Three dimensional confocal and electron microscopy imaging define the dynamics and mechanisms of diploidisation at early stages of barley microspore-derived embryogenesis

Microspore embryogenesis induction by mannitol and TSA results in a complex regulation of epigenetic dynamics and gene expression in bread wheat

Reprogramming of microspores development towards embryogenesis mediated by stress treatment constitutes the basis of doubled haploid production. Recently, compounds that alter histone post-translational modifications (PTMs) have been reported to enhance microspore embryogenesis (ME), by altering histones acetylation or methylation. However, epigenetic mechanisms underlying ME induction efficiency are poorly understood. In this study, the epigenetic dynamics and the expression of genes associated with histone PTMs and ME induction were studied in two bread wheat cultivars with different ME response. Microspores isolated at 0, 3 and 5 days, treated with 0.7M mannitol (MAN) and 0.7M mannitol plus 0.4µM trichostatin A (TSA), which induced ME more efficiently, were analyzed. An additional control of gametophytic development was included. Microspores epigenetic state at the onset of ME induction was distinctive between cultivars by the ratio of H3 variants and their acetylated forms, the localization and percentage of labeled microspores with H3K9ac, H4K5ac, H4K16ac, H3K9me2 and H3K27me3, and the expression of genes related to pollen development. These results indicated that microspores of the high responding cultivar could be at a less advanced stage in pollen development. MAN and TSA resulted in a hyperacetylation of H3.2, with a greater effect of TSA. Histone PTMs were differentially affected by both treatments, with acetylation being most concerned. The effect of TSA was observed in the H4K5ac localization pattern at 3dT in the mid-low responding cultivar. Three gene networks linked to ME response were identified. TaHDT1, TaHAG2, TaYAO, TaNFD6-A, TabZIPF1 and TaAGO802-B, associated with pollen development, were down-regulated. TaHDA15, TaHAG3, TaHAM, TaYUC11D, Ta-2B-LBD16 TaMS1 and TaDRM3 constituted a network implicated in morphological changes by auxin signaling and cell wall modification up-regulated at 3dT. The last network included TaHDA18, TaHAC1, TaHAC4, TaABI5, TaATG18fD, TaSDG1a-7A and was related to ABA and ethylene hormone signaling pathways, DNA methylation and autophagy processes, reaching the highest expression at 5dT. The results indicated that TSA mainly modified the regulation of genes related to pollen and auxin signaling. This study represents a breakthrough in identifying the epigenetic dynamics and the molecular mechanisms governing ME induction efficiency, with relevance to recalcitrant wheat genotypes and other crops.

The Nuclear 35S rDNA World in Plant Systematics and Evolution: A Primer of Cautions and Common Misconceptions in Cytogenetic Studies

The ubiquitous presence of rRNA genes in nuclear, plastid, and mitochondrial genomes has provided an opportunity to use genomic markers to infer patterns of molecular and organismic evolution as well as to assess systematic issues throughout the tree of life. The number, size, location, and activity of the 35S rDNA cistrons in plant karyotypes have been used as conventional cytogenetic landmarks. Their scrutiny has been useful to infer patterns of chromosomal evolution and the data have been used as a proxy for assessing species discrimination, population differentiation and evolutionary relationships. The correct interpretation of rDNA markers in plant taxonomy and evolution is not free of drawbacks given the complexities derived from the lability of the genetic architecture, the diverse patterns of molecular change, and the fate and evolutionary dynamics of the rDNA units in hybrids and polyploid species. In addition, the terminology used by independent authors is somewhat vague, which often complicates comparisons. To date, no efforts have been reported addressing the potential problems and limitations involved in generating, utilizing, and interpreting the data from the 35S rDNA in cytogenetics. This review discusses the main technical and conceptual limitations of these rDNA markers obtained by cytological and karyological experimental work, in order to clarify biological and evolutionary inferences postulated in a systematic and phylogenetic context. Also, we provide clarification for some ambiguity and misconceptions in terminology usually found in published work that may help to improve the usage of the 35S ribosomal world in plant evolution.

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.

Imaging Techniques to Study Plant Virus Replication and Vertical Transmission

Plant viruses are obligate parasites that need to usurp plant cell metabolism in order to infect their hosts. Imaging techniques have been used for quite a long time to study plant virus-host interactions, making it possible to have major advances in the knowledge of plant virus infection cycles. The imaging techniques used to study plant-virus interactions have included light microscopy, confocal laser scanning microscopy, and scanning and transmission electron microscopies. Here, we review the use of these techniques in plant virology, illustrating recent advances in the area with examples from plant virus replication and virus plant-to-plant vertical transmission processes.

