NO, ROS, and cell death associated with caspase-like activity increase in stress-induced microspore embryogenesis of barley

The different response of Brassica napus genotypes to microspore embryogenesis induced by heat shock and trichostatin A is not determined by changes in cell wall structure and composition but by different stress tolerance

During microspore embryogenesis, microspores are induced to develop into haploid embryos. In Brassica napus, microspore embryogenesis is induced by a heat shock (HS), which initially produces embryogenic structures with different cell wall architectures and compositions, and with different potentials to develop into embryos. The B. napus DH4079 and DH12075 genotypes have high and very low embryo yields, respectively. In DH12075, embryo yield is greatly increased by combining HS and the histone deacetylase (HDAC) inhibitor trichostatin A (TSA). However, we show that HS + TSA inhibits embryogenesis in the highly embryogenic DH4079 line. To ascertain why TSA has such different effects in these lines, we treated DH4079 and DH12075 microspore cultures with TSA and compared the cell wall structure and composition of the different embryogenic structures in both lines, specifically the in situ levels and distribution of callose, cellulose, arabinogalactan proteins and high and low methyl-esterified pectin. For both lines, HS + TSA led to the formation of cell walls unfavorable for embryogenesis progression, with reduced levels of arabinogalactan proteins, reduced cell adhesion of inner walls and altered pectin composition. Thus, TSA effects on cell walls cannot explain their different embryogenic response to TSA. We also applied TSA to DH4079 cultures at different times and concentrations before HS application, with no negative effects on embryogenic induction. These results indicate that DH4079 microspores are hypersensitive to combined TSA and HS treatments, and open up new hypotheses about the causes of such hypersensitivity.

New Prospects for Improving Microspore Embryogenesis Induction in Highly Recalcitrant Winter Wheat Lines

Among various methods stimulating biological progress, double haploid (DH) technology, which utilizes the process of microspore embryogenesis (ME), is potentially the most effective. However, the process depends on complex interactions between many genetic, physiological and environmental variables, and in many cases, e.g., winter wheat, does not operate with the efficiency required for commercial use. Stress associated with low-temperature treatment, isolation and transfer to in vitro culture has been shown to disturb redox homeostasis and generate relatively high levels of reactive oxygen species (ROS), affecting microspore vitality. The aim of this study was to investigate whether controlled plant growth, specific tiller pre-treatment and culture conditions could improve the potential of microspores to cope with stress and effectively induce ME. To understand the mechanism of the stress response, hydrogen peroxide levels, total activity and the content of the most important low-molecular-weight antioxidants (glutathione and ascorbate), as well as the content of selected macro- (Mg, Ca, NA, K) and micronutrients (Mn, Zn, Fe, Cu, Mo) were determined. These analyses, combined with the cytological characteristics of the microspore suspensions, allowed us to demonstrate that an increased microspore vitality and stronger response to ME induction were associated with higher stress resistance based on more efficient ROS scavenging and nutrient management. It was shown that a modified procedure, combining a low temperature with mannitol and sodium selenate tiller pre-treatment, reduced oxidative stress and improved the effectiveness of ME in winter wheat lines.

Transcriptomics integrated with metabolomics reveals the effect of cold stress on rice microspores

BACKGROUND

Microspore culture is one of the important biotechnological tools in plant breeding. The induction of microspore embryogenesis is a critical factor that affects the yield of microspore-derived embryo productions. Cold treatment has been reported to reprogram the gametophytic pathway in various plant species. However, the exact mechanism(s) underlying the effect of cold pre-treatment of floral buds on the efficiency of ME is still not clear.

RESULTS

In this study, the effects of cold stress on the microspore totipotency of rice cultivar Zhonghua 11 were investigated. Our results revealed that a 10-day cold treatment is necessary for microspore embryogenesis initiation. During this period, the survival rate of microspores increased and reached a peak at 7 days post treatment (dpt), before decreasing at 10 dpt. RNA-seq analysis showed that the number of DEGs increased from 3 dpt to 10 dpt, with more downregulated DEGs than upregulated ones at the same time point. GO enrichment analysis showed a shift from 'Response to abiotic stimulus' at 3 dpt to 'Metabolic process' at 7 and 10 dpt, with the most significant category in the cellular component being 'cell wall'. KEGG analysis of the pathways revealed changes during cold treatment. Mass spectrometry was used to evaluate the variations in metabolites at 10 dpt compared to 0 dpt, with more downregulated DEMs being determined in both GC-MS and LC-MS modes. These DEMs were classified into 11 categories, Most of the DEMs belonged to 'lipids and lipid-like molecules'. KEGG analysis of DEMs indicates pathways related to amino acid and nucleotide metabolism being upregulated and those related to carbohydrate metabolism being downregulated. An integration analysis of transcriptomics and metabolomics showed that most pathways belonged to 'Amino acid metabolism' and 'Carbohydrate metabolism'. Four DEMs were identified in the interaction network, with stearidonic acid involving in the most correlations, suggesting the potential role in microspore totipotency.

CONCLUSIONS

Our findings exhibited the molecular events occurring during stress-induced rice microspore. Pathways related to 'Amino acid metabolism' and 'Carbohydrate metabolism' may play important roles in rice microspore totipotency. Stearidonic acid was identified, which may participate in the initiation of microspore embryogenesis.

The Roads to Haploid Embryogenesis

Although zygotic embryogenesis is usually studied in the field of seed biology, great attention has been paid to the methods used to generate haploid embryos due to their applications in crop breeding. These mainly include two methods for haploid embryogenesis: in vitro microspore embryogenesis and in vivo haploid embryogenesis. Although microspore culture systems and maize haploid induction systems were discovered in the 1960s, little is known about the molecular mechanisms underlying haploid formation. In recent years, major breakthroughs have been made in in vivo haploid induction systems, and several key factors, such as the matrilineal (MTL), baby boom (BBM), domain of unknown function 679 membrane protein (DMP), and egg cell-specific (ECS) that trigger in vivo haploid embryo production in both the crops and Arabidopsis models have been identified. The discovery of these haploid inducers indicates that haploid embryogenesis is highly related to gamete development, fertilization, and genome stability in ealry embryos. Here, based on recent efforts to identify key players in haploid embryogenesis and to understand its molecular mechanisms, we summarize the different paths to haploid embryogenesis, and we discuss the mechanisms of haploid generation and its potential applications in crop breeding. Although these haploid-inducing factors could assist egg cells in bypassing fertilization to initiate embryogenesis or trigger genome elimination in zygotes after fertilization to form haploid embryos, the fertilization of central cells to form endosperms is a prerequisite step for haploid formation. Deciphering the molecular and cellular mechanisms for haploid embryogenesis, increasing the haploid induction efficiency, and establishing haploid induction systems in other crops are critical for promoting the application of haploid technology in crop breeding, and these should be addressed in further studies.

Effects of ROS and caspase-3-like protein on the growth and aerenchyma formation of Potamogeton perfoliatus stem

Aerenchyma formation plays an important role in the survival of Potamogeton perfoliatus in submerged environment. To understand the regulatory role of reactive oxygen species (ROS) and caspase 3-like protein signaling molecules in aerenchyma formation, we investigated the effects of exogenous NADPH oxidase inhibitor (diphenyleneiodonium chloride, DPI), catalase inhibitor (3-amino-1,2,4-triazole, AT), and caspase-3-like protein inhibitor (AC-DEVD-CHO, DEVD) on morphological and physiological characteristics and aerenchyma formation in P. perfoliatus. The results showed that after DPI treatment, caspase-3-like protein activity decreased, ROS-related enzyme activities increased, and H₂O₂ content decreased, thereby inhibiting aerenchyma formation. When the concentration of DPI was approximately 1 μmol/L, the inhibitory effect was the most obvious. On the contrary, after the AT treatment, caspase-3-like protein activity increased, ROS-related enzyme activities decreased, and the H₂O₂ content increased, ultimately promoting aerenchyma formation, and the promotion was the most obvious under treatment with approximately 500 μmol/L AT. After DEVD treatment, the inhibition of vegetative growth caused by DPI or AT treatment was alleviated, significantly reducing caspase-3-like activity and inhibiting aerenchyma development. The results of this study show that ROS has a positive regulatory effect on aerenchyma formation, and caspase-3-like protein is activated to promote ROS-mediated aerenchyma formation. This experiment provides a new theoretical basis for further exploration of the signal transduction effects of ROS and caspase-3-like protein in plant cells and their roles in plant development.

