SOMATIC EMBRYOGENESIS

The establishment of efficient embryogenic cultures has become an integral part of plant biotechnology as regeneration of transgenic plants in most of the important crops—canola, cassava, cereals, cotton, soybean, woody tree species, etc.—is dependent on the formation of somatic embryos. One of the most attractive features of embryogenic cultures is that plants derived from them are predominantly normal and devoid of any phenotypic or genotypic variation, possibly because they are derived from single cells and there is stringent selection during embryogenesis in favor of normal cells (see Vasil 1999). Embryogenic cultures were first described in callus and suspension cultures of carrot, grown on coconut milk-containing media, by Reinert (1958) and Steward et al. (1958), respectively. With increasing understanding of the physiological and genetic regulation of zygotic as well as somatic embryogenesis, embryogenic cultures can now be obtained on chemically defined media in a wide variety of species (Thorpe 1995; Raghavan 1997; Vasil 1999; Braybrook et al. 2006). In most instances the herbicidal synthetic auxin 2,4-dichlorophenoxyacetic acid (2,4-D) is required for the initiation of embryogenic cultures; somatic embryos develop when such cultures are transferred to media containing very low amounts of 2,4-D or no 2,4-D at all.
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