Embryogenic cells in plant tissue cultures: Occurrence and behavior

Transcriptomic profiles of non-embryogenic and embryogenic callus cells in a highly regenerative upland cotton line (Gossypium hirsutum L.)

Background

Genotype independent transformation and whole plant regeneration through somatic embryogenesis relies heavily on the intrinsic ability of a genotype to regenerate. The critical genetic architecture of non-embryogenic callus (NEC) cells and embryogenic callus (EC) cells in a highly regenerable cotton genotype is unknown.

Results

In this study, gene expression profiles of a highly regenerable Gossypium hirsutum L. cultivar, Jin668, were analyzed at two critical developmental stages during somatic embryogenesis, non-embryogenic callus (NEC) cells and embryogenic callus (EC) cells. The rate of EC formation in Jin668 is 96%. Differential gene expression analysis revealed a total of 5333 differentially expressed genes (DEG) with 2534 genes upregulated and 2799 genes downregulated in EC. A total of 144 genes were unique to NEC cells and 174 genes were unique to EC. Clustering and enrichment analysis identified genes upregulated in EC that function as transcription factors/DNA binding, phytohormone response, oxidative reduction, and regulators of transcription; while genes categorized in methylation pathways were downregulated. Four key transcription factors were identified based on their sharp upregulation in EC tissue; LEAFY COTYLEDON 1 (LEC1), BABY BOOM (BBM), FUSCA (FUS3) and AGAMOUS-LIKE15 with distinguishable subgenome expression bias.

Conclusions

This comparative analysis of NEC and EC transcriptomes gives new insights into the genes involved in somatic embryogenesis in cotton.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12861-020-00230-4.

Changes in Cytomorphology, Expression of Retinoblastoma Related Gene, and Superoxide Dismutase Enzyme Activity in Maize Cell Culture Exposed to Silver Nanoparticles

The ever-increasing use of silver nanoparticles (nAg) in various products necessitates investigation of the behavior of biological systems encountering these particles. In this paper, considering maize as a biological model, the effects of colloidal nAg (<80nm) on its cell culture were investigated. For comparison purposes, silver nitrate was used as a representative of silver ion (Ag⁺). After stabilization of cell suspensions, they were treated with nAg and Ag⁺ (1 mg/l), then cell suspension growth was measured and the microscopic analysis and a cell viability test were performed. In addition, the activity of superoxide dismutase (SOD) enzyme was explored. Owing to the key role of retinoblastoma-related protein (RBR) in cell cycle as well as in development and differentiation processes, the relative expression of ZmRBR1 was studied in nAg and Ag⁺ exposure. Microscopic analyses revealed that cells in suspensions treated by nAg and Ag⁺ were morphologically classified into five types: embryogenic; larvae-like; long; swollen; and polarized. The results showed an increase in percentages of large and live cells in the treated suspensions. Remarkably, we observed some cells which were differentiated into trichomes along with some stages of trichome development in the treated cell suspensions. Moreover, exposure to nAg and Ag⁺ did not elevate the activity of SOD enzyme in the treated cells. Also, the relative expression of ZmRBR1 was slightly reduced in the treated cells. The findings of these experimentations indicated that nAg affected maize suspension-cultured cells in the same manner as Ag⁺.

A Comparison of In Vitro and In Vivo Asexual Embryogenesis

In plants, embryogenesis generally occurs through the sexual process of double fertilization, which involves a haploid sperm cell fusing with a haploid egg cell to ultimately give rise to a diploid embryo. Embryogenesis can also occur asexually in the absence of fertilization, both in vitro and in vivo. Somatic or gametic cells are able to differentiate into embryos in vitro following the application of plant growth regulators or stress treatments. Asexual embryogenesis also occurs naturally in some plant species in vivo, from either ovule cells as part of a process defined as apomixis, or from somatic leaf tissue in other species. In both in vitro and in vivo asexual embryogenesis, the embryo precursor cells must attain an embryogenic fate without the act of fertilization. This review compares the processes of in vitro and in vivo asexual embryogenesis including what is known regarding the genetic and epigenetic regulation of each process, and considers how the precursor cells are able to change fate and adopt an embryogenic pathway.