The wild plant Gnaphalium lavandulifolium as a sentinel for biomonitoring the effects of environmental heavy metals in the metropolitan area of México Valley

Biomonitoring is a valuable tool for assessing the presence and effects of air pollutants such as heavy metals (HM); due to their toxicity and stability, these compounds can affect human health and the balance of ecosystems. To assess its potential as a sentinel organism of HM pollution, the wild plant Gnaphalium lavandulifolium was exposed to four sites in the metropolitan area of México Valley (MAMV): Altzomoni (ALT) Coyoacán (COY), Ecatepec (ECA), and Tlalnepantla (TLA) during 2, 4, and 8 weeks, between October and November 2019. Control plants remained under controlled conditions. The chemical analysis determined twelve HM (Al, As, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, V, and Zn) in the leaves. Macroscopic damage to the leaves, later determined in semi-thin sections under light microscopy, lead to a finer analysis. Transmission electron microscope (TEM) showed major structural changes: chromatin condensation, protoplast shrinkage, cytoplasm vacuolization, cell wall thinning, decreased number and size of starch grains, and plastoglobules in chloroplasts. All these characteristics of stress-induced programed cell death (sPCD) were related to the significant increase of toxic HM in the leaves of the exposed plants compared to the control (p < 0.05). Immunohistochemistry revealed a significant amount of proteases with caspase 3-like activity in ECA and TLA samples during long exposure times. Ultrastructural changes and sPCD features detected confirmed the usefulness of G. lavandulifolium as a good biomonitor of HM contamination. They supported the possibility of considering subcellular changes as markers of abiotic stress conditions in plants.

Proteomic Analysis of S-Nitrosation Sites During Somatic Embryogenesis in Brazilian Pine, Araucaria angustifolia (Bertol.) Kuntze

Cysteine S-nitrosation is a redox-based post-translational modification that mediates nitric oxide (NO) regulation of various aspects of plant growth, development and stress responses. Despite its importance, studies exploring protein signaling pathways that are regulated by S-nitrosation during somatic embryogenesis have not been performed. In the present study, endogenous cysteine S-nitrosation site and S-nitrosated proteins were identified by iodo-TMT labeling during somatic embryogenesis in Brazilian pine, an endangered native conifer of South America. In addition, endogenous -S-nitrosothiol (SNO) levels and S-nitrosoglutathione reductase (GSNOR) activity were determined in cell lines with contrasting embryogenic potential. Overall, we identified an array of proteins associated with a large variety of biological processes and molecular functions with some of them already described as important for somatic embryogenesis (Class IV chitinase, pyruvate dehydrogenase E1 and dehydroascorbate reductase). In total, our S-nitrosoproteome analyses identified 18 endogenously S-nitrosated proteins and 50 in vitro S-nitrosated proteins (after GSNO treatment) during cell culture proliferation and embryo development. Furthermore, SNO levels and GSNOR activity were increased during embryo formation. These findings expand our understanding of the Brazilian pine proteome and shed novel insights into the potential use of pharmacological manipulation of NO levels by using NO inhibitors and donors during somatic embryogenesis.

Au-courant and novel technologies for efficient doubled haploid development in barley (Hordeum vulgare L.)

Bounteous modern and innovative biotechnological tools have resulted in progressive development in the barley breeding program. Doubled haploids developed (homozygous lines) in a single generation is significant. Since the first discovery of haploid plants in 1920 and, in particular, after discovering in vitro androgenesis in 1964 by Guha and Maheshwari, the doubled haploidy techniques have been progressively developed and constantly improved. It has shortened the cultivar development time and has been extensively used in: genetic studies, gene mapping, marker/trait association, and QTL studies. In barley, the haploid occurrence developed gradually from being a sporadic and random process (spontaneous) to haploid development by in vivo method of modified pollination or by in vitro culture of immature male or female gametophytes. Although significant improvement in DH induction protocols has been made, challenges still exist for improvement in areas such as: low efficiency, albinism, genotypic specificity etc. Here, the paper focuses on: haploidization via different in vitro, in vivo techniques, the recent advances technologies like centromere-mediated haploidization, hap induction gene, and Doubled haploid CRISPR. The au-courant work of different researchers in barley using these technologies is reviewed. Studies on different factors affecting haploid induction and work on genome doubling of barley haploids to produce DH lines via spontaneous and induced technologies has also been highlighted.

Small molecule inhibitors of mammalian GSK-3β promote in vitro plant cell reprogramming and somatic embryogenesis in crop and forest species

Plant in vitro regeneration systems, such as somatic embryogenesis, are essential in breeding; they permit propagation of elite genotypes, production of doubled-haploids, and regeneration of whole plants from gene editing or transformation events. However, in many crop and forest species, somatic embryogenesis is highly inefficient. We report a new strategy to improve in vitro embryogenesis using synthetic small molecule inhibitors of mammalian glycogen synthase kinase 3β (GSK-3β), never used in plants. These inhibitors increased in vitro embryo production in three different systems and species, microspore embryogenesis of Brassica napus and Hordeum vulgare, and somatic embryogenesis of Quercus suber. TDZD-8, a representative compound of the molecules tested, inhibited GSK-3 activity in microspore cultures, and increased expression of embryogenesis genes FUS3, LEC2, and AGL15. Plant GSK-3 kinase BIN2 is a master regulator of brassinosteroid (BR) signalling. During microspore embryogenesis, BR biosynthesis and signalling genes CPD, GSK-3-BIN2, BES1, and BZR1 were up-regulated and the BAS1 catabolic gene was repressed, indicating activation of the BR pathway. TDZD-8 increased expression of BR signalling elements, mimicking BR effects. The findings support that the small molecule inhibitors promoted somatic embryogenesis by activating the BR pathway, opening up the way for new strategies using GSK-3β inhibitors that could be extended to other species.

A Novel Deoxynivalenol-Activated Wheat Arl6ip4 Gene Encodes an Antifungal Peptide with Deoxynivalenol Affinity and Protects Plants against Fusarium Pathogens and Mycotoxins

Deoxynivalenol (DON) is one of the most widespread trichothecene mycotoxins in contaminated cereal products. DON plays a vital role in the pathogenesis of Fusarium graminearum, but the molecular mechanisms of DON underlying Fusarium–wheat interactions are not yet well understood. In this study, a novel wheat ADP-ribosylation factor-like protein 6-interacting protein 4 gene, TaArl6ip4, was identified from DON-treated wheat suspension cells by suppression subtractive hybridization (SSH). The qRT-PCR result suggested that TaArl6ip4 expression is specifically activated by DON in both the Fusarium intermediate susceptible wheat cultivar Zhengmai9023 and the Fusarium resistant cultivar Sumai3. The transient expression results of the TaARL6IP4::GFP fusion protein indicate that TaArl6ip4 encodes a plasma membrane and nucleus-localized protein. Multiple sequence alignment using microscale thermophoresis showed that TaARL6IP4 comprises a conserved DON binding motif, ₆₇HXXXG₇₁, and exhibits DON affinity with a dissociation constant (K_D) of 91 ± 2.6 µM. Moreover, TaARL6IP4 exhibited antifungal activity with IC₅₀ values of 22 ± 1.5 µM and 25 ± 2.6 µM against Fusarium graminearum and Alternaria alternata, respectively. Furthermore, TaArl6ip4 interacted with the plasma membrane of Fusarium graminearum spores, resulting in membrane disruption and the leakage of cytoplasmic materials. The heterologous over-expression of TaArl6ip4 conferred greater DON tolerance and Fusarium resistance in Arabidopsis. Finally, we describe a novel DON-induced wheat gene, TaArl6ip4, exhibiting antifungal function and DON affinity that may play a key role in Fusarium–wheat interactions.

Structural Equation Modeling (SEM) Analysis of Sequence Variation and Green Plant Regeneration via Anther Culture in Barley

The process of anther culture involves numerous abiotic stresses required for cellular reprogramming, microspore developmental switch, and plant regeneration. These stresses affect DNA methylation patterns, sequence variation, and the number of green plants regenerated. Recently, in barley (Hordeum vulgare L.), mediation analysis linked DNA methylation changes, copper (Cu2+) and silver (Ag+) ion concentrations, sequence variation, β-glucans, green plants, and duration of anther culture (Time). Although several models were used to explain particular aspects of the relationships between these factors, a generalized complex model employing all these types of data was not established. In this study, we combined the previously described partial models into a single complex model using the structural equation modeling approach. Based on the evaluated model, we demonstrated that stress conditions (such as starvation and darkness) influence β-glucans employed by cells for glycolysis and the tricarboxylic acid cycle. Additionally, Cu2+ and Ag+ ions affect DNA methylation and induce sequence variation. Moreover, these ions link DNA methylation with green plants. The structural equation model also showed the role of time in relationships between parameters included in the model and influencing plant regeneration via anther culture. Utilization of structural equation modeling may have both scientific and practical implications, as it demonstrates links between biological phenomena (e.g., culture-induced variation, green plant regeneration and biochemical pathways), and provides opportunities for regulating these phenomena for particular biotechnological purposes.

