Zygotic and somatic embryo morphogenesis in Pinus pinaster: comparative histological and histochemical study

Characterization of hybrid larch somatic embryo maturation by biochemical analyses and by a novel, fast mid-infrared approach

The morphology of somatic embryos (SE) is not a sufficient criterion to determine the level of maturation and the optimal stage to transfer embryos for germination, unlike the biochemical components. This composition characterization in the laboratory is too restrictive to be considered at each maturation cycle, as would be necessary. It is, therefore, essential to consider alternative methods. The objectives of this work were to achieve a complete biochemical characterization of the embryos during their development, to serve as a reference and develop a characterization based on infrared spectrometry and chemometrics. During the precotyledonary stage (0-3 weeks of maturation), water content and glucose and fructose levels were high, which is consistent with SE development. After 4 weeks, the cotyledonary SE had a metabolism oriented towards the storage accumulation of lipids, proteins and starch, whereas raffinose only appeared from 8 weeks. Mid-infrared calibration models were developed for water, proteins, lipids, carbohydrates, glucose, fructose, inositols, raffinose, stachyose and starch contents with an r2 average of 0.84. A model was also developed to discriminate the weeks of SE maturation. Different classes of age were discriminated with at least 72% of accuracy. Infrared analysis of the SE based on their full biochemical spectral fingerprint revealed a very slight variation in composition between 7 and 9 weeks, information that is very difficult to obtain by conventional analysis methods. These results provide novel insights into the maturation of conifer SE and indicate that mid-infrared spectrometry could be an easy and effective method for SE characterization.

Desiccation as a Post-maturation Treatment Helps Complete Maturation of Norway Spruce Somatic Embryos: Carbohydrates, Phytohormones and Proteomic Status

Exposure of Norway spruce (Picea abies) somatic embryos and those of many other conifers to post-maturation desiccation treatment significantly improves their germination. An integration analysis was conducted to understand the underlying processes induced during the desiccation phase at the molecular level. Carbohydrate, protein and phytohormone assays associated with histological and proteomic studies were performed for the evaluation of markers and actors in this phase. Multivariate comparison of mature somatic embryos with mature desiccated somatic embryos and/or zygotic embryos provided new insights into the processes involved during the desiccation step of somatic embryogenesis. Desiccated embryos were characterized by reduced levels of starch and soluble carbohydrates but elevated levels of raffinose family oligosaccharides. Desiccation treatment decreased the content of abscisic acid and its derivatives but increased total auxins and cytokinins. The content of phytohormones in dry zygotic embryos was lower than in somatic embryos, but their profile was mostly analogous, apart from differences in cytokinin profiles. The biological processes "Acquisition of desiccation tolerance", "Response to stimulus", "Response to stress" and "Stored energy" were activated in both the desiccated somatic embryos and zygotic embryos when compared to the proteome of mature somatic embryos before desiccation. Based on the specific biochemical changes of important constituents (abscisic acid, raffinose, stachyose, LEA proteins and cruciferins) induced by the desiccation treatment and observed similarities between somatic and zygotic P. abies embryos, we concluded that the somatic embryos approximated to a state of desiccation tolerance. This physiological change could be responsible for the reorientation of Norway spruce somatic embryos toward a stage suitable for germination.

Effects of Medium Supplements on Somatic Embryo Maturation and DNA Methylation in Pseudotsuga gaussenii Flous, a Species under Protection

