Embryo proliferation

Accessory embryos on zygotic embryos. Occasionally new somatic embryos are formed directly on zygotic embryos that have been transferred to in vitro culture. Such adventitious embryos have been reported, for example, in:

Cuscuta reflexa (Maheshwari and Baldev, 1961); barley (Norstog, 1970); Ilex aquifolium (Hu and Sussex, 1972; Hu, 1977; Hu et al., 1978); Thuja orientalis (Konar and Oberoi, 1965); Trifolium repens (Maheswaran and Williams, 1985); Zamia integrifolia (Norstog, 1965b; Norstog and Rhamstine, 1967); Theobroma cacao (Pence et al., 1980a,b); Linum usitatissimum (Pretova and Williams, 1986); Vitis vinifera (Stamp and Meredith (1988).

When direct embryogenesis occurs on pre-formed embryonic tissue, the newly formed embryos are sometimes termed direct secondary embryos or accessory embryos.

Accessory embryos on somatic embryos The in vitro induction of somatic embryogenesis starts a highly repetitive process, lacking some of the controls which must exist in nature during the formation of zygotic embryos. This results in the frequent development of small additional embryos on somatic embryos which have arisen directly on explants, or indirectly in callus and suspension cultures. Accessory embryos can occur along the whole axis of the original embryo, or grow preferentially from certain sites (e.g. the hypocotyl region or the scutellum of monocot. embryoids). In walnut, accessory embryos appear to arise from single cells of the epidermis of somatic embryos (McGranahan et al., 1988).

Sometimes the term polyembryony is used to describe the formation of accessory, or secondary, embryos (Radojevic, 1988) (c.f. the term polyembryogenesis in Chapter 1). The process has also been called repetitive embryogenesis (Tulecke and McGranahan, 1985) or recurrent somatic embryogenesis (Lupotto, 1986). Such additional embryos are liable to be developed during all kinds of in vitro embryogenesis. They have been noted for example, on the somatic embryos formed in:

• Anther cultures: Atropa belladonna (Rashid and Street, 1973); Brassica napus (Thomas et al., 1976); Carica papaya (Tsay and Su, 1985); Citrus aurantifolia (Chaturvedi and Sharma, 1985); Datura innoxia (Geier and Kohlenbach, 1973); Vitis hybrids (Rajasekaran and Mullins, 1979);

• Suspension cultures: Daucus carota (Ammirato and Steward, 1971; McWilliam et al., 1974); Ranunculus scleratus (Konar and Nataraja, 1965b; Konar et al., 1972a);

• Callus cultures: Aesculus hippocastanum (Radojevic, 1988); alfalfa (when individual embryoids were transferred to a fresh medium) (Saunders and Bingham, 1972); carrot (Petru, 1970); Citrus (Button and Kochba, 1977); parsley (Vasil and

Hildebrandt, 1966b); Pennisetum purpureum (Wang and Vasil, 1982); Ranunculus sceleratus (Konar and Nataraja, 1965a,b), Theobroma cacao (Li et al., 1998).

Protocorms arising directly on explanted shoot tips or leaf pieces of orchids, frequently produce other adventive 'daughter' protocorms in culture, in a fashion that is similar to the adventive formation of somatic embryos. Somatic embryos formed in callus of oil palm have been reported to give rise to protocorm-like bodies, which regenerated shoots repeatedly as subculture was continued (Paranjothy and Rohani, 1982).

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