Understanding In Vitro Tissue Culture-Induced Variation Phenomenon in Microspore System

In vitro tissue culture plant regeneration is a complicated process that requires stressful conditions affecting the cell functioning at multiple levels, including signaling pathways, transcriptome functioning, the interaction between cellular organelles (retro-, anterograde), compounds methylation, biochemical cycles, and DNA mutations. Unfortunately, the network linking all these aspects is not well understood, and the available knowledge is not systemized. Moreover, some aspects of the phenomenon are poorly studied. The present review attempts to present a broad range of aspects involved in the tissue culture-induced variation and hopefully would stimulate further investigations allowing a better understanding of the phenomenon and the cell functioning.

Mitochondrial bioenergetics and enzymatic antioxidant defense differ in Paraná pine cell lines with contrasting embryogenic potential

Araucaria angustifolia is classified as a critically endangered species by the International Union for Conservation of Nature. This threat is worsened by the inefficiency of methods for ex-situ conservation and propagation. In conifers, somatic embryogenesis (SE) associated with cryopreservation is an efficient method to achieve germplasm conservation and mass clonal propagation. However, the efficiency of SE is highly dependent on genotype responsivity to the artificial stimulus used in vitro during cell line proliferation and later during somatic embryo development. In this study, we evaluated the activity of antioxidant enzymes and characterized mitochondrial functions during the proliferation of embryogenic cells of A. angustifolia responsive (SE1) and non-responsive (SE6) to the development of somatic embryos. The activities of the antioxidant enzymes GR (EC 1.6.4.2), MDHAR (EC 1.6.5.4), and POX (EC 1.11.1.7) were increased in SE1 culture, while in SE6 culture, only the activity of DHAR (EC 1.8.5.1) was significantly higher. Additionally, SE6 culture presented a higher number of mitochondria, which agreed with the increased rate of oxygen consumption compared to responsive SE1 culture; however, the mitochondrial volume was lower. Although the ATP levels did not differ, the NAD(P)H levels were higher in SE1 cells. NDs, AOX, and UCP were less active in responsive SE1 than in non-responsive cells. Our results show significant differences between SE1 and SE6 embryogenic cells regarding mitochondrial functions and antioxidant enzyme activities, which may be intrinsic to the in vitro proliferation phase of both cell lines, possessing a crucial role for the induction of in vitro maturation process.

Plant Nitric Oxide Signaling under Drought Stress

Water deficit caused by drought is a significant threat to crop growth and production. Nitric oxide (NO), a water- and lipid-soluble free radical, plays an important role in cytoprotection. Apart from a few studies supporting the role of NO in drought responses, little is known about this pivotal molecular amendment in the regulation of abiotic stress signaling. In this review, we highlight the knowledge gaps in NO roles under drought stress and the technical challenges underlying NO detection and measurements, and we provide recommendations regarding potential avenues for future investigation. The modulation of NO production to alleviate abiotic stress disturbances in higher plants highlights the potential of genetic manipulation to influence NO metabolism as a tool with which plant fitness can be improved under adverse growth conditions.

Suppression of Metacaspase- and Autophagy-Dependent Cell Death Improves Stress-Induced Microspore Embryogenesis in Brassica napus

Microspore embryogenesis is a biotechnological process that allows us to rapidly obtain doubled-haploid plants for breeding programs. The process is initiated by the application of stress treatment, which reprograms microspores to embark on embryonic development. Typically, a part of the microspores undergoes cell death that reduces the efficiency of the process. Metacaspases (MCAs), a phylogenetically broad group of cysteine proteases, and autophagy, the major catabolic process in eukaryotes, are critical regulators of the balance between cell death and survival in various organisms. In this study, we analyzed the role of MCAs and autophagy in cell death during stress-induced microspore embryogenesis in Brassica napus. We demonstrate that this cell death is accompanied by the transcriptional upregulation of three BnMCA genes (BnMCA-Ia, BnMCA-IIa and BnMCA-IIi), an increase in MCA proteolytic activity and the activation of autophagy. Accordingly, inhibition of autophagy and MCA activity, either individually or in combination, suppressed cell death and increased the number of proembryos, indicating that both components play a pro-cell death role and account for decreased efficiency of early embryonic development. Therefore, MCAs and/or autophagy can be used as new biotechnological targets to improve in vitro embryogenesis in Brassica species and doubled-haploid plant production in crop breeding and propagation programs.

β-1,3-Glucanases and chitinases participate in the stress-related defence mechanisms that are possibly connected with modulation of arabinogalactan proteins (AGP) required for the androgenesis initiation in rye (Secale cereale L.)

This work presents the biochemical, cytochemical and molecular studies on two groups of PR proteins, β-1,3-glucanases and chitinases, and the arabinogalactan proteins (AGP) during the early stages of androgenesis induction in two breeding lines of rye (Secale cereale L.) with different androgenic potential. The process of androgenesis was initiated by tillers pre-treatments with low temperature, mannitol and/or reduced glutathione and resulted in microspores reprogramming and formation of androgenic structures what was associated with high activity of β-1,3-glucanases and chitinases. Some isoforms of β-1,3-glucanases, namely several acidic isoforms of about 26 kDa; appeared to be anther specific. Chitinases were well represented but were less variable. RT-qPCR revealed that the cold-responsive chitinase genes Chit1 and Chit2 were expressed at a lower level in the microspores and whole anthers while the cold-responsive Glu2 and Glu3 were not active. The stress pre-treatments modifications promoted the AGP accumulation. An apparent dominance of some AGP epitopes (LM2, JIM4 and JIM14) was detected in the androgenesis-responsive rye line. An abundant JIM13 epitopes in the vesicles and inner cell walls of the microspores and in the cell walls of the anther cell layers appeared to be the most specific for embryogenesis.

In vivo light-sheet microscopy resolves localisation patterns of FSD1, a superoxide dismutase with function in root development and osmoprotection

Superoxide dismutases (SODs) are enzymes detoxifying superoxide to hydrogen peroxide while temporal developmental expression and subcellular localisation are linked to their functions. Therefore, we aimed here to reveal in vivo developmental expression, subcellular, tissue- and organ-specific localisation of iron superoxide dismutase 1 (FSD1) in Arabidopsis using light-sheet and Airyscan confocal microscopy. FSD1-GFP temporarily accumulated at the site of endosperm rupture during seed germination. In emerged roots, it showed the highest abundance in cells of the lateral root cap, columella, and endodermis/cortex initials. The largest subcellular pool of FSD1-GFP was localised in the plastid stroma, while it was also located in the nuclei and cytosol. The majority of the nuclear FSD1-GFP is immobile as revealed by fluorescence recovery after photobleaching. We found that fsd1 knockout mutants exhibit reduced lateral root number and this phenotype was reverted by genetic complementation. Mutant analysis also revealed a requirement for FSD1 in seed germination during salt stress. Salt stress tolerance was coupled with the accumulation of FSD1-GFP in Hechtian strands and superoxide removal. It is likely that the plastidic pool is required for acquiring oxidative stress tolerance in Arabidopsis. This study suggests new developmental and osmoprotective functions of SODs in plants.

Impedance flow cytometry allows the early prediction of embryo yields in wheat (Triticum aestivum L.) microspore cultures

Haplomethods are key biotechnological tools that make it possible to rapidly produce perfectly homozygous lines, speeding up plant breeding programs. Under specific stress conditions, microspores are reprogrammed toward sporophytic pathways, leading to embryo formation. Various endogenous and exogenous factors affect embryo yield in androgenesis, so the improvement of androgenesis efficiency requires the development of early, reliable and robust reactivity markers. During the last decade, numerous cytological, cellular and biochemical approaches were carried out to finely characterize microspore development and fate during androgenesis. However, the different available markers are often species-dependent, and their development and application are time-consuming and cumbersome. In this study, we show the suitable use of impedance flow cytometry (IFC) to develop new robust, reliable and strong markers of androgenesis reactivity in wheat, leading to: (i) routine monitoring of the viability of heterogeneous cell cultures; (ii) quick and simple evaluation of stress treatment efficiency; and (iii) early prediction of embryo yields from microspore suspensions. IFC can therefore provide the fine characterization of all of the microspore developmental pathways that occur in a cell suspension, for embryogenic microspores as well as pollen-like microspores. IFC technology has become a very useful tool to track and characterize wheat microspores in androgenesis, but can also be adapted to other species and other in vitro cell culture systems.