The commitment of barley microspores into embryogenesis correlates with miRNA-directed regulation of members of the SPL, GRF and HD-ZIPIII transcription factor families

Microspore embryogenesis is a model for developmental plasticity and cell fate decisions. To investigate the role of miRNAs in this development, we sequenced sRNAs and the degradome of barley microspores collected prior to (day 0) and after (days 2 and 5) the application of a stress treatment known to induce embryogenesis. Microspores isolated at these timepoints were uniform in both appearance and in their complements of sRNAs. We detected 68 miRNAs in microspores. The abundance of 51 of these miRNAs differed significantly during microspore development. One group of miRNAs was induced when the stress treatment was applied, prior to being repressed when microspores transitioned to embryogenesis. Another group of miRNAs were up-regulated in day-2 microspores and their abundance remained stable or increased in day-5 microspores, a timepoint at which the first clear indications of the transition toward embryogenesis were visible. Collectively, these miRNAs might play a role in the modulation of the stress response, the repression of gametic development, and/or the gain of embryogenic potential. A degradome analysis allowed us to validate the role of miRNAs in regulating 41 specific transcripts. We showed that the transition of microspores toward the embryogenesis pathway involves miRNA-directed regulation of members of the ARF, SPL, GRF, and HD-ZIPIII transcription factor families. We noted that 41.5% of these targets were shared between day-2 and day-5 microspores while 26.8% were unique to day-5 microspores. The former set may act to disrupt transcripts involved in pollen development while the latter set may drive the commitment to embryogenesis.

Trichostatin A Affects Developmental Reprogramming of Bread Wheat Microspores towards an Embryogenic Route

Microspores can be developmentally reprogrammed by the application of different stress treatments to initiate an embryogenic pathway leading to the production of doubled haploid (DH) plants. Epigenetic modifications are involved in cell reprogramming and totipotency in response to stress. To increase microspore embryogenesis (ME) efficiency in bread wheat, the effect of the histone deacetylase inhibitor trichostatin A (TSA) has been examined in two cultivars of wheat with different microspore embryogenesis response. Diverse strategies were assayed using 0-0.4 µM TSA as a single induction treatment and after or simultaneously with cold or mannitol stresses. The highest efficiency was achieved when 0.4 µM TSA was applied to anthers for 5 days simultaneously with a 0.7 M mannitol treatment, producing a four times greater number of green DH plants than mannitol. Ultrastructural studies by transmission electron microscopy indicated that mannitol with TSA and mannitol treatments induced similar morphological changes in early stages of microspore reprogramming, although TSA increased the number of microspores with 'star-like' morphology and symmetric divisions. The effect of TSA on the transcript level of four ME marker genes indicated that the early signaling pathways in ME, involving the TaTDP1 and TAA1b genes, may be mediated by changes in acetylation patterns of histones and/or other proteins.

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.

Differential Expression Profiling of Microspores During the Early Stages of Isolated Microspore Culture Using the Responsive Barley Cultivar Gobernadora

In barley, it is possible to induce embryogenesis in the haploid and uninucleate microspore to obtain a diploid plant that is perfectly homozygous. To change developmental fates in this fashion, microspores need to engage in cellular de-differentiation, interrupting the pollen formation, and restore totipotency prior to engaging in embryogenesis. In this work, we used the barley cultivar Gobernadora to characterize the transcriptome of microspores prior to (day 0) and immediately after (days 2 and 5) the application of a stress pretreatment. A deep RNA-seq analysis revealed that microspores at these three time points exhibit a transcriptome of ∼14k genes, ∼90% of which were shared. An expression analysis identified a total of 3,382 differentially expressed genes (DEGs); of these, 2,155 and 2,281 DEGs were respectively identified when contrasting expression at days 0 and 2 and at days 2 and 5. These define 8 expression profiles in which DEGs share a common up- or down-regulation at these time points. Up-regulation of numerous glutathione S-transferase and heat shock protein genes as well as down-regulation of ribosomal subunit protein genes was observed between days 0 and 2. The transition from microspores to developing embryos (days 2 vs. 5) was marked by the induction of transcription factor genes known to play important roles in early embryogenesis, numerous genes involved in hormone biosynthesis and plant hormonal signal transduction in addition to genes involved in secondary metabolism. This work sheds light on transcriptional changes accompanying an important developmental shift and provides candidate biomarkers for embryogenesis in barley.

The low molecular weight fraction of compounds released from immature wheat pistils supports barley pollen embryogenesis

Pollen embryogenesis provides a useful means of generating haploid plants for plant breeding and basic research. Although it is well-established that the efficacy of the process can be enhanced by the provision of immature pistils as a nurse tissue, the origin and compound class of the signal molecule(s) involved is still elusive. Here, a micro-culture system was established to enable the culturing of populations of barley pollen at a density too low to allow unaided embryogenesis to occur, and this was then exploited to assess the effect of using various parts of the pistil as nurse tissue. A five-fold increase in the number of embryogenic calli formed was obtained by simply cutting the pistils in half. The effectiveness of the pistil-conditioned medium was transitory, since it needed replacement at least every 4 days to measurably ensure embryogenic development. The differential effect of various size classes of compounds present in the pistil-conditioned medium showed that the relevant molecule(s) was of molecular weight below 3 kDa. This work narrows down possible feeder molecules to lower molecular weight compounds and showed that the cellular origin of the active compound(s) is not specific to any tested part of the pistil. Furthermore, the increased recovery of calli during treatment with cut pistils may provide a useful tool for plant breeders and researchers using haploid technology in barley and other plant species.