Pseudotsuga gaussenii is a forest species under protection in China. Propagation via somatic embryogenesis is efficient and needed for recovering this species. In this research, effects of medium supplements, i.e., methylglyoxal-bis (guanylhydrazone) (MGBG), polyethylene glycol (PEG), and maltose, on somatic embryo (SE) development were studied. More mature SEs developed with the pre-maturation treatment medium supplemented with 50 μM or 100 μM MGBG. During SE maturation, when the medium was supplemented with both PEG and maltose, the over-proliferation of embryogenic tissue was suppressed. When maltose was eliminated from the maturation medium, SEs were slightly smaller but developed better than other treatments due to the supply of lower carbon source, whereas when PEG was taken out from the maturation medium, no mature SE was obtained, only the heavily-proliferated plant tissue. Meanwhile, DNA methylation levels differed in these SE maturation cultures at the late culture stage of SE maturation. The level increased sharply with no maltose treatment (20.28%) while decreasing without PEG supplement (10.35%). DNA methyltransferase genes MET1-1, MET1-2, MET1-3, CMT3, DRM1, and DRM2 were partially cloned in this study to detect their expression level via qPCR. Expression of these genes, except MET1-3, in the culture of no PEG was higher significantly than the cultures of other treatments at weeks 1, 3, and 10. Our results suggested that MGBG and/or PEG play an important role in stimulating SE development and maturation. Furthermore, sugar supplements at a lower level benefited SE maturation in Pseudotsuga gaussenii.

Similarities and Differences in the GFP Movement in the Zygotic and Somatic Embryos of Arabidopsis

Intercellular signaling during embryo patterning is not well understood and the role of symplasmic communication has been poorly considered. The correlation between the symplasmic domains and the development of the embryo organs/tissues during zygotic embryogenesis has only been described for a few examples, including Arabidopsis. How this process occurs during the development of somatic embryos (SEs) is still unknown. The aim of these studies was to answer the question: do SEs have a restriction in symplasmic transport depending on the developmental stage that is similar to their zygotic counterparts? The studies included an analysis of the GFP distribution pattern as expressed under diverse promoters in zygotic embryos (ZEs) and SEs. The results of the GFP distribution in the ZEs and SEs showed that 1/the symplasmic domains between the embryo organs and tissues in the SEs was similar to those in the ZEs and 2/the restriction in symplasmic transport in the SEs was correlated with the developmental stage and was similar to the one in their zygotic counterparts, however, with the spatio-temporal differences and different PDs SEL value between these two types of embryos.

In Vitro Plant Regeneration in Conifers: The Role of WOX and KNOX Gene Families

Conifers are a group of woody plants with an enormous economic and ecological importance. Breeding programs are necessary to select superior varieties for planting, but they have many limitations due to the biological characteristics of conifers. Somatic embryogenesis (SE) and de novo organogenesis (DNO) from in vitro cultured tissues are two ways of plant mass propagation that help to overcome this problem. Although both processes are difficult to achieve in conifers, they offer advantages like a great efficiency, the possibilities to cryopreserve the embryogenic lines, and the ability of multiplying adult trees (the main bottleneck in conifer cloning) through DNO. Moreover, SE and DNO represent appropriate experimental systems to study the molecular bases of developmental processes in conifers such as embryogenesis and shoot apical meristem (SAM) establishment. Some of the key genes regulating these processes belong to the WOX and KNOX homeobox gene families, whose function has been widely described in Arabidopsis thaliana. The sequences and roles of these genes in conifers are similar to those found in angiosperms, but some particularities exist, like the presence of WOXX, a gene that putatively participates in the establishment of SAM in somatic embryos and plantlets of Pinus pinaster.

Cell Suspension Culture and In Vitro Screening for Drought Tolerance in Soybean Using Poly-Ethylene Glycol

Soybean (Glycine max (L) Merrill) is used in India mostly as a substantial fund of protein and oil, which makes the crop significantly important. Somaclonal variation has been researched as a base of additional variability for drought in soybean. In the present experiment calli/cell clumps/embryoids rose from immature and mature embryonic axis and cotyledons explants were exposed to different concentrations of polyethylene glycol (PEG6000). A discontinuous method proved to be superior as it permitted the calli/embryoids/cell clumps to regain their regeneration competence. A total of 64 (12.21%) plantlets of genotype JS335 and 78 (13.13%) of genotype JS93-05 were regenerated after four consequent subcultures on the selection medium with an effective lethal concentration of 20% PEG6000, and proliferated calli/embryoids/cell clumps were further subcultured on Murashige and Skoog regeneration medium supplemented with 0.5 mgL^-1 each of α-napthalene acetic acid (NAA), 6-benzyladenine (BA) and Kinetin (Kn), 20.0 gL^-1 sucrose and 7.5 gL^-1 agar. Putative drought-tolerant plantlets were acquired from genotype JS93-05 (38) in more numbers compared to genotype JS335 (26). Random decamer primers confirmed the presence of variability between mother plants and regenerated plants from both the genotypes. Since these plantlets recovered from tolerant calli/embryoids/cell clumps selected from the medium supplemented with PEG6000, the possibility exists that these plants may prove to be tolerant against drought stress.