Advances in Plant Regeneration: Shake, Rattle and Roll

Some plant cells are able to rebuild new organs after tissue damage or in response to definite stress treatments and/or exogenous hormone applications. Whole plants can develop through de novo organogenesis or somatic embryogenesis. Recent findings have enlarged our understanding of the molecular and cellular mechanisms required for tissue reprogramming during plant regeneration. Genetic analyses also suggest the key role of epigenetic regulation during de novo plant organogenesis. A deeper understanding of plant regeneration might help us to enhance tissue culture optimization, with multiple applications in plant micropropagation and green biotechnology. In this review, we will provide additional insights into the physiological and molecular framework of plant regeneration, including both direct and indirect de novo organ formation and somatic embryogenesis, and we will discuss the key role of intrinsic and extrinsic constraints for cell reprogramming during plant regeneration.

Stress-Induced Microspore Embryogenesis Requires Endogenous Auxin Synthesis and Polar Transport in Barley

Stress-induced microspore embryogenesis is a model in vitro system of cell reprogramming, totipotency acquisition, and embryo development. After induction, responsive microspores abandon their developmental program to follow an embryogenic pathway, leading to in vitro embryo formation. This process is widely used to produce doubled-haploid lines, essential players to create new materials in modern breeding programs, particularly in cereals, although its efficiency is still low in many crop species, because the regulating mechanisms are still elusive. Stress signaling and endogenous hormones, mainly auxin, have been proposed as determinant factors of microspore embryogenesis induction in some eudicot species; however, much less information is available in monocot plants. In this study, we have analyzed the dynamics and possible role of endogenous auxin during stress-induced microspore embryogenesis in the monocot Hordeum vulgare, barley. The results showed auxin accumulation in early proembryo cells, from embryogenesis initiation and a further increase with embryo development and differentiation, correlating with the induction and expression pattern of the auxin biosynthesis gene HvTAR2-like. Pharmacological treatments with kynurenine, inhibitor of auxin biosynthesis, and α-(p-chlorophenoxy)-isobutyric acid (PCIB), auxin antagonist, impaired embryogenesis initiation and development, indicating that de novo auxin synthesis and its activity were required for the process. Efflux carrier gene HvPIN1-like was also induced with embryogenesis initiation and progression; auxin transport inhibition by N-1-naphthylphthalamic acid significantly reduced embryo development at early and advanced stages. The results indicate activation of auxin biosynthesis with microspore embryogenesis initiation and progression, in parallel with the activation of polar auxin transport, and reveal a central role of auxin in the process in a monocot species. The findings give new insights into the complex regulation of stress-induced microspore embryogenesis, particularly in monocot plants for which information is still scarce, and suggest that manipulation of endogenous auxin content could be a target to improve in vitro embryo production.

Proteases with caspase 3-like activity participate in cell death during stress-induced microspore embryogenesis of Brassica napus

Microspore embryogenesis is a model system of plant cell reprogramming, totipotency acquisition, stress response and embryogenesis initiation. This in vitro system constitutes an important biotechnological tool for haploid and doubled-haploid plant production, very useful for crop breeding. In this process, microspores (cells that produce pollen grains in planta) are reprogrammed toward embryogenesis by specific stress treatment, but many microspores die after the stress. The occurrence of cell death is a serious limiting problem that greatly reduces microspore embryogenesis yield. In animals, increasing evidence has revealed caspase proteolytic activities as essential executioners of programmed cell death (PCD) processes, however, less is known in plants. Although plant genomes do not contain caspase homologues, caspase-like proteolytic activities have been detected in many plant PCD processes. In the present study, we have analysed caspase 3-like activity and its involvement in stress-induced cell death during initial stages of microspore embryogenesis of Brassica napus. After stress treatment to induce embryogenesis, isolated microspore cultures showed high levels of cell death and caspase 3-like proteolytic activity was induced. Treatments with specific inhibitor of caspase 3-like activity reduced cell death and increased embryogenesis induction efficiency. Our findings indicate the involvement of proteases with caspase 3-like activity in the initiation and/or execution of cell death at early microspore embryogenesis in B. napus, giving new insights into the pathways of stress-induced cell death in plants and opening a new way to improve in vitro embryogenesis efficiency by using chemical modulators of cell death proteases.

Microspore embryogenesis: targeting the determinant factors of stress-induced cell reprogramming for crop improvement

Under stress, isolated microspores are reprogrammed in vitro towards embryogenesis, producing doubled haploid plants that are useful biotechnological tools in plant breeding as a source of new genetic variability, fixed in homozygous plants in only one generation. Stress-induced cell death and low rates of cell reprogramming are major factors that reduce yield. Knowledge gained in recent years has revealed that initiation and progression of microspore embryogenesis involve a complex network of factors, whose roles are not yet well understood. Here, I review recent findings on the determinant factors underlying stress-induced microspore embryogenesis, focusing on the role of autophagy, cell death, auxin, chromatin modifications, and the cell wall. Autophagy and cell death proteases are crucial players in the response to stress, while cell reprogramming and acquisition of totipotency are regulated by hormonal and epigenetic mechanisms. Auxin biosynthesis, transport, and action are required for microspore embryogenesis. Initial stages involve DNA hypomethylation, H3K9 demethylation, and H3/H4 acetylation. Cell wall remodelling, with pectin de-methylesterification and arabinogalactan protein expression, is necessary for embryo development. Recent reports show that treatments with small modulators of autophagy, proteases, and epigenetic marks reduce cell death and enhance embryogenesis initiation in several crops, opening up new possibilities for improving in vitro embryo production in breeding programmes.

Barley Anther Culture

The production of doubled haploid (DH) barley plants through anther culture is a very useful yet simple in vitro technique. DH plants derive from divisions of haploid microspores that have undergone a developmental switch under the appropriate conditions. The successive divisions lead to the formation of an embryo or callus rather than the formation of mature pollen grains. Plants that regenerate from these embryos are often either haploid, in which case their chromosome set can be doubled by treatment with colchicine, or spontaneous double haploids. The efficiency of DH plant production is highly variable depending on the genotype of the source material. Despite this limitation, DH plants have been widely used in breeding and research programs. Compared to conventional approaches, breeding strategies that makes use of DH plants achieve a homozygous state, allowing transgene or mutation stabilization in the genome, within a considerably shorter time, thus accelerating workflow or reducing work volume.

Enhancement of microspore embryogenesis induction and plantlet regeneration of sweet pepper (Capsicum annuum L.) using putrescine and ascorbic acid

Production of doubled haploid (DH) plants is an efficient tool in genetic and plant breeding programs; however, sweet pepper (Capsicum annuum L.) is recalcitrant to microspore embryogenesis and DH production. Trying to break the barrier of DH production, three independent experiments were conducted on microspore embryogenesis of sweet pepper. In the first experiment, the effect of cold (4 °C) and heat (32 °C) pretreatments were investigated on microspore embryogenesis of three genotypes of sweet pepper including "Inspiration F1," "Maratus F1," and "Magno F1" cultivars in a factorial design with three replications. Heat shock (32 °C for 7 days), applied to mannitol-starved anthers of "Inspiration F1," showed higher multinuclear microspore percent, number of multicellular structures, total embryos, cotyledonary embryos, and regenerants. In the second experiment, the effect of different concentrations of putrescine (0, 0.5, 1, 2, and 5 mg l^-1) was evaluated on microspore embryogenesis of the three aforementioned cultivars of sweet pepper. The highest mean number of multicellular structures, cotyledonary embryos, and regenerants were achieved by applying 0.5-1 mg l^-1 putrescine during the mannitol starvation and heat shock (32 °C) treatments of isolated microspore culture of "Inspiration F1" cultivar. Significant decrease in microspore embryogenesis efficiency was observed when high levels of putrescine (2 and 5 mg l^-1) were used. Microspore embryogenesis was prevented completely at 5.0 mg l^-1 putrescine. In the third experiment, the effect of different concentrations of ascorbic acid (0, 20, 50, 100, and 200 mg l^-1) was investigated and the results showed that the application of ascorbic acid (20 and 50 mg l^-1) during mannitol starvation and heat shock treatment (32 °C) caused remarkable improvement in the number of produced cotyledonary embryos and their regeneration ability compared to control treatment. However, the application of higher levels of ascorbic acid (100 and 200 mg l^-1) inhibited microspore cell divisions and embryogenesis. In conclusion, the results indicated that both putrescine and ascorbic acid have significant effect on microspore embryogenesis efficiency of sweet pepper when they are used in appropriate concentrations.