Effect of ovary induction on bread wheat anther culture: ovary genotype and developmental stage, and candidate gene association

Ovary pre-conditioned medium and ovary co-culture increased the efficiency of green doubled haploid plant production in bread wheat anther culture. The positive effect of this medium led to a 6- and 11-fold increase in the numbers of embryos and green plants, respectively, having a greater effect on a medium-low responding cultivar. Ovary genotype and developmental stage significantly affected microspore embryogenesis. By the use of Caramba ovaries it was possible to reach a 2-fold increase in the number of embryos and green plants, and to decrease the rate of albinism. Mature ovaries from flowers containing microspores at a late binucleate stage raised the number of embryos and green plants by 25-46% as compared to immature ovaries (excised from flowers with microspores at a mid-late uninucleate stage). The highest numbers of embryos and green plants were produced when using mature Caramba ovaries. Ovaries from Galeón, Tigre, and Kilopondio cultivars successfully induced microspore embryogenesis at the same rate as Caramba ovaries. Moreover, Tigre ovaries raised the percentage of spontaneous chromosome doubling up to 71%. Attempts were made to identify molecular mechanisms associated to the inductive effect of the ovaries on microspore embryogenesis. The genes TAA1b, FLA26, and WALI6 associated to wheat microspore embryogenesis, the CGL1 gene involved in glycan biosynthesis or degradation, and the FER gene involved in the ovary signaling process were expressed and/or induced at different rates during ovary culture. The expression pattern of FLA26 and FER could be related to the differences between genotypes and developmental stages in the inductive effect of the ovary. Our results open opportunities for new approaches to increase bread wheat doubled haploid production by anther culture, and to identify the functional components of the ovary inductive effect on microspore embryogenesis.

Induction of Embryogenesis in Brassica Napus Microspores Produces a Callosic Subintinal Layer and Abnormal Cell Walls with Altered Levels of Callose and Cellulose

The induction of microspore embryogenesis produces dramatic changes in different aspects of the cell physiology and structure. Changes at the cell wall level are among the most intriguing and poorly understood. In this work, we used high pressure freezing and freeze substitution, immunolocalization, confocal, and electron microscopy to analyze the structure and composition of the first cell walls formed during conventional Brassica napus microspore embryogenesis, and in cultures treated to alter the intracellular Ca(2+) levels. Our results revealed that one of the first signs of embryogenic commitment is the formation of a callose-rich, cellulose-deficient layer beneath the intine (the subintinal layer), and of irregular, incomplete cell walls. In these events, Ca(2+) may have a role. We propose that abnormal cell walls are due to a massive callose synthesis and deposition of excreted cytoplasmic material, and the parallel inhibition of cellulose synthesis. These features were absent in pollen-like structures and in microspore-derived embryos, few days after the end of the heat shock, where abnormal cell walls were no longer produced. Together, our results provide an explanation to a series of relevant aspects of microspore embryogenesis including the role of Ca(2+) and the occurrence of abnormal cell walls. In addition, our discovery may be the explanation to why nuclear fusions take place during microspore embryogenesis.

Arabinogalactan proteins mark stigmatic receptivity in the protogynous flowers of Magnolia virginiana (Magnoliaceae)

PREMISE OF THE STUDY

Factors affecting floral receptivity in angiosperms remain opaque, but recent studies suggest that the acquisition of stigmatic receptivity associated with cell-wall-related arabinogalactan proteins (AGPs) may be a widespread feature of flowering plants. Here, the time during which a stigma is receptive is evaluated and related to the secretion of AGPs in Magnolia virginiana, a protogynous member of an early-divergent angiosperm clade (magnoliids) with a clearly discernible female receptive phase.

METHODS

Magnolia virginiana flower phenology was documented, and histochemical changes in the stigma before and after pollination were examined. Stigmatic receptivity was evaluated in relation to the secretion of AGPs detected in whole mounts and immunolocalized in sectioned stigmas.

KEY RESULTS

Protogynous Magnolia flowers had a precise window of stigmatic receptivity, which is concomitant with the secretion of two AGPs labeled for different epitopes. After pollen germination and tube growth, these two AGPs could no longer be detected in the stigmas, suggesting that these AGPs interact with the growing male gametophytes and could be markers of stigmatic receptivity.

