Plant RNA and Picopure RNA Isolation kit extraction methods resulted in RNA with high yield and quality. Furthermore, the use of polyvinylpolypyrrolidone for seed coats and endosperm at green stages resulted in high-quality RNA. As a result of the introduced modifications to established RNA extraction methods, the RNA from all the above-mentioned tissues presented clear 28S and 18S bands and high RIN values, ranging from 7.0 to 10.0. The protocols reported in this study are not only suitable for different and challenging seed tissue types, but also enable the extraction of high-quality RNA using only 2 to 3 mg of starting tissue.

CONCLUSIONS

Here, we present efficient, reproducible and reliable high-quality RNA extraction methods for diverse oilseed Brassica spp reproductive tissue types including pre-fertilization and developing seed tissues for diploid and polyploid species. The high-quality RNA obtained is suitable for RNA-Sequencing and subsequent gene expression analysis.

Cell Type-Specific Transcriptomics in the Plant Embryo Using an Adapted INTACT Protocol

Cells differentiate from undifferentiated precursors in order to establish the tissues of vascular plants. The different cell types and stem cells are first specified in the early embryo. How cell type specification is instructed by transcriptional control on a genome-wide level is poorly understood. A major hurdle has been the technical challenge associated with obtaining cellular transcriptomes in this inaccessible tissue. Recently, we adapted a two-component genetic labeling system called INTACT to isolate nuclei and generate a microarray-based expression atlas of the cell types in the early Arabidopsis thaliana embryo. Here we present a step-by-step description of the adapted INTACT protocol, as well as the approach to generate transcriptomic profiles. This protocol has been adapted to account for using seeds with embryos of various developmental stages as a starting material, and the relatively few cell type-specific nuclei that can be isolated from embryos.

Using Giant Scarlet Runner Bean (Phaseolus coccineus) Embryos to Dissect the Early Events in Plant Embryogenesis

The giant embryo of the scarlet runner bean (Phaseolus coccineus) has been used historically to investigate the molecular and developmental processes that control the early events of plant embryo development. In more recent years, our laboratory has been using scarlet runner bean embryos to uncover the genes and regulatory events that control embryo proper and suspensor region differentiation shortly after fertilization. In this chapter we describe methods that we have developed to isolate scarlet runner bean embryos at the globular stage of development, and capture embryo proper and suspensor regions by either hand dissection or laser capture microdissection (LCM) for use in downstream genomic analysis. These methods are also applicable for use in investigating the early events of common bean (Phaseolus vulgaris) embryo development, a close relative of scarlet runner bean, which also has a giant embryo in addition to a sequenced genome.

Constitutive signaling activity of a receptor-associated protein links fertilization with embryonic patterning in Arabidopsis thaliana

In flowering plants, the asymmetrical division of the zygote is the first hallmark of apical-basal polarity of the embryo and is controlled by a MAP kinase pathway that includes the MAPKKK YODA (YDA). In Arabidopsis, YDA is activated by the membrane-associated pseudokinase SHORT SUSPENSOR (SSP) through an unusual parent-of-origin effect: SSP transcripts accumulate specifically in sperm cells but are translationally silent. Only after fertilization is SSP protein transiently produced in the zygote, presumably from paternally inherited transcripts. SSP is a recently diverged, Brassicaceae-specific member of the BRASSINOSTEROID SIGNALING KINASE (BSK) family. BSK proteins typically play broadly overlapping roles as receptor-associated signaling partners in various receptor kinase pathways involved in growth and innate immunity. This raises two questions: How did a protein with generic function involved in signal relay acquire the property of a signal-like patterning cue, and how is the early patterning process activated in plants outside the Brassicaceae family, where SSP orthologs are absent? Here, we show that Arabidopsis BSK1 and BSK2, two close paralogs of SSP that are conserved in flowering plants, are involved in several YDA-dependent signaling events, including embryogenesis. However, the contribution of SSP to YDA activation in the early embryo does not overlap with the contributions of BSK1 and BSK2. The loss of an intramolecular regulatory interaction enables SSP to constitutively activate the YDA signaling pathway, and thus initiates apical-basal patterning as soon as SSP protein is translated after fertilization and without the necessity of invoking canonical receptor activation.

