A proteomics study of the induction of somatic embryogenesis in Medicago truncatula using 2DE and MALDI-TOF/TOF

Comparative proteomic analysis and antioxidant enzyme activity provide new insights into the embryogenic competence of Guadua chacoensis (Bambusoideae, Poaceae)

Somatic embryogenesis (SE) involves modifications of cellular, biochemical, genetic, and epigenetic patterns. Our work investigated proteins as markers of embryogenic response and characterized the redox state of embryogenic cultures (EC) of Guadua chacoensis. We identified a total of 855 proteins; 129 were up- and 136 down-accumulated in EC as compared with non-embryogenic culture (NEC). Additionally, 37 and 22 proteins were identified as unique in EC and NEC, respectively. Heat-shock proteins as unique proteins and increased activity in Superoxide Dismutase and Guaiacol Peroxidase in EC suggest that the embryogenic response requires activation of the stress response mechanism. Ribosomal, translational, and glycolytic proteins in EC seem to be associated with protein synthesis and energy sources for embryo development, respectively. Accumulation of cell wall-related proteins, such as Arabinogalactan and Polygalacturonase inhibitors, and signaling transduction proteins, including Chitinase, Phospholipase, and Guanine nucleotide-binding proteins in EC seems to be associated with embryogenic response. Enhancement of H₂O₂ content in EC compared to NEC suggests a possible role as a secondary messenger in SE. Altogether, the present study identified marker proteins of embryogenic response in G. chacoensis and revealed the activation of ROS scavenging enzymes to assure cell redox homeostasis and SE responses. SIGNIFICANCE: Somatic embryogenesis is a promising technique for the propagation and conservation of bamboo species; however, this route has been the least understood and studied until now. This study corresponds to the first work approaching proteomics complemented with biochemical analyses in the somatic embryogenesis of bamboo, bringing robust and precise information that can improve our understanding of this complex morphogenetic route.

The source, level, and balance of nitrogen during the somatic embryogenesis process drive cellular differentiation

Since the discovery of somatic embryogenesis (SE), it has been evident that nitrogen (N) metabolism is essential during morphogenesis and cell differentiation. Usually, N is supplied to cultures in vitro in three forms, ammonium (NH₄⁺), nitrate (NO₃^-), and amino N from amino acids (AAs). Although most plants prefer NO₃^- to NH₄⁺, NH₄⁺ is the primary form route to be assimilated. The balance of NO₃^- and NH₄⁺ determines if the morphological differentiation process will produce embryos. That the N reduction of NO₃^- is needed for both embryo initiation and maturation is well-established in several models, such as carrot, tobacco, and rose. It is clear that N is indispensable for SE, but the mechanism that triggers the signal for embryo formation remains unknown. Here, we discuss recent studies that suggest an optimal endogenous concentration of auxin and cytokinin is closely related to N supply to plant tissue. From a molecular and biochemical perspective, we explain N's role in embryo formation, hypothesizing possible mechanisms that allow cellular differentiation by changing the nitrogen source.

Current Proteomic and Metabolomic Knowledge of Zygotic and Somatic Embryogenesis in Plants

Embryogenesis is the primary developmental program in plants. The mechanisms that underlie the regulation of embryogenesis are an essential research subject given its potential contribution to mass in vitro propagation of profitable plant species. Somatic embryogenesis (SE) refers to the use of in vitro techniques to mimic the sexual reproduction program known as zygotic embryogenesis (ZE). In this review, we synthesize the current state of research on proteomic and metabolomic studies of SE and ZE in angiosperms (monocots and dicots) and gymnosperms. The most striking finding was the small number of studies addressing ZE. Meanwhile, the research effort focused on SE has been substantial but disjointed. Together, these research gaps may explain why the embryogenic induction stage and the maturation of the somatic embryo continue to be bottlenecks for efficient and large-scale regeneration of plants. Comprehensive and integrative studies of both SE and ZE are needed to provide the molecular foundation of plant embryogenesis, information which is needed to rationally guide experimental strategies to solve SE drawbacks in each species.

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.

Peroxidase Enzyme Fractions as Markers of Somatic Embryogenesis Capacities in Olive (Olea europaea L.)

