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Somatic embryogenesis
This is a discussion on Somatic embryogenesis within the Biotechnology Engineering forums, part of the ENGINEERING WORLD category; Somatic embryogenesis is the process of single cell or group of cells that undergo the sequence of events of embryogensis ...
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08-29-2008, 06:51 PM
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Somatic embryogenesis
Somatic embryogenesis is the process of single cell or group of cells
that undergo the sequence of events of embryogensis (i.e., all the stages of embryogenesis) leading to the regeneration of plantlets. The capability of the somatic plant cell of a culture to produce embryoids is k/a embryogenic potential Embryoid is a small ,well-organized structure comparable to the sexual embryo, which is produced in tissue culture of dividing embryogenic potential In somatic embryogenesis, the embryos regenerate from somatic cells, tissues or organs either de novo or directly from the tissues (adventive origin) Somatic embryogenesis is a useful regeneration pathway for many monocots and dicots, but is especially useful for the grasses Types of embryogenesis zygotic embryogenesis – the result of normal pollination and fertilization somatic embryogenesis – embryos from (cultured) sporophytic cells, that is embryos arise indirectly Somatic embryogenesis differs from organogenesis in the embryo being a bipolar structure rather than monopolar The embryo arises from a single cell and has no vascular connections with the maternal callus tissue or the cultured explant Further, induction of somatic embryogenesis requires a single hormonal signal, while in the organogenesis two different hormonal signals are needed to induce first a shoot organ, then a root organ Two routes to somatic embryogenesis: 1. Direct embryogenesis the embryo initiate directly from the explant tissue through ″pre-embryogenic determined cells ″ (PEDC) such cells are found in embryonic tissues (e.g. scutellum of cereals), hypocotyls and nucellus 2. Indirect embryogenesis cell proliferation i.e. callus from explant takes place from which embryos are developed the embryo arise from ″induced embryogenic determined cells ″ (IEDC) For some species any part of the plant body serves as an explant for embryogenesis (e.g. carrot) whereas in some species only certain regions of the plant body may respond in culture Floral and reproductive tissues in general have proven to be excellent source of embryogenic material The presence of auxin always essential, cytokinins in some species The composition of the culture medium controls the process auxin (usu. 2,4-D) added causes induction, the formation of embryogenic clumps or proembryogenic masses (PEMs) (induction medium) auxin is deleted and the clumps become mature embryos (maturation medium) L-glutamine play a special role Substantial amount of reduced nitrogen ( NH4 +) Addition of activated charcoal to the medium is useful (→ lower phenylacetic acid and benzoic acid compounds, which inhibit somatic embryogenesis Culturing methods solid medium liquid medium (often called suspension cultures) Two cell types are usually present in embryogenic suspension cultures large, highly vacuolate, freely dispersed cells small, densely cytoplasmic, aggregated cell clumps Stages of development early cell division doesn't follow a fixed pattern, unlike with zygotic embryogenesis later stages are very similar to zygotic embryos (dicot pattern) globular stage (multicellular) heart-shaped stage (bilateral symmetry) – bipolarity torpedo-shaped stage – consists of initial cells for the shoot/root meristem Following fertilization, the zygote undergoes an asymmetrical cell division that gives rise to a small apical cell that becomes the embryo and a large basal cell (called the suspensor) that functions to provide nutrients from the endosperm to the growing embryo. Asymmetrical cell division also seems to be important in the development of somatic embryos, and while failure to form the suspensor cell is lethal to zygotic embryos, it is not lethal for somatic embryos. From the eight cell stage (octant) in the zygotic embryo, 'embryo patterning' is apparent; however, somatic embryos at this stage may be quite variable, therefore zygotic and somatic embryos become most comparable from the globular stage. In the globular stage, the embryo develops radial patterning through a series of cell divisions. The globular embryo can be thought of as two layers of inner cells with distinct developmental fates; the apical layer will go on to produce cotyledons and shoot meristem, while the lower layer produces the hypocotyl and root meristem. Bilateral symmetry is apparent from the heart stage; provascular cells will also differentiate at this stage. In the subsequent torpedo and cotyledonary stages of embryogenesis, the embryo completes its growth by elongating and enlarging. Artificial seeds (Synthetic seeds) Encapsulated embryos Redenbaught et al.,1984 discovered hydrogel, like, sodium alginate could be use to produce single embryoid artificial seeds. Method for making artificial seeds Establishment of callus cultures Induction of somatic embryogenesis Maturation of SEs Encapsulation of SEs Test for embryoid to plant conversion Green house and field planting To coat SEs 2 standard methods – 1. Gel complexation via dropping procedure Isolated SEs are mixed with 0.5% to 5%(w/v) sodium alginate and dropped into 30-100μM Calcium nitrate solution. Surface complexation begins immediately and the drops are gelled completely within 30 minutes 2.Molding:Isolated SEs are mixed in a temperaturedependent gel such as Gel-riteTM and placed in the well of a microliter plate and it forms gel when it cools down storage is a great limitation Potential applications to crop improvement haploids for breeding, mutant screening germplasm storage and preservation 𐂫 "artificial seeds "Artificial seeds" clonal propagation – theoretically much faster, easier to mechanize than micropropagation Results have not lived up to the promise somatic embryos are highly susceptible to desiccation "conversion" rates are low shelf life is short encapsulation techniques have yet to be perfected Not used often in plant propagation, because there is a high probability of mutations arising the method is usually rather difficult the changes of losing regenerative capacity become greater with repeated subcultures induction of embryogenesis is often very difficult or impossible with many plant species a deep dormancy often occurs     
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