Plantlets can be formed by two different methods:
a. Regeneration of shoots followed by the regeneration of root (organogenesis)
b. Regeneration of somatic embryo followed by its germination (somatic embryogenesis) Regeneration means the development of an organised structure such as root, shoot or somatic embryo from cultured cell.
Method # 1. Regeneration of Shoot and Root:
The generation of shoot and root are controlled by the amount of auxin and cytokine in the growing medium. Cytokine like BAP promotes the regeneration of shoot and root regeneration is promoted by auxin like naphthalene acetic acid. To regenerate the shoot first, the callus culture is kept in a medium having cytokine.
After sometime, shoots arise from the callus cells. When these shoots attain a height of 2-3 cm, they are excised and transferred to an auxin-containing medium. Presence of auxin helps in regeneration of root at the lower end of the shoot. This results in the generation of a complete plantlet.
Method # 2. Somatic Embryo Regeneration:
Under natural conditions embryos are formed in flowering plants. When gametes fuse they form zygote, which differentiate into embryo. Under laboratory conditions, totipotent cells at certain condition in culture undergo embryonic pathways and form somatic embryos or embryoids.
An embryo developing from a somatic cell is known as somatic embryo. In 1958, F.C. Steward (USA) reported somatic embryogenesis in suspension culture of carrot. In 1959, J. Reinert reported somatic embryogenesis in callus of carrot grown on agar medium.
According to Sharp in 1980, somatic embryogenesis is initiated either by induced pre-embryonic determined cells which are programmed to form embryo, or by induced embryonic cells within the callus. Auxin 2, 4-D is the most commonly used auxin for somatic embryogenesis. In several cultures supplementing the embryo with activated charcoal facilitates embryogenesis (Fig. 9). Later an embryo will germinate into a complete plantlet.
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