As part of the search for biochemical markers of somatic embryogenesis in tissue cultures of olive (Olea europaea L.), peroxidases (POXs) in both the soluble and ionically wall-bound fractions were studied in two reputed olive cultivars (cvs.): "Picholine Marocaine" and "Dahbia". In order to carry out embryogenesis induction, proximal cotyledons were cultured in modified olive medium (OMc) supplemented with 25 μM indole-3-butylic acid (IBA) and 2.5 μM 2-isopentenyladenine (2iP), while distal leaf fragments (somatic explants) were cultured in OMc supplemented with 4.56 µM zeatin riboside (ZR) and 10.25 µM 1-naphthaleneacetic acid (NAA). Regarding embryogenic potentials, the zygotic explants (cv. Picholine Marocaine: 43.39%; cv. Dahbia: 53.41%) were more regenerative than the somatic explants (cv. Picholine Marocaine: 13.05%; cv. Dahbia: 19.51%). The enzyme assay showed a higher POX activity in embryogenic calluses (ECs) than in nonembryogenic calluses (NECs) for the zygotic explants in both studied cultivars. When expressed as units per milligram of proteins (U mg^-1 proteins), the highest total POXs activities (soluble POXs + ionically wall-bound POXs) were found in the ECs derived from the zygotic explants; for cv. Dahbia, 65% of the enzyme activities came from the ionically wall-bound fractions. Polyacrylamide gel electrophoresis showed that the ECs of the highly active cv. Dahbia were characterized by highly active isoperoxidases that were revealed in four migration zones, particularly a doublet in the A4 zone (R_f 0.70-0.73) present in the ionically wall-bound POXs. The fast-moving anodic POXs of the ionically wall-bound fractions could be adopted as an early electrophoretic test to determine the embryogenesis capacities in olive tissue culture materials. As biochemical markers, the POX enzyme and its profile in fractions, i.e., as soluble POXs and ionically wall-bound POXs, can offer a valuable tool for improving the tissue culture of olive via somatic embryogenesis.

Proteomic variations after short-term heat shock treatment reveal differentially expressed proteins involved in early microspore embryogenesis in cabbage (Brassica oleracea)

Microspore embryogenesis (ME), a widely used haploid breeding method that can considerably shorten the breeding cycle, provides an efficient mean of cultivating many important Brassica crops, such as cabbage, Chinese cabbage, and oilseed rape. For cabbage, in many cases, short-term heat shock treatment can strongly increase the embryogenesis rate, however, the underlying mechanism of this effect has not been elucidated. In this study, we compared the proteomics of isolated microspores with samples pretreated at 32 °C for 24 h and 25 °C for 24 h using two cabbage accessions (Zhonggan 628 and 87–534) showing highly different embryogenic rates. The embryo yield was 19.7 embryos/bud in Zhonggan 628 after 32 °C treatment, while no embryoid was observed in Zhonggan 628 after 25 °C treatment as well as in 87–534 at both temperatures. We identified a total of 363 and 282 differentially expressed proteins (DEPs) for Zhonggan 628 and 87–534 via a label-free proteomics technology. There were 97 DEPs specifically identified only in Zhonggan 628 but not in 87–534 after 32 °C heat-shock treatment that may be related to heat shock-induced embryogenesis in vitro culture. These DEPs were primarily enriched in carbon metabolic process, protein synthesis and degradation process, and signal transduction. Based on protein-protein interaction and pathway enrichment analyses, we proposed that SGT1 homolog A and B(SGT1), heat shock 70 kDa protein 5 (HSP70), cell division control protein 48 homolog A (CDC48) and fatty acyl-CoA reductase (FAR) might play important roles in microspore embryogenesis. This proteomic study may contribute to our molecular understanding of cabbage microspore embryogenesis and help to build a high-efficiency haploid breeding system.

NaCl-responsive ROS scavenging and energy supply in alkaligrass callus revealed from proteomic analysis

BACKGROUND

Salinity has obvious effects on plant growth and crop productivity. The salinity-responsive mechanisms have been well-studied in differentiated organs (e.g., leaves, roots and stems), but not in unorganized cells such as callus. High-throughput quantitative proteomics approaches have been used to investigate callus development, somatic embryogenesis, organogenesis, and stress response in numbers of plant species. However, they have not been applied to callus from monocotyledonous halophyte alkaligrass (Puccinellia tenuifora).