Enhancement of microspore embryogenesis induction and plantlet regeneration of sweet pepper (Capsicum annuum L.) using putrescine and ascorbic acid

Production of doubled haploid (DH) plants is an efficient tool in genetic and plant breeding programs; however, sweet pepper (Capsicum annuum L.) is recalcitrant to microspore embryogenesis and DH production. Trying to break the barrier of DH production, three independent experiments were conducted on microspore embryogenesis of sweet pepper. In the first experiment, the effect of cold (4 °C) and heat (32 °C) pretreatments were investigated on microspore embryogenesis of three genotypes of sweet pepper including "Inspiration F1," "Maratus F1," and "Magno F1" cultivars in a factorial design with three replications. Heat shock (32 °C for 7 days), applied to mannitol-starved anthers of "Inspiration F1," showed higher multinuclear microspore percent, number of multicellular structures, total embryos, cotyledonary embryos, and regenerants. In the second experiment, the effect of different concentrations of putrescine (0, 0.5, 1, 2, and 5 mg l^-1) was evaluated on microspore embryogenesis of the three aforementioned cultivars of sweet pepper. The highest mean number of multicellular structures, cotyledonary embryos, and regenerants were achieved by applying 0.5-1 mg l^-1 putrescine during the mannitol starvation and heat shock (32 °C) treatments of isolated microspore culture of "Inspiration F1" cultivar. Significant decrease in microspore embryogenesis efficiency was observed when high levels of putrescine (2 and 5 mg l^-1) were used. Microspore embryogenesis was prevented completely at 5.0 mg l^-1 putrescine. In the third experiment, the effect of different concentrations of ascorbic acid (0, 20, 50, 100, and 200 mg l^-1) was investigated and the results showed that the application of ascorbic acid (20 and 50 mg l^-1) during mannitol starvation and heat shock treatment (32 °C) caused remarkable improvement in the number of produced cotyledonary embryos and their regeneration ability compared to control treatment. However, the application of higher levels of ascorbic acid (100 and 200 mg l^-1) inhibited microspore cell divisions and embryogenesis. In conclusion, the results indicated that both putrescine and ascorbic acid have significant effect on microspore embryogenesis efficiency of sweet pepper when they are used in appropriate concentrations.

Modulation of autophagy and protease activities by small bioactive compounds to reduce cell death and improve stress-induced microspore embryogenesis initiation in rapeseed and barley

Microspore embryogenesis is a powerful biotechnological tool that is very useful in crop breeding for the rapid production of haploid and double-haploid embryos and plants. In this in vitro system, the haploid microspore is reprogrammed by the application of specific stress treatments. A high level of cell death after the stress is a major factor that greatly reduces embryogenesis yield at its initial stages. Autophagy is a degradation pathway that is present in all eukaryotes and plays key roles in a range of processes, including stress responses. Many proteases participate in autophagy and cell death; among them, cathepsins are the most abundant enzymes with a role in plant senescence and programmed cell death (PCD). Moreover, although plant genomes do not contain homologues of caspases, caspase 3-like activity (main executioner protease of animal cell death) has been detected in many plant PCD processes. Recent studies by our group in barley microspore cultures reported that the stress treatment required for inducing microspore embryogenesis (cold treatment), also produced reactive oxygen species (ROS) and cell death, concomitantly with the induction of autophagy, as well as cathepsin-like and caspase 3-like proteolytic activities. In the present study, we report new data on microspore embryogenesis of rapeseed that indicate, as in barley, activation of cell death and autophagy processes after the inductive stress. The results revealed that treatments modulating autophagy and proteases produced the same effect in the two plant systems, regardless of the stress applied, cold in barley or heat in rapeseed. Pharmacological treatments with small bioactive compounds that inhibit ROS, autophagy and specific cell death-proteases led to reduced cell death and an increased embryogenesis initiation rate in both, barley and rapeseed. Taken together, these findings open up new intervention pathways by modulating autophagy and proteases, which are very promising in terms of increasing the efficiency of in vitro microspore embryogenesis systems for biotechnological applications in crop breeding.

Microspore embryogenesis in Brassica: calcium signaling, epigenetic modification, and programmed cell death

Stress induction followed by excessive calcium influx causes multiple changes in microspores resulting in chromatin remodeling, epigenetic modifications, and removal of unwanted gametophytic components via autophagy, switching microspores towards ME. In Brassica, isolated microspores that are placed under specific external stresses can switch their default developmental pathway towards an embryogenic state. Microspore embryogenesis is a unique system that speeds up breeding programs and, in the context of developmental biology, provides an excellent tool for embryogenesis to be investigated in greater detail. The last few years have provided ample evidence that has allowed Brassica researchers to markedly increase their understanding of the molecular and sub-cellular changes underlying this process. We review recent advances in this field, focusing mainly on the perception to inductive stresses, signal transduction, molecular and structural alterations, and the involvement of programmed cell death at the onset of embryogenic induction.

The effects of organophosphorus insecticides and heavy metals on DNA damage and programmed cell death in two plant models

The ubiquity of pollutants, such as agrochemicals and heavy metals, constitute a serious risk to human health. To evaluate the induction of DNA damage and programmed cell death (PCD), root cells of Allium cepa and Vicia faba were treated with two organophosphate insecticides (OI), fenthion and malathion, and with two heavy metal (HM) salts, nickel nitrate and potassium dichromate. An alkaline variant of the comet assay was performed to identify DNA breaks; the results showed comets in a dose-dependent manner, while higher concentrations induced clouds following exposure to OIs and HMs. Similarly, treatments with higher concentrations of OIs and HMs were analyzed by immunocytochemistry, and several structural characteristics of PCD were observed, including chromatin condensation, cytoplasmic vacuolization, nuclear shrinkage, condensation of the protoplast away from the cell wall, and nuclei fragmentation with apoptotic-like corpse formation. Abiotic stress also caused other features associated with PCD, such as an increase of active caspase-3-like protein, changes in the location of cytochrome C (Cyt C) toward the cytoplasm, and decreases in extracellular signal-regulated protein kinase (ERK) expression. Genotoxicity results setting out an oxidative via of DNA damage and evidence the role of the high affinity of HM and OI by DNA molecule as underlying cause of genotoxic effect. The PCD features observed in root cells of A. cepa and V. faba suggest that PCD takes place through a process that involves ERK inactivation, culminating in Cyt C release and caspase-3-like activation. The sensitivity of both plant models to abiotic stress was clearly demonstrated, validating their role as good biosensors of DNA breakage and PCD induced by environmental stressors.

Autophagy is activated and involved in cell death with participation of cathepsins during stress-induced microspore embryogenesis in barley

Microspores are reprogrammed towards embryogenesis by stress. Many microspores die after this stress, limiting the efficiency of microspore embryogenesis. Autophagy is a degradation pathway that plays critical roles in stress response and cell death. In animals, cathepsins have an integral role in autophagy by degrading autophagic material; less is known in plants. Plant cathepsins are papain-like C1A cysteine proteases involved in many physiological processes, including programmed cell death. We have analysed the involvement of autophagy in cell death, in relation to cathepsin activation, during stress-induced microspore embryogenesis in Hordeum vulgare. After stress, reactive oxygen species (ROS) and cell death increased and autophagy was activated, including HvATG5 and HvATG6 up-regulation and increase of ATG5, ATG8, and autophagosomes. Concomitantly, cathepsin L/F-, B-, and H-like activities were induced, cathepsin-like genes HvPap-1 and HvPap-6 were up-regulated, and HvPap-1, HvPap-6, and HvPap-19 proteins increased and localized in the cytoplasm, resembling autophagy structures. Inhibitors of autophagy and cysteine proteases reduced cell death and promoted embryogenesis. The findings reveal a role for autophagy in stress-induced cell death during microspore embryogenesis, and the participation of cathepsins. Similar patterns of activation, expression, and localization suggest a possible connection between cathepsins and autophagy. The results open up new possibilities to enhance microspore embryogenesis efficiency with autophagy and/or cysteine protease modulators.

