Microspore-derived Embryos Research Articles

Current Status of Haploidization in Cool-Season Grain Legume Crop Species

Doubled haploid technology is, so far, the fastest route to induce a true homozygous state in plants. True homozygous plants are particularly important for breeders, as they can facilitate hybrid breeding and are useful in fixing traits in a breeding line. Fabaceae species are of great importance in food and feed production; however, they are far behind other families with respect to the development of effective haploidization protocols. Here, we present the most recent status of research on haploidization protocols in cool-season grain legume crops, including dry peas, chickpeas, faba beans, lentils, lupines, and grass peas. The first four species are primarily for human consumption; the latter are utilized as forage. All the mentioned species have been subject to haploidization trials; however, repeatable protocols, including the regeneration of confirmed haploid or doubled haploid plants, have not been elaborated. Research in field pea, chickpea, grass pea, and lupine is promising, with the reported regeneration of microspore-derived embryos in all four species. Repeatable plant regeneration has been reported only in field peas and chickpeas. The most recent achievements on haploidization through male and female gametophytes in faba bean are also presented. The key factors for the effective stimulation of haploid cell development in cool-season legumes are reviewed, providing a useful basis for future efforts toward haploidization in this group.

Anther-derived microspore embryogenesis in pepper hybrids orobelle and Bomby

BackgroundTraditional breeding methods have long been employed worldwide for the evaluation and development of pepper cultivars. However, these methods necessitate multiple generations of screening, line development, evaluation, recognition, and crossing to obtain highly homozygous lines. In contrast, in vitro anther-derived microspore culture represents a rapid method to generate homozygous lines within a single generation. In the present study, we have optimized a protocol for microspore embryogenesis from anther cultures of pepper hybrids Orobelle and Bomby.ResultsWe achieved early and successful embryo formation from both genotypes by subjecting the buds to a cold pretreatment at 4 °C for 4 days. Our optimized culture medium, comprised of MS medium supplemented with 4 mg/L NAA, 1 mg/L BAP, 0.25% activated charcoal, 2.6 g/L gelrite, 30 g/L sucrose, and 15 mg/L silver nitrate, exhibited the highest efficiency in embryo formation (1.85% and 1.46%) for Orobelle and Bomby, respectively. Furthermore, successful plant regeneration from the anther derived microspore embryos was accomplished using half-strength MS medium fortified with 2% sucrose and 0.1 mg/L 6-benzylaminopurine (BA), solidified with 2.6 g/L gelrite. The ploidy status of the microspore-derived plantlets was analyzed using flow cytometry technique. Notably, the haploid plants exhibited distinct characteristics such as reduced plant height, leaf length, leaf width, and shorter internode length when compared to their diploid counterparts derived from seeds.ConclusionOur findings highlight the potential of anther culture and microspore embryogenesis as an advanced method for accelerating pepper breeding programs, enabling the rapid production of superior homozygous lines.

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

BackgroundMicrospore culture is one of the important biotechnological tools in plant breeding. The induction of microspore embryogenesis is a critical factor that affects the yield of microspore-derived embryo productions. Cold treatment has been reported to reprogram the gametophytic pathway in various plant species. However, the exact mechanism(s) underlying the effect of cold pre-treatment of floral buds on the efficiency of ME is still not clear.ResultsIn this study, the effects of cold stress on the microspore totipotency of rice cultivar Zhonghua 11 were investigated. Our results revealed that a 10-day cold treatment is necessary for microspore embryogenesis initiation. During this period, the survival rate of microspores increased and reached a peak at 7 days post treatment (dpt), before decreasing at 10 dpt. RNA-seq analysis showed that the number of DEGs increased from 3 dpt to 10 dpt, with more downregulated DEGs than upregulated ones at the same time point. GO enrichment analysis showed a shift from ‘Response to abiotic stimulus’ at 3 dpt to ‘Metabolic process’ at 7 and 10 dpt, with the most significant category in the cellular component being ‘cell wall’. KEGG analysis of the pathways revealed changes during cold treatment. Mass spectrometry was used to evaluate the variations in metabolites at 10 dpt compared to 0 dpt, with more downregulated DEMs being determined in both GC-MS and LC-MS modes. These DEMs were classified into 11 categories, Most of the DEMs belonged to ‘lipids and lipid-like molecules’. KEGG analysis of DEMs indicates pathways related to amino acid and nucleotide metabolism being upregulated and those related to carbohydrate metabolism being downregulated. An integration analysis of transcriptomics and metabolomics showed that most pathways belonged to ‘Amino acid metabolism’ and ‘Carbohydrate metabolism’. Four DEMs were identified in the interaction network, with stearidonic acid involving in the most correlations, suggesting the potential role in microspore totipotency.ConclusionsOur findings exhibited the molecular events occurring during stress-induced rice microspore. Pathways related to ‘Amino acid metabolism’ and ‘Carbohydrate metabolism’ may play important roles in rice microspore totipotency. Stearidonic acid was identified, which may participate in the initiation of microspore embryogenesis.

