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

Generic Workflow of a Highly Effective and Easy Anther Culture Method for Both Japonica and Indica Rice

As one of the most important staple crops in the world, rice plays a pivotal role in world food security. The creation of doubled haploids based on anther culture is an important technology for rice breeding. However, at present, rice anther culture technology still faces many problems, such as genotype dependency, especially genotypes of indica rice. In this study, fifteen rice genotypes, including twelve japonica rice genotypes and three indica rice genotypes, were randomly selected and used to study anther culture by using a modified M8 medium. The results showed that the total callus induction rates of these different rice genotypes ranged from 0.81 to 13.95%, with an average of 6.64%, while the callus induction rates calculated for the top ten highest callus inductions for each rice genotype ranged from 2.75 to 17.00%, with an average of 10.56%. There were varying gaps between the total callus induction rates and the callus induction rates in these different rice genotypes. The fact that the gaps for some rice genotypes were relatively large indicated that standard tiller or anther collection was not applicable to all rice genotypes and that there was still a lot of room for improvement in the callus induction rate of some rice genotypes through optimization of the sampling method. The plantlet regeneration rates ranged from 12.55 to 456.54%, with an average of 200.10%. Although there were many albinos from anther culture for some rice genotypes, these would still meet the requirement if the rice genotypes had higher callus induction rates or regeneration rates. The percentages of seed setting of regenerated green seedlings ranged from 14% to 84%, with an average of 48.73%. Genetic diversity analysis showed that the genetic background of these different rice genotypes was representative, and the phylogenetic tree and Principal Component Analysis (PCA) divided them into indica and japonica types. Therefore, in this study, an anther culture method suitable for both indica and japonica rice genotypes was established, which could improve doubled haploid breeding in rice.

The maternal embrace: the protection of plant embryos

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

Metabolic changes during wheat microspore embryogenesis induction using the highly responsive cultivar Svilena

Androgenetically-derived haploids can be obtained by inducing embryogenesis in microspores. Thus, full homozygosity is achieved in a single generation, oppositely to conventional plant breeding programs. Here, the metabolite profile of embryogenic microspores of Triticum aestivum was acquired and integrated with transcriptomic existing data from the same samples in an effort to identify the key metabolic processes occurring during the early stages of microspore embryogenesis. Primary metabolites and transcription profiles were identified at three time points: prior to and immediately following a low temperature pre-treatment given to uninuclear microspores, and after the first nuclear division. This is the first time an integrative -omics analysis is reported in microspore embryogenesis in T. aestivum. The key findings were that the energy produced during the pre-treatment was obtained from the tricarboxylic acid (TCA) cycle and from starch degradation, while starch storage resumed after the first nuclear division. Intermediates of the TCA cycle were highly demanded from a very active amino acid metabolism. The transcription profiles of genes encoding enzymes involved in amino acid synthesis differed from the metabolite profiles. The abundance of glutamine synthetase was correlated with that of glutamine. Cytosolic glutamine synthetase isoform 1 was found predominantly after the nuclear division. Overall, energy production was shown to represent a major component of the de-differentiation process induced by the pre-treatment, supporting a highly active amino acid metabolism.

Distinct chromatin signatures in the Arabidopsis male gametophyte

Epigenetic reprogramming in the germline contributes to the erasure of epigenetic inheritance across generations in mammals but remains poorly characterized in plants. Here we profiled histone modifications throughout Arabidopsis male germline development. We find that the sperm cell has widespread apparent chromatin bivalency, which is established by the acquisition of H3K27me3 or H3K4me3 at pre-existing H3K4me3 or H3K27me3 regions, respectively. These bivalent domains are associated with a distinct transcriptional status. Somatic H3K27me3 is generally reduced in sperm, while dramatic loss of H3K27me3 is observed at only ~700 developmental genes. The incorporation of the histone variant H3.10 facilitates the establishment of sperm chromatin identity without a strong impact on resetting of somatic H3K27me3. Vegetative nuclei harbor thousands of specific H3K27me3 domains at repressed genes, while pollination-related genes are highly expressed and marked by gene body H3K4me3. Our work highlights putative chromatin bivalency and restricted resetting of H3K27me3 at developmental regulators as key features in plant pluripotent sperm.

Effects of genotype and culture conditions on microspore embryogenesis in radish (Raphanus sativus L.)

Radish (Raphanus sativus L.), an important annual or biennial root vegetable crop, is widely cultivated in the world for its high nutritive value. Isolated microspore culture (IMC) is one of the most effective methods for rapid development of homozygous lines. Due to imperfection of the IMC technology system, it is particularly important to establish an efficient IMC system in radish. In this study, the effects of different factors on radish microspore embryogenesis were investigated with 23 genotypes. Buds with the largest population of late-uninucleate-stage microspores were most suitable for embryogenesis, with a ratio of petal length to anther length (P/A) in buds of about 3/4 ~ 1. Cold pretreatment was found to be genotype specific, and the highest microspore-derived embryoid (MDE) yield occurred for treatment of the heat shock of 48 h. In addition, the supplement of 0.75 g/L activated charcoal (AC) could increase the yield of embryoids. It was found that genotypes, bud size, as well as temperature treatments had significant effects on microspore embryogenesis. Furthermore, somatic embryogenesis-related kinase (SERK) genes were profiled by reverse transcription quantitative polymerase chain reaction (RT-qPCR) analysis, which indicated that they are involved in the process of MDE formation and plantlet regeneration. The ploidy of microspore-derived plants was identified by chromosome counting and flow cytometry, and the microspore-derived plants were further proved as homozygous plants through expressed sequence tags-simple sequence repeats (EST-SSR) and genetic-SSR markers. The results would facilitate generating the large-scale double haploid (DH) from various genotypes, and promoting further highly efficient genetic improvement in radish.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-022-01312-w.