IMMUNEPOTENT CRP induces cell cycle arrest and caspase-independent regulated cell death in HeLa cells through reactive oxygen species production

BACKGROUND

Regulated cell death (RCD) is a mechanism by which the cell activates its own machinery to self-destruct. RCD is important for the maintenance of tissue homeostasis and its deregulation is involved in diseases such as cervical cancer. IMMUNEPOTENT CRP (I-CRP) is a dialyzable bovine leukocyte extract that contains transfer factors and acts as an immunomodulator, and can be cytotoxic to cancer cell lines and reduce tumor burden in vivo. Although I-CRP has shown to improve or modulate immune response in inflammation, infectious diseases and cancer, its widespread use has been limited by the absence of conclusive data on the molecular mechanism of its action.

METHODS

In this study we analyzed the mechanism by which I-CRP induces cytotoxicity in HeLa cells. We assessed cell viability, cell death, cell cycle, nuclear morphology and DNA integrity, caspase dependence and activity, mitochondrial membrane potential, and reactive oxygen species production.

RESULTS

I-CRP diminishes cell viability in HeLa cells through a RCD pathway and induces cell cycle arrest in the G2/M phase. We show that the I-CRP induces caspase activation but cell death induction is independent of caspases, as observed by the use of a pan-caspase inhibitor, which blocked caspase activity but not cell death. Moreover, we show that I-CRP induces DNA alterations, loss of mitochondrial membrane potential, and production of reactive-oxygen species. Finally, pretreatment with N-acetyl-L-cysteine (NAC), a ROS scavenger, prevented both ROS generation and cell death induced by I-CRP.

CONCLUSIONS

Our data indicate that I-CRP treatment induced cell cycle arrest in G2/M phase, mitochondrial damage, and ROS-mediated caspase-independent cell death in HeLa cells. This work opens the way to the elucidation of a more detailed cell death pathway that could potentially work in conjunction with caspase-dependent cell death induced by classical chemotherapies.

The Chemical Characteristics and Immune-Modulating Activity of Polysaccharides Isolated from Cold-Brew Coffee

To elucidate new biological ingredients in cold-brew coffee extracted with cold water, crude polysaccharide (CCP-0) was isolated by ethanol precipitation, and its immune-stimulating activities were assayed. CCP-0 mainly comprised galactose (53.6%), mannose (15.7%), arabinose (11.9%), and uronic acid (12.4%), suggesting that it might exist as a mixture of galactomannan and arabinogalactan. CCP-0 significantly increased cell proliferation on both murine peritoneal macrophages and splenocytes in a dose dependent manner. CCP-0 also significantly augmented nitric oxide and reactive oxygen species production by murine peritoneal macrophages. In addition, macrophages stimulated by CCP-0 enhanced production of various cytokines such as tumor necrosis factor-α, interleukin (IL)-6, and IL-12. In an in vitro assay for intestinal immune-modulating activity, CCP-0 showed higher bone-marrow cell-proliferation activity through Peyer's patch cells at 100 μg/mL than the negative control. These results suggest that CCP-0 may potentially enhance macrophage functions and the intestinal immune system.

A Genetically Modified attenuated Listeria Vaccine Expressing HPV16 E7 Kill Tumor Cells in Direct and Antigen-Specific Manner

Attenuated Listeria monocytogenes (L. monocytogenes, LM) induces specific CD8⁺ and CD4⁺ T cell responses, and has been identified as a promising cancer vaccine vector. Cervical cancer is the third most common cancer in women worldwide, with human papillomavirus (HPV), particularly type 16, being the main etiological factor. The therapeutic HPV vaccines are urgently needed. The E7 protein of HPV is necessary for maintaining malignancy in tumor cells. Here, a genetically modified attenuated LM expressing HPV16 E7 protein was constructed. Intraperitoneal vaccination of LM4Δhly::E7 significantly reduced tumor size and even resulted in complete regression of established tumors in a murine model of cervical cancer. We provided evidence that recombinant LM strains could enter the tumor tissue and induce non-specific tumor cell death, probably via activation of reactive oxygen species and increased intracellular Ca²⁺ levels. LM4Δhly::E7 effectively triggered a strong antigen-specific cellular immunity in tumor-bearing mice, and elicited significant infiltration of T cells in the intratumoral milieu. In summary, these data showed LM4Δhly::E7 to be effective in a cervical cancer model and LM4Δhly::E7 induced an antitumor effect by antigen-specific cellular immune responses and direct killing of tumor cells, indicating a potential application against cervical cancer.

NO and H2O2 contribute to SO2 toxicity via Ca2+ signaling in Vicia faba guard cells

NO and H₂O₂ have been implicated as important signals in biotic and abiotic stress responses of plants to the environment. Previously, we have shown that SO₂ exposure increased the levels of NO and H₂O₂ in plant cells. We hypothesize that, as signaling molecules, NO and H₂O₂ mediate SO₂-caused toxicity. In this paper, we show that SO₂ hydrates caused guard cell death in a concentration-dependent manner in the concentration range of 0.25 to 6 mmol L^-1, which was associated with elevation of intracellular NO, H₂O₂, and Ca²⁺ levels in Vicia faba guard cells. NO donor SNP enhanced SO₂ toxicity, while NO scavenger c-PTIO and NO synthesis inhibitors L-NAME and tungstate significantly prevented SO₂ toxicity. ROS scavenger ascorbic acid (AsA) and catalase (CAT), Ca²⁺ chelating agent EGTA, and Ca²⁺ channel inhibitor LaCl₃ also markedly blocked SO₂ toxicity. In addition, both c-PTIO and AsA could completely block SO₂-induced elevation of intracellular Ca²⁺ level. Moreover, c-PTIO efficiently blocked SO₂-induced H₂O₂ elevation, and AsA significantly blocked SO₂-induced NO elevation. These results indicate that extra NO and H₂O₂ are produced and accumulated in SO₂-treated guard cells, which further activate Ca²⁺ signaling to mediate SO₂ toxicity. Our findings suggest that both NO and H₂O₂ contribute to SO₂ toxicity via Ca²⁺ signaling.

Small RNA Sequencing Reveals Differential miRNA Expression in the Early Development of Broccoli (Brassica oleracea var. italica) Pollen

Pollen development is an important and complex biological process in the sexual reproduction of flowering plants. Although the cytological characteristics of pollen development are well defined, the regulation of its early stages remains largely unknown. In the present study, miRNAs were explored in the early development of broccoli (Brassica oleracea var. italica) pollen. A total of 333 known miRNAs that originated from 235 miRNA families were detected. Fifty-five novel miRNA candidates were identified. Sixty of the 333 known miRNAs and 49 of the 55 predicted novel miRNAs exhibited significantly differential expression profiling in the three distinct developmental stages of broccoli pollen. Among these differentially expressed miRNAs, miRNAs that would be involved in the developmental phase transition from uninucleate microspores to binucleate pollen grains or from binucleate to trinucleate pollen grains were identified. miRNAs that showed significantly enriched expression in a specific early stage of broccoli pollen development were also observed. In addition, 552 targets for 127 known miRNAs and 69 targets for 40 predicted novel miRNAs were bioinformatically identified. Functional annotation and GO (Gene Ontology) analysis indicated that the putative miRNA targets showed significant enrichment in GO terms that were related to plant organ formation and morphogenesis. Some of enriched GO terms were detected for the targets directly involved in plant male reproduction development. These findings provided new insights into the functions of miRNA-mediated regulatory networks in broccoli pollen development.

Inhibition of Histone H3K9 Methylation by BIX-01294 Promotes Stress-Induced Microspore Totipotency and Enhances Embryogenesis Initiation

Microspore embryogenesis is a process of cell reprogramming, totipotency acquisition and embryogenesis initiation, induced in vitro by stress treatments and widely used in plant breeding for rapid production of doubled-haploids, but its regulating mechanisms are still largely unknown. Increasing evidence has revealed epigenetic reprogramming during microspore embryogenesis, through DNA methylation, but less is known about the involvement of histone modifications. In this study, we have analyzed the dynamics and possible role of histone H3K9 methylation, a major repressive modification, as well as the effects on microspore embryogenesis initiation of BIX-01294, an inhibitor of histone methylation, tested for the first time in plants, in Brassica napus and Hordeum vulgare. Results revealed that microspore reprogramming and initiation of embryogenesis involved a low level of H3K9 methylation. With the progression of embryogenesis, methylation of H3K9 increased, correlating with gene expression profiles of BnHKMT SUVR4-like and BnLSD1-like (writer and eraser enzymes of H3K9me2). At early stages, BIX-01294 promoted cell reprogramming, totipotency and embryogenesis induction, while diminishing bulk H3K9 methylation. DNA methylation was also reduced by short-term BIX-01294 treatment. By contrast, long BIX-01294 treatments hindered embryogenesis progression, indicating that H3K9 methylation is required for embryo differentiation. These findings open up new possibilities to enhance microspore embryogenesis efficiency in recalcitrant species through pharmacological modulation of histone methylation by using BIX-01294.