Somatic Embryogenesis: Still a Relevant Technique in Citrus Improvement

The genus Citrus contains numerous fresh and processed fruit cultivars that are economically important worldwide. New cultivars are needed to battle industry threatening diseases and to create new marketing opportunities. Citrus improvement by conventional methods alone has many limitations that can be overcome by applications of emerging biotechnologies, generally requiring cell to plant regeneration. Many citrus genotypes are amenable to somatic embryogenesis, which became a key regeneration pathway in many experimental approaches to cultivar improvement. This chapter provides a brief history of plant somatic embryogenesis with focus on citrus, followed by a discussion of proven applications in biotechnology-facilitated citrus improvement techniques, such as somatic hybridization, somatic cybridization, genetic transformation, and the exploitation of somaclonal variation. Finally, two important new protocols that feature plant regeneration via somatic embryogenesis are provided: protoplast transformation and Agrobacterium-mediated transformation of embryogenic cell suspension cultures.

From Stress to Embryos: Some of the Problems for Induction and Maturation of Somatic Embryos

Although somatic embryogenesis has been successfully achieved in numerous plant species, little is known about the mechanism(s) underlying this process. Changes in the balance of growth regulators of the culture medium, osmolarity, or amino acids as well as the genotype and developmental stage of the tissue used as initial explant may have a pivotal influence on the induction of somatic embryogenic cultures. Moreover, different stress agents (ethylene, activated charcoal, cold or heat or electrical shocks), as well as abscisic acid, can also foster the induction or further development of somatic embryos. In the process, cells first return to a stem cell-like status and then either enter their new program or dye when the stress level exceeds cell tolerance. Recalcitrance to differentiation of somatic cells into embryos is frequently observed, and problems such as secondary or recurrent embryogenesis, embryo growth arrest (at the globular stage or during the transition from torpedo to cotyledonary stage), and development of only the aerial part of somatic embryos can appear, interfering with normal germination and conversion of embryos to plants. Some solutions to solve these problems associated to embryogenesis are proposed and two very efficient somatic embryogenesis protocols for two model plant species are detailed.

Highly efficient protocol for callogenesis, somagenesis and regeneration of Indica rice plants

In the present study, we have reported a simple, fast and efficient regeneration protocol using mature embryos as explants, and discovered its effective applicability to a range of Indica rice genotypes. We have considered the response of six varieties in the steps of the regeneration procedure. The results showed that calli were variably developed from the scutellar region of seeds and visible within 6-20 days. The highest and lowest calli induction frequency (70% and 51.66%) and number of induced calli from seeds (14 and 10.33) were observed in MR269 and MRQ74, respectively. The maximum and minimum number (7.66 and 4) and frequency of embryogenic calli (38.33% and 20%) were recorded in MR219 and MRQ74, respectively. However, the highest browning rate was observed in MR84 (87%) and the lowest rate in MRQ50 (46%). The majority of plants regenerated from embryogenic calli were obtained from MRQ50 (54%) and the minimum number of plants from MR84. In this study, the maximum numbers of plantlets were regenerated from the varieties with highest rate of embryogenic calli. Also, various varieties, including MRQ50, MR269, MR276 and MR219, were satisfactorily responding, while MRQ74 and MR84 weakly responded to the procedure. Such a simple, successful and generalized method possesses the potential to become an important tool for crop improvement and functional studies of genes in rice as a model monocot plant.