Isolation of living apical and basal cell lineages of early proembryos for transcriptome analysis

Plant zygotes usually undergo asymmetrical cell division, giving rise to the formation of two daughter cells with distinct developmental cell fate. The small apical cell will develop into the major part of embryo proper, whereas the larger basal cell will divide to form a transient suspensor. Thus, the apical and basal cell lineages are an excellent model to study cell fate determination in relation to zygote polarity. However, the molecular mechanism underlying the differentiation of two distinct cell lineages is not yet understood, possibly due to the technique limitations. Previously, we have established a protocol for isolating apical cell and basal cell for cDNA library construction in tobacco. However, the method for isolating tiny Arabidopsis embryos has long been considered much more difficult. Here, we present a detailed protocol for isolating early Arabidopsis proembryos and separating apical and basal cell lineages of proembryos, which allow us to establish cell lineage-specific transcriptomes of early proembryos.

Gene expression atlas of embryo development in Arabidopsis

Embryogenesis represents a critical phase in the life cycle of flowering plants. Here, we characterize transcriptome landscapes associated with key stages of embryogenesis by combining an optimized method for the isolation of developing Arabidopsis embryos with high-throughput RNA-seq. The resulting RNA-seq datasets identify distinct overlapping patterns of gene expression, as well as temporal shifts in gene activity across embryogenesis. Network analysis revealed stage-specific and multi-stage gene expression clusters and biological functions associated with key stages of embryo development. Methylation-related gene expression was associated with early- and middle-stage embryos, initiation of photosynthesis components with the late embryogenesis stage, and storage/energy-related protein activation with late and mature embryos. These results provide a comprehensive understanding of transcriptome programming in Arabidopsis embryogenesis and identify modules of gene expression corresponding to key stages of embryo development. This dataset and analysis are a unique resource to advance functional genetic analysis of embryo development in plants.

Adapting INTACT to analyse cell-type-specific transcriptomes and nucleocytoplasmic mRNA dynamics in the Arabidopsis embryo

In the early embryo of vascular plants, the different cell types and stem cells of the seedling are specified as the embryo develops from a zygote towards maturity. How the key steps in cell and tissue specification are instructed by genome-wide transcriptional activity is poorly understood. Progress in defining transcriptional regulation at the genome-wide level in plant embryos has been hampered by difficulties associated with capturing cell-type-specific transcriptomes in this small and inaccessible structure. We recently adapted a two-component genetic nucleus labelling system called INTACT to isolate nuclei from distinct cell types at different stages of Arabidopsis thaliana embryogenesis. We have used these to generate a transcriptomic atlas of embryo development following microarray-based expression profiling. Here, we present a general description of the adapted INTACT procedure, including the two-component labelling system, seed isolation, nuclei preparation and purification, as well as transcriptomic profiling. We also compare nuclear and cellular transcriptomes from the early Arabidopsis embryo to assess nucleocytoplasmic differences and discuss how these differences can be used to infer regulation of gene activity.

Allele-specific expression and genetic determinants of transcriptomic variations in response to mild water deficit in tomato

Characterizing the natural diversity of gene expression across environments is an important step in understanding how genotype-by-environment interactions shape phenotypes. Here, we analyzed the impact of water deficit onto gene expression levels in tomato at the genome-wide scale. We sequenced the transcriptome of growing leaves and fruit pericarps at cell expansion stage in a cherry and a large fruited accession and their F₁ hybrid grown under two watering regimes. Gene expression levels were steadily affected by the genotype and the watering regime. Whereas phenotypes showed mostly additive inheritance, ~80% of the genes displayed non-additive inheritance. By comparing allele-specific expression (ASE) in the F₁ hybrid to the allelic expression in both parental lines, respectively, 3005 genes in leaf and 2857 genes in fruit deviated from 1:1 ratio independently of the watering regime. Among these genes, ~55% were controlled by cis factors, ~25% by trans factors and ~20% by a combination of both types of factors. A total of 328 genes in leaf and 113 in fruit exhibited significant ASE-by-watering regime interaction, among which ~80% presented trans-by-watering regime interaction, suggesting a response to water deficit mediated through a majority of trans-acting loci in tomato. We cross-validated the expression levels of 274 transcripts in fruit and leaves of 124 recombinant inbred lines (RILs) and identified 163 expression quantitative trait loci (eQTLs) mostly confirming the divergences identified by ASE. Combining phenotypic and expression data, we observed a complex network of variation between genes encoding enzymes involved in the sugar metabolism.