ФАК ТОРЫ ПРЯМОГО ПРОРАСТАНИЯ МИКРОСПОРОГЕННЫХ ЭМБРИОИДОВ BRASSICA NAPUS L.

Isolated microspore culture is the main method of producing doubled rapeseed haploids and is widely used in research institutions and commercial companies. The protocol of rapeseed embryo production is well developed and efficient for many genotypes, but some issues remain due to the low regeneration frequency of plantlets from embryos. When the standard protocol is applied, regeneration of plantlets from microspore-derived embryos usually involves a callus-forming stage followed by regeneration of adventitious shoots or secondary embryos, which prolong the period of plantlet regeneration and makes production of doubled haploids complicated. Identifying factors, which affect the frequency of direct embryo germination will increase the frequency of plantlet formation and reduce the period of DH plant production. In this work, we studied the effect of medium pH on the duration of plantlet regeneration from rapeseed microspore-derived embryos and effect of their low temperature treatment of +1 and +5℃ for 3, 6, 8, 9, 12 days in complete darkness on the embryo maturation and germination. Rising the pH of the nutrient medium from 5.8 to 6.1 increased the frequency of direct embryos germination up to 18% and the overall frequency of plantlet regeneration up to 76%. Culturing embryos at low temperatures effected the frequency of direct germination of embryos into plantlets. The maximum frequency of 44–53% direct embryo germination was observed when cultured at +1℃ for 6 and 9 days, when embryos were cultured at +5℃ the frequency of direct germination was 0–10%. In the control variant without cold treatment it was 16%.

Quantitative and qualitative study of endogenous and exogenous growth regulators in eggplant (Solanum melongena) microspore cultures

In eggplant microspore embryogenesis, embryos are produced and then transformed into undifferentiated calli, instead of developing as true embryos. This is the main current bottleneck that precludes this process from being efficient. In this work we aimed to shed light on the factors involved in the successful in vitro development of eggplant haploid embryos by evaluating the role of growth regulators (GRs) in this process. We analyzed the endogenous levels of different GRs, including auxins, cytokinins and gibberelins, as well as salicylic, jasmonic and abscisic acid, in microspores and microspore-derived embryos at different culture stages. We also analyzed the same GR profiles in leaf and anther wall tissues of different eggplant backgrounds. Finally, we assessed the application of different GR combinations to the culture medium. Our results showed that in eggplant there are no genotype-specific endogenous GR profiles that can be associated to a high embryogenic response. Instead, the embryogenic response seems related to different GR accumulation patterns during in vitro culture. The changes observed in the endogenous levels of salicylic and abscisic acid were not related to the embryo transition. There were, however, changes in the levels of indole acetic acid and dihydrozeatin. The best GR combination to promote callus production was 0.5 mg/L 1-naphthaleneacetic acid (NAA) and 0.5 mg/L 6-benzylaminopurine (BAP). A 20% reduction of NAA and BAP reduced embryo production but produced structures more anatomically similar to embryos. These results shed light on the role of GRs during the development of microspore-derived embryos in eggplant microspore cultures.

The Resistance of Oilseed Rape Microspore-Derived Embryos to Osmotic Stress Is Associated With the Accumulation of Energy Metabolism Proteins, Redox Homeostasis, Higher Abscisic Acid, and Cytokinin Contents.