Characterization of the KNOTTED1-LIKE HOMEOBOX (KNOX) gene family in Pinus pinaster Ait

KNOTTED1-LIKE HOMEOBOX (KNOX) genes are a family of plant-specific homeobox transcription factors with important roles in plant development that have been classified into two subfamilies with differential expression domains and functions. Studies in angiosperms have shown that class I members are related to the maintenance of meristem homeostasis and leaf development, whereas class II members promote differentiation of tissues and organs. However, little is known about its diversification and function in gymnosperms. By combining PCR-based detection and transcriptome data analysis, we identified four class I and two class II KNOX genes in Pinus pinaster. Expression analyses showed that class I members were mainly expressed in meristematic regions and differentiating tissues, with practically no expression in lateral organs, whereas expression of class II members was restricted to lateral organs. Furthermore, overexpression of P. pinaster KNOX genes in Arabidopsis thaliana caused similar phenotypic effects to those described for their angiosperms counterparts. This is the first time to our knowledge that functional analyses of class II members are reported in a conifer species. These results suggest a high conservation of the KNOX gene family throughout seed plants, as the functional differentiation of both subfamilies observed in angiosperms might be partially conserved in gymnosperms.

Progress in Somatic Embryogenesis of Japanese Pines

Somatic embryogenesis (SE) in not only one of the most promising techniques for mass propagation of selected trees, but also is a valuable tool for basic research studies in cell biology and genetic engineering, and it allows the long-term ex situ conservation of genetic resources by cryopreservation techniques. This review reports the most recent progress in SE, protoplast culture, and cryopreservation of four important Japanese pines (Pinus thunbergii, Pinus densiflora, Pinus armandii var. amamiana, and Pinus luchuensis). Induction of embryogenic tissues (ET), embryogenic culture maintenance/proliferation, production of somatic embryos, germination, and conversion to plants are described focusing on the protocols most commonly reported for plant production in Pinus species through to SE.

New Approaches to Optimize Somatic Embryogenesis in Maritime Pine

Maritime pine (Pinus pinaster Aiton) is a coniferous native of the Mediterranean basin. Because of its adaptability to a wide range of environmental conditions, the species have become a model for studies in coniferous forest management and functional genomics. Somatic embryogenesis (SE) has been so far, the preferred biotechnological strategy for maritime pine breeding programs initiated at the middle-end of the 20th century. To overcome the limitations of the induction and maturation phases in maritime pine SE, we analyzed the possible maternal influence on the embryogenic capability of megagametophytes from controlled crosses, as well as the effect of the temperature and water availability during SE process on the production of plants. A strong maternal effect on the embryogenic potential of maritime pine megagametophytes was observed in our experiments using half-sib and full-sib progenies, while paternal effect was almost undetectable. Besides, it seems possible to improve somatic embryo production of maritime pine megagametophytes by adjusting optimal temperature throughout the process: 28°C during induction and proliferation, and 23°C during the maturation phase. Using induction and proliferation media with reduced water availability (6 g/L Gelrite) can also increase embryo production. Since other limitation of maritime pine SE is culture decline of embryogenic masses (EMs), that reduces embryo yield and germination, we assessed the profile of ABA and IAA and the expression of two embryogenesis-related genes (LEC1 and WOX2) during maturation of EMs of two morphotypes that differed in their maturation capability. Spiky morphotype (SK), with high maturation capability, had a steady increase in both hormones along the 12 weeks of the maturation, whereas ABA content in smooth morphotype picked at the 4th week and dropped. EMs with this morphotype also had a higher IAA content at the beginning of the maturation. A decrease of LEC1 and WOX2 gene expression over the course of embryo development was found to be characteristic of the SK with high maturation capability.