Global DNA Methylation and mRNA-miRNA Variations Activated by Heat Shock Boost Early Microspore Embryogenesis in Cabbage (Brassica oleracea)

Microspore culture, a type of haploid breeding, is extensively used in the cultivation of cruciferous crops such as cabbage. Heat shock (HS) treatment is essential to improve the embryo rate during the culture process; however, its molecular role in boosting early microspore embryogenesis (ME) remains unknown. Here we combined DNA methylation levels, miRNAs, and transcriptome profiles in isolated microspores of cabbage '01-88' under HS (32 °C for 24 h) and normal temperature (25 °C for 24 h) to investigate the regulatory roles of DNA methylation and miRNA in early ME. Global methylation levels were significantly different in the two pre-treatments, and 508 differentially methylated regions (DMRs) were identified; 59.92% of DMRs were correlated with transcripts, and 39.43% of miRNA locus were associated with methylation levels. Significantly, the association analysis revealed that 31 differentially expressed genes (DEGs) were targeted by methylation and miRNA and were mainly involved in the reactive oxygen species (ROS) response and abscisic acid (ABA) signaling, indicating that HS induced DNA methylation, and miRNA might affect ME by influencing ROS and ABA. This study revealed that DNA methylation and miRNA interfered with ME by modulating key genes and pathways, which could broaden our understanding of the molecular regulation of ME induced by HS pre-treatment.

Genome-wide characterization of Histone gene family and expression profiling during microspore development in radish (Raphanus sativus L.)

Histone, a predominant protein component of chromatin, participates in DNA packaging and transcriptional regulation. However, the available information of Histone gene family is limited in radish. In this study, a total of 42 Histone gene family members were identified from the radish genome. Sequence alignment and phylogenetic analyses classified the Histone family into three groups (H2A, H2B and H3). Motif analysis showed that the functions of some motifs shared by H3 subfamily genes were related to chromosome regulation and cell development activities, such as motif 5 containing Cks1 and PPR region. Analysis of intron/exon structure indicated that RsCENH3 (RsHistone 18) has the characteristics of variant Histone. Furthermore, several motifs, including the LTR, G-box and TC-elements, were found in the promoters of RsHistone genes, which involved in cell development or various abiotic stresses responses. Transcriptome analysis indicated that the RsHistone genes exhibited higher expression level in floral buds than in roots and leaves. Subcellular localization showed that the RsCENH3 was localized on the nucleus, and it was highly expressed in the floral bud of 3.0-4.0 mm in radish. These findings would provide valuable information for characterization and potential utilization of Histone genes, and facilitate the efficient induction of double haploid plants in radish.

Proteins, Small Peptides and Other Signaling Molecules Identified as Inconspicuous but Possibly Important Players in Microspores Reprogramming Toward Embryogenesis

In this review, we describe and integrate the latest knowledge on the signaling role of proteins and peptides in the stress-induced microspore embryogenesis (ME) in some crop plants with agricultural importance (i.e., oilseed rape, tobacco, barley, wheat, rice, triticale, rye). Based on the results received from the most advanced omix analyses, we have selected some inconspicuous but possibly important players in microspores reprogramming toward embryogenic development. We provide an overview of the roles and downstream effect of stress-related proteins (e.g., β-1,3-glucanases, chitinases) and small signaling peptides, especially cysteine—(e.g., glutathione, γ-thionins, rapid alkalinization factor, lipid transfer, phytosulfokine) and glycine-rich peptides and other proteins (e.g., fasciclin-like arabinogalactan protein) on acclimation ability of microspores and the cell wall reconstruction in a context of ME induction and haploids/doubled haploids (DHs) production. Application of these molecules, stimulating the induction and proper development of embryo-like structures and green plant regeneration, brings significant improvement of the effectiveness of DHs procedures and could result in its wider incorporation on a commercial scale. Recent advances in the design and construction of synthetic peptides–mainly cysteine-rich peptides and their derivatives–have accelerated the development of new DNA-free genome-editing techniques. These new systems are evolving incredibly fast and soon will find application in many areas of plant science and breeding.

Cell Wall Composition and Structure Define the Developmental Fate of Embryogenic Microspores in Brassica napus

Microspore cultures generate a heterogeneous population of embryogenic structures that can be grouped into highly embryogenic structures [exine-enclosed (EE) and loose bicellular structures (LBS)] and barely embryogenic structures [compact callus (CC) and loose callus (LC) structures]. Little is known about the factors behind these different responses. In this study we performed a comparative analysis of the composition and architecture of the cell walls of each structure by confocal and quantitative electron microscopy. Each structure presented specific cell wall characteristics that defined their developmental fate. EE and LBS structures, which are responsible for most of the viable embryos, showed a specific profile with thin walls rich in arabinogalactan proteins (AGPs), highly and low methyl-esterified pectin and callose, and a callose-rich subintinal layer not necessarily thick, but with a remarkably high callose concentration. The different profiles of EE and LBS walls support the development as suspensorless and suspensor-bearing embryos, respectively. Conversely, less viable embryogenic structures (LC) presented the thickest walls and the lowest values for almost all of the studied cell wall components. These cell wall properties would be the less favorable for cell proliferation and embryo progression. High levels of highly methyl-esterified pectin are necessary for wall flexibility and growth of highly embryogenic structures. AGPs seem to play a role in cell wall stiffness, possibly due to their putative role as calcium capacitors, explaining the positive relationship between embryogenic potential and calcium levels.

Doubled Haploid Production in High- and Low-Response Genotypes of Rapeseed (Brassica napus) Through Isolated Microspore Culture

Rapeseed (Brassica napus) is one of the most important oilseed crops worldwide. It is also a model system to study the process of microspore embryogenesis, due to the high response of some B. napus lines, and to the refinements of the protocols. This chapter presents a protocol for the induction of haploid and DH embryos in B. napus through isolated microspore culture in two specific backgrounds widely used in DH research, the high response DH4079 line and the low response DH12075 line. We also present methods to identify the best phenological window to identify buds with microspores/pollen at the right developmental stage to induce this process. Methods to determine microspore/pollen viability and to check the ploidy by flow cytometry are also described.

