Principles of Somatic Embryogenesis
Somatic Embryogenesis may be Initiated in two Different Ways:
1. In some cultures somatic embryogenesis occurs in absence of any callus production from “pro-embryogenic determined cells” that are already programmed for embryo differentiation. For instances somatic embryos have been developed directly from leaf mesophyll cells of orchard grass without an intervening callus tissue. Explants made from the basal portions of two innermost leaves of orchard grass were cultured on Schenk and Hildebrandt medium supplemented with 30µm, 3, 6-dichloro-0-anisic acid, plant formation occurred after subculturing the embryos on the same medium without decamba.
2. The second types of somatic embryo development needs some prior callus formation and embryoids originate from “Induced embryogenic determined cells” ( IEDCs) within the callus tissue.
In most cases direct embryogenesis occurs. For direct somatic embryogenesis where it, has been induced under in vitro condition, two distinctly different types of media may be required- one medium for the initiation of the embryonic cells and another for the subsequent development of these cells into embryoids. The first or the induction medium must contain auxin in case of carrot tissue and somatic embryogenesis can be initiated in the second medium by removing the hormone or lowering its concentration. With some plants, however, both embryo embryo initiation and subsequent maturation occur on the first medium and second medium is required for plantlet development.
Embroids are generally initiated in callus tissue from the superficial clumps of cells associated with enlarged with enlarged vacuolated cells that do not take part in embryogenesis. The embryogenic cells are generally characterised by dense cytoplasmic contents, large starch grains, relatively large nucleus with a darkly stained nucleolus. In suspension culture, embryoids do not form from suspended single cell, but form from cells lying at or near the surface of the small cell aggregates.
Each developing embryoid of carrot passes through three sequential stages of embryo formation such as globular stage heart shaped stage and torpedo- stage. The torpedo stage is a bipolar structure which ultimately gives rise to complete plantlet. The culture of other plants may not follow such sequential stages of embryo development.
In general, somatic embryogenesis occurs in short term culture and this ability decreases with increasing duration of culture. The loss of embryogenic potential of culture may be due to changes in Ploidy and lose of certain biochemical properties of cultured cells.
In callus culture or in suspension culture, embryoids formation occurs asynchronously. A high degree of synchronization has been achieved in carrot suspension culture by sieving the initial cell population.