The role of arginine metabolic pathway during embryogenesis and germination in maritime pine (Pinus pinaster Ait.)

Vegetative propagation through somatic embryogenesis is critical in conifer biotechnology towards multivarietal forestry that uses elite varieties to cope with environmental and socio-economic issues. An important and still sub-optimal process during in vitro maturation of somatic embryos (SE) is the biosynthesis and deposition of storage proteins, which are rich in amino acids with high nitrogen (N) content, such as arginine. Mobilization of these N-rich proteins is essential for the germination and production of vigorous somatic seedlings. Somatic embryos accumulate lower levels of N reserves than zygotic embryos (ZE) at a similar stage of development. To understand the molecular basis for this difference, the arginine metabolic pathway has been characterized in maritime pine (Pinus pinaster Ait.). The genes involved in arginine metabolism have been identified and GFP-fusion constructs were used to locate the enzymes in different cellular compartments and clarify their metabolic roles during embryogenesis and germination. Analysis of gene expression during somatic embryo maturation revealed high levels of transcripts for genes involved in the biosynthesis and metabolic utilization of arginine. By contrast, enhanced expression levels were only observed during the last stages of maturation and germination of ZE, consistent with the adequate accumulation and mobilization of protein reserves. These results suggest that arginine metabolism is unbalanced in SE (simultaneous biosynthesis and degradation of arginine) and could explain the lower accumulation of storage proteins observed during the late stages of somatic embryogenesis.

Analysis of the WUSCHEL-RELATED HOMEOBOX gene family in Pinus pinaster: New insights into the gene family evolution

WUSCHEL-RELATED HOMEOBOX (WOX) genes are key players controlling stem cells in plants and can be divided into three clades according to the time of their appearance during plant evolution. Our knowledge of stem cell function in vascular plants other than angiosperms is limited, they separated from gymnosperms ca 300 million years ago and their patterning during embryogenesis differs significantly. For this reason, we have used the model gymnosperm Pinus pinaster to identify WOX genes and perform a thorough analysis of their gene expression patterns. Using transcriptomic data from a comprehensive range of tissues and stages of development we have shown three major outcomes: that the P. pinaster genome encodes at least fourteen members of the WOX family spanning all the major clades, that the genome of gymnosperms contains a WOX gene with no homologues in angiosperms representing a transitional stage between intermediate- and WUS-clade proteins, and that we can detect discrete WUS and WOX5 transcripts for the first time in a gymnosperm.

Carbohydrate-mediated responses during zygotic and early somatic embryogenesis in the endangered conifer, Araucaria angustifolia

Three zygotic developmental stages and two somatic Araucaria angustifolia cell lines with contrasting embryogenic potential were analyzed to identify the carbohydrate-mediated responses associated with embryo formation. Using a comparison between zygotic and somatic embryogenesis systems, the non-structural carbohydrate content, cell wall sugar composition and expression of genes involved in sugar sensing were analyzed, and a network analysis was used to identify coordinated features during embryogenesis. We observed that carbohydrate-mediated responses occur mainly during the early stages of zygotic embryo formation, and that during seed development there are coordinated changes that affect the development of the different structures (embryo and megagametophyte). Furthermore, sucrose and starch accumulation were associated with the responsiveness of the cell lines. This study sheds light on how carbohydrate metabolism is influenced during zygotic and somatic embryogenesis in the endangered conifer species, A. angustifolia.