RESULTS

The alkaligrass callus growth, viability and membrane integrity were perturbed by 50 mM and 150 mM NaCl treatments. Callus cells accumulated the proline, soluble sugar and glycine betaine for the maintenance of osmotic homeostasis. Importantly, the activities of ROS scavenging enzymes (e.g., SOD, APX, POD, GPX, MDHAR and GR) and antioxidants (e.g., ASA, DHA and GSH) were induced by salinity. The abundance patterns of 55 salt-responsive proteins indicate that salt signal transduction, cytoskeleton, ROS scavenging, energy supply, gene expression, protein synthesis and processing, as well as other basic metabolic processes were altered in callus to cope with the stress.

CONCLUSIONS

The undifferentiated callus exhibited unique salinity-responsive mechanisms for ROS scavenging and energy supply. Activation of the POD pathway and AsA-GSH cycle was universal in callus and differentiated organs, but salinity-induced SOD pathway and salinity-reduced CAT pathway in callus were different from those in leaves and roots. To cope with salinity, callus mainly relied on glycolysis, but not the TCA cycle, for energy supply.

Dynamic TMT-Based Quantitative Proteomics Analysis of Critical Initiation Process of Totipotency during Cotton Somatic Embryogenesis Transdifferentiation

The somatic embryogenesis (SE) process of plants, as one of the typical responses to abiotic stresses with hormone, occurs through the dynamic expression of different proteins that constitute a complex regulatory network in biological activities and promotes plant totipotency. Plant SE includes two critical stages: primary embryogenic calli redifferentiation and somatic embryos development initiation, which leads to totipotency. The isobaric labels tandem mass tags (TMT) large-scale and quantitative proteomics technique was used to identify the dynamic protein expression changes in nonembryogenic calli (NEC), primary embryogenic calli (PEC) and globular embryos (GEs) of cotton. A total of 9369 proteins (6730 quantified) were identified; 805, 295 and 1242 differentially accumulated proteins (DAPs) were identified in PEC versus NEC, GEs versus PEC and GEs versus NEC, respectively. Eight hundred and five differentially abundant proteins were identified, 309 of which were upregulated and 496 down regulated in PEC compared with NEC. Of the 295 DAPs identified between GEs and PEC, 174 and 121 proteins were up- and down regulated, respectively. Of 1242 differentially abundant proteins, 584 and 658 proteins were up- and down regulated, respectively, in GEs versus NEC. We have also complemented the authenticity and accuracy of the proteomic analysis. Systematic analysis indicated that peroxidase, photosynthesis, environment stresses response processes, nitrogen metabolism, phytohormone response/signal transduction, transcription/posttranscription and modification were involved in somatic embryogenesis. The results generated in this study demonstrate a proteomic molecular basis and provide a valuable foundation for further investigation of the roles of DAPs in the process of SE transdifferentiation during cotton totipotency.

iTRAQ-Based Quantitative Proteomic Analysis of Embryogenic and Non-embryogenic Calli Derived from a Maize (Zea mays L.) Inbred Line Y423

Somatic embryos (SE) have potential to rapidly form a whole plant. Generally, SE is thought to be derived from embryogenic calli (EC). However, in maize, not only embryogenic calli (EC, can generate SE) but also nonembryogenic calli (NEC, can’t generate SE) can be induced from immature embryos. In order to understand the differences between EC and NEC and the mechanism of EC, which can easily form SE in maize, differential abundance protein species (DAPS) of EC and NEC from the maize inbred line Y423 were identified by using the isobaric tags for relative and absolute quantification (iTRAQ) proteomic technology. We identified 632 DAPS in EC compared with NEC. The results of bioinformatics analysis showed that EC development might be related to accumulation of pyruvate caused by the DAPS detected in some pathways, such as starch and sucrose metabolism, glycolysis/gluconeogenesis, tricarboxylic acid (TCA) cycle, fatty acid metabolism and phenylpropanoid biosynthesis. Based on the differentially accumulated proteins in EC and NEC, a series of DAPS related with pyruvate biosynthesis and suppression of acetyl-CoA might be responsible for the differences between EC and NEC cells. Furthermore, we speculate that the decreased abundance of enzymes/proteins involved in phenylpropanoid biosynthesis pathway in the EC cells results in reducing of lignin substances, which might affect the maize callus morphology.