The present study aims to investigate the response of rapeseed microspore-derived embryos (MDE) to osmotic stress at the proteome level. The PEG-induced osmotic stress was studied in the cotyledonary stage of MDE of two genotypes: Cadeli (D) and Viking (V), previously reported to exhibit contrasting leaf proteome responses under drought. Two-dimensional difference gel electrophoresis (2D-DIGE) revealed 156 representative protein spots that have been selected for MALDI-TOF/TOF analysis. Sixty-three proteins have been successfully identified and divided into eight functional groups. Data are available via ProteomeXchange with identifier PXD024552. Eight selected protein accumulation trends were compared with real-time quantitative PCR (RT-qPCR). Biomass accumulation in treated D was significantly higher (3-fold) than in V, which indicates D is resistant to osmotic stress. Cultivar D displayed resistance strategy by the accumulation of proteins in energy metabolism, redox homeostasis, protein destination, and signaling functional groups, high ABA, and active cytokinins (CKs) contents. In contrast, the V protein profile displayed high requirements of energy and nutrients with a significant number of stress-related proteins and cell structure changes accompanied by quick downregulation of active CKs, as well as salicylic and jasmonic acids. Genes that were suitable for gene-targeting showed significantly higher expression in treated samples and were identified as phospholipase D alpha, peroxiredoxin antioxidant, and lactoylglutathione lyase. The MDE proteome profile has been compared with the leaf proteome evaluated in our previous study. Different mechanisms to cope with osmotic stress were revealed between the genotypes studied. This proteomic study is the first step to validate MDE as a suitable model for follow-up research on the characterization of new crossings and can be used for preselection of resistant genotypes.

Changes in plastid biogenesis leading to the formation of albino regenerants in barley microspore culture

BackgroundMicrospore embryogenesis is potentially the most effective method of obtaining doubled haploids (DH) which are utilized in breeding programs to accelerate production of new cultivars. However, the regeneration of albino plants significantly limits the exploitation of androgenesis for DH production in cereals. Despite many efforts, the precise mechanisms leading to development of albino regenerants have not yet been elucidated. The objective of this study was to reveal the genotype-dependent molecular differences in chloroplast differentiation that lead to the formation of green and albino regenerants in microspore culture of barley.ResultsWe performed a detailed analysis of plastid differentiation at successive stages of androgenesis in two barley cultivars, ‘Jersey’ and ‘Mercada’ that differed in their ability to produce green regenerants. We demonstrated the lack of transition from the NEP-dependent to PEP-dependent transcription in plastids of cv. ‘Mercada’ that produced mostly albino regenerants in microspore culture. The failed NEP-to-PEP transition was associated with the lack of activity of Sig2 gene encoding a sigma factor necessary for transcription of plastid rRNA genes. A very low level of 16S and 23S rRNA transcripts and impaired plastid translation machinery resulted in the inhibition of photomorphogenesis in regenerating embryos and albino regenerants. Furthermore, the plastids present in differentiating ‘Mercada’ embryos contained a low number of plastome copies whose replication was not always completed. Contrary to ‘Mercada’, cv. ‘Jersey’ that produced 90% green regenerants, showed the high activity of PEP polymerase, the highly increased expression of Sig2, plastid rRNAs and tRNAGlu, which indicated the NEP inhibition. The increased expression of GLKs genes encoding transcription factors required for induction of photomorphogenesis was also observed in ‘Jersey’ regenerants.ConclusionsProplastids present in microspore-derived embryos of albino-producing genotypes did not pass the early checkpoints of their development that are required for induction of further light-dependent differentiation of chloroplasts. The failed activation of plastid-encoded RNA polymerase during differentiation of embryos was associated with the genotype-dependent inability to regenerate green plants in barley microspore culture. The better understanding of molecular mechanisms underlying formation of albino regenerants may be helpful in overcoming the problem of albinism in cereal androgenesis.

ВЛИЯНИЕ УСЛОВИЙ КУЛЬТИВИРОВАНИЯ НА ЧАСТОТУ ПРОРАСТАНИЯ/РЕГЕНЕРАЦИИ МИКРОСПОРОГЕННЫХ ЭМБРИОИДОВ BRASSICA OLERACEA L.

