Synthetic Seed of Rice: An Emerging Avenue of Applied Biotechnology 
,
Surje Dinesh Tulsiram 
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
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.
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.
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).
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2.1 Desiccated synthetic seed
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