Identification, Gene Structure, and Expression of BnMicEmUP: A Gene Upregulated in Embryogenic Brassica napus Microspores

Microspores of Brassica napus can be diverted from normal pollen development into embryogenesis by treating them with a mild heat shock. As microspore embryogenesis closely resembles zygotic embryogenesis, it is used as model for studying the molecular mechanisms controlling embryo formation. A previous study comparing the transcriptomes of three-day-old sorted embryogenic and pollen-like (non-embryogenic) microspores identified a gene homologous to AT1G74730 of unknown function that was upregulated 8-fold in the embryogenic cells. In the current study, the gene was isolated and sequenced from B. napus and named BnMicEmUP (B. napus microspore embryogenesis upregulated gene). Four forms of BnMicEmUP mRNA and three forms of genomic DNA were identified. BnMicEmUP2,3 was upregulated more than 7-fold by day 3 in embryogenic microspore cultures compared to non-induced cultures. BnMicEmUP1,4 was highly expressed in leaves. Transient expression studies of BnMicEmUP3::GFP fusion protein in Nicotiana benthamiana and in stable Arabidopsis transgenics showed that it accumulates in chloroplasts. The features of the BnMicEmUP protein, which include a chloroplast targeting region, a basic region, and a large region containing 11 complete leucine-rich repeats, suggest that it is similar to a bZIP PEND (plastid envelope DNA-binding protein) protein, a DNA binding protein found in the inner envelope membrane of developing chloroplasts. Here, we report that the BnMicEmUP3 overexpression in Arabidopsis increases the sensitivity of seedlings to exogenous abscisic acid (ABA). The BnMicEmUP proteins appear to be transcription factors that are localized in plastids and are involved in plant responses to biotic and abiotic environmental stresses; as well as the results obtained from this study can be used to improve crop yield.

Production of Homozygous Carrot (Daucus carota L.) Plants by Anther Culture

Carrot is a vegetable of increasing economic importance. New hybrid cultivars are constantly required to meet the changing market needs. The application of anther culture significantly shortens the difficult and long-lasting breeding of carrot. We examined all the stages of the process of generating androgenic plants: induction of embryos in anther cultures, regeneration and acclimatization of produced plants, their evaluation, ploidy and homozygosity, and many other factors affecting their effectiveness. Every factor has been optimized by experimentally selecting the optimal level. As a result, a full protocol of producing homozygous plants using anther cultures was developed, which is presented in this chapter.

Isolation of Staged and Viable Maize Microspores for DH Production

Isolated microspore culture systems have been designed in maize by several groups, mainly from the late 1980s to early 2000s. However, even with optimized protocols, microspore embryogenesis induction has remained very dependent on the genotype in maize, with elite germplasm generally displaying no response or very low response. Yet, these last few years, significant progress has been accomplished in understanding and controlling microspore embryogenesis induction in model dicot and monocot species. This knowledge may be transferred to maize, and isolated microspore culture may gain new interest in this crop, at least for embryogenesis research. The methods we hereby present in detail permit the purification of 3-12 × 10⁵ viable microspores per maize tassel, at the favorable stage for microspore embryogenesis. When cultured in appropriate liquid media, microspores from responsive genotypes give rise to androgenic embryos, which can then be regenerated into fertile doubled haploid 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.

Isolated Microspore Culture in Brassica napus

Isolated microspore culture is the most efficient technique among those used to induce microspore embryogenesis. In the particular case of Brassica napus, it is also the most widely used and optimized. In this chapter, we describe a protocol for microspore culture in B. napus which includes the steps necessary to isolate and culture microspores, to induce microspore-derived embryos, to produce doubled haploid plants from them, as well as to check for the developmental stage of the microspores isolated, their viability, and the ploidy level of regenerated plantlets.

Modeling callus induction and regeneration in an anther culture of tomato (Lycopersicon esculentum L.) using image processing and artificial neural network method

Doubled haploids, subsequent to haploid induction, have wide range of applications in basic and applied plant studies. Various parameters can affect the efficiency of haploid induction through an anther culture of tomato. The hybrid system of image processing-artificial neural network (ANN) was used to better understand callus induction and regeneration in an anther culture of tomato. The effect of parameters such as plant genotype, the concentrations of 2,4-dichlorophenoxyacetic acid (2,4-D) and kinetin (Kin) plant growth regulators, the concentration of gum arabic (GA) additive, the cold pretreatment duration, and flower length on callus induction percentage and number of regenerated calli in an anther culture of tomato were studied using multiple linear regression (MLR) and ANN models. The precise flower bud length was measured using an image processing technique. The 4',6-diamidino-2-phenylindole (DAPI) analysis showed that the flowers with 5-6.9 mm length had the highest percentage of the mid- to late-uninucleate microspore stage. The best ANN model for both callus induction percentage and number of regenerated calli was a model with one hidden layer, 12-15 neurons in the first hidden layer, Levenberg-Marquardt learning algorithm, and Tan-Sigmoid transfer function in hidden layer, based on the root mean square error (RMSE), mean absolute error (MAE), and coefficient of determination (R²) statistics. The scatter plot of measured values versus the predicted values showed the superiority of the ANN to MLR model to predict the callus induction percentage in an anther culture of tomato. The sensitivity analysis of MLR and ANN models revealed the plant genotype and 2,4-D concentration as the most important factors affecting both callus induction percentage and number of regenerated calli. Since tomato is a recalcitrant plant to androgenesis-based pathway of haploid induction, therefore the results of the present study can be helpful to develop an efficient haploid induction protocol in tomato through an anther culture pathway.

Genetic Loci Governing Androgenic Capacity in Perennial Ryegrass (Lolium perenne L.)

Immature pollen can be induced to switch developmental pathways from gametogenesis to embryogenesis and subsequently regenerate into homozygous, diploid plants. Such androgenic production of doubled haploids is particularly useful for species where inbreeding is hampered by effective self-incompatibility systems. Therefore, increasing the generally low androgenic capacity of perennial ryegrass (Lolium perenne L.) germplasm would enable the efficient production of homozygous plant material, so that a more effective exploitation of heterosis through hybrid breeding schemes can be realized. Here, we present the results of a genome-wide association study in a heterozygous, multiparental population of perennial ryegrass (n = 391) segregating for androgenic capacity. Genotyping-by-sequencing was used to interrogate gene- dense genomic regions and revealed over 1,100 polymorphic sites. Between one and 10 quantitative trait loci (QTL) were identified for anther response, embryo and total plant production, green and albino plant production and regeneration. Most traits were under polygenic control, although a major QTL on linkage group 5 was associated with green plant regeneration. Distinct genetic factors seem to affect green and albino plant recovery. Two intriguing candidate genes, encoding chromatin binding domains of the developmental phase transition regulator, Polycomb Repressive Complex 2, were identified. Our results shed the first light on the molecular mechanisms behind perennial ryegrass microspore embryogenesis and enable marker-assisted introgression of androgenic capacity into recalcitrant germplasm of this forage crop of global significance.