Most genotypes of white cabbage (Brassica oleracea L.) have a low frequency of microspore-derived embryos germination and regeneration through adventitious shoots or secondary embryogenesis. Increasing the frequency of regeneration and the formation of seedlings can ensure the production of a large number of doubled haploids. This work studied the effects of gelling agents, such as agar (11 g/l), agargel (4.5 g/l), and phytagel (2 g/l), on the frequency of regeneration/germination of microspore-derived embryos of white cabbage (B. oleracea L.), as well as the effect of a cold incubation of kohlrabi cabbage embryos (B. oleracea var. gongylodes L.) at 5°C for 3, 6, 9, 12 and 15 days in complete darkness. The studies showed that when culturing embryos on regeneration medium B5 solidified with agar, the embryo germination and seedling regeneration were higher than those on media solidified with agargel and phytagel. No embryo germination but vitrification of embryo tissues was observed on media containing agargel and phytagel. Cold incubation of Kohlrabi cabbage embryos at 5°C for 3, 6 and 9 days in complete darkness twice increased the frequency of embryo germination compared to the control sample. More extended periods of cold incubation for 12 days in the dark, on the other hand, twice reduced the rate of direct embryo germination compared to the control sample, and for 15 days – 6 times.

Zygotic Embryogenesis in Flowering Plants.

In the context of plant regeneration, in vitro systems to produce embryos are frequently used. In many of these protocols, nonzygotic embryos are initiated that will produce shoot-like structures but may lack a primary root. By increasing the auxin-to-cytokinin ratio in the growth medium, roots are then regenerated in a second step. Therefore, in vitro systems might not or only partially execute a similar developmental program as employed during zygotic embryogenesis. There are, however, in vitro systems that can remarkably mimic zygotic embryogenesis such as Brassica microspore-derived embryos. In this case, the patterning process of these haploid embryos closely follows zygotic embryogenesis and all fundamental tissue types are generated in a rather similar manner. In this review, we discuss the most fundamental molecular events during early zygotic embryogenesis and hope that this brief summary can serve as a reference for studying and developing in vitro embryogenesis systems in the context of doubled haploid production.

Live Imaging of embryogenic structures in Brassica napus microspore embryo cultures highlights the developmental plasticity of induced totipotent cells

Key messageIn vitro embryo development is highly plastic; embryo cell fate can be re-established in tissue culture through different pathways.In most angiosperms, embryo development from the single-celled zygote follows a defined pattern of cell divisions in which apical (embryo proper) and basal (root and suspensor) cell fates are established within the first cell divisions. By contrast, embryos that are induced in vitro in the absence of fertilization show a less regular initial cell division pattern yet develop into histodifferentiated embryos that can be converted into seedlings. We used the Brassica napus microspore embryogenesis system, in which the male gametophyte is reprogrammed in vitro to form haploid embryos, to identify the developmental fates of the different types of embryogenic structures found in culture. Using time-lapse imaging of LEAFY COTYLEDON1-expressing cells, we show that embryogenic cell clusters with very different morphologies are able to form haploid embryos. The timing of surrounding pollen wall (exine) rupture is a major determinant of cell fate in these clusters, with early exine rupture leading to the formation of suspensor-bearing embryos and late rupture to suspensorless embryos. In addition, we show that embryogenic callus, which develops into suspensor-bearing embryos, initially expresses transcripts associated with both basal- and apical-embryo cell fates, suggesting that these two cell fates are fixed later in development. This study reveals the inherent plasticity of in vitro embryo development and identifies new pathways by which embryo cell fate can be established.

In vitro Culture of Heat-treated Anthers Induces Embryogenic Callus in Cassava (Manihot esculenta Crantz)

Microspore embryogenesis is the developmental plasticity of juvenile male gametophytes to switch from pollen to embryo development upon exposure to in vitro stress. It is a common method for obtaining haploid and doubled haploid plants in breeding programs for development of superior varieties via haploid-diploidization which allows fast development of homozygous lines from heterozygous parents. In cassava, obtaining haploidization through traditional methods of successive selfing is difficult because of cassava’s long reproductive cycle, high heterozygosity, and inbreeding depression. As a first step towards microspore embryogenesis, callus induction following heat treatment has not been investigated in cassava. We used two elite Ugandan cassava varieties, hereafter called “genotypes”, NASE3 and NASE14, for the study of callus induction. Heat stress of 40°C for 0, 6, 12, 18 and 24hrs and Murashige and Skoog medium supplemented with 2-9% sucrose, and 2,4-dichlorophenoxy acetic acid were used. Heating anthers of NASE3 at 40°C for 6 hrs resulted in a significantly higher percentage of callus induction on MS medium supplemented with 2% sucrose. Callus emerged from inside of the anthers with production influenced by genotype, sucrose concentration, anther density and duration in culture (P≤0.001). Limited in vitro callus differentiation was observed on auxin-and cytokinin-supplemented media. In both genotypes, embryo genic callus was obtained in liquid medium, while green callus was achieved on solid medium. This is a significant step upstream of double haploid plant production pathway in cassava: rigorous optimization of protocols downstream of callus induction are needed for regeneration of microspore-derived embryos and haploid plants.