Taxonomy and Biogeography of Apomixis in Angiosperms and Associated Biodiversity Characteristics

Apomixis in angiosperms is asexual reproduction from seed. Its importance to angiospermous evolution and biodiversity has been difficult to assess mainly because of insufficient taxonomic documentation. Thus, we assembled literature reporting apomixis occurrences among angiosperms and transferred the information to an internet database (http://www.apomixis.uni-goettingen.de). We then searched for correlations between apomixis occurrences and well-established measures of taxonomic diversity and biogeography. Apomixis was found to be taxonomically widespread with no clear tendency to specific groups and to occur with sexuality at all taxonomic levels. Adventitious embryony was the most frequent form (148 genera) followed by apospory (110) and diplospory (68). All three forms are phylogenetically scattered, but this scattering is strongly associated with measures of biodiversity. Across apomictic-containing orders and families, numbers of apomict-containing genera were positively correlated with total numbers of genera. In general, apomict-containing orders, families, and subfamilies of Asteraceae, Poaceae, and Orchidaceae were larger, i.e., they possessed more families or genera, than non-apomict-containing orders, families or subfamilies. Furthermore, many apomict-containing genera were found to be highly cosmopolitan. In this respect, 62% occupy multiple geographic zones. Numbers of genera containing sporophytic or gametophytic apomicts decreased from the tropics to the arctic, a trend that parallels general biodiversity. While angiosperms appear to be predisposed to shift from sex to apomixis, there is also evidence of reversions to sexuality. Such reversions may result from genetic or epigenetic destabilization events accompanying hybridization, polyploidy, or other cytogenetic alterations. Because of increased within-plant genetic and genomic heterogeneity, range expansions and diversifications at the species and genus levels may occur more rapidly upon reversion to sexuality. The significantly-enriched representations of apomicts among highly diverse and geographically-extensive taxa, from genera to orders, support this conclusion.

Cellular and molecular changes associated with somatic embryogenesis induction in Agave tequilana

In spite of the importance of somatic embryogenesis for basic research in plant embryology as well as for crop improvement and plant propagation, it is still unclear which mechanisms and cell signals are involved in acquiring embryogenic competence by a somatic cell. The aim of this work was to study cellular and molecular changes involved in the induction stage in calli of Agave tequilana Weber cultivar azul in order to gain more information on the initial stages of somatic embryogenesis in this species. Cytochemical and immunocytochemical techniques were used to identify differences between embryogenic and non-embryogenic cells from several genotypes. Presence of granular structures was detected after somatic embryogenesis induction in embryogenic cells; composition of these structures as well as changes in protein and polysaccharide distribution was studied using Coomassie brilliant blue and Periodic Acid-Schiff stains. Distribution of arabinogalactan proteins (AGPs) and pectins was investigated in embryogenic and non-embryogenic cells by immunolabelling using anti-AGP monoclonal antibodies (JIM4, JIM8 and JIM13) as well as an anti-methyl-esterified pectin-antibody (JIM7), in order to evaluate major modifications in cell wall composition in the initial stages of somatic embryogenesis. Our observations pointed out that induction of somatic embryogenesis produced accumulation of proteins and polysaccharides in embryogenic cells. Presence of JIM8, JIM13 and JIM7 epitopes were detected exclusively in embryogenic cells, which supports the idea that specific changes in cell wall are involved in the acquisition of embryogenic competence of A. tequilana.

Cloning and molecular characterisation of a potato SERK gene transcriptionally induced during initiation of somatic embryogenesis

Somatic embryogenesis offers great potential in plant propagation, long-term germplasm conservation, and as a suitable model system for deciphering early events during embryogenesis. The up-regulation and ectopic expression of a SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASE (SERK) gene has been shown to mark and enhance embryogenic competence in somatic cells of model plant species. We have cloned and characterised a SERK gene (StSERK1) from potato (Solanum tuberosum L.), an important crop plant. Sequence analysis of StSERK1 revealed high levels of similarity to other plant SERKs, as well as a conserved intron/exon structure which is unique to members of the SERK family. Furthermore, StSERK clustered most closely with SERK gene family members such as MtSERK1, CuSERK1, AtSERK1, and DcSERK, implicated in evoking somatic embryogenesis. Monitoring of SERK expression during progression of potato somatic embryogenesis revealed increased StSERK expression during the induction phase. Subsequently, during the embryo transition phases, StSERK expression was unchanged and did not vary among embryo-forming and inhibitory conditions. However, in isolated somatic embryos StSERK expression was again up-regulated. In other plant parts (leaves, true potato seeds, microtubers and flower buds), StSERK showed different levels of expression. Expression analysis suggests that the isolated StSERK could be a functional SERK orthologue. The possible role of SERK as a marker of pluripotency, rather than embryogenesis alone, is discussed.