CONCLUSIONS

These results show that the period of stigmatic receptivity is finely coordinated with the secretion of two arabinogalactan proteins on stigmas of flowers of M. virginiana. This first report of AGP presence in stigmatic tissues in a member of the magnoliids, together with recently described similar patterns in eudicots, monocots, and members of early-divergent lineages of flowering plants, suggests an ancient and widespread role for AGPs on stigmatic receptivity in angiosperms.

Changes in DNA methylation levels and nuclear distribution patterns after microspore reprogramming to embryogenesis in barley

Under specific stress treatments, the microspore can be induced in vitro to deviate from its gametophytic development and to reprogram towards embryogenesis, becoming a totipotent cell and forming haploid embryos. These can further regenerate homozygous plants for production of new isogenic lines, an important biotechnological tool for crop breeding. DNA methylation constitutes a prominent epigenetic modification of the chromatin fiber which regulates gene expression. Changes in DNA methylation accompany the reorganization of the nuclear architecture during plant cell differentiation and proliferation; however, the relationship between global DNA methylation and genome-wide expression patterns is still poorly understood. In this work, the dynamics of global DNA methylation levels and distribution patterns were analyzed during microspore reprogramming to embryogenesis and during pollen development in Hordeum vulgare. Quantification of global DNA methylation levels and 5-methyl-deoxycytidine (5mdC) immunofluorescence were conducted at specific stages of pollen development and after reprogramming to embryogenesis to analyze the epigenetic changes that accompany the change of developmental program and cell fate. The results showed low DNA methylation levels in microspores and a high increase along pollen development and maturation; an intense 5mdC signal was concentrated in the generative and sperm nuclei whereas the vegetative nucleus exhibited a weaker DNA methylation signal. After inductive stress treatment, low methylation levels and faint 5mdC signals were observed in nuclei of reprogrammed microspores and 2-4-cell proembryos. This data revealed a global DNA hypomethylation during the change of the developmental program and first embryogenic divisions. This is in contrast with the hypermethylation of generative and sperm cells of the male germline during pollen maturation, suggesting an epigenetic regulation after induction of microspore embryogenesis. At later embryogenesis stages, global DNA methylation progressively increased, accompanying embryo development and differentiation events like in zygotic embryos, corroborating that DNA methylation is critical for the regulation of gene expression in microspore embryogenesis.

True-breeding targeted gene knock-out in barley using designer TALE-nuclease in haploid cells

Transcription activator-like effector nucleases (TALENs) are customizable fusion proteins able to cleave virtually any genomic DNA sequence of choice, and thereby to generate site-directed genetic modifications in a wide range of cells and organisms. In the present study, we expressed TALENs in pollen-derived, regenerable cells to establish the generation of instantly true-breeding mutant plants. A gfp-specific TALEN pair was expressed via Agrobacterium-mediated transformation in embryogenic pollen of transgenic barley harboring a functional copy of gfp. Thanks to the haploid nature of the target cells, knock-out mutations were readily detected, and homozygous primary mutant plants obtained following genome duplication. In all, 22% of the TALEN transgenics proved knocked out with respect to gfp, and the loss of function could be ascribed to the deletions of between four and 36 nucleotides in length. The altered gfp alleles were transmitted normally through meiosis, and the knock-out phenotype was consistently shown by the offspring of two independent mutants. Thus, here we describe the efficient production of TALEN-mediated gene knock-outs in barley that are instantaneously homozygous and non-chimeric in regard to the site-directed mutations induced. This TALEN approach has broad applicability for both elucidating gene function and tailoring the phenotype of barley and other crop species.

Cellular dynamics during early barley pollen embryogenesis revealed by time-lapse imaging

Plants display a remarkable capacity for cellular totipotency. An intriguing and useful example is that immature pollen cultured in vitro can pass through embryogenic development to form haploid or doubled haploid plants. However, a lack of understanding the initial mechanisms of pollen embryogenesis hampers the improvement and more effective and widespread employment of haploid technology in plant research and breeding. To investigate the cellular dynamics during the onset of pollen embryogenesis, we used time-lapse imaging along with transgenic barley expressing nuclear localized Green Fluorescent Protein. The results enabled us to identify nine distinct embryogenic and non-embryogenic types of pollen response to the culture conditions. Cell proliferation in embryogenic pollen normally started via a first symmetric mitosis (54.3% of pollen observed) and only rarely did so via asymmetric pollen mitosis I (4.3% of pollen observed). In the latter case, proliferation generally originated from the vegetative-like cell, albeit the division of the generative-like cell was observed in few types of pollen. Under the culture conditions used, fusion of cell nuclei was the only mechanism of genome duplication observed.

Validation of doubled haploid plants by enzymatic mismatch cleavage

BACKGROUND

Doubled haploidy is a fundamental tool in plant breeding as it provides the fastest way to generate populations of meiotic recombinants in a genetically fixed state. A wide range of methods has been developed to produce doubled haploid (DH) plants and recent advances promise efficient DH production in otherwise recalcitrant species. Since the cellular origin of the plants produced is not always certain, rapid screening techniques are needed to validate that the produced individuals are indeed homozygous and genetically distinct from each other. Ideal methods are easily implemented across species and in crops where whole genome sequence and marker resources are limited.