Triple synergistic effect of maltose, silver nitrate and activated charcoal on high embryo yield of eggplant (Solanum melongena L.) anther cultures

More efficient haploidy protocols have the power to speed up breeding studies. We aimed to increase microspore-derived embryo and in vitro plantlet yields in eggplant anther cultures. To achieve this, a comprehensive optimization study was carried out on the commonly used two-step DDV (Dumas de Vaulx et al., 1982) eggplant anther culture system. For optimizations of both induction and differentiation culture media, the effects of carbohydrate sources (maltose and sucrose) were investigated alone or in combination with two kinds of additives consisting of silver nitrate (AgNO3) and activated charcoal (AC). They were done in the presence of macro-micro salts, vitamins and PGRs of original DDV protocol. During the optimizations, we also compared the effects of genotype, the growth season of donor plants, anther culture type, and culture media, on embryo and in vitro plantlet yields. Regarding culture media, single effect of maltose use was considerable only at the highest concentration. The effects of maltose + AgNO3 combinations were not noteworthy while the effect of maltose + AC combination was negative. However, triple use of maltose (90 g/L) + AgNO3 (10 mg/L) + AC (1 g/L) had a remarkably positive synergistic effect depending on the genotype, season, and concentrations of optimization chemicals. We observed a triple synergistic effect of this aSC24-encoded modified induction medium on longer viability of anthers, embryo quality, direct embryogenesis, faster regeneration and increased embryo and in vitro plantlet yields. Through this extremely efficient modified induction medium, we achieved 3.9 times more embryo yields than the original DDV medium in two genotypes. The highest embryo and in vitro plantlet yields (320 embryos and 200 plantlets/100 anthers) were obtained from ‘A117’ eggplant anthers cultured in solid aSC24 + DDV-R combined cultures in the autumn season. In addition, as far as we know, embryos (up to 42 embryos/100 anthers) were produced for the first time in a liquid anther culture in eggplant.

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.

Behavior of Opuntia ficus-indica (L.) Mill. Heat-Stressed Microspores Under In Vitro Culture Conditions as Evidenced by Microscopic Analysis

When constrained by in vitro culture conditions, microspores from Opuntia ficus-indica (L.) Mill. (Barbary fig) anthers were forced to stall out their gametophytic pathway and switch towards androgenesis. Five microspore stages were characterized based on cellular architecture. A relationship was also established between anthers and bud flower features. Anthers were cultured on three culture media containing 2,4-dichlorophenoxyacetic acid, thidiazuron and gibberellic acid at 22°C for 4 wk, followed by heat shock treatment at 32°C or 42°C for 2 wk, and the stressed material was recultured at 22°C for 24 wk in the dark. When anthers were heat shocked at 32°C and 42°C, the cultivated anthers swelled and burst, followed by the microspores flowing onto the explant surface. Androgenesis occurred directly and indirectly at uni- and binucleate stages. The micromorphology of multicellular, proembryos, globular, torpedo and cotyledonary-like-shaped structures from dehisced anthers was confirmed by environmental scanning electron microscopy. Furthermore, at 42°C, microscopic analysis demonstrated that the non-responsive microspores achieved pollen maturation and more rarely, the emission of a pollen tube. However, 11.4 to 14.4% of the mature pollen grains were converted into pollen embryos. The established system may serve, for the first time, as a protocol to produce microspore embryos in O. ficus-indica. Nevertheless, more efforts are needed to complete the development of diplo-haploid plantlets.