Advanced Proteomic Approaches to Elucidate Somatic Embryogenesis

Somatic embryogenesis (SE) is a cell differentiation process by which a somatic cell changes its genetic program and develops into an embryonic cell. Investigating this process with various explant sources in vitro has allowed us to trace somatic embryo development from germination to plantlets and has led to the generation of new technologies, including genetic transformation, endangered species conservation, and synthetic seed production. A transcriptome data comparison from different stages of the developing somatic embryo has revealed a complex network controlling the somatic cell's fate, suggesting that an interconnected network acts at the protein level. Here, we discuss the current progress on SE using proteomic-based data, focusing on changing patterns of proteins during the establishment of the somatic embryo. Despite the advanced proteomic approaches available so far, deciphering how the somatic embryo is induced is still in its infancy. The new proteomics techniques that lead to the quantification of proteins with different abundances during the induction of SE are opening this area of study for the first time. These quantitative differences can elucidate the different pathways involved in SE induction. We envisage that the application of these proteomic technologies can be pivotal to identifying proteins critical to the process of SE, demonstrating the cellular localization, posttranslational modifications, and turnover protein events required to switch from a somatic cell to a somatic embryo cell and providing new insights into the molecular mechanisms underlying SE. This work will help to develop biotechnological strategies for mass production of quality crop material.

Gene expression and metabolite profiling of gibberellin biosynthesis during induction of somatic embryogenesis in Medicago truncatula Gaertn

Gibberellins (GAs) are involved in the regulation of numerous developmental processes in plants including zygotic embryogenesis, but their biosynthesis and role during somatic embryogenesis (SE) is mostly unknown. In this study we show that during three week- long induction phase, when cells of leaf explants from non-embryogenic genotype (M9) and embryogenic variant (M9-10a) were forming the callus, all the bioactive gibberellins from non-13-hydroxylation (GA4, GA7) and 13-hydroxylation (GA1, GA5, GA3, GA6) pathways were present, but the contents of only a few of them differed between the tested lines. The GA53 and GA19 substrates synthesized by the 13-hydroxylation pathway accumulated specifically in the M9-10a line after the first week of induction; subsequently, among the bioactive gibberellins detected, only the content of GA3 increased and appeared to be connected with acquisition of embryogenic competence. We fully annotated 20 Medicago truncatula orthologous genes coding the enzymes which catalyze all the known reactions of gibberellin biosynthesis. Our results indicate that, within all the genes tested, expression of only three: MtCPS, MtGA3ox1 and MtGA3ox2, was specific to embryogenic explants and reflected the changes observed in GA53, GA19 and GA3 contents. Moreover, by analyzing expression of MtBBM, SE marker gene, we confirmed the inhibitory effect of manipulation in GAs metabolism, applying exogenous GA3, which not only impaired the production of somatic embryos, but also significantly decreased expression of this gene.

Proteomics in commercial crops: An overview

Proteomics is a rapidly growing area of biological research that is positively affecting plant science. Recent advances in proteomic technology, such as mass spectrometry, can now identify a broad range of proteins and monitor their modulation during plant growth and development, as well as during responses to abiotic and biotic stresses. In this review, we highlight recent proteomic studies of commercial crops and discuss the advances in understanding of the proteomes of these crops. We anticipate that proteomic-based research will continue to expand and contribute to crop improvement.

SIGNIFICANCE

Plant proteomics study is a rapidly growing area of biological research that is positively impacting plant science. With the recent advances in new technologies, proteomics not only allows us to comprehensively analyses crop proteins, but also help us to understand the functions of the genes. In this review, we highlighted recent proteomic studies in commercial crops and updated the advances in our understanding of the proteomes of these crops. We believe that proteomic-based research will continue to grow and contribute to the improvement of crops.

