RNA and mRNA Nitration as a Novel Metabolic Link in Potato Immune Response to Phytophthora infestans

ROS and RNS production, subcellular localization, and signaling triggered by immunogenic danger signals

Plants continuously monitor the environment to detect changing conditions and to properly respond, avoiding deleterious effects on their fitness and survival. An enormous number of cell surface and intracellular immune receptors are deployed to perceive danger signals associated with microbial infections. Ligand binding by cognate receptors represents the first essential event in triggering plant immunity and determining the outcome of the tissue invasion attempt. Reactive oxygen and nitrogen species (ROS/RNS) are secondary messengers rapidly produced in different subcellular localizations upon the perception of immunogenic signals. Danger signal transduction inside the plant cells involves cytoskeletal rearrangements as well as several organelles and interactions between them to activate key immune signaling modules. Such immune processes depend on ROS and RNS accumulation, highlighting their role as key regulators in the execution of the immune cellular program. In fact, ROS and RNS are synergic and interdependent intracellular signals required for decoding danger signals and for the modulation of defense-related responses. Here we summarize current knowledge on ROS/RNS production, compartmentalization, and signaling in plant cells that have perceived immunogenic danger signals.

Histone (de)acetylation in epigenetic regulation of Phytophthora pathobiology

Phytophthora species are oomycetes that have evolved a broad spectrum of biological processes and improved strategies to cope with host and environmental challenges. A growing body of evidence indicates that the high pathogen plasticity is based on epigenetic regulation of gene expression linked to Phytophthora's rapid adjustment to endogenous cues and various stresses. As 5mC DNA methylation has not yet been identified in Phytophthora, the reversible processes of acetylation/deacetylation of histone proteins seem to play a pivotal role in the epigenetic control of gene expression in oomycetes. To explore this issue, we review the structure, diversity, and phylogeny of histone acetyltransferases (HATs) and histone deacetylases (HDACs) in six plant-damaging Phytophthora species: P. capsici, P. cinnamomi, P. infestans, P. parasitica, P. ramorum, and P. sojae. To further integrate and improve our understanding of the phylogenetic classification, evolutionary relationship, and functional characteristics, we supplement this review with a comprehensive view of HATs and HDACs using recent genome- and proteome-level databases. Finally, the potential functional role of transcriptional reprogramming mediated by epigenetic changes during Phytophthora species saprophytic and parasitic phases under nitro-oxidative stress is also briefly discussed.

The role of epigenetic and epitranscriptomic modifications in plants exposed to non-essential metals

Contamination of the soil with non-essential metals and metalloids is a serious problem in many regions of the world. These non-essential metals and metalloids are toxic to all organisms impacting crop yields and human health. Crop plants exposed to high concentrations of these metals leads to perturbed mineral homeostasis, decreased photosynthesis efficiency, inhibited cell division, oxidative stress, genotoxic effects and subsequently hampered growth. Plants can activate epigenetic and epitranscriptomic mechanisms to maintain cellular and organism homeostasis. Epigenetic modifications include changes in the patterns of cytosine and adenine DNA base modifications, changes in cellular non-coding RNAs, and remodeling histone variants and covalent histone tail modifications. Some of these epigenetic changes have been shown to be long-lasting and may therefore contribute to stress memory and modulated stress tolerance in the progeny. In the emerging field of epitranscriptomics, defined as chemical, covalent modifications of ribonucleotides in cellular transcripts, epitranscriptomic modifications are postulated as more rapid modulators of gene expression. Although significant progress has been made in understanding the plant's epigenetic changes in response to biotic and abiotic stresses, a comprehensive review of the plant's epigenetic responses to metals is lacking. While the role of epitranscriptomics during plant developmental processes and stress responses are emerging, epitranscriptomic modifications in response to metals has not been reviewed. This article describes the impact of non-essential metals and metalloids (Cd, Pb, Hg, Al and As) on global and site-specific DNA methylation, histone tail modifications and epitranscriptomic modifications in plants.

Do Reactive Oxygen and Nitrogen Species Have a Similar Effect on Digestive Processes in Carnivorous Nepenthes Plants and Humans?