Improving the efficiency of wheat microspore culture methodology: evaluation of pretreatments, gradients, and epigenetic chemicals

The objective of the study was to develop efficient widely adaptable microspore culture methodology for spring and winter wheat genotypes. Current wheat microspore culture methods are still inadequate for routine application, especially for winter genotypes. The present study involved increasing the frequency of microspore embryogenesis and improving the conversion of microspore-derived embryos to fertile green plants. A systematic evaluation was conducted testing numerous factors at all phases of the microspore embryogenesis process. Results from pretreatments, maltose gradients, and histone deacetylase inhibitors are presented. A 28 days cold pretreatment of spikes was optimal for winter wheat genotypes whereas a 21 days cold pretreatment was best for spring genotypes. Microspore suspensions containing a high frequency of embryogenic microspores were collected by resuspension in a 23% maltose gradient with subsequent pelleting with a mannitol containing solution. The application of trichostatin A (TSA) enhanced embryogenesis and/or green plant regeneration. Other epigenetic chemicals like scriptaid, BIX-01294, and sodium butyrate were also evaluated. Scriptaid enhanced embryogenesis but regenerated plantlets showed abnormal shoot and root development. The improved methodology was validated using a range of spring and winter wheat genotypes. Enhanced microspore embryogenesis and green plant regeneration in spring and winter wheat was observed with 21 or 28 days cold pretreatments, a 23% maltose gradient, and the application of trichostatin A (TSA).

Early in vitro selection of winter oilseed rape (Brassica napus L.) plants with the fertility restorer gene for CMS Shaan 2A via non-destructive molecular analysis of microspore-derived embryos

Cotyledons of 201 microspore-derived oilseed rape (Brassica napus) embryos were analysed for the presence of the cytoplasmic male sterility Shaan 2A restorer gene using the primer pair 3F/4R. In total, 172 plants regenerated and were grown to the generative stage. Among phenotypically non-haploid flowering plants, there was a broad spectrum of different flower types from normally developed flowers to flowers with short anthers, stamens, or deformed flowers with atypically developed floral organs, often with traces of pollen. For this reason, only 115 typically fertile or sterile accessions were selected for further phenotype-genotype comparisons. In 14 plants (12.2%) the phenotype did not match the genotype: two plants were marked as fertile, but without the Rf gene. Vice versa, in 12 sterile plants the Rf gene was determined. Nevertheless, the two fertile plants did not produce any pods after self-pollination. In total, 47 doubled haploid (DH) lines with good seed yield, carrying the Rf gene were harvested.

Towards artificial seeds from microspore derived embryos of Brassica napus

Microspore derived embryos are haploid and their immediate diploidization generates doubled haploid homozygous plants, whereas a normal breeding process would take 7–8 generations to attain homozygosity. However, the flexibility available in conventional seeds—storage, transport, variable planting time, and handling—is not possible with microspore derived embryos in breeding programs; they are continuously growing from induction of embryogenesis to planting in the soil, without a pause. Artificial seed technology can by-pass the expensive and time-consuming process of acclimatizing and loss of in vitro derived embryos in the green house. The doubled haploid technology in Brassica species has advanced considerably on many fronts—reliable induction of embryogenesis, in vitro diploidization, desiccation and conversion of embryos to plantlets. Although microspore derived embryos are bipolar, they are not considered within the scheme of artificial seeds. The development of artificial seeds in brassica, however, needs the input from other biotechnologies to develop vigorous embryos capable of conversion to plants. It is now necessary for empirical research on microspore derived embryos of Brassica to expand the applications available to the plant breeder by including the biotechnologies of encapsulation and embryo priming to develop artificial seeds. The objective of this review is to draw the attention of researchers to make the transition from microspore-derived embryos to artificial seeds in Brassica crop species, by bringing together relevant studies from the relevant biotechnologies. This would expand the present scope of artificial seeds and thus provide more options and flexibility to the Brassica plant breeder. Artificial seeds from microspore derived embryos enhance breeding of Brassica species.

Dynamic Changes in Arabinogalactan-Protein, Pectin, Xyloglucan and Xylan Composition of the Cell Wall During Microspore Embryogenesis in Brassica napus.