Norway spruce embryogenesis: changes in carbohydrate profile, structural development and response to polyethylene glycol

Two unrelated, geographically distinct, highly embryogenic lines of Norway spruce (Picea abies (L.) Karst.) were analysed to identify metabolic traits characteristic for lines with good yields of high-quality embryos. The results were compared with corresponding characteristics of a poorly productive line (low embryo yield, scarce high-quality embryos). The following carbohydrate profiles and spectra during maturation, desiccation and germination were identified as promising characteristics for line evaluation: a gradual decrease in total soluble carbohydrates with an increasing sucrose : hexose ratio during maturation; accumulation of raffinose family oligosaccharides resulting from desiccation and their rapid degradation at the start of germination; and a decrease in sucrose, increase in hexoses and the appearance of pinitol with proceeding germination. We propose that any deviation from this profile in an embryonic line is a symptom of inferior somatic embryo development. We further propose that a fatty acid spectrum dominated by linoleic acid (18 : 2) was a common feature of healthy spruce somatic embryos, although it was quite different from zygotic embryos mainly containing oleic acid (18 : 1). The responses of the lines to osmotic stress were evaluated based on comparison of control (without osmoticum) and polyethylene glycol (PEG)-exposed (PEG 4000) variants. Although genetically distinct, both highly embryogenic lines responded in a very similar manner, with the only difference being sensitivity to high concentrations of PEG. At an optimum PEG concentration (3.75 and 5%), which was line specific, negative effects of PEG on embryo germination were compensated for by a higher maturation efficiency so that the application of PEG at an appropriate concentration improved the yield of healthy germinants per gram of initial embryonal mass and accelerated the process. Polyethylene glycol application, however, resulted in no improvement of the poorly productive line.

Toward establishing a morphological and ultrastructural characterization of proembryogenic masses and early somatic embryos of Araucaria angustifolia (Bert.) O. Kuntze

Somatic embryogenesis is a morphogenetic route useful for the study of embryonic development, as well as the large-scale propagation of endangered species, such as the Brazilian pine (Araucaria angustifolia). In the present study, we investigated the morphological and ultrastructural organization of A. angustifolia somatic embryo development by means of optical and electron microscopy. The proembryogenic stage was characterized by the proliferation of proembryogenic masses (PEMs), which are cellular aggregates composed of embryogenic cells (ECs) attached to suspensor-like cells (SCs). PEMs proliferate through three developmental stages, PEM I, II, and III, by changes in the number of ECs and SCs. PEM III-to-early somatic embryo (SE) transition was characterized by compact clusters of ECs growing out of PEM III, albeit still connected to it by SCs. Early SEs showed a dense globular embryonic mass (EM) and suspensor region (SR) connected by embryonic tube cells (TCs). By comparison, early somatic and zygotic embryos showed similar morphology. ECs are round with a large nucleus, nucleoli, and many cytoplasmic organelles. In contrast, TCs and SCs are elongated and vacuolated with cellular dismantling which is associated with programmed cell death of SCs. Abundant starch grains were observed in the TCs and SCs, while proteins were more abundant in the ECs. Based on the results of this study, a fate map of SE development in A. angustifolia is, for the first time, proposed. Additionally, this study shows the cell biology of SE development of this primitive gymnosperm which may be useful in evolutionary studies in this area.

Somatic Versus Zygotic Embryogenesis: Learning from Seeds

Plant embryogenesis is a fascinating developmental program that is very successfully established in nature in seeds. In case of in vitro somatic embryogenesis this process is subjected to several limitations such as asynchronous differentiation and further development of somatic embryos, malformations and disturbed polarity, precocious germination, lack of maturity, early loss of embryogenic potential, and strong genotypic differences in the regeneration efficiency. Several studies have shown the similarity of somatic and zygotic embryos in terms of morphological, histological, biochemical, and physiological aspects. However, pronounced differences have also been reported and refer to much higher stress levels, less accumulation of storage compounds and a missing distinction of differentiation and germination by a quiescent phase in somatic embryos. Here, an overview on recent literature describing both embryogenesis pathways, comparing somatic and zygotic embryos and analyzing the role of the endosperm is presented. By taking zygotic embryos as the reference and learning from the situation in seeds, somatic embryogenesis can be improved and optimized in order to make use of the enormous potential this regeneration pathway offers for plant propagation and breeding.