AgriSeqDB: an online RNA-Seq database for functional studies of agriculturally relevant plant species

BACKGROUND

The genome-wide expression profile of genes in different tissues/cell types and developmental stages is a vital component of many functional genomic studies. Transcriptome data obtained by RNA-sequencing (RNA-Seq) is often deposited in public databases that are made available via data portals. Data visualization is one of the first steps in assessment and hypothesis generation. However, these databases do not typically include visualization tools and establishing one is not trivial for users who are not computational experts. This, as well as the various formats in which data is commonly deposited, makes the processes of data access, sharing and utility more difficult. Our goal was to provide a simple and user-friendly repository that meets these needs for data-sets from major agricultural crops.

DESCRIPTION

AgriSeqDB ( https://expression.latrobe.edu.au/agriseqdb ) is a database for viewing, analysing and interpreting developmental and tissue/cell-specific transcriptome data from several species, including major agricultural crops such as wheat, rice, maize, barley and tomato. The disparate manner in which public transcriptome data is often warehoused and the challenge of visualizing raw data are both major hurdles to data reuse. The popular eFP browser does an excellent job of presenting transcriptome data in an easily interpretable view, but previous implementation has been mostly on a case-by-case basis. Here we present an integrated visualisation database of transcriptome data-sets from six species that did not previously have public-facing visualisations. We combine the eFP browser, for gene-by-gene investigation, with the Degust browser, which enables visualisation of all transcripts across multiple samples. The two visualisation interfaces launch from the same point, enabling users to easily switch between analysis modes. The tools allow users, even those without bioinformatics expertise, to mine into data-sets and understand the behaviour of transcripts of interest across samples and time. We have also incorporated an additional graphic download option to simplify incorporation into presentations or publications.

CONCLUSION

Powered by eFP and Degust browsers, AgriSeqDB is a quick and easy-to-use platform for data analysis and visualization in five crops and Arabidopsis. Furthermore, it provides a tool that makes it easy for researchers to share their data-sets, promoting research collaborations and data-set reuse.

Regulation of Division and Differentiation of Plant Stem Cells

A major challenge in developmental biology is unraveling the precise regulation of plant stem cell maintenance and the transition to a fully differentiated cell. In this review, we highlight major themes coordinating the acquisition of cell identity and subsequent differentiation in plants. Plant cells are immobile and establish position-dependent cell lineages that rely heavily on external cues. Central players are the hormones auxin and cytokinin, which balance cell division and differentiation during organogenesis. Transcription factors and miRNAs, many of which are mobile in plants, establish gene regulatory networks that communicate cell position and fate. Small peptide signaling also provides positional cues as new cell types emerge from stem cell division and progress through differentiation. These pathways recruit similar players for patterning different organs, emphasizing the modular nature of gene regulatory networks. Finally, we speculate on the outstanding questions in the field and discuss how they may be addressed by emerging technologies.

Layers of regulation - Insights into the role of transcription factors controlling mucilage production in the Arabidopsis seed coat

A polysaccharide-rich mucilage is released from the seed coat epidermis of numerous plant species and has been intensively studied in the model plant Arabidopsis. This has led to the identification of a large number of genes involved in the synthesis, secretion and modification of cell wall polysaccharides such as pectin, hemicellulose and cellulose being identified. These genes include a small network of transcription factors (TFs) and transcriptional co-regulators, that not only regulate mucilage production, but epidermal cell differentiation and in some cases flavonoid biosynthesis in the internal endothelial layer of the seed coat. Here we focus on the function of these regulators and propose a simplified model where they are assigned to a hierarchical gene network with three regulatory levels (tiers) as a means of assisting in the interpretation of the complexity. We discuss limitations of current methodologies and highlight some of the problems associated with defining the function of TFs, particularly those that perform different functions in adjacent layers of the seed coat. We suggest approaches that should provide a more accurate picture of the function of transcription factors involved with mucilage production and release.

