Morphological analyses and variation in carbohydrate content during the maturation of somatic embryos of Carica papaya

Functional analysis of the eTM-miR171-SCL6 module regulating somatic embryogenesis in Lilium pumilum DC. Fisch

Somatic embryogenesis (SE) is of great significance in Lilium bulb production, germplasm preservation and genetic improvement. miRNAs are important regulators of plant growth and development at the transcriptional level. Previous research by our group has shown that lpu-miR171 and its target gene SCARECROW-LIKE 6 (SCL6) play an important regulatory role in lily SE, and we predicted and identified that endogenous target mimics (eTMs) can regulate lpu-miR171. However, the associated mechanism and internal regulatory network are not yet clear. In the present study, lpu-miR171 was used as an entry point to explore the regulatory network between its upstream eTMs and its downstream target gene LpSCL6, as well as to identify the mechanism of this regulatory network in Lilium SE. Tobacco transient transformation confirmed that miRNA171 significantly inhibited the expression of LpSCL6. On this basis, the Lilium stable genetic transformation system was used to demonstrate that silencing lpu-miR171a and lpu-miR171b and overexpressing LpSCL6-II and LpSCL6-I promoted starch accumulation in calli and the expression of key cell cycle genes, thus providing energy to meet preconditions for SE and accelerate the formation and development of Lilium somatic embryos. LpSCL6-II and LpSCL6-I are nuclear proteins with self-activation activity in yeast cells. In addition, we confirmed in Lilium that lpu-eTM171 is the eTM of lpu-miR171 that binds lpu-miR171 to prevent cleavage of the target gene LpSCL6, thereby promoting SE. Therefore, the present study established a new mechanism whereby the eTM-miR171-SCL6 module regulates SE in Lilium pumilum DC. Fisch. and provided new insights clarifying the mechanism of SE.

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.

Mitochondrial proteomics reveals new insights into embryogenic competence acquisition in Carica papaya L. callus

Understanding the mechanisms that endow a somatic cell with the ability to differentiate into a somatic embryo, which could result in numerous biotechnological applications, is still a challenge. The objective of this work was to identify some of the molecular and physiological mechanisms responsible for the acquisition of embryogenic competence during somatic embryogenesis in Carica papaya L. We performed a broad characterization of embryogenic (EC) and nonembryogenic calli (NEC) of using global and mitochondrial proteomic approaches, histomorphology, histochemistry, respiratory activity, and endogenous hormonal and hydrogen peroxide (H₂O₂) contents. EC and NEC presented remarkable differences in anatomical and histochemical characteristics, with EC showing a higher reactivity for the presence of proteins and neutral polysaccharides. Our results demonstrate that mitochondrial metabolism affects the embryogenic competence of C. papaya callus. The EC presented higher participation of alternative oxidase (AOX) enzymes, higher total cell respiration and presented a stronger accumulation of mitochondrial stress response proteins. Differential accumulation of auxin-responsive Gretchen Hagen 3 (GH3) family proteins in EC was related to a decrease in the content of free 2,4-dichlorophenoxyacetic acid (2,4-D). EC also showed higher endogenous H₂O₂ contents. H₂O₂ is a promising molecule for further investigation in differentiation protocols for C. papaya somatic embryos. SIGNIFICANCE: To further advance the understanding of somatic embryogenesis, we performed a broad characterization of embryogenic and nonembryogenic callus, through global and mitochondrial proteomic approaches, histomorphology, histochemistry, respiratory activity, and endogenous hormonal and hydrogen peroxide contents. Based on these results, we propose a working model for the competence of papaya callus. This model suggests that GH3 proteins play an important role in the regulation of auxins. In addition, embryogenic callus showed a greater abundance of stress response proteins and folding proteins. Embryogenic callus respiration occurs predominantly via AOX, and the inhibition of its activity is capable of inhibiting callus differentiation. Although the embryogenic callus presented greater total respiration and a greater abundance of oxidative phosphorylation proteins, they had less COX participation and less coupling efficiency, indicating less ATP production.

Direct and Indirect Somatic Embryogenesis Induction in Camellia oleifera Abel

Camellia oleifera Abel. is an important woody oil species; however, the shortage of rapid and industrialized seedling culture is a large constraint on the development of the tea oil industry. Somatic embryogenesis (SE) is one of the main powerful biotechnological tools for plant mass regeneration, but the largely unknown SE in C. oleifera limits the scale production of clonal plants. In this study, we described a high-efficiency SE system via direct and indirect pathways in C. oleifera and investigated the effect of genotype, explant age and phytohormones on SE. In the direct pathway, somatic embryos were highly induced from immature seeds 220 days after full blossom, and the development of embryoids was achieved with a combination of 0.19 mg/L 2,4-dichlorophenoxyacetic acid (2,4-D) and 0.05 mg/L thidiazuron (TDZ). In the indirect pathway, embryogenic calli were induced from the same explants in medium containing 1.5 mg/L 2,4-D, while 0.75 mg/L 2,4-D treatment led to high proliferation rates for embryogenic calli. The addition of 0.19 mg/L 2,4-D alone stimulated the production of globular embryos while causing a 75% loss of the induction rate in the heart embryo stage. Upon transfer of the globular embryos to phytohormone-free medium, an optimal induction rate of 62.37% from globular embryos to cotyledonary embryos was obtained. These data suggest that the subsequent differentiation process after the globular embryo stage in ISE is more similar to an endogenous phytohormones-driven process. Mature embryos germinated to produce intact plantlets on half-strength MS basal medium with a regeneration rate of 63.67%. Histological analysis confirmed the vascular bundle isolation of embryoids from the mother tissue. We further studied the different varieties and found that there were no significant genotype differences for SE induction efficiency in C. oleifera. Thus, we established a high-efficiency induction system for direct and indirect somatic embryogenesis (ISE) in C. oleifera and regenerated intact plantlets via SE, not organogenesis. ISE has a more complicated induction and regulatory mechanism than direct somatic embryogenesis. The improved protocol of SE would benefit mass propagation and genetic manipulation in C. oleifera.