RESULTS

We have adapted enzymatic mismatch cleavage techniques commonly used for TILLING (Targeting Induced Local Lesions IN Genomes) for the evaluation of heterozygosity in parental, F1 and putative DH plants. We used barley as a model crop and tested 26 amplicons previously developed for TILLING. Experiments were performed using self-extracted single-strand-specific nuclease and standard native agarose gels. Eleven of the twenty-six tested primers allowed unambiguous assignment of heterozygosity in material from F1 crosses and loss of heterozygosity in the DH plants. Through parallel testing of previously developed Simple Sequence Repeat (SSR) markers, we show that 3/32 SSR markers were suitable for screening. This suggests that enzymatic mismatch cleavage approaches can be more efficient than SSR based screening, even in species with well-developed markers.

CONCLUSIONS

Enzymatic mismatch cleavage has been applied for mutation discovery in many plant species, including those with little or no available genomic DNA sequence information. Here, we show that the same methods provide an efficient system to screen for the production of DH material without the need of specialized equipment. This gene target based approach further allows discovery of novel nucleotide polymorphisms in candidate genes in the parental lines.

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

Under specific stress treatments (cold, starvation), in vitro microspores can be induced to deviate from their gametophytic development and switch to embryogenesis, forming haploid embryos and homozygous breeding lines in a short period of time. The inductive stress produces reactive oxygen species (ROS) and nitric oxide (NO), signalling molecules mediating cellular responses, and cell death, modifying the embryogenic microspore response and therefore, the efficiency of the process. This work analysed cell death, caspase 3-like activity, and ROS and NO production (using fluorescence probes and confocal analysis) after inductive stress in barley microspore cultures and embryogenic suspension cultures, as an in vitro system which permitted easy handling for comparison. There was an increase in caspase 3-like activity and cell death after stress treatment in microspore and suspension cultures, while ROS increased in non-induced microspores and suspension cultures. Treatments of the cultures with a caspase 3 inhibitor, DEVD-CHO, significantly reduced the cell death percentages. Stress-treated embryogenic suspension cultures exhibited high NO signals and cell death, while treatment with S-nitrosoglutathione (NO donor) in control suspension cultures resulted in even higher cell death. In contrast, in microspore cultures, NO production was detected after stress, and, in the case of 4-day microspore cultures, in embryogenic microspores accompanying the initiation of cell divisions. Subsequent treatments of stress-treated microspore cultures with ROS and NO scavengers resulted in a decreasing cell death during the early stages, but later they produced a delay in embryo development as well as a decrease in the percentage of embryogenesis in microspores. Results showed that the ROS increase was involved in the stress-induced programmed cell death occurring at early stages in both non-induced microspores and embryogenic suspension cultures; whereas NO played a dual role after stress in the two in vitro systems, one involved in programmed cell death in embryogenic suspension cultures and the other in the initiation of cell division leading to embryogenesis in reprogrammed microspores.

Correlative microscopy: providing new understanding in the biomedical and plant sciences

Correlative microscopy is the application of two or more distinct microscopy techniques to the same region of a sample, generating complementary morphological, structural and chemical information that exceeds what is possible with any single technique. As a variety of complementary microscopy approaches rather than a specific type of instrument, correlative microscopy has blossomed in recent years as researchers have recognised that it is particularly suited to address the intricate questions of the modern biological sciences. Specialised technical developments in sample preparation, imaging methods, visualisation and data analysis have also accelerated the uptake of correlative approaches. In light of these advances, this critical review takes the reader on a journey through recent developments in, and applications of, correlative microscopy, examining its impact in biomedical research and in the field of plant science. This twin emphasis gives a unique perspective into use of correlative microscopy in fields that often advance independently, and highlights the lessons that can be learned from both fields for the future of this important area of research.

Gametic embryogenesis and haploid technology as valuable support to plant breeding

Plant breeding is focused on continuously increasing crop production to meet the needs of an ever-growing world population, improving food quality to ensure a long and healthy life and address the problems of global warming and environment pollution, together with the challenges of developing novel sources of biofuels. The breeders' search for novel genetic combinations, with which to select plants with improved traits to satisfy both farmers and consumers, is endless. About half of the dramatic increase in crop yield obtained in the second half of the last century has been achieved thanks to the results of genetic improvement, while the residual advance has been due to the enhanced management techniques (pest and disease control, fertilization, and irrigation). Biotechnologies provide powerful tools for plant breeding, and among these ones, tissue culture, particularly haploid and doubled haploid technology, can effectively help to select superior plants. In fact, haploids (Hs), which are plants with gametophytic chromosome number, and doubled haploids (DHs), which are haploids that have undergone chromosome duplication, represent a particularly attractive biotechnological method to accelerate plant breeding. Currently, haploid technology, making possible through gametic embryogenesis the single-step development of complete homozygous lines from heterozygous parents, has already had a huge impact on agricultural systems of many agronomically important crops, representing an integral part in their improvement programmes. The aim of this review was to provide some background, recent advances, and future prospective on the employment of haploid technology through gametic embryogenesis as a powerful tool to support plant breeding.