Microspore embryogenesis is a manifestation of plant cell totipotency whereby new cell walls are formed as a consequence of the embryogenic switch. In particular, the callose-rich subintinal layer created immediately upon induction of embryogenesis was recently related to protection against stress. However, little is currently known about the functional significance of other compositional changes undergone by the walls of embryogenic microspores. We characterized these changes in Brassica napus at different stages during induction of embryogenic microspores and development of microspore-derived embryos (MDEs) by using a series of monoclonal antibodies specific for cell wall components, including arabinogalactan-proteins (AGPs), pectins, xyloglucan and xylan. We used JIM13, JIM8, JIM14 and JIM16 for AGPs, CCRC-M13, LM5, LM6, JIM7, JIM5 and LM7 for pectins, CCRC-M1 and LM15 for xyloglucan, and LM11 for xylan. By transmission electron microscopy and quantification of immunogold labeling on high-pressure frozen, freeze-substituted samples, we profiled the changes in cell wall ultrastructure and composition at the different stages of microspore embryogenesis. As a reference to compare with, we also studied in vivo microspores and maturing pollen grains. We showed that the cell wall of embryogenic microspores is a highly dynamic structure whose architecture, arrangement and composition changes dramatically as microspores undergo embryogenesis and then transform into MDEs. Upon induction, the composition of the preexisting microspore intine walls is remodeled, and unusual walls with a unique structure and composition are formed. Changes in AGP composition were related to developmental fate. In particular, AGPs containing the JIM13 epitope were massively excreted into the cell apoplast, and appeared associated to cell totipotency. According to the ultrastructure and the pectin and xyloglucan composition of these walls, we deduced that commitment to embryogenesis induces the formation of fragile, plastic and deformable cell walls, which allow for cell expansion and microspore growth. We also showed that these special walls are transient, since cell wall composition in microspore-derived embryos was completely different. Thus, once adopted the embryogenic developmental pathway and far from the effects of heat shock exposure, cell wall biosynthesis would approach the structure, composition and properties of conventional cell walls.

Glutathione provides antioxidative defence and promotes microspore-derived embryo development in isolated microspore cultures of triticale (\xd7\u2009Triticosecale Wittm.)

Key messageDepending on the capability for stress adaptation, the role played by glutathione in microspore embryogenesis consists of both antioxidative activity and stimulation of embryo-like structure development.The efficiency of microspore embryogenesis (ME) is determined by the complex network of internal and environmental factors. Among them, the efficient defence against oxidative stress seems to be one of the most important. The present study confirms this hypothesis showing the positive effect of glutathione—the most abundant cellular antioxidant—on ME in isolated microspore cultures of triticale (× Triticosecale Wittm.). For the first time, low temperature (LT) pre-treatment of tillers was combined with the exogenous application of glutathione and associated with the total activity of low-molecular weight antioxidants, the endogenous content and redox status of glutathione, and the effectiveness of ME. The results indicate that efficient antioxidative defence is the first, although not the only, prerequisite for effective ME. In responsive genotypes, LT alone stimulated antioxidative defence and decreased cell redox status, which was associated with increased cell viability and high frequency (ca. 20%) of microspore reprogramming. Application of glutathione had no effect either on the microspore viability or on the initial number of embryogenic microspores. However, it increased the number of embryo-like structures, probably by stimulating the next phases of its development. In recalcitrant genotypes, the main role of glutathione seems to be its participation in cell protection from oxidative stress. However, even enhanced antioxidative activity, which sustained cell viability and increased the number of embryogenic microspores, was insufficient for efficient haploid/doubled haploid plant production. Evidently, there are still other defective elements in the complex network of factors that regulate the process of ME.

Mapping of quantitative trait loci for microspore embryogenesis-related traits in the oilseed rape doubled haploid population DH4069 × Express 617

Oilseed rape (Brassica napus L.) is among dicotyledonous plants, a model species for microspore embryogenesis. Tremendous differences exist among oilseed rape genotypes in their embryogenic response and direct embryo to plant conversion. Despite some attempts to identify relevant genes, the genetic basis of these traits remains largely unknown. The objective of this work was to develop and to provide to the scientific community a doubled haploid (DH) population derived from a cross of the reported highly embryogenic genotype DH4079 and the low embryogenic inbred line Express 617. A population of 198 DH-lines was generated and genotyped with the Brassica 60 K Illumina Infinium™ SNP array. The parental and the F1 genotypes as well as between 81 and 107 DH-lines were characterized for their number of microspores, number of microspore-derived embryos, embryo survival rate, direct embryo to shoot conversion, and related traits. The results obtained for the F1 genotype were mostly in between the two parents. SNP markers in the DH population showed to 49% distorted segregation and of those 63% were in favor of DH4079. Significant genotypic differences were found for all traits and heritabilities ranged from 66 to 88%. Together, 13 quantitative trait loci (QTL) for the different traits were identified on linkage groups A01, A02, A05, A10, C04, and C06, and candidate genes were identified within their QTL confidence intervals.