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Rice Genomics and Genetics 2013, Vol. 4, No. 4, 14-27
http://rgg.biopublisher.ca
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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).
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 development.
The embryoids were subsequently air dried slowly
(over 7 days) or rapidly (over 1 day) to moisture
contents of less than 15% and remained fully viable.
Dry somatic embryos were stored with no loss of
viability for 8 months at room temperature and
humidity. Bornman et al. (2003) reported that the
percentage germination of fresh or somatic embryos
partially desiccated at relative humidities of 97% and
63% to moisture content approaching those of the
seed was substantially lower. This sensitivity to drying
suggests that the somatic embryo seed may behave
either as an orthodox seed with limited ability to
withstand desiccation or as a recalcitrant seed that
cannot survive drying below a moisture content that is
relatively high.
Figure 1 Synthetic seed, gel encapsulated embryo with
hydrophobic membrane
Note: A: Cross-section of a dicotyledonous true seed; B:
Cross-section of a synthetic seed