Haploids in flowering plants: origins and exploitation

The first haploid angiosperm, a dwarf form of cotton with half the normal chromosome complement, was discovered in 1920, and in the ninety years since then such plants have been identified in many other species. They can occur either spontaneously or can be induced by modified pollination methods in vivo, or by in vitro culture of immature male or female gametophytes. Haploids represent an immediate, one-stage route to homozygous diploids and thence to F(1) hybrid production. The commercial exploitation of heterosis in such F(1) hybrids leads to the development of hybrid seed companies and subsequently to the GM revolution in agriculture. This review describes the range of techniques available for the isolation or induction of haploids and discusses their value in a range of areas, from fundamental research on mutant isolation and transformation, through to applied aspects of quantitative genetics and plant breeding. It will also focus on how molecular methods have been used recently to explore some of the underlying aspects of this fascinating developmental phenomenon.

Haploid technology allows for the efficient and rapid generation of homozygous antibody-accumulating transgenic tobacco plants

The large-scale production of plant-derived recombinant proteins requires the breeding of lines homozygous for the transgene(s). These can be selected by progeny testing over multiple sexual generations, but a more efficient means is to fix homozygosity in a single generation using doubled haploid technology. In this study, transgenic tobacco plants, hemizygous for both of the independently inherited genes encoding the light and heavy chains of the anti-human immunodeficiency virus monoclonal antibody 2F5, were used to establish embryogenic pollen cultures. The improved protocol employed in this study guaranteed a very high regeneration efficiency, with more than 50% of the regenerants being spontaneously doubled haploids. Hence, there was no requirement to chemically induce chromosome doubling to recover sufficient entirely homozygous recombinants. As expected, approximately 25% of the regenerants were homozygous for both transgenes. Thus, the employment of haploid technology allowed for the efficient and rapid generation of true-breeding tobacco lines accumulating functional immunoglobulins.

Transformation of isolated barley (Hordeum vulgare L.) microspores: I. the influence of pretreatments and osmotic treatment on the time of DNA synthesis

The objective of this study was to determine when DNA synthesis occurred during pretreatments of cultured barley (Hordeum vulgare L.) microspores and during their preparation for particle bombardment. Based on this information, an investigation of the influence of cell cycle stage on the ability to obtain homozygous transgenic plants by particle bombardment will be presented in paper II of this series. It was hypothesized that the introduction of foreign genes at the G1 cell cycle stage in cultured uninucleate microspores would produce homozygous transgenic plants. Experiments were conducted with two different commonly used pretreatments to induce microspore embryogenesis: cold (4 degrees C) for 21days and cold plus 0.3 mol/L mannitol for 4 days. After pretreatment, the microspores were placed in a higher osmotic medium for 4 h prior to and for 18 h following bombardment. It was confirmed that during the cold plus mannitol pretreatment, there was no apparent change in the cell cycle stage, with the majority of the microspores remaining at the G1 stage. While in the cold for 21 days, the microspores progressed slowly through to G2, with a few progressing further into the mitosis and binucleate stages. Hourly DNA density measurements that were taken during the 4 h osmotic adjustment period following the cold plus mannitol pretreatment indicated that DNA synthesis began during this period at 25 degrees C, while at 4 degrees C, there was no apparent change in cell cycle stage or in DNA density. Thus, one might expect to find a higher frequency of homozygous doubled haploids by maintaining the temperature low during the 4 h osmotic adjustment period following the cold plus mannitol pretreatment than following the 21 day cold pretreatment. However, it is also not known what effect the temperatures during the whole high-osmotic treatments will have on the rate and time of incorporation of the transgene.

Transformation of isolated barley (Hordeum vulgare L.) microspores: II. Timing of pretreatment and temperatures relative to results of bombardment

