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

Comparative transcriptome analysis of high- and low-embryogenic Hevea brasiliensis genotypes reveals involvement of phytohormones in somatic embryogenesis

BACKGROUND

Rubber plant (Hevea brasiliensis) is one of the major sources of latex. Somatic embryogenesis (SE) is a promising alterative to its propagation by grafting and seed. Phytohormones have been shown to influence SE in different plant species. However, limited knowledge is available on the role of phytohormones in SE in Hevea. The anther cultures of two Hevea genotypes (Yunyan 73477-YT and Reken 628-RT) with contrasting SE rate were established and four stages i.e., anthers (h), anther induced callus (y), callus differentiation state (f), and somatic embryos (p) were studied. UPLC-ESI-MS/MS and transcriptome analyses were used to study phytohormone accumulation and related expression changes in biosynthesis and signaling genes.

RESULTS

YT showed higher callus induction rate than RT. Of the two genotypes, only YT exhibited successful SE. Auxins, cytokinins (CKs), abscisic acid (ABA), jasmonic acid (JA), salicylic acid (SA), gibberellins (GAs), and ethylene (ETH) were detected in the two genotypes. Indole-3-acetic acid (IAA), CKs, ABA, and ETH had notable differences in the studied stages of the two genotypes. The differentially expressed genes identified in treatment comparisons were majorly enriched in MAPK and phytohormone signaling, biosynthesis of secondary metabolites, and metabolic pathways. The expression changes in IAA, CK, ABA, and ETH biosynthesis and signaling genes confirmed the differential accumulation of respective phytohormones in the two genotypes.

CONCLUSION

These results suggest potential roles of phytohormones in SE in Hevea.

Transcriptome Analysis of Embryogenic and Non-Embryogenic Callus of Picea Mongolica

Picea mongolica is a rare tree species in China, which is of great significance in combating desertification and improving the harsh ecological environment. Due to the low rate of natural regeneration, high mortality, and susceptibility to pests and cold springs, Picea mongolica has gradually become extinct. At present, somatic embryogenesis (SE) is the most effective method of micro-proliferation in conifers, but the induction rate of embryogenic callus (EC) is low, and EC is difficult to differentiate from non-embryonic callus (NEC). Therefore, the EC and NEC of Picea mongolica were compared from the morphology, histological, physiological, and transcriptional levels, respectively. Morphological observation showed that the EC was white and transparent filamentous, while the NEC was compact and brownish-brown lumpy. Histological analyses showed that the NEC cells were large and loosely arranged; the nuclei attached to the edge of the cells were small; the cytoplasm was low; and the cell gap was large and irregular. In the EC, small cells, closely arranged cells, and a large nucleus and nucleolus were observed. Physiological studies showed significant differences in ROS-scavenging enzymes between the EC and NEC. Transcriptome profiling revealed that 13,267 differentially expressed genes (DEGs) were identified, 3682 were up-regulated, and 9585 were down-regulated. In total, 63 GO terms had significant enrichment, 32 DEGs in plant hormone signal transduction pathway were identified, and 502 different transcription factors (TFs) were characterized into 38 TF families. Meanwhile, we identified significant gene expression trends associated with somatic embryo development in plant hormones (AUX/IAA, YUCCA, LEA, etc.), stress (GST, HSP, GLP, etc.), phenylpropanoid metabolism (4CL, HCT, PAL, etc.), and transcription factors (AP2/ERF, MYB, WOX, etc.). In addition, nine genes were chosen for RT-qPCR, and the results were consistent with RNA-Seq data. This study revealed the changes in morphology, histology, physiology, and gene expression in the differentiation of NEC into EC and laid the foundation for finding the key genes to promote EC formation.

Integrated proteome and acetylome analyses provide novel insights into early somatic embryogenesis of Dimocarpus longan

Longan (Dimocarpus longan) is a precious subtropical fruit with high nutritional value. The somatic embryogenesis (SE) affects the quality and yield of fruit. Apart from clonal propagation, SE has extensive applications in genetic improvement and mutation. Thus, understanding the molecular basis of embryogenesis in longan will help to develop strategies for mass production of quality planting material. Lysine acetylation (Kac) plays an important role in diverse cellular processes, but limited knowledge is available regarding acetylation modifications in plant early SE. In this study, the proteome and acetylome of longan embryogenic callus (ECs) and globular embryos (GEs) were investigated. In total, 7232 proteins and 14,597 Kac sites were identified, and this resulted in the discovery of 1178 differentially expressed proteins and 669 differentially expressed acetylated proteins. KEGG and GO analysis showed that glucose metabolism, carbon metabolism, fatty acid degradation, and oxidative phosphorylation pathways were influenced by Kac modification. Furthermore, sodium butyrate (Sb, a deacetylase inhibitor) led to reduced the proliferation and delayed the differentiation of ECs by regulating the homeostasis of reactive oxygen species (ROS) andindole-3-acetic acid (IAA). Our study provides a comprehensive proteomic and acetylomic analysis to aid in understanding the molecular mechanisms involved in early SE, representing a potential tool for genetic improvement of longan.