Dynamics of Calcium during In vitro Microspore Embryogenesis and In vivo Microspore Development in Brassica napus and Solanum melongena

Calcium is widely known to have a role as a signaling molecule in many different processes, including stress response and activation of the embryogenic program. However, there are no direct clues about calcium levels during microspore embryogenesis, an experimental process that combines a developmental switch toward embryogenesis and the simultaneous application of different stressing factors. In this work, we used FluoForte, a calcium-specific fluorescent vital dye, to track by confocal microscopy the changes in levels and subcellular distribution of calcium in living rapeseed (B. napus) and eggplant (S. melongena) microspores and pollen grains during in vivo development, as well as during the first stages of in vitro-induced microspore embryogenesis in rapeseed. During in vivo development, a clear peak of cytosolic Ca2+ was observed in rapeseed vacuolate microspores and young pollen grains, the stages more suitable for embryogenesis induction. However, the Ca2+ levels observed in eggplant were dramatically lower than in rapeseed. Just after in vitro induction, Ca2+ levels increased specifically in rapeseed embryogenic microspores at levels dramatically higher than during in vivo development. The increase was observed in the cytosol, but predominantly in vacuoles. Non-embryogenic forms such as callus-like and pollen-like structures presented remarkably different calcium patterns. After the heat shock-based inductive treatment, Ca2+ levels progressively decreased in all cases. Together, our results reveal unique calcium dynamics in in vivo rapeseed microspores, as well as in those reprogrammed to in vitro embryogenesis, establishing a link between changes in Ca2+ level and subcellular distribution, and microspore embryogenesis.

Dynamics of post-translationally modified histones during barley pollen embryogenesis in the presence or absence of the epi-drug trichostatin A

Improving pollen embryogenesis. Despite the agro-economic importance of pollen embryogenesis, the mechanisms underlying this process are still poorly understood. We describe the dynamics of chromatin modifications (histones H3K4me2, H3K9ac, H3K9me2, and H3K27me3) and chromatin marks (RNA polymerase II CDC phospho-Ser5, and CENH3) during barley pollen embryogenesis. Immunolabeling results show that, in reaction to stress, immature pollen rapidly starts reorganizing several important chromatin modifications indicative of a change in cell fate. This new chromatin modification pattern was accomplished within 24 h from whereon it remained unaltered during subsequent mitotic activity. This indicates that cell fate transition, the central element of pollen embryogenesis, is completed early on during the induction process. Application of the histone deacetylase inhibitor trichostatin A stimulated pollen embryogenesis when used on pollen with a gametophytic style chromatin pattern. However, when this drug was administered to embryogenic pollen, the chromatin markers reversed toward a gametophytic profile, embryogenesis was halted and all pollen invariably died.

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.

Global DNA methylation variations after short-term heat shock treatment in cultured microspores of Brassica napus cv. Topas

Heat stress can induce the cultured microspores into embryogenesis. In this study, whole genome bisulphite sequencing was employed to study global DNA methylation variations after short-term heat shock (STHS) treatments in cultured microspores of Brassica napus cv. Topas. Our results indicated that treatment on cultured Topas microspores at 32 °C for 6 h triggered DNA hypomethylation, particularly in the CG and CHG contexts. And the total number of T32 (Topas 32 °C for 6 h) vs. T0 (Topas 0 h) differentially methylated region-related genes (DRGs) was approximately two-fold higher than that of T18 (Topas 18 °C for 6 h) vs. T0 DRGs, which suggested that 32 °C might be a more intense external stimulus than 18 °C resulting in more changes in the DNA methylation status of cultured microspores. Additionally, 32 °C treatment for 6 h led to increased CHG differential methylations of transposons (DMTs), which were mainly constituted by overlaps between the hypomethylated differentially methylated regions (hypo-DMRs) and transposon elements (TEs). Further analysis demonstrated that the DRGs and their paralogs exhibited differential methylated/demethylated patterns. To summarize, the present study is the first methylome analysis of cultured microspores in response to STHS and may provide valuable information on the roles of DNA methylation in heat response.

Analysis of wheat microspore embryogenesis induction by transcriptome and small RNA sequencing using the highly responsive cultivar "Svilena"

BACKGROUND

Microspore embryogenesis describes a stress-induced reprogramming of immature male plant gametophytes to develop into embryo-like structures, which can be regenerated into doubled haploid plants after whole genome reduplication. This mechanism is of high interest for both research as well as plant breeding. The objective of this study was to characterize transcriptional changes and regulatory relationships in early stages of cold stress-induced wheat microspore embryogenesis by transcriptome and small RNA sequencing using a highly responsive cultivar.

RESULTS

Transcriptome and small RNA sequencing was performed in a staged time-course to analyze wheat microspore embryogenesis induction. The analyzed stages were freshly harvested, untreated uninucleate microspores and the two following stages from in vitro anther culture: directly after induction by cold-stress treatment and microspores undergoing the first nuclear divisions. A de novo transcriptome assembly resulted in 29,388 contigs distributing to 20,224 putative transcripts of which 9,305 are not covered by public wheat cDNAs. Differentially expressed transcripts and small RNAs were identified for the stage transitions highlighting various processes as well as specific genes to be involved in microspore embryogenesis induction.

CONCLUSION

This study establishes a comprehensive functional genomics resource for wheat microspore embryogenesis induction and initial understanding of molecular mechanisms involved. A large set of putative transcripts presumably specific for microspore embryogenesis induction as well as contributing processes and specific genes were identified. The results allow for a first insight in regulatory roles of small RNAs in the reprogramming of microspores towards an embryogenic cell fate.