Carnivorous plants attract animals, trap and kill them, and absorb nutrients from the digested bodies. This unusual (for autotrophs) type of nutrient acquisition evolved through the conversion of photosynthetically active leaves into specialised organs commonly called traps. The genus Nepenthes (pitcher plants) consists of approximately 169 species belonging to the group of carnivorous plants. Pitcher plants are characterised by specialised passive traps filled with a digestive fluid. The digestion that occurs inside the traps of carnivorous plants depends on the activities of many enzymes. Reactive oxygen species (ROS) and reactive nitrogen species (RNS) also participate in the digestive process, but their action is poorly recognised. ROS and RNS, named together as RONS, exhibit concentration-dependent bimodal functions (toxic or signalling). They act as antimicrobial agents, participate in protein modification, and are components of signal transduction cascades. In the human stomach, ROS are considered as the cause of different diseases. RNS have multifaceted functions in the gastrointestinal tract, with both positive and negative impacts on digestion. This review describes the documented and potential impacts of RONS on the digestion in pitcher plant traps, which may be considered as an external stomach.

The emerging role of epitranscriptome in shaping stress responses in plants

KEY MESSAGE

RNA modifications and editing changes constitute 'epitranscriptome' and are crucial in regulating the development and stress response in plants. Exploration of the epitranscriptome and associated machinery would facilitate the engineering of stress tolerance in crops. RNA editing and modifications post-transcriptionally decorate almost all classes of cellular RNAs, including tRNAs, rRNAs, snRNAs, lncRNAs and mRNAs, with more than 170 known modifications, among which m6A, Ψ, m5C, 8-OHG and C-to-U editing are the most abundant. Together, these modifications constitute the "epitranscriptome", and contribute to changes in several RNA attributes, thus providing an additional structural and functional diversification to the "cellular messages" and adding another layer of gene regulation in organisms, including plants. Numerous evidences suggest that RNA modifications have a widespread impact on plant development as well as in regulating the response of plants to abiotic and biotic stresses. High-throughput sequencing studies demonstrate that the landscapes of m6A, m5C, Am, Cm, C-to-U, U-to-G, and A-to-I editing are remarkably dynamic during stress conditions in plants. GO analysis of transcripts enriched in Ψ, m6A and m5C modifications have identified bonafide components of stress regulatory pathways. Furthermore, significant alterations in the expression pattern of genes encoding writers, readers, and erasers of certain modifications have been documented when plants are grown in challenging environments. Notably, manipulating the expression levels of a few components of RNA editing machinery markedly influenced the stress tolerance in plants. We provide updated information on the current understanding on the contribution of RNA modifications in shaping the stress responses in plants. Unraveling of the epitranscriptome has opened new avenues for designing crops with enhanced productivity and stress resilience in view of global climate change.

An activated form of NB-ARC protein RLS1 functions with cysteine-rich receptor-like protein RMC to trigger cell death in rice

A key event that follows pathogen recognition by a resistance (R) protein containing an NB-ARC (nucleotide-binding adaptor shared by Apaf-1, R proteins, and Ced-4) domain is hypersensitive response (HR)-type cell death accompanied by accumulation of reactive oxygen species and nitric oxide. However, the integral mechanisms that underlie this process remain relatively opaque. Here, we show that a gain-of-function mutation in the NB-ARC protein RLS1 (Rapid Leaf Senescence 1) triggers high-light-dependent HR-like cell death in rice. The RLS1-mediated defense response is largely independent of salicylic acid accumulation, NPR1 (Nonexpressor of Pathogenesis-Related Gene 1) activity, and RAR1 (Required for Mla12 Resistance 1) function. A screen for suppressors of RLS1 activation identified RMC (Root Meander Curling) as essential for the RLS1-activated defense response. RMC encodes a cysteine-rich receptor-like secreted protein (CRRSP) and functions as an RLS1-binding partner. Intriguingly, their co-expression resulted in a change in the pattern of subcellular localization and was sufficient to trigger cell death accompanied by a decrease in the activity of the antioxidant enzyme APX1. Collectively, our findings reveal an NB-ARC-CRRSP signaling module that modulates oxidative state, the cell death process, and associated immunity responses in rice.