Proteomic analysis of stipe explants reveals differentially expressed proteins involved in early direct somatic embryogenesis of the tree fern Cyathea delgadii Sternb

Using cyto-morphological analysis of somatic embryogenesis (SE) in the tree fern Cyathea delgadii as a guide, we performed a comparative proteomic analysis in stipe explants undergoing direct SE. Plant material was cultured on hormone-free medium supplemented with 2% sucrose. Phenol extracted proteins were separated using two-dimensional gel electrophoresis (2-DE) and mass spectrometry was performed for protein identification. A total number of 114 differentially regulated proteins was identified during early SE, i.e. when the first cell divisions started and several-cell pro-embryos were formed. Proteins were assigned to seven functional categories: carbohydrate metabolism, protein metabolism, cell organization, defense and stress responses, amino acid metabolism, purine metabolism, and fatty acid metabolism. Carbohydrate and protein metabolism were found to be the most sensitive SE functions with the greatest number of alterations in the intensity of spots in gel. Differences, especially in non-enzymatic and structural protein abundance, are indicative for cell organization, including cytoskeleton rearrangement and changes in cell wall components. The highest induced changes concern those enzymes related to fatty acid metabolism. Global analysis of the proteome reveals several proteins that can represent markers for the first 16days of SE induction and expression in fern. The findings of this research improve the understanding of molecular processes involved in direct SE in C. delgadii.

Gene expression and metabolite profiling of gibberellin biosynthesis during induction of somatic embryogenesis in Medicago truncatula Gaertn

Gibberellins (GAs) are involved in the regulation of numerous developmental processes in plants including zygotic embryogenesis, but their biosynthesis and role during somatic embryogenesis (SE) is mostly unknown. In this study we show that during three week- long induction phase, when cells of leaf explants from non-embryogenic genotype (M9) and embryogenic variant (M9-10a) were forming the callus, all the bioactive gibberellins from non-13-hydroxylation (GA4, GA7) and 13-hydroxylation (GA1, GA5, GA3, GA6) pathways were present, but the contents of only a few of them differed between the tested lines. The GA53 and GA19 substrates synthesized by the 13-hydroxylation pathway accumulated specifically in the M9-10a line after the first week of induction; subsequently, among the bioactive gibberellins detected, only the content of GA3 increased and appeared to be connected with acquisition of embryogenic competence. We fully annotated 20 Medicago truncatula orthologous genes coding the enzymes which catalyze all the known reactions of gibberellin biosynthesis. Our results indicate that, within all the genes tested, expression of only three: MtCPS, MtGA3ox1 and MtGA3ox2, was specific to embryogenic explants and reflected the changes observed in GA53, GA19 and GA3 contents. Moreover, by analyzing expression of MtBBM, SE marker gene, we confirmed the inhibitory effect of manipulation in GAs metabolism, applying exogenous GA3, which not only impaired the production of somatic embryos, but also significantly decreased expression of this gene.

An iTRAQ-based proteomics approach to clarify the molecular physiology of somatic embryo development in Prince Rupprecht's larch (Larix principis-rupprechtii Mayr)

Prince Rupprecht's larch (Larix principis-rupprechtii Mayr) is a native high-value forest tree species in North China whose clonal propagation through somatic embryogenesis (SE) has the potential to rapidly capture the benefits of breeding or genetic engineering programs and to improve raw material uniformity and quality. To date, research has focused on clarifying the molecular mechanism of SE, but proteomic studies are still in the early stages. In this study, isobaric tags for relative and absolute quantitation (iTRAQ) analysis was performed on three developmental stages of SE in L. principis-rupprechtii in an attempt to identify a wide range of proteins that are regulated differentially during this process. Proteins were extracted and analyzed from the pro-embryogenic mass (PEM), globular embryo (GE), and cotyledon embryo (CE) stages of embryo development. We detected 503 proteins in total and identified 96 proteins expressed differentially during different developmental stages. The identified proteins were analyzed further to provide information about their expression patterns and functions during SE. Four clusters of proteins based on shared expression profiles were generated. Functional analysis showed that proteins involved in primary metabolism, phosphorylation, and oxidation reduction were upregulated during somatic embryo development. This work provides novel insights into the process of larch embryo development in vitro and a basis for further study of the biological process and opportunities for practical application of this knowledge.