Molecular characterization and expression analysis of SERK1 and SERK2 in Brassica napus L.: implication for microspore embryogenesis and plant regeneration

The BnSERK1 and BnSERK2 are involved in the process of microspore embryogenesis induction, development, and plantlet regeneration. Little is known about regulatory role of somatic embryogenesis-related kinase (SERK) genes family in the induction of microspore embryogenesis, development and plant regeneration. In this study, the expression of two SERK genes (SERK1 and SERK2) was assessed during the microspore embryogenesis and plantlet regeneration in Brassica napus L. The BnSERK1 was severely up-regulated 1-5 days following microspore culture and its expression drastically decreased in the globular-heart and also torpedo staged microspore-derived embryos (MDEs). In addition, high levels of BnSERK1 transcript were detected in the MDE maturation phase and in the roots and shoots of the regenerated plantlets which indicates a broader role(s) of BnSERK1 in the organ formation, rather than being specific to the embryogenesis. Results of partial sequencing indicated that the BnSERK1 shares a conserved serine-threonine kinase catalytic domain and exhibited 95 % similarity with AtSERK1, CsSERK1, BrSERK1, NaSERK1, and NbSERK1. A steady increase in the expression of BnSERK2 was observed during the MDE initiation and development so that, the highest expression was noted in the MDE maturation phase i.e., late cotyledonary MDEs. Our results also indicated low amounts of BnSERK2 transcript at the onset of rhyzogenesis but significantly higher expression in the developing roots. In contrast, the BnSERK2 strongly up-regulated during the both initially and developed shoots. The BnSERK2 shares highly conserved LRR-RLK domain when compared with different species tested so that, high homology (100 %) was noticed with BrSERK2. Based on our findings, MDE formation and plantlet regeneration seem to be correlated with both BnSERK1 and BnSERK2 expression.

Androgenesis in Solanaceae

The Solanaceae is one of the most important families for global agriculture. Among the different solanaceous species, tobacco (Nicotiana tabacum), potato (Solanum tuberosum), tomato (Solanum lycopersicum), eggplant (Solanum melongena), and pepper (Capsicum annuum) are five crops of outstanding importance worldwide. In these crops, maximum yields are produced by hybrid plants created by crossing pure (homozygous) lines with the desired traits. Pure lines may be produced by conventional breeding methods, which is time consuming and costly. Alternatively, it is possible to accelerate the production of pure lines by creating doubled haploid (DH) plants derived from (haploid) male gametophytes or their precursors (androgenesis). In this way, the different steps for the production of pure lines can be reduced to only one generation, which implies important time and cost savings. This and other advantages make androgenic DHs the choice in a number of important crops where any of the different experimental in vitro techniques (anther culture or isolated microspore culture) is well set up. The Solanaceae family is an excellent example of heterogeneity in terms of response to these techniques, including highly responding species such as tobacco, considered a model system, and tomato, one of the most recalcitrant species, where no reliable and reproducible methods are yet available. Interestingly, the first evidence of androgenesis, particularly through in vitro anther culture, was demonstrated in a solanaceous species, Datura innoxia. In this chapter, we report the state of the art of the research about androgenic DHs in Solanaceae, paying special attention to datura, tobacco, potato, tomato, eggplant, and pepper.

Anther Culture in Pepper (Capsicum annuum L.)

Anther culture is the most popular of the techniques used to induce microspore embryogenesis. This technique is well set up in a wide range of crops, including pepper. In this chapter, a protocol for anther culture in pepper is described. The protocol presented hereby includes the steps from the selection of buds from donor plants to the regeneration and acclimatization of doubled haploid plants derived from the embryos, as well as a description of how to analyze the ploidy level of the regenerated plants.

Haploids: Constraints and opportunities in plant breeding

The discovery of haploids in higher plants led to the use of doubled haploid (DH) technology in plant breeding. This article provides the state of the art on DH technology including the induction and identification of haploids, what factors influence haploid induction, molecular basis of microspore embryogenesis, the genetics underpinnings of haploid induction and its use in plant breeding, particularly to fix traits and unlock genetic variation. Both in vitro and in vivo methods have been used to induce haploids that are thereafter chromosome doubled to produce DH. Various heritable factors contribute to the successful induction of haploids, whose genetics is that of a quantitative trait. Genomic regions associated with in vitro and in vivo DH production were noted in various crops with the aid of DNA markers. It seems that F2 plants are the most suitable for the induction of DH lines than F1 plants. Identifying putative haploids is a key issue in haploid breeding. DH technology in Brassicas and cereals, such as barley, maize, rice, rye and wheat, has been improved and used routinely in cultivar development, while in other food staples such as pulses and root crops the technology has not reached to the stage leading to its application in plant breeding. The centromere-mediated haploid induction system has been used in Arabidopsis, but not yet in crops. Most food staples are derived from genomic resources-rich crops, including those with sequenced reference genomes. The integration of genomic resources with DH technology provides new opportunities for the improving selection methods, maximizing selection gains and accelerate cultivar development. Marker-aided breeding and DH technology have been used to improve host plant resistance in barley, rice, and wheat. Multinational seed companies are using DH technology in large-scale production of inbred lines for further development of hybrid cultivars, particularly in maize. The public sector provides support to national programs or small-medium private seed for the exploitation of DH technology in plant breeding.

Effects of Heat Shock and 2,4-D Treatment on Morphological and Physiological Characteristics of Microspores and Microspore-Derived Doubled Haploid Plants in Brassica napus L

BACKGROUND

Stresses such as heat shock, starvation, or osmotic is essential to lead isolated microspores towards embryogenesis. Despite the effectiveness of stresses in embryogenesis, they exert adverse effects on metabolism and growth of the regenerated plants.

OBJECTIVES

The effects of heat shock and 2,4-D treatment on total protein content of treated microspores, morphological and physiological characteristics of the doubled haploid (DH) plants were assessed.

MATERIALS AND METHODS

Buds containing mid- to late- uninucleate microspores were used for microspore culture. Microspores were isolated and cultured in NLN-13 medium and incubated at 30ºC for 14 days or treated with 2,4-D (35 mg.L^-1) for 30 min to induce embryogenesis. Microspore-derived embryos were transferred onto B5 medium for plantlet regeneration. Ploidy level of the regenerated plantlets was determined using Partec flow cytometry. Spectrophotometric readings were carried out at 490, 663 and 645 nm to determine Chl a-b and carotenoids contents. TRIzol and cetyl-threeethyl-ammonium bromide (CTAB) were used for protein extraction from microspores and leaves. Length and width of stomata and pollen grains were also photographed using light microscope (Olympus).

RESULTS

Applied stressors significantly reduced total protein content of treated microspores however, protein content and concentration of chlorophyll a and b of the DH plants were only increased by heat shock treatment when compared with the donor plant 'Hyola 420'. In contrast, carotenoids were not affected by applied stressors. Longer and wider stomata were observed by 2,4-D treatment but, the length of pollen grains was significantly decreased following heat shock and 2,4-D treatment.