Nitric Oxide in Seed Biology

Nitric oxide (NO) has been recognized as a gasotransmitter in the mainstream of plant research since the beginning of the 21st century. It is produced in plant tissue and the environment. It influences plant physiology during every ontogenetic stage from seed germination to plant senescence. In this review, we demonstrate the increased interest in NO as a regulatory molecule in combination with other signalling molecules and phytohormones in the information network of plant cells. This work is a summary of the current knowledge on NO action in seeds, starting from seed pretreatment techniques applied to increase seed quality. We describe mode of action of NO in the regulation of seed dormancy, germination, and aging. During each stage of seed physiology, NO appears to act as a key agent with a predominantly beneficial effect.

Dynamics of nitration during dark-induced leaf senescence in Arabidopsis reveals proteins modified by tryptophan nitration

Nitric oxide (NO) is a critical molecule that links plant development with stress responses. Herein, new insights into the role of NO metabolism during leaf senescence in Arabidopsis are presented. A gradual decrease in NO emission accompanied dark-induced leaf senescence (DILS), and a transient wave of peroxynitrite (ONOO-) formation was detected by day 3 of DILS. The boosted ONOO- did not promote tryptophan (Trp) nitration, while the pool of 6-nitroTrp-containing proteins was depleted as senescence progressed. Immunoprecipitation combined with mass spectrometry was used to identify 63 and 4 characteristic 6-nitroTrp-containing proteins in control and individually darkened leaves, respectively. The potential in vivo targets of Trp nitration were mainly related to protein biosynthesis and carbohydrate metabolism. In contrast, nitration of tyrosine-containing proteins was intensified 2-fold on day 3 of DILS. Also, nitrative modification of RNA and DNA increased significantly on days 3 and 7 of DILS, respectively. Taken together, ONOO- can be considered a novel pro-senescence regulator that fine-tunes the redox environment for selective bio-target nitration. Thus, DILS-triggered nitrative changes at RNA and protein levels promote developmental shifts during the plant's lifespan and temporal adjustment in plant metabolism under suboptimal environmental conditions.

Nitric Oxide Implication in Potato Immunity to Phytophthora infestans via Modifications of Histone H3/H4 Methylation Patterns on Defense Genes

Nitric oxide (NO) is an essential redox-signaling molecule operating in many physiological and pathophysiological processes. However, evidence on putative NO engagement in plant immunity by affecting defense gene expressions, including histone modifications, is poorly recognized. Exploring the effect of biphasic NO generation regulated by S-nitrosoglutathione reductase (GNSOR) activity after avr Phytophthora infestans inoculation, we showed that the phase of NO decline at 6 h post-inoculation (hpi) was correlated with the rise of defense gene expressions enriched in the TrxG-mediated H3K4me3 active mark in their promoter regions. Here, we report that arginine methyltransferase PRMT5 catalyzing histone H4R3 symmetric dimethylation (H4R3sme2) is necessary to ensure potato resistance to avr P. infestans. Both the pathogen and S-nitrosoglutathione (GSNO) altered the methylation status of H4R3sme2 by transient reduction in the repressive mark in the promoter of defense genes, R3a and HSR203J (a resistance marker), thereby elevating their transcription. In turn, the PRMT5-selective inhibitor repressed R3a expression and attenuated the hypersensitive response to the pathogen. In conclusion, we postulate that lowering the NO level (at 6 hpi) might be decisive for facilitating the pathogen-induced upregulation of stress genes via histone lysine methylation and PRMT5 controlling potato immunity to late blight.

Nitric oxide signalling in plant interactions with pathogenic fungi and oomycetes

Nitric oxide (NO) and reactive nitrogen species have emerged as crucial signalling and regulatory molecules across all organisms. In plants, fungi, and fungi-like oomycetes, NO is involved in the regulation of multiple processes during their growth, development, reproduction, responses to the external environment, and biotic interactions. It has become evident that NO is produced and used as a signalling and defence cue by both partners in multiple forms of plant interactions with their microbial counterparts, ranging from symbiotic to pathogenic modes. This review summarizes current knowledge on the role of NO in plant-pathogen interactions, focused on biotrophic, necrotrophic, and hemibiotrophic fungi and oomycetes. Actual advances and gaps in the identification of NO sources and fate in plant and pathogen cells are discussed. We review the decisive role of time- and site-specific NO production in germination, oriented growth, and active penetration by filamentous pathogens of the host tissues, as well in pathogen recognition, and defence activation in plants. Distinct functions of NO in diverse interactions of host plants with fungal and oomycete pathogens of different lifestyles are highlighted, where NO in interplay with reactive oxygen species governs successful plant colonization, cell death, and establishment of resistance.

