Here is a compilation of term papers on ‘Somatic Embryogenesis’ for class 11 and 12. Find paragraphs, long and short term papers on ‘Somatic Embryogenesis’ especially written for school and college students.
Term Paper on Somatic Embryogenesis
Term Paper Contents:
- Term Paper on the Introduction to Somatic Embryogenesis
- Term Paper on the Induction of Somatic Embryogenesis
- Term Paper on the Development of Somatic Embryogenesis
- Term Paper on the Factors Affecting Somatic Embryogenesis
- Term Paper on the Potential Applications of Somatic Embryogenesis
- Term Paper on the Advantages of Somatic Embryogenesis
- Term Paper on the Disadvantages of Somatic Embryogenesis
Term Paper # 1. Introduction to Somatic Embryogenesis:
It leads to production of bipolar structure containing a root-shoot axis, with a closed independent vascular system. Somatic embryogenesis can be produced directly from the explant (direct embryogenesis) or can be produced via callus formation from explant (indirect embryogenesis). Embryoids have also been artificially induced in cultured plant tissues, besides zygote. Somatic embryogenesis was first induced in suspension culture and callus culture of carrot.
Somatic embryogenesis may be initiated in two ways:
(i) By inducing embryogenic cells within the pre-formed callus, and
(ii) Directly from pre-embryogenic determined cells (without callus), which are ready to differentiate into embryoids.
The nutritional media of different composition are needed to obtain embryoids. The first medium contains auxin to initiate embryogenic cells. The second medium lacks auxin or reduced level of auxin is required for subsequent development of embryonic cells into embryoids and plantlets. In both the cases a reduced amount of nitrogen is required. Increased osmotic stress (by using 2 – 6% carbohydrate) in the medium often enhances somatic embryogenesis.
Two theories about induction of somatic embryogenesis have been proposed. According to predetermination theory for somatic embryogenesis, cells undergoing embryo initiation are embryogenic to start with and that the in vitro culture conditions simply cause embryogenesis to occur.
Further Sharp et al. (1980) and Evans et al. (1981) suggested that explanted tissues contain cells that are already determined for embryogenic development, namely pre-embryogenic determined cells (PEDC’s) and those which require redetermination through a period in culture i.e. induced embryogenic determined cells (IEDC’s).
The process occurs naturally in a wide range of species from both reproductive and somatic tissues. Somatic embryos can be found on callus in cell suspensions and protoplast cultures, or directly from cells of organized structures (such as stem segments or zygotic embryo). Around 200 species from angiosperms and gymnosperms have been reported to produce somatic embryos in culture.
The first plantlet formation in vitro was reported as early as 1940s; Ernest Ball (1946) reported it in Tropaeolum and Lupinus. There are several plants in which embryogenesis has been induced in vitro, some of them are – Atropa belladona, Brassica oleracea, Coffea arabica, Nicotiana tabacum, Saccharum officinarum.
There are several advantages of plantlet regeneration through organogenesis or embryogenesis. These include the efficiency of process (the formation of plantlet in fewer steps, simultaneously with reduction in labour, time and cost), the capacity to produce higher number of plantlets, and the morphological and cytological uniformity of the plantlets.
When embryos regenerate from somatic cells or tissues (haploid, diploid etc.), it is termed as somatic embryogenesis. Embryos can be obtained either directly from cultured explants (the organized structures – like leaf, hypocotyl, stem and other plant parts) and anthers (or pollen) or indirectly from callus and isolated single cells in culture. This is different from zygotic embryogenesis which results from fertilization of an egg cell.
The process of embryogenesis involves various stages of differentiation and development such as pro-embryo, globular, heart shape and torpedo embryo. This phenomenon is most common in Ranunculaceae, Solanaceae, Rutaceae, Umbellifere and Gramineae.
Stewart et al. (1958) and Reneirt (1959) reported somatic embryo formation in carrot cell suspension cultures. These somatic embryos were similar to zygotic embryos in development and structure (Fig. 2.1). Although the origin of somatic embryos produced in carrot cell suspension cultures was uncertain, these somatic embryos morphologically developed through the globular, heart and torpedo stages. The development of cotyledons is used to distinguish between mature cotyledonary stage and heart/torpedo shaped embryo. At torpedo stage, cell differentiation occurs establishing root and shoots meristem.
Massive efforts have been put to define the medium conditions, which permit embryogenesis, and characterize the morphological and developmental events leading to embryo formation. Embryogenesis is a two-step process. The first step is the induction of embryogenesis while the second step is the development of embryo, ultimately leading to germination.