Detection of Epigenetic Modifications During Microspore Embryogenesis: Analysis of DNA Methylation Patterns Dynamics

Methylation of 5-deoxy-cytidines of DNA constitutes a prominent epigenetic modification of the chromatin fiber which is locked in a transcriptionally inactive conformation. Changes in global DNA methylation are involved in many plant developmental processes during proliferation and differentiation events. The analysis of the changes of global DNA methylation distribution patterns during microspore embryogenesis induction and progression will inform on the regulatory mechanisms of the process, helping in the design of protocols to improve its efficiency in different species. To investigate the DNA methylation dynamics during microspore embryogenesis in the different cell types present in the cultures, the analysis of spatial and temporal pattern of nuclear distribution of 5-methyl-deoxy-cytidine (5mdC) constitutes a potent approach. The immunolocalization of 5mdC on sections and subsequent confocal laser microscopy analysis have been developed for in situ cellular analysis of a variety of plant samples, including embryogenic microspore and anther cultures. Quantification of 5mdC immunofluorescence intensity by image analysis software also permits to estimate differences in global DNA methylation levels among different cell types during development.

Reactive Carbonyl Species Activate Caspase-3-Like Protease to Initiate Programmed Cell Death in Plants

Reactive oxygen species (ROS)-triggered programmed cell death (PCD) is a typical plant response to biotic and abiotic stressors. We have recently shown that lipid peroxide-derived reactive carbonyl species (RCS), downstream products of ROS, mediate oxidative signal to initiate PCD. Here we investigated the mechanism by which RCS initiate PCD. Tobacco Bright Yellow-2 cultured cells were treated with acrolein, one of the most potent RCS. Acrolein at 0.2 mM caused PCD in 5 h (i.e. lethal), but at 0.1 mM it did not (sublethal). Specifically, these two doses caused critically different effects on the cells. Both lethal and sublethal doses of acrolein exhausted the cellular glutathione pool in 30 min, while the lethal dose only caused a significant ascorbate decrease and ROS increase in 1-2 h. Prior to such redox changes, we found that acrolein caused significant increases in the activities of caspase-1-like protease (C1LP) and caspase-3-like protease (C3LP), the proteases which trigger PCD. The lethal dose of acrolein increased the C3LP activity 2-fold more than did the sublethal dose. In contrast, C1LP activity increments caused by the two doses were not different. Acrolein and 4-hydroxy-(E)-2-nonenal, another RCS, activated both proteases in a cell-free extract from untreated cells. H2O2 at 1 mM added to the cells increased C1LP and C3LP activities and caused PCD, and the RCS scavenger carnosine suppressed their activation and PCD. However, H2O2 did not activate the proteases in a cell-free extract. Thus the activation of caspase-like proteases, particularly C3LP, by RCS is an initial biochemical event in oxidative signal-stimulated PCD in plants.

Improvement of adventitious root formation in flax using hydrogen peroxide

Flax (Linum usitatissimum L.) is an important crop for the production of oil and fiber. In vitro manipulations of flax are used for genetic improvement and breeding while improvements in adventitious root formation are important for biotechnological programs focused on regeneration and vegetative propagation of genetically valuable plant material. Additionally, flax hypocotyl segments possess outstanding morphogenetic capacity, thus providing a useful model for the investigation of flax developmental processes. Here, we investigated the crosstalk between hydrogen peroxide and auxin with respect to reprogramming flax hypocotyl cells for root morphogenetic development. Exogenous auxin induced the robust formation of adventitious roots from flax hypocotyl segments while the addition of hydrogen peroxide further enhanced this process. The levels of endogenous auxin (indole-3-acetic acid; IAA) were positively correlated with increased root formation in response to exogenous auxin (1-Naphthaleneacetic acid; NAA). Histochemical staining of the hypocotyl segments revealed that hydrogen peroxide and peroxidase, but not superoxide, were positively correlated with root formation. Measurements of antioxidant enzyme activities showed that endogenous levels of hydrogen peroxide were controlled by peroxidases during root formation from hypocotyl segments. In conclusion, hydrogen peroxide positively affected flax adventitious root formation by regulating the endogenous auxin levels. Consequently, this agent can be applied to increase flax regeneration capacity for biotechnological purposes such as improved plant rooting.

Dying with Style: Death Decision in Plant Embryogenesis

Embryogenesis is a fascinating event during the plant life cycle encompassing several steps whereby the zygote develops into a fully developed embryo which, in angiosperms, is composed of an axis separating the apical meristems, and two cotyledons. Recapitulation of embryogenesis can also occur in vitro through somatic embryogenesis, where somatic cells are induced to form embryos, and androgenesis, in which embryos originate from immature male gametophytes. Besides cell division and differentiation, embryo patterning in vivo and in vitro requires the dismantling and selective elimination of cells and tissues via programmed cell death (PCD). While the manifestation of the death program has long been acknowledged in vivo, especially in relation to the elimination of the suspensor during the late phases of embryo development, PCD during in vitro embryogenesis has only been described in more recent years. Independent studies using the gymnosperm Norway spruce and the angiosperm maize have shown that the death program is crucial for the proper formation and further development of immature somatic embryos. This chapter summarizes the recent advances in the field of PCD during embryogenesis and proposes novel regulatory mechanisms activating the death program in plants.

Recent Advances on Genetic and Physiological Bases of In Vitro Somatic Embryo Formation

Somatic embryogenesis involves a broad repertoire of genes, and complex expression patterns controlled by a concerted gene regulatory network. The present work describes this regulatory network focusing on the main aspects involved, with the aim of providing a deeper insight into understanding the total reprogramming of cells into a new organism through a somatic way. To the aim, the chromatin remodeling necessary to totipotent stem cell establishment is described, as the activity of numerous transcription factors necessary to cellular totipotency reprogramming. The eliciting effects of various plant growth regulators on the induction of somatic embryogenesis is also described and put in relation with the activity of specific transcription factors. The role of programmed cell death in the process, and the related function of specific hemoglobins as anti-stress and anti-death compounds is also described. The tools for biotechnology coming from this information is highlighted in the concluding remarks.

Bcl-2△21 and Ac-DEVD-CHO Inhibit Death of Wheat Microspores

Microspore cell death and low green plant production efficiency are an integral obstacle in the development of doubled haploid production in wheat. The aim of the current study was to determine the effect of anti-apoptotic recombinant human B-cell lymphoma-2 (Bcl-2△21) and caspase-3-inhibitor (Ac-DEVD-CHO) in microspore cell death in bread wheat cultivars AC Fielder and AC Andrew. Induction medium containing Bcl-2△21 and Ac-DEVD-CHO yielded a significantly higher number of viable microspores, embryo-like structures and total green plants in wheat cultivars AC Fielder and AC Andrew. Total peroxidase activity was lower in Bcl-2△21 treated microspore cultures at 96 h of treatment compared to control and Ac-DEVD-CHO. Electron paramagnetic resonance study of total microspore protein showed a different scavenging activity for Bcl-2△21 and Ac-DEVD-CHO. Bcl-2△21 scavenged approximately 50% hydroxyl radical (HO^•) formed, whereas Ac-DEVD-CHO scavenged approximately 20% of HO^•. Conversely, reduced caspase-3-like activities were detected in the presence of Bcl-2△21 and Ac-DEVD-CHO, supporting the involvement of Bcl-2△21 and Ac-DEVD-CHO in increasing microspore viability by reducing oxidative stress and caspase-3-like activity. Our results indicate that Bcl-2△21 and Ac-DEVD-CHO protects cells from cell death following different pathways. Bcl-2△21 prevents cell damage by detoxifying HO^• and suppressing caspase-3-like activity, while Ac-DEVD-CHO inhibits the cell death pathways by modulating caspase-like activity.