CONCLUSIONS

Total protein content of cultured microspores, concentration of chlorophyll a and b, length and width of stomata of microspore-derived doubled haploid plants were significantly affected by the type of inductive stresses. However, carotenoids were more stable and not affected by applied stressors.

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.

Formation and excretion of autophagic plastids (plastolysomes) in Brassica napus embryogenic microspores

The change in developmental fate of microspores reprogrammed toward embryogenesis is a complex but fascinating experimental system where microspores undergo dramatic changes derived from the developmental switch. After 40 years of study of the ultrastructural changes undergone by the induced microspores, many questions are still open. In this work, we analyzed the architecture of DNA-containing organelles such as plastids and mitochondria in samples of B. napus isolated microspore cultures covering the different stages before, during, and after the developmental switch. Mitochondria presented a conventional oval or sausage-like morphology for all cell types studied, similar to that found in vivo in other cell types from vegetative parts. Similarly, plastids of microspores before induction and of non-induced cells showed conventional architectures. However, approximately 40% of the plastids of embryogenic microspores presented atypical features such as curved profiles, protrusions, and internal compartments filled with cytoplasm. Three-dimensional reconstructions confirmed that these plastids actually engulf cytoplasm regions, isolating them from the rest of the cell. Acid phosphatase activity was found in them, confirming the lytic activity of these organelles. In addition, digested plastid-like structures were found excreted to the apoplast. All these phenomena seemed transient, since microspore-derived embryos (MDEs) showed conventional plastids. Together, these results strongly suggested that under special circumstances, such as those of the androgenic switch, plastids of embryogenic microspores behave as autophagic plastids (plastolysomes), engulfing cytoplasm for digestion, and then are excreted out of the cytoplasm as part of a cleaning program necessary for microspores to become embryos.

A roadmap to embryo identity in plants

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

Timing of the G1/S transition in tobacco pollen vegetative cells as a primary step towards androgenesis in vitro

KEY MESSAGE

Mid-bicellular pollen vegetative cells in tobacco escape from G1 arrest and proceed to the G1/S transition towards androgenesis within 1 day under glutamine starvation conditions in vitro. In the Nicotiana tabacum pollen culture system, immature pollen grains at the mid-bicellular stage can mature in the presence of glutamine; however, if glutamine is absent, they deviate from their native cell fate in a few days. The glutamine-starved pollen grains cannot undergo maturation, even when supplied with glutamine later. Instead, they undergo cell division towards androgenesis slowly within 10 days in a medium containing appropriate nutrients. During the culture period, they ought to escape from G1 arrest to proceed into S phase as the primary step towards androgenesis. However, this event has not been experimentally confirmed. Here, we demonstrated that the pollen vegetative cells proceeded to the G1/S transition within approximately 15-36 h after the start of culture. These results were obtained by analyzing transgenic pollen possessing a fusion gene encoding nuclear-localizing GFP under the control of an E2F motif-containing promoter isolated from a gene encoding one of DNA replication licensing factors. Observations using a 5-ethynyl-2'-deoxyuridine DNA labeling and detection technique uncovered that the G1/S transition was soon followed by S phase. These hallmarks of vegetative cells undergoing dedifferentiation give us new insights into upstream events causing the G1/S transition and also provide a novel strategy to increase the frequency of the androgenic response in tobacco and other species, including recalcitrants.

Somatic embryogenesis - Stress-induced remodeling of plant cell fate

Plants as sessile organisms have remarkable developmental plasticity ensuring heir continuous adaptation to the environment. An extreme example is somatic embryogenesis, the initiation of autonomous embryo development in somatic cells in response to exogenous and/or endogenous signals. In this review I briefly overview the various pathways that can lead to embryo development in plants in addition to the fertilization of the egg cell and highlight the importance of the interaction of stress- and hormone-regulated pathways during the induction of somatic embryogenesis. Somatic embryogenesis can be initiated in planta or in vitro, directly or indirectly, and the requirement for dedifferentiation as well as the way to achieve developmental totipotency in the various systems is discussed in light of our present knowledge. The initiation of all forms of the stress/hormone-induced in vitro as well as the genetically provoked in planta somatic embryogenesis requires extensive and coordinated genetic reprogramming that has to take place at the chromatin level, as the embryogenic program is under strong epigenetic repression in vegetative plant cells. Our present knowledge on chromatin-based mechanisms potentially involved in the somatic-to-embryogenic developmental transition is summarized emphasizing the potential role of the chromatin to integrate stress, hormonal, and developmental pathways leading to the activation of the embryogenic program. The role of stress-related chromatin reorganization in the genetic instability of in vitro cultures is also discussed. This article is part of a Special Issue entitled: Stress as a fundamental theme in cell plasticity.

Characterization of polarity development through 2- and 3-D imaging during the initial phase of microspore embryogenesis in Brassica napus L

Isolated microspores of B. napus in culture change their developmental pathway from gametophytic to sporophytic and form embryo-like structures (ELS) upon prolonged heat shock treatment (5 days at 32 °C). ELS express polarity during the initial days of endosporic development. In this study, we focussed on the analysis of polarity development of ELS without suspensor. Fluorescence microscopy and 3-D confocal laser scanning microscopy (CLSM) without tissue interfering enabled us to get a good insight in the distribution of nuclei, mitochondria and endoplasmic reticulum (ER), the architecture of microtubular (MT) cytoskeleton and the places of 5-bromo-2'-deoxy-uridine (BrdU) incorporation in successive stages of microspore embryogenesis. Scanning electron microscopy (SEM) analysis revealed, for the first time, the appearance of a fibrillar extracellular matrix-like structure (ECM-like structure) in androgenic embryos without suspensor. Two types of endosporic development were distinguished based upon the initial location of the microspore nucleus. The polarity of dividing and growing cells was recognized by the differential distributions of organelles, by the organization of the MT cytoskeleton and by the visualization of DNA synthesis in the cell cycle. The directional location of nuclei, ER, mitochondria and starch grains in relation to the MTs configurations were early polarity indicators. Both exine rupture and ECM-like structure on the outer surfaces of ELS are supposed to stabilize ELS's morphological polarity. As the role of cell polarity during early endosporic microspore embryogenesis in apical-basal cell fate determination remains unclear, microspore culture system provides a powerful in vitro tool for studying the developmental processes that take place during the earliest stages of plant embryogenesis.