Stage-specific protein regulation during somatic embryo development of Carica papaya L. 'Golden'

Somatic embryogenesis is an important biotechnological technique for large-scale propagation of elite genotypes. Identifying stage-specific compounds associated with somatic embryo development can help elucidate the ontogenesis of Carica papaya L. somatic embryos and improve tissue culture protocols. To identify the stage-specific proteins that are present during the differentiation of C. papaya somatic embryos, proteomic analyses of embryos at the globular, heart, torpedo and cotyledonary developmental stages were performed. Mass spectrometry data have been deposited in the ProteomeXchange with the dataset identifier PXD021107. Comparative proteomic analyses revealed a total of 801 proteins, with 392 classified as differentially accumulated proteins in at least one of the developmental stages. The globular-staged presented a higher number of unique proteins (16), and 7 were isoforms of 60S ribosomal proteins, suggesting high translational activity at the beginning of somatic embryogenesis. Proteins related to mitochondrial metabolism accumulated to a high degree at the early developmental stages and then decreased with increasing development, and they contributed to cell homeostasis in early somatic embryos. A progressive increase in the accumulation of vicilin, late embryogenesis abundant proteins and chloroplastic proteins that lead to somatic embryo maturation was also observed. The differential accumulation of acetylornithine deacetylase and S-adenosylmethionine synthase 2 proteins was correlated with increases in putrescine and spermidine contents, which suggests that both polyamines should be tested to determine whether they increase the conversion rates of globular- to cotyledonary-staged somatic embryos. Taken together, the results showed that somatic embryo development in C. papaya is regulated by the differential accumulation of proteins, with ribosomal and mitochondrial proteins more abundant during the early somatic embryo stages and seed maturation proteins more abundant during the late stages.

Physiological and Biochemical Traits in Korean Pine Somatic Embryogenesis

Korean pine broadleaf mixed forest is an important ecosystem for maintaining biodiversity in Northeast China. Korean pine is also an important species for the production of timber and nuts in the mountainous areas of Northeast China. In this study, we compared three types of Korean pine callus and found that embryogenic callus had high amounts of storage substances (protein, sugar and starch). Non-embryonic callus had high levels of polyphenols and polyphenol oxidation, while callus that lost somatic embryogenesis potential had lower levels of storage substances (protein, sugar and starch) and higher contents of peroxidase and catalase. These results indicate that high contents of storage substances (protein, sugar and starch), and low levels of polyphenols and polyphenol oxidase can be used as physiological markers of callus with somatic embryogenic potential. During the development process of Korean pine somatic embryos, fresh weight and dry weight gradually increased, while water content gradually decreased. Soluble protein, starch, soluble sugar and superoxide dismutase also increased during development, while peroxidase and catalase levels reduced over time. These results indicate that somatic embryogenesis involves energy storage, and antioxidant enzymes cooperate to regulate the occurrence and development of embryos. These results provide physiological markers for identification of embryogenic callus with somatic embryogenesis, to evaluate callus suitable for somatic embryogenesis, and provide basis for further research on the molecular mechanisms of somatic embryogenesis.

Explant Type, Culture System, 6-Benzyladenine, Meta-Topolin and Encapsulation Affect Indirect Somatic Embryogenesis and Regeneration in Carica papaya L

A protocol to propagate papaya hybrid plants through indirect somatic embryogenesis was developed considering the effect of explant type, culture system, particular cytokinins and encapsulation, in different stages of the process. Optimal 2,4-dichlorophenoxyacetic acid (2,4-D) concentrations for non-embryogenic callus formation ranged between 9.0 and 27.1 μM in half-cut seeds, while higher concentrations were harmful. Non-embryogenic callus was also obtained with 22-158 μM 2,4-D from hypocotyl segments. Callus with embryogenic structures was only obtained in half-cut seeds cultured in the darkness on half-strength Murashige and Skoog culture medium supplemented with 2,4-D, while hypocotyl segments and isolated zygotic embryos failed to produce this type of callus regardless of the 2,4-D and sucrose (30 and 70 g l-1) concentrations tested in this study. Both, embryogenic callus development and quantity of somatic embryos formed per embryogenic callus, which ranged between 11 and 31 units after 14 months, required 2,4-D, but without any effect of the concentration. Histological studies confirmed the multicellular origin of the somatic embryos. In further steps, liquid medium induced over four times more somatic embryos than agar-gelled medium and showed significantly higher production of globular somatic embryos (85 vs. 57%). Both, 6-benzyladenine (BA) and meta-topolin (Mtop) stimulated sprouting (40-45%) of the somatic embryos (development of shoots only) in concentrations of up to 2.7 and 10 μM, respectively. Sprouting probability showed a 2nd order polynomial trend despite the range of concentration used for each cytokinin. This is the first report about the positive effect of Mtop on the apical shoot development of Carica papaya somatic embryos known to the authors. Radicle growth was observed in 5% or less of the cultivated embryos, regardless of the BA concentration. Finally, all encapsulation conditions tested (2.5, 3.5, and 4.5% sodium alginate, combined with 50 and 100 mM CaCl2) reduced sprouting of somatic embryos when compared to the non-encapsulated ones, whereas capsule hardness showed low correlation with embryo sprouting. Embryos were further cultivated until they became plantlets approximately 5 cm long. They were acclimatized and afterward planted in the field, where they flowered and produced fruit.