A shared cis-regulatory module activates transcription in the suspensor of plant embryos

The mechanisms controlling the transcription of gene sets in specific regions of a plant embryo shortly after fertilization remain unknown. Previously, we showed that G564 mRNA, encoding a protein of unknown function, accumulates to high levels in the giant suspensor of both Scarlet Runner Bean (SRB) and Common Bean embryos, and a cis-regulatory module containing three unique DNA sequences, designated as the 10-bp, Region 2, and Fifth motifs, is required for G564 suspensor-specific transcription [Henry KF, et al. (2015) Plant Mol Biol 88:207-217; Kawashima T, et al. (2009) Proc Natl Acad Sci USA 106:3627-3632]. We tested the hypothesis that these motifs are also required for transcription of the SRB GA 20-oxidase gene, which encodes a gibberellic acid hormone biosynthesis enzyme and is coexpressed with G564 at a high level in giant bean suspensors. We used deletion and gain-of-function experiments in transgenic tobacco embryos to show that two GA 20-oxidase DNA regions are required for suspensor-specific transcription, one in the 5' UTR (+119 to +205) and another in the 5' upstream region (-341 to -316). Mutagenesis of sequences in these two regions determined that the cis-regulatory motifs required for G564 suspensor transcription are also required for GA 20-oxidase transcription within the suspensor, although the motif arrangement differs. Our results demonstrate the flexibility of motif positioning within a cis-regulatory module that activates gene transcription within giant bean suspensors and suggest that G564 and GA 20-oxidase comprise part of a suspensor gene regulatory network.

Combining laser-assisted microdissection (LAM) and RNA-seq allows to perform a comprehensive transcriptomic analysis of epidermal cells of Arabidopsis embryo

BACKGROUND

Genome-wide characterization of tissue- or cell-specific gene expression is a recurrent bottleneck in biology. We have developed a sensitive approach based on ultra-low RNA sequencing coupled to laser assisted microdissection for analyzing different tissues of the small Arabidopsis embryo.

METHODS AND RESULTS

We first characterized the number of genes detected according to the quantity of tissue yield and total RNA extracted. Our results revealed that as low as 0.02 mm² of tissue and 50 pg of total RNA can be used without compromising the number of genes detected. The optimised protocol was used to compare the epidermal versus mesophyll cell transcriptomes of cotyledons at the torpedo-shaped stage of embryo development. The approach was validated by the recovery of well-known epidermal genes such AtML1 or AtPDF2 and genes involved in flavonoid and cuticular waxes pathways. Moreover, the interest and sensitivity of this approach were highlighted by the characterization of several transcription factors preferentially expressed in epidermal cells.

CONCLUSION

This technical advance unlocks some current limitations of transcriptomic analyses and allows to investigate further and efficiently new biological questions for which only a very small amounts of cells need to be isolated. For instance, it paves the way to increasing the spatial accuracy of regulatory networks in developing small embryo of Arabidopsis or other plant tissues.

Transcriptome dynamics revealed by a gene expression atlas of the early Arabidopsis embryo

During early plant embryogenesis, precursors for all major tissues and stem cells are formed. While several components of the regulatory framework are known, how cell fates are instructed by genome-wide transcriptional activity remains unanswered-in part because of difficulties in capturing transcriptome changes at cellular resolution. Here, we have adapted a two-component transgenic labelling system to purify cell-type-specific nuclear RNA and generate a transcriptome atlas of early Arabidopsis embryo development, with a focus on root stem cell niche formation. We validated the dataset through gene expression analysis, and show that gene activity shifts in a spatio-temporal manner, probably signifying transcriptional reprogramming, to induce developmental processes reflecting cell states and state transitions. This atlas provides the most comprehensive tissue- and cell-specific description of genome-wide gene activity in the early plant embryo, and serves as a valuable resource for understanding the genetic control of early plant development.