Based on paper I in this series, our goals in this paper were to determine the relationship between prebombardment pretreatments and temperatures, microspore cell cycle when bombarded, and the frequencies of homozygous and hemizygous transgenic progeny in barley (Hordeum vulgare L.). Of the 104 fluorescent plants selected when using the GFP fluorescence transgene, 28 were albino and 76 plants were green. Thirty-one green plants were confirmed to be transgenic; the others were either transient green fluorescent protein expression or selected due to autofluorescence. Of the 31 plants, 23 came from embryos expressing a high level of fluorescence during selection and eight from 51 plants exhibiting a low level of fluorescence. Of the two pretreatments used to induce embryogenesis, 24 of 31 plants were from the cold pretreatment for 21 days (C) versus seven from the 4 day cold plus mannitol pretreatment. Following pretreatment, the microspores were subjected to a high-osmotic period (0.5 mol/L mannitol plus sorbitol) of 4 h prebombardment and 18 h postbombardment at either 25 or 4 degrees C. Of the 31 transgenic plants, 19 were produced following the 25 degrees C 4 h prebombardment. Sixteen of the 19 were doubled haploid plants (seven being homozygous for the transgene) and the other three plants were haploid. Of the remaining 12 plants recovered following the 4 h 4 degrees C prebombardment treatment, nine were haploid and three were doubled haploid plants, two of the latter being homozygous for the transgene. All 12 haploid plants obtained were treated with colchicine and produced homozygous transgenic doubled haploids. Of the two promoters compared, 30 plants had the actin promoter and only one had the 35S promoter. The use of arabinogalactan protein in the culture medium was very beneficial, giving rise to 29 of the 31 plants. The best procedure for obtaining transgenic barley plants from this study was pretreatment C, leaving the cultures at either 4 or 25 degrees C during the 4 h prebombardment high-osmotic period, using the actin promoter and having arabinogalactan protein in the microspore culture medium. With this procedure, the transgenic frequency was improved 8- to10-fold over previous reports on bombardment of microspores. It yielded about one transgenic plant per Petri dish and is comparable with Agrobacterium frequencies on structures derived from microspores.

Calcium phosphate-based particles influence osteogenic maturation of human mesenchymal stem cells

Biphasic calcium phosphates (BCPs) consist of a mixture of hydroxyapatite and beta-tricalcium phosphate and are recommended as alternatives or additives to autogenous bone for orthopaedic and dental applications. There is clinical evidence showing particle release from bioceramics, which might impair the ability of human mesenchymal stem cells (hMSC) from bone marrow to proliferate or mature into a functional osteoblast phenotype. This study analyses the influence of BCP particles and their precursors, calcium-deficient apatite (CDA) particles, on in vitro hMSC behaviour. Both types of particles were efficiently internalized by hMSC. Cell viability, morphology and actin cytoskeleton reorganization were unaffected by exposure of hMSC to BCP or CDA particles. Direct exposure to BCP particles impaired hMSC osteogenic differentiation and bone matrix mineralization to a lesser extent than CDA, as assayed by evaluation of alkaline phosphatase activity, osteopontin secretion and mineralized nodule formation. The ability of bioceramic particles to affect osteogenic maturation through modification of soluble factors in media was assayed in an in vitro system that avoids direct cell-particle contact. Indirect exposure to CDA particles severely impaired hMSC osteogenic maturation owing to the uptake of Ca2+ from the culture media. Lower textural properties of BCP and the lack of calcium deficiency in its composition prevented Ca2+ uptake, allowing the development of a functional osteoblast phenotype.

How microspores transform into haploid embryos: changes associated with embryogenesis induction and microspore-derived embryogenesis

Microspore embryogenesis is the most powerful androgenic pathway to produce haploid and doubled haploid plants. To deviate a microspore toward embryogenesis, a number of factors, different for each species, must concur at the same time and place. Once induced, the microspore undergoes numerous changes at different levels, from overall morphology to gene expression. Induction of microspore embryogenesis not only implies the expression of an embryogenic program, but also a stress-related cellular response and a repression of the gametophytic program to revert the microspore to a totipotent status. In this review, we compile the most recent advances in the understanding of the changes undergone by the induced microspore to readapt to the new developmental scenario. We devote special attention to the efforts made to uncover changes in the transcriptome of the induced microspore and microspore-derived embryo (MDE). Finally, we discuss the influence that an in vitro environment exerts over the MDE, as compared with its zygotic counterpart.

Enhanced induction of microspore embryogenesis after n-butanol treatment in wheat (Triticum aestivum L.) anther culture

The aim of this study was the improvement of embryo production in wheat anther culture. Three butanol alcohols, n-butanol, sec-butanol and tert-butanol, were evaluated for their effect on microspore embryogenesis in two spring cultivars of wheat, Pavon and Caramba. Application of n-butanol, at 0.1 and 0.2% (v/v) in the induction media for 5 h, highly improved embryo production in both cultivars. Sec- and tert-butanol performed similarly to control plates. Regeneration ability was unaffected by any butyl-alcohol treatment. As a consequence of the higher embryo production after n-butanol treatment, the number of green regenerated plants increased up to five times in cultivar Pavon and up to three times in cultivar Caramba. The percentage of green plants was improved or unaffected by the treatment. Doubled haploid plant production was between 2 and 4 times higher after n-butanol treatment than in control plates. Therefore, n-butanol was successfully applied in the production of wheat doubled haploids. This primary alcohol is known as an activator of phospholipase D and has been previously reported to disrupt cortical microtubules and detach them from the plasma membrane in plants. Its effects on androgenetic induction could confirm the importance of microtubule regulation in plant cell fate, specifically in microspore development. A possible implication of phospholipase D is discussed.