The requirements for embryo induction and embryo development are different, and thus separate media are used for each step. Ammirato (1986) described four stages viz. induction, early growth, embryo maturation and germination or conversion. These stages not only differ in their morphological structure but also in their physico-chemical requirements.
Term Paper # 2. Induction of Somatic Embryogenesis:
With regard to place of origin, embryogenesis is of two types – direct embryogenesis, and indirect embryogenesis. Juvenile explants like hypocotyls, cotyledons and young immature zygotic embryos are best materials to initiate embryogenic cultures. When embryogenesis occurs directly on the explants without production of callus, it is known as direct embryogenesis. When explants produce callus and the callus forms embryos then it is called as indirect embryogenesis. An exogenously supplied auxin is needed in appropriate concentration for the induction of somatic embryogenesis from callus or explant.
In certain cases (like carrot), exogenous supply of auxin may not be required for embryogenesis to occur. Sharp et al. (1982) proposed that in case of direct embryogenesis cells of explanted tissues are already determined for embryogenic development, and these are termed as pre-embryogenic determined cells (PEDC’s). The starting material is completely rejuvenated, e.g., nucellus of citrus, epidermal cells of hypocotyls (Ranunculus sp., Brassica napus).
In case of indirect embryogenesis cells require redetermination through a period in culture and this is termed as induced embryogenic determined cells (IEDC’s). It has been suggested that this phenomenon is determined by epigenetic factors. Here, differentiated cells must firstly be dedifferentiated and then re-determined as embryogenic cells after cell division. Complete rejuvenation must take place, e.g., secondary phloem of carrot, leaf explants of Petunia, Asparagus, Coffea.
Embryogenesis occurs from a single cell or from a group of cells. The earliest cell divisions in embryo-genetically determined cells follow various patterns, but finally produce embryos of similar shape. Embryogenic cells are small, isodiametric in shape, occupied with dense cytoplasm and have a peculier nucleus. The non-embryogenic cells are relatively large, vacuolated and lack dense cytoplasm. Embryogenic cells from an auxin-containing medium when transferred into a medium having low auxin concentration or without auxin, develop pro-embryos.
Usually media containing 2, 4-D, 2, 4, 5-trichlorophenoxy acetic acid and picloram are used as embryo induction medium. Several species of monocotyledons and dicotyledons have been regenerated through somatic embryo formation by this method. Other auxins like IAA or IBA in combination with a cytokinin have also been found equally good for embryo induction in many dicotyledonous species.
When somatic embryos are transferred on induction medium they give rise to secondary somatic embryos. This method of obtaining embryos recurrently is termed as repetitive or cyclic embryogenesis. This method is useful for continuously obtaining embryos in large number e.g. in Atropa belladonna, carrot, Ranunculus, Pennisetum purpurium and Pannicum maxicana.
When young cotyledonary ’embryos of white clover (Trifolium repens) are explanted in presence of cytokinin BAP (6-benzylamino purine), the hypocotyl responds with the formation of somatic embryo. When 2, 4-D is used instead of BA, cotyledons form somatic embryos.
Likewise, pea shows a positive response from hypocotyl cells in the presence of cytokinin, but it is the cotyledon cells that respond in the presence of auxin. Genotype, tissue type, and developmental stage may influence the comparative ability in responding to auxin or cytokinin.
Term Paper # 3. Development of Somatic Embryogenesis:
During somatic embryogenesis in cell suspension cultures embryos of different sizes are produced. Embryos of uniform size are selected. This can be achieved by sieving or fractionation of suspension with appropriate sieve size. Such cultures may be fully synchronized for their growth. The development and maturation of somatic embryos is similar to that of zygotic embryos.
Cell differentiation is most evident in the formation of vascular tissues especially vis ible in the hypocotyl and cotyledon. Differentiated cells should be de-differentiated and then divide, leading to formation of non-organized mass of vacuolated parenchyma cells. This is transformed into cytoplasm-rich cells that become embryogenic in presence of auxin. The embryoids are formed from embryogenic cells which are small in size, have dense cytoplasmic contents, large nuclei, starch grains and small vacuoles.
Term Paper # 4. Factors Affecting Somatic Embryogenesis:
The composition of culture medium is critical, specially the levels of sucrose and nitrogen:
(1) Nitrate nitrogen alone in high amount is required for induction of somatic embryogenesis. Reduced nitrogen in the embryo development medium supports embryo development.