The histone deacetylase inhibitor trichostatin a promotes totipotency in the male gametophyte

The haploid male gametophyte, the pollen grain, is a terminally differentiated structure whose function ends at fertilization. Plant breeding and propagation widely use haploid embryo production from in vitro-cultured male gametophytes, but this technique remains poorly understood at the mechanistic level. Here, we show that histone deacetylases (HDACs) regulate the switch to haploid embryogenesis. Blocking HDAC activity with trichostatin A (TSA) in cultured male gametophytes of Brassica napus leads to a large increase in the proportion of cells that switch from pollen to embryogenic growth. Embryogenic growth is enhanced by, but not dependent on, the high-temperature stress that is normally used to induce haploid embryogenesis in B. napus. The male gametophyte of Arabidopsis thaliana, which is recalcitrant to haploid embryo development in culture, also forms embryogenic cell clusters after TSA treatment. Genetic analysis suggests that the HDAC protein HDA17 plays a role in this process. TSA treatment of male gametophytes is associated with the hyperacetylation of histones H3 and H4. We propose that the totipotency of the male gametophyte is kept in check by an HDAC-dependent mechanism and that the stress treatments used to induce haploid embryo development in culture impinge on this HDAC-dependent pathway.

A milestone in the doubled haploid pathway of cassava: a milestone in the doubled haploid pathway of cassava (Manihot esculenta Crantz): cellular and molecular assessment of anther-derived structures

This study was aimed at inducing androgenesis in cultured anthers of cassava (Manihot esculenta Crantz) to develop a protocol for the production of doubled haploids. Microspore reprogramming was induced in cassava by cold or heat stress of anthers. Since the anthers contain both haploid microspores and diploid somatic cells, it was essential to verify the origin of anther-derived calli. The origin of anther-derived calli was assessed by morphological screening followed by histological analysis and flow cytometry (FCM). Additionally, simple sequence repeat (SSR) and amplified fragmented length polymorphism (AFLP) assays were used for the molecular identification of the microspore-derived calli. The study clearly demonstrated the feasibility of producing microspore-derived calli using heat- or cold-pretreated anthers. Histological studies revealed reprogramming of the developmental pathway of microspores by symmetrical division of the nucleus. Flow cytometry analysis revealed different ploidy level cell types including haploids, which confirmed their origin from the microspores. The SSR and AFLP marker assays independently confirmed the histological and FCM results of a haploid origin of the calli at the DNA level. The presence of multicellular microspores in the in vitro system indicated a switch of developmental program, which constitutes a crucial step in the design of protocols for the regeneration of microspore-derived embryos and plants. This is the first detailed report of calli, embryos, and abnormal shoots originated from the haploid cells in cassava, leading to the development of a protocol for the production of doubled haploid plants in cassava.

Microspore embryogenesis in wheat: new marker genes for early, middle and late stages of embryo development

Microspore embryogenesis involves reprogramming of the pollen immature cell towards embryogenesis. We have identified and characterized a collection of 14 genes induced along different morphological phases of microspore-derived embryo development in wheat (Triticum aestivum L.) anther culture. SERKs and FLAs genes previously associated with somatic embryogenesis and reproductive tissues, respectively, were also included in this analysis. Genes involved in signalling mechanisms such as TaTPD1-like and TAA1b, and two glutathione S-transferase (GSTF2 and GSTA2) were induced when microspores had acquired a 'star-like' morphology or had undergone the first divisions. Genes associated with control of plant development and stress response (TaNF-YA, TaAGL14, TaFLA26, CHI3, XIP-R; Tad1 and WALI6) were activated before exine rupture. When the multicellular structures have been released from the exine, TaEXPB4, TaAGP31-like and an unknown embryo-specific gene TaME1 were induced. Comparison of gene expression, between two wheat cultivars with different response to anther culture, showed that the profile of genes activated before exine rupture was shifted to earlier stages in the low responding cultivar. This collection of genes constitutes a value resource for study mechanism of intra-embryo communication, early pattern formation, cell wall modification and embryo differentiation.

Novel features of Brassica napus embryogenic microspores revealed by high pressure freezing and freeze substitution: evidence for massive autophagy and excretion-based cytoplasmic cleaning

Induction of embryogenesis from isolated microspore cultures is a complex experimental system where microspores undergo dramatic changes in developmental fate. After ~40 years of application of electron microscopy to the study of the ultrastructural changes undergone by the induced microspore, there is still room for new discoveries. In this work, high pressure freezing and freeze substitution (HPF/FS), the best procedures known to date for ultrastructural preservation, were used to process Brassica napus microspore cultures covering all the stages of microspore embryogenesis. Analysis of these cultures by electron microscopy revealed massive processes of autophagy exclusively in embryogenic microspores, but not in other microspore-derived structures also present in cultures. However, a significant part of the autophagosomal cargo was not recycled. Instead, it was transported out of the cell, producing numerous deposits of extracytoplasmic fibrillar and membranous material. It was shown that commitment of microspores to embryogenesis is associated with both massive autophagy and excretion of the removed material. It is hypothesized that autophagy would be related to the need for a profound cytoplasmic cleaning, and excretion would be a mechanism to avoid excessive growth of the vacuolar system. Together, the results also demonstrate that the application of HPF/FS to the study of the androgenic switch is the best option currently available to identify the complex and dramatic ultrastructural changes undergone by the induced microspore. In addition, they provide significant insights to understand the cellular basis of induction of microspore embryogenesis, and open a new door for the investigation of this intriguing developmental pathway.