Comparative quantitative proteomic analysis of embryogenic and non-embryogenic calli in maize suggests the role of oxylipins in plant totipotency

Totipotency, the ability of somatic plant cell to generate whole plant through somatic embryogenesis, is still not well understood. In this study, maize immature zygotic embryos were used to generate embryogenic (EC) and non-embryogenic (NEC) calli. In order to compare proteomes of EC and NEC, two-dimensional electrophoresis (2-DE) in combination with mass spectrometry was used. This approach resulted into 361 quantified 2-DE spots out of which 44 were found statistically significantly differentially abundant between EC and NEC. Mass spectrometry provided the identity for 23 proteins that were classified into 8 metabolic categories. The most abundant were proteins associated with energy followed by proteins associated with disease and defense. Based on the abundances of identified proteins in this and other studies, working model for plant totipotency was proposed. One aspect of this working model suggests that increased abundances of proteins associated with pyruvate biosynthesis and suppression of embryogenic genes might be responsible for differences between EC and NEC cells. Furthermore we speculate that the increased abundance of lipoxygenase in the NEC cells results in changes in the equilibrium levels of one or more signaling molecules and is at least partly responsible for somatic cell reprogramming during totipotency.

BIOLOGICAL SIGNIFICANCE

Totipotency, the ability of somatic plant cell to generate whole plant through somatic embryogenesis, is still not well understood. In order to further advance understanding of this biological phenomenon, proteomes of embryogenic and non-embryogenic callus, derived from immature zygotic embryos of inbred maize line A19, were compared using 2-DE based proteomic technology. Based on the abundances of identified proteins in this and other studies, working model for plant totipotency was proposed. One aspect of this working model suggests that increased abundances of proteins associated with pyruvate biosynthesis and suppression of embryogenic genes might be responsible for differences between EC and NEC cells. Furthermore we speculate that the increased abundance of lipoxygenase in the NEC cells results in changes in the equilibrium levels of one or more signaling molecules and is at least partly responsible for somatic cell reprogramming during totipotency. This article is part of a Special Issue entitled: Environmental and structural proteomics.

Involvement of peroxidase activity in developing somatic embryos of Medicago arborea L. Identification of an isozyme peroxidase as biochemical marker of somatic embryogenesis

The legume Medicago arborea L. is very interesting as regards the regeneration of marginal arid soils. The problem is that it does not have a good germinative yield. It was therefore decided to regenerate via somatic embryogenesis and find a marker of embryogenic potential. In this study, peroxidase activity was evaluated in non-embryogenic and embryogenic calli from M. arborea L. A decrease in soluble peroxidase activity is observed in its embryonic calli at the time at which the somatic embryos begin to appear. This activity is always lower in embryonic calli than in non-embryonic ones (unlike what happens in the case of wall-bound peroxidases). These results suggest that peroxidases can be considered to be enzymes involved in somatic embryogenesis in M. arborea. In addition, isozyme analyses were carried out on protein extracts using polyacrylamide gel electrophoresis. The band called P5 was detected only in embryogenic cultures at very early stages of development. This band was digested with trypsin and analyzed using linear ion trap (LTQ) mass spectrometer. In P5 isoform a peroxidase-L-ascorbate peroxidase was identified. It can be used as a marker that allows the identification of embryological potential.

Proteomic analysis and polyamines, ethylene and reactive oxygen species levels of Araucaria angustifolia (Brazilian pine) embryogenic cultures with different embryogenic potential