Effect of light conditions on anatomical and biochemical aspects of somatic and zygotic embryos of hybrid larch (Larix × marschlinsii)

BACKGROUND AND AIMS

In conifers, mature somatic embryos and zygotic embryos appear to resemble one another physiologically and morphologically. However, phenotypes of cloned conifer embryos can be strongly influenced by a number of in vitro factors and in some instances clonal variation can exceed that found in nature. This study examines whether zygotic embryos that develop within light-opaque cones differ from somatic embryos developing in dark/light conditions in vitro. Embryogenesis in larch is well understood both in situ and in vitro and thus provides a suitable system for addressing this question.

METHODS

Features of somatic and zygotic embryos of hybrid larch, Larix × marschlinsii, were quantified, including cotyledon numbers, protein concentration and phenol chemistry. Somatic embryos were placed either in light or darkness for the entire maturation period. Embryos at different developmental stages were embedded and sectioned for histological analysis.

KEY RESULTS

Light, and to a lesser degree abscisic acid (ABA), influenced accumulation of protein and phenolic compounds in somatic and zygotic embryos. Dark-grown mature somatic embryos had more protein (91·77 ± 11·26 µg protein mg(-1) f.wt) than either dark-grown zygotic embryos (62·40 ± 5·58) or light-grown somatic embryos (58·15 ± 10·02). Zygotic embryos never accumulated phenolic compounds at any stage, whereas somatic embryos stored phenolic compounds in the embryonal root caps and suspensors. Light induced the production of quercetrin (261·13 ± 9·2 µg g(-1) d.wt) in somatic embryos. Mature zygotic embryos that were removed from seeds and placed on medium in light rapidly accumulated phenolics in the embryonal root cap and hypocotyl. Delaying germination with ABA delayed phenolic compound accumulation, restricting it to the embryonal root cap.

CONCLUSIONS

In larch embryos, light has a negative effect on protein accumulation, but a positive effect on phenol accumulation. Light did not affect morphogenesis, e.g. cotyledon number. Somatic embryos produced different amounts of phenolics, such as quercetrin, depending on light conditions. The greatest difference was seen in the embryonal root cap in all embryo types and conditions.

Cotyledonary somatic embryos of Pinus pinaster Ait. most closely resemble fresh, maturing cotyledonary zygotic embryos: biological, carbohydrate and proteomic analyses

Cotyledonary somatic embryos (SEs) of maritime pine are routinely matured for 12 weeks before being germinated and converted to plantlets. Although regeneration success is highly dependent on SEs quality, the date of harvesting is currently determined mainly on the basis of morphological features. This empirical method does not provide any accurate information about embryo quality with respect to storage compounds (proteins, carbohydrates). We first analyzed SEs matured for 10, 12 and 14 weeks by carrying out biological (dry weight, water content) and biochemical measurements (total protein and carbohydrate contents). No difference could be found between collection dates, suggesting that harvesting SEs after 12 weeks is appropriate. Cotyledonary SEs were then compared to various stages, from fresh to fully desiccated, in the development of cotyledonary zygotic embryos (ZEs). We identified profiles that were similar using hierarchical ascendant cluster analysis (HCA). Fresh and dehydrated ZEs could be distinguished, and SEs clustered with fresh ZEs. Both types of embryo exhibited similar carbohydrate and protein contents and signatures. This high level of similarity (94.5 %) was further supported by proteome profiling. Highly expressed proteins included storage, stress-related, late embryogenesis abundant and energy metabolism proteins. By comparing overexpressed proteins in developing and cotyledonary SEs or ZEs, some (23 proteins) could be identified as candidate biomarkers for the late, cotyledonary stage. This is the first report of useful generic protein markers for monitoring embryo development in maritime pine. Our results also suggest that improvements of SEs quality may be achieved if the current maturation conditions are refined.