Nitric oxide signalling in plant nanobiology: current status and perspectives

Plant nanobiology as a novel research field provides a scientific basis for the agricultural use of nanoparticles (NPs). Plants respond to the presence of nanomaterials by synthesizing signal molecules, such as the multifunctional gaseous nitric oxide (NO). Several reports have described the effects of different nanomaterials (primarily chitosan NPs, metal oxide NPs, and carbon nanotubes) on endogenous NO synthesis and signalling in different plant species. Other works have demonstrated the ameliorating effect of exogenous NO donor (primarily sodium nitroprusside) treatments on NP-induced stress. NO-releasing NPs are preferred alternatives to chemical NO donors, and evaluating their effects on plants has recently begun. Previous studies clearly indicate that endogenous NO production in the presence of nanomaterials or NO levels increased by exogenous treatments (NO-releasing NPs or chemical NO donors) exerts growth-promoting and stress-ameliorating effects in plants. Furthermore, an NP-based nanosensor for NO detection in plants has been developed, providing a new and excellent perspective for basic research and also for the evaluation of plants' health status in agriculture.

Nitric oxide and hydrogen sulfide modulate the NADPH-generating enzymatic system in higher plants

Nitric oxide (NO) and hydrogen sulfide (H2S) are two key molecules in plant cells that participate, directly or indirectly, as regulators of protein functions through derived post-translational modifications, mainly tyrosine nitration, S-nitrosation, and persulfidation. These post-translational modifications allow the participation of both NO and H2S signal molecules in a wide range of cellular processes either physiological or under stressful circumstances. NADPH participates in cellular redox status and it is a key cofactor necessary for cell growth and development. It is involved in significant biochemical routes such as fatty acid, carotenoid and proline biosynthesis, and the shikimate pathway, as well as in cellular detoxification processes including the ascorbate-glutathione cycle, the NADPH-dependent thioredoxin reductase (NTR), or the superoxide-generating NADPH oxidase. Plant cells have diverse mechanisms to generate NADPH by a group of NADP-dependent oxidoreductases including ferredoxin-NADP reductase (FNR), NADP-glyceraldehyde-3-phosphate dehydrogenase (NADP-GAPDH), NADP-dependent malic enzyme (NADP-ME), NADP-dependent isocitrate dehydrogenase (NADP-ICDH), and both enzymes of the oxidative pentose phosphate pathway, designated as glucose-6-phosphate dehydrogenase (G6PDH) and 6-phosphogluconate dehydrogenase (6PGDH). These enzymes consist of different isozymes located in diverse subcellular compartments (chloroplasts, cytosol, mitochondria, and peroxisomes) which contribute to the NAPDH cellular pool. We provide a comprehensive overview of how post-translational modifications promoted by NO (tyrosine nitration and S-nitrosation), H2S (persulfidation), and glutathione (glutathionylation), affect the cellular redox status through regulation of the NADP-dependent dehydrogenases.

Nitric oxide: A radical molecule with potential biotechnological applications in fruit ripening

Nitric oxide (NO) is a short-life and free radical molecule involved in a wide range of cellular, physiological and stressful processes in higher plants. In recent years it has been observed that exogenous NO application can palliate adverse damages against abiotic and biotic stresses. Conversely, there is accumulating information indicating that endogenous NO participates significantly in the mechanism of modulation of the ripening in climacteric and non-climacteric fruits. Even more, when NO is exogenously applied, it can mediate beneficial effects during ripening and postharvest storage being one of the main effects the increase of antioxidant systems. Consequently, NO could be a promising biotechnological tool to improve crops through ameliorating nutritional indexes and to alleviate damages during fruit ripening and postharvest management. Thus, this approach should be complementary to previous strategies to allow preserving the quality and healthiness of fruits with a view of enhancing their added value. The present mini-review aims to provide an overview of NO biochemistry in plants and updated information on the relevance of NO in fruit ripening and postharvest stages with a view to its biotechnological applications.