Somatic embryogenesis is an important biotechnological tool in the large-scale propagation of elite genotypes and ex situ conservation of conifer species. Protocols for the induction and proliferation of embryogenic cultures (ECs) of Brazilian pine (Araucaria angustifolia (Bert.) O. Ktze) are well established, although the proper formation of mature somatic embryos (SEs) is still problematic. Thus, the identification of molecular markers for the screening of ECs able to respond to maturation conditions (abscisic acid and osmotic agents) is highly desirable. To develop molecular markers for the early detection of ECs able to develop well-formed SEs under maturation conditions, we analyzed the proteins found during the proliferation phase of A. angustifolia cell lines with different embryogenic capabilities, with one cell line being responsive to maturation conditions (R cell line), and one cell line that presented blocked development of SEs (B cell line). In addition, based on the peptides identified, polyamine levels (free and conjugate), ethylene production and reactive oxygen species (ROS) emission were analyzed using both EC lines (R and B cell lines). A marked difference in the biochemistry of ECs between these two cell lines was observed. Eleven proteins that were differentially expressed in the cell lines were identified by the combination of two-dimensional electrophoresis (2-DE) and MALDI-TOF/TOF mass spectrometry. Among these, S-adenosylmethionine synthase, the enzyme associated with polyamines and ethylene biosynthesis, was observed exclusively in the R cell line, while a protein linked to the oxidative stress subunit F of NADH dehydrogenase was observed exclusively in the B cell lines. Additionally, B cell lines showed higher levels of diamine putrescine and lower levels of ethylene. Higher values of ethylene and ROS were observed for the cell line that showed normal development of SEs. Altogether, our results open new perspectives in the optimization of culture conditions for A. angustifolia somatic embryogenesis, as well as establishing biochemical markers for the early selection of ECs during maturation trials.

Comparative proteomic analysis of off-type and normal phenotype somatic plantlets derived from somatic embryos of Feijoa (Acca sellowiana (O. Berg) Burret)

Morphological disorders in a relevant portion of emerged somatic embryos have been a limiting factor in the true-to-type plantlet formation in Acca sellowiana. In this sense, the present study undertook a comparison between normal phenotype and off-type somatic plantlets protein profiles by means of the 2-D DIGE proteomics approach. Off-type and normal phenotype somatic plantlets obtained at 10 and 20 days conversion were evaluated. Results indicated 12 exclusive spots between normal and off-type plantlets at 10 days conversion, and 17 exclusive spots at 20 days conversion. Also at 20 days conversion, 4 spots were differentially expressed, up- or down-regulated. Two proteins related to carbohydrate metabolism were only expressed in off-types at 10 days conversion, suggesting a more active respiratory pathway. A vicilin-like storage protein was only found in off-types at 20 days conversion, indicating that plantlets may present an abnormality in the mobilization of storage compounds, causing reduced vigor in the development of derived plantlets. The presence of heat shock proteins were only observed during formation of normal phenotype somatic plantlets, indicating that these proteins may be involved in normal morphogenesis of plantlets formed. These new findings shed light on possible genetic or epigenetic mechanisms governing A. sellowiana morphogenesis.

Potential link between biotic defense activation and recalcitrance to induction of somatic embryogenesis in shoot primordia from adult trees of white spruce (Picea glauca)

BACKGROUND

Among the many commercial opportunities afforded by somatic embryogenesis (SE), it is the ability to clonally propagate individual plants with rare or elite traits that has some of the most significant implications. This is particularly true for many long-lived species, such as conifers, but whose long generation times pose substantive challenges, including increased recalcitrance for SE as plants age. Identification of a clonal line of somatic embryo-derived trees whose shoot primordia have remained responsive to SE induction for over a decade, provided a unique opportunity to examine the molecular aspects underpinning SE within shoot tissues of adult white spruce trees.

RESULTS

Microarray analysis was used to conduct transcriptome-wide expression profiling of shoot explants taken from this responsive genotype following one week of SE induction, which when compared with that of a nonresponsive genotype, led to the identification of four of the most differentially expressed genes within each genotype. Using absolute qPCR to expand the analysis to three weeks of induction revealed that differential expression of all eight candidate genes was maintained to the end of the induction treatment, albeit to differing degrees. Most striking was that both the magnitude and duration of candidate gene expression within the nonresponsive genotype was indicative of an intense physiological response. Examining their putative identities further revealed that all four encoded for proteins with similarity to angiosperm proteins known to play prominent roles in biotic defense, and that their high-level induction over an extended period is consistent with activation of a biotic defense response. In contrast, the more temperate response within the responsive genotype, including induction of a conifer-specific dehydrin, is more consistent with elicitation of an adaptive stress response.

CONCLUSIONS

While additional evidence is required to definitively establish an association between SE responsiveness and a specific physiological response, these results suggest that biotic defense activation may be antagonistic, likely related to the massive transcriptional and metabolic reprogramming that it elicits. A major issue for future work will be to determine how and if suppressing biotic defense activation could be used to promote a physiological state more conducive to SE induction.