Evolutionarily diverse SYP1 Qa-SNAREs jointly sustain pollen tube growth in Arabidopsis

Intracellular membrane fusion is effected by SNARE proteins that reside on adjacent membranes and form bridging trans-SNARE complexes. Qa-SNARE members of the Arabidopsis SYP1 family are involved in membrane fusion at the plasma membrane or during cell plate formation. Three SYP1 family members have been classified as pollen-specific as inferred from gene expression profiling studies, and two of them, SYP124 and SYP125, are confined to angiosperms. The SYP124 gene appears genetically unstable, whereas its sister gene SYP125 shows essentially no variation among Arabidopsis accessions. The third pollen-specific member SYP131 is sister to SYP132, which appears evolutionarily conserved in the plant lineage. Although evolutionarily diverse, the three SYP1 proteins are functionally overlapping in that only the triple mutant syp124 syp125 syp131 shows a specific and severe male gametophytic defect. While pollen development and germination appear normal, pollen tube growth is arrested during passage through the style. Our results suggest that angiosperm pollen tubes employ a combination of ancient and modern Qa-SNARE proteins to sustain their growth-promoting membrane dynamics during the reproductive process.

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.

Cell Type-Specific Gene Expression Profiling Using Fluorescence-Activated Nuclear Sorting

Fluorescence-activated cell sorting (FACS) is a powerful method for the analysis of cell type-specific transcriptome profiles, DNA or histone modifications, and chemical compounds. In plants, it has been employed mainly with root and shoot tissue in combination with cell wall digestion on cellular and nuclear content. However, many tissues are recalcitrant to cell separation and are therefore not readily accessible for FACS analysis. Here, we lay out a detailed protocol for the generation of transcriptional profiles from fluorescently labeled nuclei. The protocol described in this chapter has been used successfully to generate a transcriptional map of the early Arabidopsis thaliana embryo.

Staining and Clearing of Arabidopsis Reproductive Tissue for Imaging of Fluorescent Proteins

Imaging of fluorescent proteins in whole-mount tissue is a powerful tool to understand growth and developmental processes, not only in plants. With the advent of genetically encoded fluorescent reporters, which specifically label reproductive cells in Arabidopsis, deep tissue imaging has become increasingly important for the study of plant reproduction. To penetrate the surrounding layers of maternal tissue, however, the tissue has to be cleared by homogenizing the refractive index of the sample, often leading to inactivation of fluorescent proteins. 2,2'-thiodiethanol (TDE) has recently been introduced as a clearing agent that allows the imaging of fluorescent proteins in a cleared plant tissue. Here, we describe a simple protocol that combines TDE-based tissue clearing with cell wall staining to outline cells that enable deep tissue imaging in reproductive structures of Arabidopsis thaliana.

Tissue and Organ Initiation in the Plant Embryo: A First Time for Everything

Land plants can grow to tremendous body sizes, yet even the most complex architectures are the result of iterations of the same developmental processes: organ initiation, growth, and pattern formation. A central question in plant biology is how these processes are regulated and coordinated to allow for the formation of ordered, 3D structures. All these elementary processes first occur in early embryogenesis, during which, from a fertilized egg cell, precursors for all major tissues and stem cells are initiated, followed by tissue growth and patterning. Here we discuss recent progress in our understanding of this phase of plant life. We consider the cellular basis for multicellular development in 3D and focus on the genetic regulatory mechanisms that direct specific steps during early embryogenesis.

A Versatile Optical Clearing Protocol for Deep Tissue Imaging of Fluorescent Proteins in Arabidopsis thaliana

Confocal microscopy is widely used to visualize gene expression patterns and developmental processes in plants. However, the imaging of plant tissue can be challenging due to its opacity, which often makes previous immersion in a clearing agent necessary. Many commonly-used chemicals suffer either from their incompatibility with fluorescent proteins or their complex and lengthy application. 2,2'-thiodiethanol (TDE) has recently been described as a clearing agent with an emphasis on high resolution microscopy due to its potential to adjust the refractive index. Here, we evaluate the use of TDE-based clearing for confocal as well as two-photon microscopy in various Arabidopsis thaliana tissue types. We demonstrate that tissue fixation is a mandatory prerequisite for the use of TDE, in order to preserve tissue integrity and fluorescent protein activity. TDE concentrations between 50-70% are a good compromise for imaging of technically challenging tissue to achieve good clearing without affecting fluorescent protein activity. TDE-based clearing is simple and rapid to use and allows for a flexible experimental setup while facilitating high quality imaging.