Androgenesis-inducing stress treatments change phytohormone levels in anthers of three legume species (Fabaceae)

Legumes are recalcitrant to androgenesis and induction protocols were only recently developed for pea (Pisum sativum L.) and chickpea (Cicer arietinum L.), albeit with low regeneration frequencies. Androgenesis is thought to be mediated through abscisic acid (ABA) but other phytohormones, such as auxins, cytokinins, and gibberellins, have also been implicated. In view of improving induction protocols, the hormone content of pea, chickpea, and lentil anthers was measured after exposure to cold, centrifugation, electroporation, sonication, osmotic shock, or various combinations thereof using an analytical mass spectrometer. Indole-3-acetic acid (IAA) had a key function during the induction process. In pea, high concentrations of IAA-asparagine (IAA-Asp), a putative IAA metabolite, accumulated during the application of the different stresses. In chickpea, the IAA-Asp concentration increased 30-fold compared to pea but only during the osmotic shock treatment and likely as a result of the presence of exogenous IAA in the medium. In contrast, no treatment in lentil (Lens culinaris) invoked such an increase in IAA-Asp content. Of the various cytokinins monitored, only cis zeatin riboside increased after centrifugation and electroporation in pea and possibly chickpea. No bioactive gibberellins were detected in any species investigated, indicating that this hormone group is likely not linked to androgenesis in legumes. In contrast to the other stresses, osmotic shock treatment caused a reduction in the levels of all hormones analyzed, with the exception of IAA-Asp in chickpea. A short period of low hormone content might be a necessary transition phase for androgenesis induction of legumes.

KEY MESSAGE

Five androgenesis-inducing stress treatments changed content of ABA, auxin and cytokinin in anthers of three legumes. Osmotic shock treatment differed because it reduced hormone content to very low levels.

Haploid and doubled haploid plants from developing male and female gametes of Gentiana triflora

Protocols were developed for the generation of haploid or doubled haploid plants from developing microspores and ovules of Gentiana triflora. Plant regeneration was achieved using flower buds harvested at the mid to late uninucleate stages of microspore development and then treated at 4°C for 48 h prior to culture. Anthers and ovaries were cultured on modified Nitsch and Nitsch medium supplemented with a combination of naphthoxyacetic acid and benzylaminopurine. The explants either regenerated new plantlets directly or produced callus that regenerated into plantlets upon transfer to basal media supplemented with benzylaminopurine. Among seven genotypes of different ploidy levels used, 0-32.6% of cultured ovary pieces and 0-18.4% of cultured anthers regenerated plants, with all the genotypes responding either through ovary or anther culture. Flow cytometry confirmed that 98% of regenerated plants were either diploid or haploid. Diploid regenerants were shown to be gamete-derived by observing parental band loss using RAPD markers. Haploid plants were propagated on a proliferation medium and then treated with oryzalin for 4 weeks before transfer back to proliferation medium. Most of the resulting plants were diploids. Over 150 independently derived diploidised haploid plants have been deflasked. The protocol has been successfully used to regenerate plants from developing gametes of seven different diploid, triploid and tetraploid G. triflora genotypes.

A change of developmental program induces the remodeling of the interchromatin domain during microspore embryogenesis in Brassica napus L

After a stress treatment, in vitro-cultured pollen changes its normal gametophytic developmental pathway towards embryogenesis producing multicellular embryos from which, finally, haploid and double haploid plants develop. The architecture of the well-organized nuclear functional domains changes in response to DNA replication, RNA transcription, processing and transport dynamics. A number of subnuclear structures present in the interchromatin region (IR, the nuclear domain between chromosome territories) have been shown as involved, either directly or indirectly, in transcriptional regulation. These structures include the interchromatin granule clusters (IGCs), perichromatin fibrils (PFs), Cajal bodies (CBs) and perichromatin granules (PGs). In this work, we present a cytochemical, immunocytochemical, quantitative and morphometric analysis at the light, confocal and electron microscopy levels to characterize the changes in the functional architecture of the nuclear interchromatin domain during two developmental programs followed by the microspore: differentiation to mature pollen grains (transcriptionally inactive), and microspore embryogenesis involving proliferation in the first stages (highly engaged in transcription). Our results revealed characteristic changes in size, shape and distribution of the different interchromatin structures as a consequence of the reprogramming of the microspore, allowing us to relate the remodeling of the interchromatin domain to the variations in transcriptional activities during proliferation and differentiation events, and suggesting that RNA-associated structures could be a regulatory mechanism in the process. In addition, we document the presence of two structurally different types of CBs, and of IGC and CB-associated regions, similar to those present in animal cells, and not yet described in plants.

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.

Microspore embryogenesis: assignment of genes to embryo formation and green vs. albino plant production

Plant microspores can be reprogrammed from their normal pollen development to an embryogenic route in a process termed microspore embryogenesis or androgenesis. Stress treatment has a critical role in this process, inducing the dedifferentiation of microspores and conditioning the following androgenic response. In this study, we have used three barley doubled haploid lines with similar genetic background but different androgenic response. The Barley1 GeneChip was used for transcriptome comparison of these lines after mannitol stress treatment, allowing the identification of 213 differentially expressed genes. Most of these genes belong to the functional categories "cell rescue, defense, and virulence"; "metabolism"; "transcription"; and "transport". These genes were grouped into clusters according to their expression profiles among lines. A principal component analysis allowed us to associate specific gene expression clusters to phenotypic variables. Genes associated with the ability of microspores to divide and form embryos were mainly involved in changes in the structure and function of membranes, efficient use of available energy sources, and cell fate. Genes related to stress response, transcription and translation regulation, and degradation of pollen-specific proteins were associated with green plant production, while expression of genes related to plastid development was associated with albino plant regeneration.

Histological comparison between wheat embryos developing in vitro from isolated zygotes and those developing in vivo

There is currently great interest shown in understanding the process of embryogenesis and, due to the relative inaccessibility of these structures in planta, extended studies are carried out in various in vitro systems. The culture of isolated zygotes in particular provides an excellent platform to study the process of in planta embryogenesis. However, very few comparisons have been made between zygotic embryos grown entirely in cultures and those grown in vivo. The present study analyses the differences and similarities between the in vitro and in vivo development of wheat zygotic embryos at the level of morphology and histology. The study was possible thanks to an efficient culture system and an appropriate method of preparing isolated wheat zygotes for microscopy. The in vitro embryos were fixed, embedded and sectioned in the two-celled, globular, club-shaped and fully differentiated stages. Embryos developing in vitro closely followed the morphology of their in planta counterparts and their cell types and tissues were also similar, demonstrating the applicability of the present culture system for studying the process of zygotic embryogenesis. However, some important differences were also detected in the case of in vitro development: the disturbance of or lack of initial polarity led to changes in the division symmetry of the zygotes and subsequently to the formation of uniform cells in the globular structures. Presumably, differences between the in vitro and in planta environments resulted in a lower level of differentiation and maturation in in vitro embryos and in abundant starch and protein accumulation in the scutellum.