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Synthetic Seed of Rice: An Emerging Avenue of Applied Biotechnology  

Bidhan Roy , Surje Dinesh  Tulsiram
Department of Genetics and Plant Breeding, Regional Research Station, Terai Zone, Uttar Banga Krishi Viswavidyalaya, Pundibari, Cooch Behar 736165, West Bengal, India;
Author    Correspondence author
Rice Genomics and Genetics, 2013, Vol. 4, No. 4   doi: 10.5376/rgg.2013.04.0004
Received: 15 Jul., 2013    Accepted: 21 Aug., 2013    Published: 20 Sep., 2013
© 2013 BioPublisher Publishing Platform
This is an open access article published under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Preferred citation for this article:
Roy and Tulsiram, 2013, Synthetic Seed of Rice: An Emerging Avenue of Applied Biotechnology, Rice Genomics and Genetics, Vol.4, No.4, 14-27 (doi:10.5376/rgg.2013.04.0004)

Progress in plant biotechnological research has opened many avenues for basic and applied research in the field of crop plants. Plant tissue culture is an important component of biotechnology, involves in the improvement of crops. Plant tissue culture led to develop synthetic seed technology. Synthetic seed technology is an exciting and rapidly growing area of research in plant cell and tissue culture. This technology is currently considered as the most effective and efficient alternative technique for propagation. The base materials for production of synthetic seeds are somatic embryos or tissue culture derived materials. This technology also facilitates the way of handling cells and tissues, protecting them from external gradients, short-term and long-term storage under low temperature and ultra-low temperature, respectively and as an efficient system of delivery. The information in the areas of synthetic seed preparation technology, its implications, achievements and limitations are lying unorganized in different articles of journals and edited books and that information were presented in this article in organized way with up-to-date citations, which will provide comprehensive literatures of recent advances.

Rice; Synthetic seed; Mass multiplication; Plant conservation

1 introduction
Rice (Oryza sativa) is the major food crop in the world. Nearly 40% of the world population consumes rice as the major staple food. Most of the people, who depend on rice as primary food, live in the less developed countries. Since the dawn of civilization, rice has served humans as a life-giving cereal in the humid regions of Asia and, to a lesser extent, in West Africa. Introduction of rice into Europe and the America has led to its increased use in human diets. There are 42 rice producing countries throughout the world but China and India are major rice production centers. Rice provides fully 60% of the food intake in Southeast Asia and about 35% in East Asia and South Asia. The highest level of per capita rice consumption (130~180 kg per year, 55%~80% of total caloric source) takes place in Bangladesh, Cambodia, Indonesia, Laos, Myanmar (Burma), Thailand, and Vietnam (Kenneth and Kriemhild, 2000). In many cultures of the world rice is the central part of people’s life and culture. Rice is an excellent food and is an excellent source of carbohydrates and energy. In 2008, international rice price rose greatly due to general upward trend in grain prices caused by droughts, increased use of grains animal feed, and so forth, has led to worldwide food crisis. This caused the domestic rice price in Malaysia increase almost double. The only way to protect and stabilize local price is to increase local rice production.

Production of synthetic seeds endowed with high germination rate under in vitro and in vivo conditions bears immense potential as an alternative of true seeds. Encapsulation technique for producing synthetic seeds has become an important asset in micropropagation. Botanically seed is a mature ovule along with its food storage in the form of either endosperm or cotyledon. The essential part is the embryo contained within the integuments, but it may be used less critically to describe planting materials. In terms of seed science, seed can be described as any propagating material used for raising a crop. Whereas, synthetic seed could be defined as artificially encapsulated somatic embryos, shoot buds, cell aggregates or any other tissue that can be used for sowing as a seed and that possesses the ability to be converted into a plant under in vitro or ex-vitro conditions, and that retains this potential even after storage (Capurno et al., 1998).

Synthetic seed technology can also help in germplasm storage and transportation of elite genotypes. A strong potential exists for propagation of high yielding, individual hybrids through somatic embryogenesis and artificial seeds (Brar et al., 1994). For potential application of seed encapsulation, technology has been demonstrated for many crop plants (Bapat and Rao, 1988, Padmaja et al., 1995; Onay et al., 1996; Shigeta and Sato, 1994; Suprasanna et al., 1996). In this endeavour efforts have been made to aggregate research findings on synthetic seed technology with particular emphasis on rice.

2 Types of Synthetic Seed
Success in production of synthetic seeds mainly depends on how best callus development and plantlet regeneration are achieved. The primary goal of synthetic seed production is to produce somatic embryos that resemble more closely to the true seed embryo in storage and handling characteristics so that they can be utilized as a unit for clonal propagation and germplasm conservation. Synthetic seeds may or may not have a synthetic seed coat, may be hydrated or dehydrated and may be quiescent or not. Encapsulation of micropropagules enables to satisfy the requirements. The gelling agents used for encapsulation for production of synthetic seeds act as protective cover. The encapsulated synthetic seeds also contain growth nutrients, plant growth promoting microorganisms (mycorrhizah, rhizobium, etc.), and/or other biochemical constituents necessary for optimal embryo-to-plant development (Figure 1).

Figure 1 Synthetic seed, gel encapsulated embryo with hydrophobic membrane

2.1 Desiccated synthetic seed
Kitto and Janick (1982) first time successfully formulated the plant synthetic seed production technology involving carrot somatic embryos. They used polyoxyethylene, which is readily soluble in water, dries to form a thin film, does not support the growth of microorganisms and is non-toxic to the embryo, leading to the production of desiccated synthetic seed. Desiccation can be achieved either slowly over a period of one or two weeks sequentially using chambers of decreasing relatively humidity, or rapidly by unsealing the Petri dishes and leaving them on the bench overnight to dry. Such types of synthetic seeds are produced only in plant species whose somatic embryos are desiccation-tolerant. Janick et al. (1989) have reported that coating a mixture of carrot somatic embryos and callus in polyoxyethelene glycol produced desiccated artificial seeds. The coating mixture was allowed to dry for several hours on a Teflon surface in a sterile hood. The dried mixture was then placed on a culture medium, allowed to rehydrate; and then scored for embryo survival. McKersie et al. (1989) induced somatic embryos to acquire desiccation tolerance by treatment with abscisic acid or any one of several environmental stresses, including water and nutrient stresses, applied to the embryoids at the cotyledonary stage of
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