Peroxynitrite induced signaling pathways in plant response to non-proteinogenic amino acids

Nitro/oxidative modifications of proteins and RNA nitration resulted from altered peroxynitrite generation are elements of the indirect mode of action of canavanine and meta-tyrosine in plants Environmental conditions and stresses, including supplementation with toxic compounds, are known to impair reactive oxygen (ROS) and reactive nitrogen species (RNS) homeostasis, leading to modification in production of oxidized and nitrated derivatives. The role of nitrated and/or oxidized biotargets differs depending on the stress factors and developmental stage of plants. Canavanine (CAN) and meta-tyrosine (m-Tyr) are non-proteinogenic amino acids (NPAAs). CAN, the structural analog of arginine, is found mostly in seeds of Fabaceae species, as a storage form of nitrogen. In mammalian cells, CAN is used as an anticancer agent due to its inhibitory action on nitric oxide synthesis. m-Tyr is a structural analogue of phenylalanine and an allelochemical found in root exudates of fescues. In animals, m-Tyr is recognized as a marker of oxidative stress. Supplementation of plants with CAN or m-Tyr modify ROS and RNS metabolism. Over the last few years of our research, we have collected the complex data on ROS and RNS metabolism in tomato (Solanum lycopersicum L.) plants exposed to CAN or m-Tyr. In addition, we have shown the level of nitrated RNA (8-Nitro-guanine) in roots of seedlings, stressed by the tested NPAAs. In this review, we describe the model of CAN and m-Tyr mode of action in plants based on modifications of signaling pathways induced by ROS/RNS with a special focus on peroxynitrite induced RNA and protein modifications.

In search of the mRNA modification landscape in plants

BACKGROUND

Precise regulation of gene expression is indispensable for the proper functioning of organisms in both optimal and challenging conditions. The most commonly known regulative mechanisms include the modulation of transcription, translation and adjustment of the transcript, and protein half-life. New players have recently emerged in the arena of gene expression regulators - chemical modifications of mRNAs.

MAIN TEXT

The latest studies show that modified ribonucleotides affect transcript splicing, localization, secondary structures, interaction with other molecules and translation efficiency. Thus far, attention has been focused mostly on the most widespread mRNA modification - adenosine methylation at the N6 position (m6A). However, initial reports on the formation and possible functions of other modified ribonucleotides, such as cytosine methylated at the 5' position (m5C), 8-hydroxyguanosine (8-OHG) and 8-nitroguanosine (8-NO2G), have started to appear in the literature. Additionally, some reports indicate that pseudouridine (Ψ) is present in mRNAs and might perform important regulatory functions in eukaryotic cells. The present review summarizes current knowledge regarding the above-mentioned modified ribonucleotides (m6A, m5C, 8-OHG, 8-NO2G) in transcripts across various plant species, including Arabidopsis, rice, sunflower, wheat, soybean and potato.

CONCLUSIONS

Chemical modifications of ribonucleotides affect mRNA stability and translation efficiency. They thus constitute a newly discovered layer of gene expression regulation and have a profound effect on the development and functioning of various organisms, including plants.

Interaction between Bean and Colletotrichum gloeosporioides: Understanding Through a Biochemical Approach

In addition to its role in animals, nowadays nitric oxide (NO) is considered as an emerging signaling molecule in plant systems. It is now believed that NO exerts its pivotal role in various plant physiological processes, such as in seed germination, plant developmental stages, and plant defense mechanisms. In this study, we have taken an initiative to show the biochemical basis of defense response activation in bean leaves during the progression of Colletotrichum gloeosporioides (Penz.) Penz. and Sacc. in detached bean leaves. Stages of pathogen penetration and colonization were successfully established in the detached bean leaves. Results showed up-regulation of different defense-related enzymes and other defense molecules, such as phenols, flavonoids, callose, and lignin molecules, along with NO at early stages of pathogen invasion. Although in the later stages of the disease, development of NO and other defense components (excluding lignin) were down-regulated, the production of reactive oxygen species in the form of H2O2 became elevated. Consequently, other stress markers, such as lipid peroxidation, proline content, and chlorophyll content, were changed accordingly. Correlation between the disease index and other defense molecules, along with NO, indicate that production of NO and reactive oxygen species (ROS) might influence the development of anthracnose in common bean.