Nanoparticles as smart treatment-delivery systems in plants: assessment of different techniques of microscopy for their visualization in plant tissues

BACKGROUND AND AIMS

The great potential of using nanodevices as delivery systems to specific targets in living organisms was first explored for medical uses. In plants, the same principles can be applied for a broad range of uses, in particular to tackle infections. Nanoparticles tagged to agrochemicals or other substances could reduce the damage to other plant tissues and the amount of chemicals released into the environment. To explore the benefits of applying nanotechnology to agriculture, the first stage is to work out the correct penetration and transport of the nanoparticles into plants. This research is aimed (a) to put forward a number of tools for the detection and analysis of core-shell magnetic nanoparticles introduced into plants and (b) to assess the use of such magnetic nanoparticles for their concentration in selected plant tissues by magnetic field gradients.

METHODS

Cucurbita pepo plants were cultivated in vitro and treated with carbon-coated Fe nanoparticles. Different microscopy techniques were used for the detection and analysis of these magnetic nanoparticles, ranging from conventional light microscopy to confocal and electron microscopy.

KEY RESULTS

Penetration and translocation of magnetic nanoparticles in whole living plants and into plant cells were determined. The magnetic character allowed nanoparticles to be positioned in the desired plant tissue by applying a magnetic field gradient there; also the graphitic shell made good visualization possible using different microscopy techniques.

CONCLUSIONS

The results open a wide range of possibilities for using magnetic nanoparticles in general plant research and agronomy. The nanoparticles can be charged with different substances, introduced within the plants and, if necessary, concentrated into localized areas by using magnets. Also simple or more complex microscopical techniques can be used in localization studies.

Differential inflammatory macrophage response to rutile and titanium particles

Titanium and its alloys are widely used as implant materials for dental and orthopaedic applications due to their advantageous bulk mechanical properties and biocompatibility, compared to other metallic biomaterials. In order to improve their wear and corrosion resistance, several surface modifications that give rise to an outer ceramic layer of rutile have been developed. The ability of rutile wear debris to stimulate the release of inflammatory cytokines from macrophages has not been addressed to date. We have compared the in vitro biocompatibility of sub-cytotoxic doses of rutile and titanium particles in THP-1 cells driven to the monocyte/macrophage differentiation pathway as well as in primary cultures of human macrophages. Confocal microscopy experiments indicated that differentiated THP-1 cells and primary macrophages efficiently internalised rutile and titanium particles. Treatment of THP-1 cells with rutile particles stimulated the release of TNF-alpha, IL-6 and IL-1beta to a lesser extent than titanium. The influence of osteoblasts on the particle-induced stimulation of TNF-alpha and IL-1beta was analysed by co-culturing differentiated THP-1 cells with human primary osteoblasts. Under these conditions, secretion levels of both cytokines after treatment of THP-1 cells with rutile particles were lower than after exposure to titanium. Finally, we observed that primary macrophages released higher amounts of TNF-alpha, IL-6 and IL-1beta after incubation with titanium particles than with rutile. Taken together, these data indicate that rutile particles are less bioreactive than titanium particles and, therefore, a higher biocompatibility of titanium-based implants modified with an outer surface layer of rutile is expected.

Genetic transformation of barley (Hordeum vulgare L.) via infection of androgenetic pollen cultures with Agrobacterium tumefaciens

A novel genetic transformation method for barley (Hordeum vulgare L.), based on infection of androgenetic pollen cultures with Agrobacterium tumefaciens, is presented. Winter-type barley cv. 'Igri' was amenable to stable integration of transgenes mediated by A. tumefaciens strain LBA4404 harbouring a vector system that confers hypervirulence, or by the non-hypervirulent strain GV3101 with a standard binary vector. The efficacy of gene transfer was substantially influenced by pollen pre-culture time, choice of Agrobacterium strain and vector system, Agrobacterium population density, medium pH and the concentrations of acetosyringone, CaCl(2) and glutamine. After co-culture, rapid removal of viable agrobacteria was crucial for subsequent development of the pollen culture. To this end, the growth of agrobacteria was suppressed by the concerted effects of appropriate antibiotics, low pH, reduced level of glutamine and high concentrations of CaCl(2) and acetosyringone. Following infection with LBA4404 and GV3101, about 31% and 69%, respectively, of the primary transgenic (T(0)) plants carried a single copy of the sequence integrated. The use of hypervirulent A. tumefaciens and hygromycin resistance as a selectable marker resulted in 3.7 T(0) plants per donor spike. About 60% of the primary transgenic plants set seed, indicating spontaneous genome doubling. An analysis of 20 T(1) populations revealed that four progenies did not segregate for reporter gene expression. This indicates that the approach pursued enables the generation of instantly homozygous primary transgenic plants. The method established will be a valuable tool in functional genomics as well as for the biotechnological improvement of barley.