Early molecular events involved in Pinus pinaster Ait. somatic embryo development under reduced water availability: transcriptomic and proteomic analyses

Maritime pine somatic embryos (SEs) require a reduction in water availability (high gellan gum concentration in the maturation medium) to reach the cotyledonary stage. This key switch, reported specifically for pine species, is not yet well understood. To facilitate the use of somatic embryogenesis for mass propagation of conifers, we need a better understanding of embryo development. Comparison of both transcriptome (Illumina RNA sequencing) and proteome [two-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis with mass spectrometry (MS) identification] of immature SEs, cultured on either high (9G) or low (4G) gellan gum concentration, was performed, together with analysis of water content, fresh and dry mass, endogenous abscisic acid (ABA; gas chromatography-MS), soluble sugars (high-pressure liquid chromatography), starch and confocal laser microscope observations. This multiscale, integrated analysis was used to unravel early molecular and physiological events involved in SE development. Under unfavorable conditions (4G), the glycolytic pathway was enhanced, possibly in relation to cell proliferation that may be antagonistic to SE development. Under favorable conditions (9G), SEs adapted to culture constraint by activating specific protective pathways, and ABA-mediated molecular and physiological responses promoting embryo development. Our results suggest that on 9G, germin-like protein and ubiquitin-protein ligase could be used as predictive markers of SE development, whereas protein phosphatase 2C could be a biomarker for culture adaptive responses. This is the first characterization of early molecular mechanisms involved in the development of pine SEs following an increase in gellan gum concentration in the maturation medium, and it is also the first report on somatic embryogenesis in conifers combining transcriptomic and proteomic datasets.

In search of markers for somatic embryo maturation in hybrid larch (Larix × eurolepis): global DNA methylation and proteomic analyses

A global DNA methylation and proteomics approach was used to investigate somatic embryo maturation in hybrid larch. Each developmental step during somatic embryogenesis was associated with a distinct and significantly different global DNA methylation level: from 45.8% mC for undifferentiated somatic embryos (1-week proliferation) to 61.5% mC for immature somatic embryos (1-week maturation), while maturation was associated with a decrease in DNA methylation to 53.4% for mature cotyledonary somatic embryos (8-weeks maturation). The presence of 5-azacytidine (hypo-methylating agent) or hydroxyurea (hyper-methylating agent) in the maturation medium altered the global DNA methylation status of the embryogenic cultures, and significantly reduced both their relative growth rate and embryogenic potential, suggesting an important role for DNA methylation in embryogenesis. Maturation was also assessed by examining changes in the total protein profile. Storage proteins, identified as legumin- and vicilin-like, appeared at the precotyledonary stage. In the proteomic study, total soluble proteins were extracted from embryos after 1 and 8 weeks of maturation, and separated by two-dimensional gel electrophoresis. There were 147 spots which showed significant differences between the stages of maturation; they were found to be involved mainly in primary metabolism and the stabilization of the resulting metabolites. This indicated that the somatic embryo was still metabolically active at 8 weeks of maturation. This is the first report of analyses of global DNA methylation (including the effects of hyper- and hypo-treatments) and proteome during somatic embryogenesis in hybrid larch, and thus provides novel insights into maturation of conifer somatic embryos.

Structural and component characterization of meristem cells in Araucaria angustifolia (Bert.) O. Kuntze zygotic embryo

Araucaria angustifolia, the Brazilian pine, is an endangered native conifer with economic and ecological importance. The female cone develops seeds containing the zygotic embryo, which, at cotyledonary stage, shows well-developed meristems. Little is known about the structure of gymnosperm meristems. In the present work, the composition and morphological organization of Araucaria angustifolia shoot and root apical meristems were studied during embryo development, using histochemical and microscope analyses. Histochemical evaluation revealed the presence of cellulose within the cell wall, cells with the presence of total proteins that react with Coomassie Brilliant Blue, starch grains, and large nuclei with evident nucleoli in the cytoplasm. Scanning electron microscopy showed apical meristem surface morphology, and both scanning and transmission microscopy revealed a thin and irregular cell wall with plasmodesmata and within the cells, mitochondria, many vacuoles, lipid bodies, Golgi bodies, and many amyloplasts with endoplasmic reticulum surrounding them and large nuclei. Similar to angiosperm cells, A. angustifolia meristem cells exhibit pluripotent characteristics, such as apparatus for intercellular communication and differentiation.