A physiological perspective on targets of nitration in NO-based signaling networks in plants

Although peroxynitrite (ONOO-) has been well documented as a nitrating cognate of nitric oxide (NO) in plant cells, modifications of proteins, fatty acids, and nucleotides by nitration are relatively under-explored topics in plant NO research. As a result, they are seen mainly as hallmarks of redox processes or as markers of nitro-oxidative stress under unfavorable conditions, similar to those observed in human and other animal systems. Protein tyrosine nitration is the best-known nitrative modification in the plant system and can be promoted by the action of both ONOO- and related NO-derived oxidants within the cell environment. Recent progress in 'omics' and modeling tools have provided novel biochemical insights into the physiological and pathophysiological fate of nitrated proteins. The nitration process can be specifically involved in various cell regulatory mechanisms that control redox signaling via nitrated cGMP or nitrated fatty acids. In addition, there is evidence to suggest that nitrative modifications of nucleotides embedded in DNA and RNA can be considered as smart switches of gene expression that fine-tune adaptive cellular responses to stress. This review highlights recent advances in our understanding of the potential implications of biotargets in the regulation of intracellular traffic and plant biological processes.

Switchable Nitroproteome States of Phytophthora infestans Biology and Pathobiology

The study demonstrates protein tyrosine nitration as a functional post-translational modification (PTM) in biology and pathobiology of the oomycete Phytophthora infestans (Mont.) de Bary, the most harmful pathogen of potato (Solanum tuberosum L.). Using two P. infestans isolates differing in their virulence toward potato cv. Sarpo Mira we found that the pathogen generates reactive nitrogen species (RNS) in hyphae and mature sporangia growing under in vitro and in planta conditions. However, acceleration of peroxynitrite formation and elevation of the nitrated protein pool within pathogen structures were observed mainly during the avr P. infestans MP 946-potato interaction. Importantly, the nitroproteome profiles varied for the pathogen virulence pattern and comparative analysis revealed that vr MP 977 P. infestans represented a much more diverse quality spectrum of nitrated proteins. Abundance profiles of nitrated proteins that were up- or downregulated were substantially different also between the analyzed growth phases. Briefly, in planta growth of avr and vr P. infestans was accompanied by exclusive nitration of proteins involved in energy metabolism, signal transduction and pathogenesis. Importantly, the P. infestans-potato interaction indicated cytosolic RXLRs and Crinklers effectors as potential sensors of RNS. Taken together, we explored the first plant pathogen nitroproteome. The results present new insights into RNS metabolism in P. infestans indicating protein nitration as an integral part of pathogen biology, dynamically modified during its offensive strategy. Thus, the nitroproteome should be considered as a flexible element of the oomycete developmental and adaptive mechanism to different micro-environments, including host cells.

Nitric Oxide-Induced Dormancy Removal of Apple Embryos Is Linked to Alterations in Expression of Genes Encoding ABA and JA Biosynthetic or Transduction Pathways and RNA Nitration

Short-term (3 h) treatment of embryos isolated from dormant apple (Malus domestica Borkh.) seeds with NO donors stimulates their transition from dormancy to germination. Seed dormancy is maintained by ABA, while germination is controlled mainly by gibberellins (GAs) and jasmonic acid (JA). NO-induced dormancy removal correlates with low ABA concentration in embryonic axes and reduced embryo sensitivity to ABA. We analyzed the expression of genes encoding key enzymes of ABA degradation (CYP707A1, CYP707A2), biosynthesis (NCED3, NCED9), and elements of the ABA transduction pathway (PYL1, PYL2, RCAR1, RCAR3, PP2CA, ABI1, ABI2, SNRK2, ABI5, AREB3, ABF). A role for JA in the regulation of germination led us to investigate the expression of genes encoding enzymes of JA biosynthesis (AOS1, JMT, JAR1) and the transduction pathway (COI1, MYC2, JAZ3, JAZ12). The expression profiles of the genes were estimated in embryonic axes isolated from dormant or NO fumigated apple embryos. The analyzed genes were differentially regulated during dormancy alleviation, the main modifications in the transcription level were detected for NCED3, NCED9, CYP707A2, RCAR1, ABF, AOS1, JMT, JAR1 and JAZ3. A regulatory role of NO in the removal of seed dormancy is associated with the stimulation of expression of genes related to ABA degradation, down-regulation of genes responsible for ABA synthesis, an increase of expression level of genes engaged in JA synthesis and modification of the expression of genes engaged in signaling pathways of the hormones. To confirm a signaling role of NO during dormancy breakage, an increased RNA nitration level in embryonic axes was demonstrated.