AtSERK1 expression precedes and coincides with early somatic embryogenesis in Arabidopsis thaliana

The Arabidopsis thaliana primordia timing (pt) mutant was transformed with an AtSERK1::GUS construct. Liquid cultures of this line were used to study the relationship between somatic embryogenesis and the expression of SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASE (AtSERK1) as a marker for cells competent to form embryos. In order to search for the expression of AtSERK1::GUS during early stages of somatic embryogenesis, histochemical as well as immunochemical approaches were used for the detection of beta-glucuronidase (GUS). Four sites of AtSERK1 expression were found in the embryogenic cultures: in embryogenic callus, where primary somatic embryos developed; in the basal parts of primary somatic embryos; in the outer layers of cotyledons of primary somatic embryos where secondary embryos were formed; and in provascular and vascular strands of developing somatic embryos. The in vitro expression of AtSERK1::GUS coincides with embryogenic development up to the heart-shaped stage. Prior to the expression in embryos, AtSERK1 was expressed in single cells and small cell clusters, indicating that AtSERK1 indeed marks embryogenic competence. Its expression in (pro)vascular strands, suggests that embryogenic cells in tissue culture retain at least in part their original identity.

Regulation of in vitro somatic embryogenesis with emphasis on to the role of endogenous hormones

Different aspects of the in vitro somatic embryogenesis regulation are reviewed in this paper.work. A description of g General aspects, such as terminology, uses, stages of development and factors associated with the somatic embryogenesis, are described. is carried out. Although a brief description ofn the effects of the addition of different plant growth regulators to the culture medium wasis given, the article is centereds itself on the effect that the endogenous hormone concentrations in the initial explants and in the tissue cultures derived from them could play oin the induction and expression of somatic embryogenesis. It is significant that few to emphasize the low amount of systematic studies have been conducted, in this subject, in which different species and hormone groups were compared in cultures with and without embryogenic capacity. Moreover, the lack of correlation between the results presented in different studies the distinct works indicates that the hormone content of the cultures is not the only factor involved.

Strategies for "minimal growth maintenance" of cell cultures: a perspective on management for extended duration experimentation in the microgravity environment of a Space station

How cells manage without gravity and how they change in the absence of gravity are basic questions that only prolonged life on a Space station will enable us to answer. We know from investigations carried out on various kinds of Space vehicles and stations that profound physiological effects can and often to occur. We need to know more of the basic biochemistry and biophysics both of cells and of whole organisms in conditions of reduced gravity. The unique environment of Space affords plant scientists an unusual opportunity to carry out experiments in microgravity, but some major challenges must be faced before this can be done with confidence. Various laboratory activities that are routine on Earth take on special significance and offer problems that need imaginative resolution before even a relatively simple experiment can be reliably executed on a Space station. For example, scientists might wish to investigate whether adaptive or other changes that have occurred in the environment of Space are retained after return to Earth-normal conditions. Investigators seeking to carry out experiments in the low-gravity environment of Space using cultured cells will need to solve the problem of keeping cultures quiescent for protracted periods before an experiment is initiated, after periodic sampling is carried out, and after the experiment is completed. This review gives an evaluation of a range of strategies that can enable one to manipulate cell physiology and curtail growth dramatically toward this end. These strategies include cryopreservation, chilling, reduced oxygen, gel entrapment strategies, osmotic adjustment, nutrient starvation, pH manipulation, and the use of mitotic inhibitors and growth-retarding chemicals. Cells not only need to be rendered quiescent for protracted periods but they also must be recoverable and further grown if it is so desired. Elaboration of satisfactory procedures for management of cells and tissues at "near zero or minimal growth" will have great value and practical consequences for experimentation on Earth as well as in Space. All of the parameters and conditions and procedural details needed to meet all the specific objectives will be the basis of the design and fabrication of cell culture units for use in the Space environment. It is expected that this will be an evolutionary process.

