somatic embryogenesis

somatic embryogenesis - Chapter 9 Somatic Embryogenesis 1....

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Chapter 9 Somatic Embryogenesis 1. INTRODUCTION During the course of evolution, many plant species have evolved different methods of asexual embryogenesis, including somatic embryogenesis, to overcome various environmental and genetic factors that prevent fertilization. Somatic embryogenesis is a process whereby somatic cells differentiate into somatic embryos. Somatic embryos resemble zygotic embryos morphologically. They are bipolar and bear typical embryonic organs. However, they develop via a different pathway. Somatic embryogenesis occurs to a limited extent under natural conditions, within ovules (e.g., Paeonia ) and more rarely on leaves (e.g. Asplenium and Kalanchoe ). Since the first observation of somatic embryo formation in Daucus carota cell suspensions by Steward et al . (1958) and Reinert (1958) the potential for somatic embryogenesis has been shown to be characteristic of a wide range of tissue culture systems in plants. During the past 40 years, somatic embryogenesis has been described in a large number of plant species. New species and modified methods are continuously reported and described and their number continuously increases. Methods for bringing about this kind of morphogenesis are also steadily being modified and improved. Somatic embryogenesis can probably be achieved for all plant species provided that the appropriate explant, culture media and environmental conditions are employed. In this chapter we shall highlight important aspects of somatic embryogenesis. We shall not give details for different species since this field of research is developing very fast and it is important to search for the latest information; this can easily be done on the web. Somatic embryos are used as a model system in embryological studies. However, the greatest interest of somatic embryos is centred in its practical application for large-scale vegetative propagation, particularly because of the possibility to scale up the propagation by using bioreactors (see Chapter 1). In addition, in most cases, somatic embryos or embryo- genic cultures can be cryopreserved, which makes it possible to establish gene banks. Embryogenic cultures are also an attractive target for gene transformation. 2. PLANT EMBRYOGENESIS Before starting to work with somatic embryogenesis, it is crucial to have a basic knowledge about the embryology of the species of interest. Plant embryogenesis begins with the zygote and passes through a stereotyped sequence of characteristic stages. Although considerable morpho- genesis occurs after seed germination, the embryonic phase is crucial as it is here that meristems and the shoot-root body pattern are specified. Angiosperms and gymnosperms became separated about 300 million years ago. Since their embryology differs in many aspects we shall briefly describe embryology in one angiosperm, Arabidopsis , and one gymnosperm, Pinus (Fig. 9.1). 2.1.
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somatic embryogenesis - Chapter 9 Somatic Embryogenesis 1....

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