A Comparative Transcriptome Analysis Reveals the Molecular Mechanisms That Underlie Somatic Embryogenesis in Peaonia ostii ‘Fengdan’

Low propagation rate is the primary problem that limits industry development of tree peony. In this study, a highly efficient regeneration system for tree peony using somatic embryogenesis (SE) was established. The transcriptomes of zygotic embryo explants (S0), non-embryonic callus (S1), embryonic callus (S2), somatic embryos (S3), and regenerated shoots (S4) were analyzed to determine the regulatory mechanisms that underlie SE in tree peony. The differentially expressed genes (DEGs) were identified in the pairwise comparisons of S1-vs-S2 and S1-vs-S3, respectively. The enriched DEGs were primarily involved in hormone signal transduction, stress response and the nucleus (epigenetic modifications). The results indicated that cell division, particularly asymmetric cell division, was enhanced in S3. Moreover, the genes implicated in cell fate determination played central roles in S3. Hormone signal pathways work in concert with epigenetic modifications and stress responses to regulate SE. SERK, WOX9, BBM, FUS3, CUC, and WUS were characterized as the molecular markers for tree peony SE. To our knowledge, this is the first study of the SE of tree peony using transcriptome sequencing. These results will improve our understanding of the molecular mechanisms that underly SE in tree peony and will benefit the propagation and genetic engineering of this plant.

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.

Polycomb Repressive Complex 2-mediated histone modification H3K27me3 is associated with embryogenic potential in Norway spruce

Epigenetic reprogramming during germ cell formation is essential to gain pluripotency and thus embryogenic potential. The histone modification H3K27me3, which is catalysed by the Polycomb repressive complex 2 (PRC2), regulates important developmental processes in both plants and animals, and defects in PRC2 components cause pleiotropic developmental abnormalities. Nevertheless, the role of H3K27me3 in determining embryogenic potential in gymnosperms is still elusive. To address this, we generated H3K27me3 profiles of Norway spruce (Picea abies) embryonic callus and non-embryogenic callus using CUT&RUN, which is a powerful method for chromatin profiling. Here, we show that H3K27me3 mainly accumulated in genic regions in the Norway spruce genome, similarly to what is observed in other plant species. Interestingly, H3K27me3 levels in embryonic callus were much lower than those in the other examined tissues, but markedly increased upon embryo induction. These results show that H3K27me3 levels are associated with the embryogenic potential of a given tissue, and that the early phase of somatic embryogenesis is accompanied by changes in H3K27me3 levels. Thus, our study provides novel insights into the role of this epigenetic mark in spruce embryogenesis and reinforces the importance of PRC2 as a key regulator of cell fate determination across different plant species.

Phenylpropanoids Are Connected to Cell Wall Fortification and Stress Tolerance in Avocado Somatic Embryogenesis

Somatic embryogenesis (SE) is a valuable model for understanding the mechanism of plant embryogenesis and a tool for the mass production of plants. However, establishing SE in avocado has been complicated due to the very low efficiency of embryo induction and plant regeneration. To understand the molecular foundation of the SE induction and development in avocado, we compared embryogenic (EC) and non-embryogenic (NEC) cultures of two avocado varieties using proteomic and metabolomic approaches. Although Criollo and Hass EC exhibited similarities in the proteome and metabolome profile, in general, we observed a more active phenylpropanoid pathway in EC than NEC. This pathway is associated with the tolerance of stress responses, probably through the reinforcement of the cell wall and flavonoid production. We could corroborate that particular polyphenolics compounds, including p-coumaric acid and t-ferulic acid, stimulated the production of somatic embryos in avocado. Exogen phenolic compounds were associated with the modification of the content of endogenous polyphenolic and the induction of the production of the putative auxin-a, adenosine, cellulose and 1,26-hexacosanediol-diferulate. We suggest that in EC of avocado, there is an enhanced phenylpropanoid metabolism for the production of the building blocks of lignin and flavonoid compounds having a role in cell wall reinforcement for tolerating stress response. Data are available at ProteomeXchange with the identifier PXD019705.

Stress and cell cycle regulation during somatic embryogenesis plays a key role in oil palm callus development

Oil palm is an oleaginous plant of relevant economic importance since its fruits are rich in vegetable oil. These plants have a single apical meristem and the main method for vegetative propagation is somatic embryogenesis. The aim of this study was to identify differentially abundant proteins from oil palm genotypes contrasting in the capacity of embryogenic competence acquisition, using shotgun proteomics. Oil palm leaves were subjected to callus induction and the material was collected in biological triplicates at 14 and 90 days of callus induction. LC-MS/MS analysis was performed and revealed a total of 4695 proteins. Responsive and non-responsive genotypes were compared at 14 and 90 days of callus induction and 221 differentially abundant proteins were obtained. The data analysis revealed several proteins mainly related to energy metabolism, stress response and regulation of cell cycle, further analyzed by qRT-PCR, which seem important for embryogenic development. We suggest some of these proteins as key factors for the success of callus formation in oil palm including antioxidant and cell division proteins as well as proteins involved in the ubiquitination pathway. These proteins may also be potential biomarkers for the acquisition of embryogenic competence. SIGNIFICANCE: Antioxidant and cell division proteins as well as proteins involved in the ubiquitination pathway are key factors for the success of callus formation in oil palm. The proteins identified in this study may be potential biomarkers for embryogenic competence acquisition.

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.