Artificial Seed Production and Cryopreservation
Department of Horticulture and Landscape Architecture, Oklahoma State University, 358, Ag Hall, Stillwater, Oklahoma, USA. 74078-6027
Tissue culture is an important process in plant propagation. After explant induction and somatic embryo/callus production, plant materials can be placed in a suitable medium to become seedlings or encapsulated and transformed to artificial/synthetic (syn) seed for short or medium-term preservation. This process is called cryopreservation and has advantages over traditional micropropagation that can result in the need for excessive subculturing, unpredictable mutations or somaclonal variations, larger space requirements, and higher labor needs, which is costly. Callus and somatic embryos are very sensitive organs and will act promptly to the loss of water, so the only way to keep them alive for short or medium term is to convert into syn seed, which has the ability to stop the growth at a certain stage of the somatic embryos’ life. Synthetic seeds containing propagules (plants reproducible organs) such as nodes, shoots, callus, somatic embryos, and protocorms are contained in a gelatinous matrix of a combination of sodium alginate and calcium chloride. The use of in-vitro derived axillary buds and shoot tips provides inexpensive, easily obtained propagules for storage. To date, cryopreservation is the only vital way to conserve plant germplasm and living cells in liquid nitrogen (LN). Cryopreservation also offers other advantages compared to other available storage approaches including stability of phenotypic and genotypic characters, freedom from contamination, and needs minimal storage space and maintenance requirements. New cryopreservation procedures are less complex and a programmable freezer is not required. This provides the suitability of these techniques to be used in any laboratory with basic facilities for tissue culture. A common feature to all the new protocols is that the critical step to achieve survival is the dehydration step, and not the freezing step, as in classical protocols. In addition, these techniques increased the applicability of cryopreservation to a wide range of horticultural crops. Seven different vitrification based procedures has been identified: (1) encapsulation-dehydration; (2) vitrification; (3) encapsulation-vitrification; (4) dehydration; (5) pregrowth; (6) pregrowth-dehydration; and (7) droplet freezing. In the past two decades, cryopreservation has been widely used as an alternative method for plant germplasm storage.
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