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Rice Genomics and Genetics 2013, Vol. 4, No. 4, 14-27
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and plantlet regeneration achieved. Long-term
maintenance of embryogenic masses in culture tubes
or mechanically stirred bioreactors requires frequent
transfer of tissue to fresh media, which is both labour
intensive and costly. However, over time the
morphogenic competence of differentiated cultures
declines (Lynch and Benson, 1991). Therefore, new
culture has to be regularly initiated and characterized
in order to maintain a constant supply of embryogenic
callus. This approach is highly cumbersome. To cope
with these difficulties, the embryo masses of Pistacia
vera have been encapsulated in sodium alginate gel
and stored at 4
℃
after treatment with BAP. The
maintenance of recurrent cycle of somatic
embryogeneis can be spontaneous as is the case with
alfalfa, Medicago sativa (Lupotto, 1983). The cycles
were maintained without growth regulators (Lupotto,
1986), and with specific growth regulator at specific
concentration (Roy, 2006; Roy and Mandal, 2011).
The initiation of recurrent culture requires that the
developing embryos be locked into a developmental
stage beyond which they cannot proceed, thereby
repeating a cycle. This can be achieved through its
initial exposure to a very high auxin concentration
such as 40 mg/L 2,4-D followed by maintenance of
the recurrent system using a lower level of auxin, such
as 5 mg/L of 2,4-D (Finer and Nagasawa, 1988),
which prevent the transition from pro-embryogenic to
embryogenic development. Onay et al. (1996) have
reported that the encapsulated embryogenic masses
recovered their original proliferate capacity after two
month storage following two subcultures. Petiard et al.
(1993) elaborated tool for large-scale propagation of
coffee (Coffee canephora) and carrot from
mass-somatic embryogenesis in bioreactors.
Subsequently they have stressed upon the economics
of mass somatic embryogenesis.
4.3 non-embryogenic synthetic seeds- microtillers
Information about production of artificial seeds from
microshoots/microtillers in rice is extremely limited.
Roy and Mandal (2006) developed a protocol for rapid
and recurrent mass-multiplication of androgenic
embryos and microtillers of indica rice var. IR 72.
Those embryo masses and microtillers were used to
prepare synthetic seeds (Roy and Mandal, 2008) and
germinated in vitro and in vivo on vermiculite.
Taha et al. (2012) induced microshoots from stem
explants on MS medium supplemented with 1.5 mg/L
BAP. The microshoots were encapsulated in 3% (w/v)
sodium alginate, 3% sucrose, 0.1 mg/L BAP, and 0.1
mg/L α-Naphthalene acetic acid (NAA). Germination
and plantlet regeneration of the encapsulated seeds
were tested by culturing them on various germination
media. They also investigated the effect of storage
period (15~30 days) was also. Maximum germination
and plantlet regeneration (100.0%) were recorded on
MS media containing 3% sucrose and 0.8% agar with
and without 0.1mg/l BAP. However, a low
germination rate (6.67%) was obtained using top soil
as a sowing substrate. The germination rate of the
encapsulated microshoots decreased from 93.33% to
3.33% after 30 days of storage at 4
℃
in the dark.
5 Implications
By combining the benefits of vegetative propagation
system with the capability of long-term storage and
with clonal multiplication, synthetic seeds have many
diverse applications in agriculture (Gray and Purohit,
1991; Redenbaugh et al., 1991; Redenbaugh, 1993). The
exact application of synthetic seeds will vary from
species to species. The possible implications of rice
synthetic seed technology have been detailed hereunder.
5.1 Clonal propagation
The potential application of synthetic seed technology
is to produce true-to-type propagules of different crop
plants. The sexually reproduced of seed in
cross-pollinated crops is undesirable because it assures
that the seeds are not alike genetically following
meiotic recombination. Synthetic seed technology is
an alternative to traditional micro-propagation for
production and delivery of cloned plants. It offers the
possibility of low cost, high-volume propagation
system that will compete with true seeds and
transplants. The explants used in synthetic technology
are somatic in origin, which satisfy the asexual means
of reproduction. There are two distinct routes for
clonal propagation through synthetic seeds- somatic
embryo and microtillers.
5.2 Mass multiplication of transgenic and elite plants
This newly emerging technology would also be useful