Genetic control of androgenetic response in Lolium perenne L

In a study of androgenesis in 90 Norwegian genotypes of perennial ryegrass (Lolium perenne L.), heritabilities ranged from h b (2) =0.46 to 0.80. Very high or completely positive genotypic correlations were found between most characters of androgenetic response (e.g. embryo-like structures per 100 anthers, plants per 100 embryo-like structures, albino plants per 100 anthers, green plants per 100 anthers). Three genotypes, 2 Norwegian (7-5 and 9-5) and 1 Danish (245), which had significantly different androgenetic responses were selected to study the genetic control of the processes. Genotypes 7-5 and 9-5 were highly embryogenie, 7-5 and 245 were relatively high producers of green plants, while 9-5 was unable to produce green plants. The six possible reciprocal crosses between these three genotypes were made, and 10 or 11 F1 plants from each cross were used for anther culture experiments. The cross 7-5 x 245 showed average superiority over both parents for total plant regeneration and green plant production, results not previously reported. The phenotypic correlations estimated among progenies from the crosses ranged from r=-0.99(***) to 0.81(***). These considerable changes, relative to the results of the screening experiment, are most likely the result of changed allele frequencies caused by the strong selection of parents in these crosses, and a relatively simple genetical control. This is also inferred from the large transgressive segregation observed.

Oxygen influences benzyladenine and 2,4-dichlorophenoxyacetic acid levels in cultured embryogenic tissue of Norway spruce

It is known that reducing the partial pressure of O₂ influences the induction of somatic embryogenesis. We tested the hypothesis that O₂ causes changes in the endogenous levels of exogenously supplied benzyladenine (BA) or 2,4-dichlorophenoxyacetic acid (2,4-D). Embryogenic tissue of Picea abies was incubated under reduced (2.5, 5 kPa) and ambient (21 kPa) levels of O₂ for 1, 3, 7 and 11 days and the endogenous concentrations of BA and 2,4-D were measured. For all treatments the concentration of BA in the tissue increased until the third day. At day 3, the ratio of BA in the tissue relative to the initial concentration in the medium, was 3.9, 2.8 and 1.9 for tissue incubated under 2.5, 5 and 21 kPa O₂ , respectively. The BA concentration then declined gradually. Uptake of 2,4-D was inhibited at low O₂ levels. However, 2,4-D gradually accumulated in tissue grown under hypoxia, so that high levels were reached by day 11. These shifts in the BA and 2,4-D levels also caused a transient increase in the BA to 2,4-D ratio in tissue incubated under hypoxia. Although relevant for the previously reported effects of oxygen on induction of embryogenic tissue, it is unlikely that oxygen-induced alterations in BA and 2,4-D levels alone suffice to explain these findings.

Initiation of somatic embryogenesis in white spruce (Picea glauca): genetic control, culture treatment effects, and implications for tree breeding

The degree of genetic control and the effects of cultural treatments on somatic embryogenesis (SE) in white spruce were investigated with material derived from six-parent diallel crosses, including reciprocals. Thirty zygotic embryos from both immature and mature cones of each family were cultured in media with either 2,4-D or Picloram immediately after the collection of cones and after 2 months of cold storage. There were significant differences in SE initiation between immature and mature explants, and fresh and cold-stored seeds, but there was no significant differences with culture media effect. Significant variances due to families and to family x treatment interactions were found. The mean percentage of explants that initiated SE in each family ranged from 3.3% to 54.6%, with an overall average of 30.5%. The partitioning of family variance revealed that 21.7% was due to general combining ability effects, 3.5% was due to maternal effects, and 5.5% was due to reciprocal effects, but that the specific combining ability (SCA) was negligible. Variance due to interactions of family x treatments collectively accounted for 32.6%, while the remaining 37.8% of variation was accounted for by random error. However, when comparing the responses obtained with the treatment combinations, the SE response for freshly excised immature embryo explants showed comparatively large SCA variance, whereas the SCA variance was very small in the other treatment combinations.