RALFL34 regulates formative cell divisions in Arabidopsis pericycle during lateral root initiation

In plants, many signalling molecules, such as phytohormones, miRNAs, transcription factors, and small signalling peptides, drive growth and development. However, very few small signalling peptides have been shown to be necessary for lateral root development. Here, we describe the role of the peptide RALFL34 during early events in lateral root development, and demonstrate its specific importance in orchestrating formative cell divisions in the pericycle. Our results further suggest that this small signalling peptide acts on the transcriptional cascade leading to a new lateral root upstream of GATA23, an important player in lateral root formation. In addition, we describe a role for ETHYLENE RESPONSE FACTORs (ERFs) in regulating RALFL34 expression. Taken together, we put forward RALFL34 as a new, important player in lateral root initiation.

Going mainstream: How is the body axis of plants first initiated in the embryo?

Vascular plants have an open body plan and continuously generate new axes of growth, such as shoot or root branches. Apical-to-basal transport of the hormone auxin is a hallmark of every axis, and the resulting pattern of auxin distribution affects plant development across scales, from overall architecture to cellular differentiation. How the first axis is initiated in the early embryo is a long-standing question. While our knowledge is still sparse, some of the key players of axialization have emerged, and recent work points to specific models for connecting cellular polarity to the asymmetric division of the zygote and domain specific gene expression to the organization of basipetal auxin flux.

Flow cytometry and single nucleus sorting for Cre-based analysis of changes in transcriptional states

The organs of eukaryotic organisms comprise complex interspersions of cell types, whose different molecular activities, and corresponding cellular states, cooperate during development to produce the final, functional organ. Dysfunction of organs in disease, particularly oncogenesis, initiates with changes of state of a minor subset of cells. It therefore is hard to detect early molecular indicators of disease within an overwhelming background of normal cells. Flow cytometry and sorting provides a convenient way to purify minority subpopulations, if a specific fluorophore can be unambiguously and exclusively associated with this subpopulation. We have generated a number of transgenic mouse lines expressing a nuclear-localized version of the Green Fluorescent Protein (GFP), within which the production of a chimeric histone 2B-GFP protein occurs under the control of a constitutively-active, actin-derived promoter, separated by a Floxed-STOP sequence. In the presence of Cre recombinase, within F1 progeny of these mouse lines, excision of the STOP sequence activates transcription which results in the emergence of cells containing green fluorescent nuclei. We describe the characterization of these lines using a combination of microscopic imaging, flow cytometry and sorting, and Reverse-Transcription polymerase chain reaction of transcripts within single sorted nuclei isolated from tissue homogenates. These lines should be particularly useful for analysis of transcriptional changes in oncogenesis. © 2016 International Society for Advancement of Cytometry.

Auxin responsiveness of the MONOPTEROS-BODENLOS module in primary root initiation critically depends on the nuclear import kinetics of the Aux/IAA inhibitor BODENLOS

Primary root formation in early embryogenesis of Arabidopsis thaliana is initiated with the specification of a single cell called hypophysis. This initial step requires the auxin-dependent release of the transcription factor MONOPTEROS (MP, also known as ARF5) from its inhibition by the Aux/IAA protein BODENLOS (BDL, also known as IAA12). Auxin-insensitive bdl mutant embryos and mp loss-of-function embryos fail to specify the hypophysis, giving rise to rootless seedlings. A suppressor screen of rootless bdl mutant seedlings yielded a mutation in the nuclear import receptor IMPORTIN-ALPHA 6 (IMPα6) that promoted primary root formation through rescue of the embryonic hypophysis defects, without causing additional phenotypic changes. Aux/IAA proteins are continually synthesized and degraded, which is essential for rapid transcriptional responses to changing auxin concentrations. Nuclear translocation of bdl:3×GFP was slowed down in impα6 mutants as measured by fluorescence recovery after photobleaching (FRAP) analysis, which correlated with the reduced inhibition of MP by bdl in transient expression assays in impα6 knock-down protoplasts. The MP-BDL module acts like an auxin-triggered genetic switch because MP activates its own expression as well as the expression of its inhibitor BDL. Using an established simulation model, we determined that the reduced nuclear translocation rate of BDL in impα6 mutant embryos rendered the auxin-triggered switch unstable, impairing the fast response to changes in auxin concentration. Our results suggest that the instability of the inhibitor BDL necessitates a fast nuclear uptake in order to reach the critical threshold level required for auxin responsiveness of the MP-BDL module in primary root initiation.