Auxin Biosynthesis, Accumulation, Action and Transport are Involved in Stress-Induced Microspore Embryogenesis Initiation and Progression in Brassica napus

Isolated microspores are reprogrammed in vitro by stress, becoming totipotent cells and producing embryos and plants via a process known as microspore embryogenesis. Despite the abundance of data on auxin involvement in plant development and embryogenesis, no data are available regarding the dynamics of auxin concentration, cellular localization and the expression of biosynthesis genes during microspore embryogenesis. This work involved the analysis of auxin concentration and cellular accumulation; expression of TAA1 and NIT2 encoding enzymes of two auxin biosynthetic pathways; expression of the PIN1-like efflux carrier; and the effects of inhibition of auxin transport and action by N-1-naphthylphthalamic acid (NPA) and α-(p-chlorophenoxy) isobutyric acid (PCIB) during Brassica napus microspore embryogenesis. The results indicated de novo auxin synthesis after stress-induced microspore reprogramming and embryogenesis initiation, accompanying the first cell divisions. The progressive increase of auxin concentration during progression of embryogenesis correlated with the expression patterns of TAA1 and NIT2 genes of auxin biosynthetic pathways. Auxin was evenly distributed in early embryos, whereas in heart/torpedo embryos auxin was accumulated in apical and basal embryo regions. Auxin efflux carrier PIN1-like gene expression was induced in early multicellular embryos and increased at the globular/torpedo embryo stages. Inhibition of polar auxin transport (PAT) and action, by NPA and PCIB, impaired embryo development, indicating that PAT and auxin action are required for microspore embryo progression. NPA also modified auxin embryo accumulation patterns. These findings indicate that endogenous auxin biosynthesis, action and polar transport are required in stress-induced microspore reprogramming, embryogenesis initiation and progression.

Nitric oxide: a multifaceted regulator of the nitrogen-fixing symbiosis

The specific interaction between legumes and Rhizobium-type bacteria leads to the establishment of a symbiotic relationship characterized by the formation of new differentiated organs named nodules, which provide a niche for bacterial nitrogen (N2) fixation. In the nodules, bacteria differentiate into bacteroids with the ability to fix atmospheric N2 via nitrogenase activity. As nitrogenase is strongly inhibited by oxygen, N2 fixation is made possible by the microaerophilic conditions prevailing in the nodules. Increasing evidence has shown the presence of NO during symbiosis, from early interaction steps between the plant and the bacterial partners to N2-fixing and senescence steps in mature nodules. Both the plant and the bacterial partners participate in NO synthesis. NO was found to be required for the optimal establishment of the symbiotic interaction. Transcriptomic analysis at an early stage of the symbiosis showed that NO is potentially involved in the repression of plant defence reactions, favouring the establishment of the plant-microbe interaction. In mature nodules, NO was shown to inhibit N2 fixation, but it was also demonstrated to have a regulatory role in nitrogen metabolism, to play a beneficial metabolic function for the maintenance of the energy status under hypoxic conditions, and to trigger nodule senescence. The present review provides an overview of NO sources and multifaceted effects from the early steps of the interaction to the senescence of the nodule, and presents several approaches which appear to be particularly promising in deciphering the roles of NO in N2-fixing symbioses.

Hydrogen peroxide promotes programmed cell death and salicylic acid accumulation during the induced production of sesquiterpenes in cultured cell suspensions of Aquilaria sinensis

Aquilaria sinensis (Lour.) Gilg produces a highly valuable agarwood characterised by a diverse array of sesquiterpenes and chromone derivatives that can protect wounded trees against potential herbivores and pathogens. A defensive reaction on the part of the plant has been proposed as the key reason for agarwood formation, but the issue of whether programmed cell death (PCD), an important process of plant immune responding, is involved in agarwood formation, still needs to be clarified. In this study, treatment of cultured cell suspensions with hydrogen peroxide (H2O2) induced the production of sesquiterpenes due to endogenous accumulation of salicylic acid (SA) and elevations in the expression of sesquiterpene biosynthetic genes. Moreover, PCD was stimulated by H2O2 in cultured cell suspensions of A. sinensis due to the induction of caspase activity, upregulated expression of metacaspases and cytochrome c, and SA accumulation. Our findings demonstrate for the first time that H2O2 stimulates PCD, SA accumulation and sesquiterpene production in cultured cell suspensions of A. sinensis. Furthermore, results from this study provide a valuable insight into investigations of the potential interactions between sesquiterpene synthesis and PCD during agarwood formation.

Interaction of nitric oxide and reactive oxygen species and associated regulation of root growth in wheat seedlings under zinc stress

The inhibition of root growth was investigated in wheat seedlings exposed to 3mM zinc (Zn). Zn treatment with or without 250 µM 2-phenyl-4,4,5,5,-tetrame-thylimidazoline-3-oxide-1-oxyl (PTIO) or 10 µM diphenylene iodonium (DPI) significantly inhibited growth, increased malondialdehyde content and lowered cell viability in roots. The most prominent changes of these three parameters at Zn+DPI treatment could be partly blocked by high PTIO concentration (1mM). The production of nitric oxide (NO) and hydrogen peroxide (H2O2) influenced each other under different treatments, with the highest NO level and the highest H2O2 accumulation in Zn+DPI-treated roots. Compared with Zn-stressed roots, catalase, soluble peroxidase (POD), ascorbate peroxidase and superoxide dismutase decreased in Zn+DPI-treated roots, suggesting that ROS generation from plasma membrane (PM) NADPH oxidase was associated with the regulation of antioxidant enzyme activities. Additionally, Zn-treated roots exhibited significant decreases in cell wall-bound POD, diamine oxidase and polyamine oxidase activities. Our results suggested that Zn-induced effects on root growth resulted from NO interaction with H2O2 and that Zn+DPI-induced strongest inhibition could be explained by the highest increase in the endogenous NO content and the reduction of extracellular ROS production.

5-azacytidine promotes microspore embryogenesis initiation by decreasing global DNA methylation, but prevents subsequent embryo development in rapeseed and barley

Microspores are reprogrammed by stress in vitro toward embryogenesis. This process is an important tool in breeding to obtain double-haploid plants. DNA methylation is a major epigenetic modification that changes in differentiation and proliferation. We have shown changes in global DNA methylation during microspore reprogramming. 5-Azacytidine (AzaC) cannot be methylated and leads to DNA hypomethylation. AzaC is a useful demethylating agent to study DNA dynamics, with a potential application in microspore embryogenesis. This work analyzes the effects of short and long AzaC treatments on microspore embryogenesis initiation and progression in two species, the dicot Brassica napus and the monocot Hordeum vulgare. This involved the quantitative analyses of proembryo and embryo production, the quantification of DNA methylation, 5-methyl-deoxy-cytidine (5mdC) immunofluorescence and confocal microscopy, and the analysis of chromatin organization (condensation/decondensation) by light and electron microscopy. Four days of AzaC treatments (2.5 μM) increased embryo induction, response associated with a decrease of DNA methylation, modified 5mdC, and heterochromatin patterns compared to untreated embryos. By contrast, longer AzaC treatments diminished embryo production. Similar effects were found in both species, indicating that DNA demethylation promotes microspore reprogramming, totipotency acquisition, and embryogenesis initiation, while embryo differentiation requires de novo DNA methylation and is prevented by AzaC. This suggests a role for DNA methylation in the repression of microspore reprogramming and possibly totipotency acquisition. Results provide new insights into the role of epigenetic modifications in microspore embryogenesis and suggest a potential benefit of inhibitors, such as AzaC, to improve the process efficiency in biotechnology and breeding programs.

Caspase-like activities accompany programmed cell death events in developing barley grains

Programmed cell death is essential part of development and cell homeostasis of any multicellular organism. We have analyzed programmed cell death in developing barley caryopsis at histological, biochemical and molecular level. Caspase-1, -3, -4, -6 and -8-like activities increased with aging of pericarp coinciding with abundance of TUNEL positive nuclei and expression of HvVPE4 and HvPhS2 genes in the tissue. TUNEL-positive nuclei were also detected in nucellus and nucellar projection as well as in embryo surrounding region during early caryopsis development. Quantitative RT-PCR analysis of micro-dissected grain tissues revealed the expression of HvVPE2a, HvVPE2b, HvVPE2d, HvPhS2 and HvPhS3 genes exclusively in the nucellus/nucellar projection. The first increase in cascade of caspase-1, -3, -4, -6 and -8-like activities in the endosperm fraction may be related to programmed cell death in the nucellus and nucellar projection. The second increase of all above caspase-like activities including of caspase-9-like was detected in the maturating endosperm and coincided with expression of HvVPE1 and HvPhS1 genes as well as with degeneration of nuclei in starchy endosperm and transfer cells. The distribution of the TUNEL-positive nuclei, tissues-specific expression of genes encoding proteases with potential caspase activities and cascades of caspase-like activities suggest that each seed tissue follows individual pattern of development and disintegration, which however harmonizes with growth of the other tissues in order to achieve proper caryopsis development.