Development of coffee somatic and zygotic embryos to plants differs in the morphological, histochemical and hydration aspects

In Coffea arabica L., the development of direct sowing of somatic embryos (SE) in planting substrate, with subsequent nursery production of plants, has promoted the industrialization of somatic embryogenesis. However, plant conversion rates are still low and require improvements to enhance the cost-effectiveness of commercial micropropagation. With the aim of improving plant regeneration from SE, we studied the morphological and histological criteria and water characteristics during germination and plant conversion of zygotic embryos (ZE) and SE. At the cotyledonary stage, SE produced in a 1 l RITA(®) temporary immersion bioreactor (area 55.8 cm(2)) were morphologically similar in size (2-3 mm) but abnormal as compared with mature ZE. Protein and starch reserve levels were extremely low throughout germination and conversion to plantlets, while the water status remained steady [water content (WC) from 76 to 87%, Ψ from -0.37 to -0.47 MPa, pressure potential from 0.69 to 0.24 MPa]. In ZE, spectacular hydration occurred during the first 3 weeks (WC from 37 to 75%; Ψ from -6.24 to -1.0 MPa). Cotyledons remained undifferentiated for 10 weeks after sowing. Conversely, after only 3 weeks under germination conditions in a RITA(®) bioreactor, spongy and palisade parenchyma and stomata formed in SE cotyledons. The ZE plant conversion was faster than that of SE (14 vs. 22 weeks) and more efficient (rates 96 vs. 55%), with much more substantial hypocotyl and cotyledon development. The use of a new 5 l MATIS(®) bioreactor (area 355 cm(2)), designed especially to favor embryo dispersion and light transmittance to SE, markedly improved the embryo-to-plantlet conversion rate (91%). These results highlight the morphological heterogeneity and lack of protein reserves in SE at the beginning of the germination phase and marked differences in water characteristics. However, they also reveal high phenotypic plasticity, leading to a highly efficient plantlet conversion rate due to better embryo dispersion and light transmittance in more horizontal bioreactors.

Transcriptome profiling and in silico analysis of somatic embryos in Japanese larch (Larix leptolepis)

Japanese larch (Larix leptolepis) is an ecologically and economically important species mainly grown in northeastern China, Japan and Europe. However, erratic flowering and poor germplasm resources caused by high embryo abortion rates have hampered breeding of Larix species. Somatic embryogenesis (SE) is an effective tool for the production of L. leptolepis with desirable characteristics, such as expression of totipotency, preparation of synthetic seeds, and genetic transformation. However, public genomic resources for this species are limited. We sequenced 591,759 raw expressed sequence tags (ESTs) from a 454 sequencing cDNA library of L. leptolepis somatic embryos, resulting in 572,403 high-quality reads. These reads were assembled into 70,927 unique sequences (UniGenes), including 32,321 contigs and 38,606 singletons. After removal of low-quality sequences, 65,115 UniGenes were annotated using the UniProtKB program. Based on their sequence similarity with known proteins, the matched 30,372 sequences from 664 species were estimated to represent approximately 19,000 unique genes. Gene ontology analysis revealed 21,324 UniGenes assigned to 51 categories. By Kyoto Encyclopedia of Genes and Genomes mapping, 25,773 transcripts were associated with 160 biochemical pathways. Further analysis screened four signal transduction pathways represented by 337 enzymes and 17 secondary metabolites. In silico analysis reveals that 207 UniESTs in Larix are homologous to MAPKs genes identified from other model plants, which may be involved in regulating SE development. This study provides an initial insight into the Larix transcriptomes of the pro-embryogenic mass and is a sound basis for future studies.

KEY MESSAGE

We constructed a large, full-length 454 sequencing cDNA library of Larix leptolepis during somatic embryogenesis. More than 590,000 sequences were obtained and a deep-coverage EST database was constructed.

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.