(2) Favourable effect of certain amino acids like proline and glutamine has been established.
(3) Increasing the osmotic concentration by using high sucrose levels or by addition of mannitol or sorbitol, has shown to affect the embryo development.
(4) Auxins, specially the 2, 4-D, appear to be required for embryo induction as in case, of cereal crops. Activated charcoal when added to the medium removes excess auxin from the somatic embryos.
(5) Cytokinins, with the exception of zeatin, suppress embryogenesis.
(6) Gibberellins and ethylene usually inhibit embryogenesis.
(7) Carrot cells produce somatic embryos below 16% dissolved oxygen while above 16% oxygen levels; roots are produced, in liquid cultures.
(8) The chances of embryogenesis are higher with a callus that has originated from a juvenile plant.
(9) Light promotes embryogenesis, though it can take place at low irradiance or even in darkness in some species.
(10) High temperature favours somatic embryogenesis.
(11) A saccharose concentration of about 2 – 3% and coconut milk promote embryogenesis.
The demonstration of somatic embryogenesis in synchronous cell culture has made this phenomenon as a model to study organ differentiation and regulation.
For instance:
(1) Callus specific and embryo specific proteins are identified. It is observed that embryo specific genes are expressed well before the morphologically visible differentiation.
(2) Rapid increases in the rates of protein and RNA synthesis after transfer of embryonically competent carrot cells to an auxin-free medium have been shown.
(3) Polyamine synthesis and its role in embryogenesis suggest embryogenic cultures have higher level of polyamines; inhibition of polyamines biosynthesis could suppress embryogenesis, whereas the addition of the polyamine spermidine could reverse this inhibition, and levels of enzymes for polyamine synthesis are higher under embryogenic conditions.
Term Paper # 5. Potential Applications of Somatic Embryogenesis:
(1) Mass propagation somatic embryos have powerful advantages for mass propagation in comparison to both conventional clonal propagation methods (rooted cutting, grafting) and other in vitro regenerated systems (e.g., in propagation). Unlimited number of embryos can be produced from a single explant.
(2) Raising somaclonal variants in tree species is possible.
(3) Source of re-generable protoplast system embryogenic callus and suspension cultures, as well as somatic embryos themselves have been employed as a source of protoplasts for a range of species, e.g., in three groups of plant species, viz., graminaceous species, citrus species, and forest trees (specially conifers).
(4) Embryo culture and gene transfer regeneration in several species, especially trees and large-seeded legumes, is limited to regeneration via direct somatic embryogenesis from immature zygotic embryos. The embryos form directly on the original explant tissue. If callus is present, it grows concomitantly with the somatic embryos.
The embryos do not originate from it, thereby by-passing any chance that a callus phase provides to sort transferred cells from non-transferring cells. So callus phase can be by-passed through repetitive somatic embryogenesis. McGranahan et al. (1990) exploited repetitive embryogenesis for agrobacterium-transformed walnut cells to obtain multiple plants of somatic embyros by-passing the callus phase.
(5) Clonal propagation through somatic embryogenesis is reported in about 60 species of woody trees.
(6) Somatic embryogenesis provides potential plantlets in the form of somatic seeds. The somatic embryo can be used for the production of synthetic seeds for direct sowing in the field. This gives impetus to increased agricultural production.
(7) Somatic embryo provides organized culture system to produce organ specific or differentiation related compounds in higher amounts as in case of Digitalis and Theobroma cacao. Repetitive somatic embryogenesis in borage ensures a good availability or supply of ¡- linolenic acid, which is used in the treatment of atopic eczema.
Term Paper # 6. Advantages of Somatic Embryogenesis:
Somatic embryo is very useful for micro-propagation of plant species. A large number of herbaceous dicots and monocots have been regenerated through somatic embryogenesis (Table 2. 1).
(1) Rapid multiplication through production of somatic embryogenesis in cell cultures, and use of bioreactors for scale-up technology.
(2) Somatic embryos grown individually make the system easy to manipulate (to sub-culture).
(3) Provides an important resource for the analysis of molecular and biochemical events that occur during induction and maturation of embryo.
(4) Presence of both root and shoot avoids the rooting step required in organogenesis.
(5) It shortens the breeding cycle of deciduous trees and increases the germination of hybrid embryos.
Term Paper # 7. Disadvantages of Somatic Embryogenesis:
(1) High risk of mutations involved.
(2) Repeated sub-culturing may weaken regenerative capacity.
(3) Induction of embryogenesis sometimes is difficult or impossible with many plant species.
No comments yet.