Page 9 - Rice Genomics and Genetics

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Rice Genomics and Genetics 2013, Vol. 4, No. 4, 14-27
powder as the encapsulating medium. Their results
showed that the germination rate reached 56.8% when
the proportion of clay: vermiculite: water was 2: 1: 2.
When this system was added with only 1.0% activated
charcoal, or 1.0% activated charcoal along with 0.5%
starch, the corresponding average germination rates of
the artificial seeds increased to 76.7% and 80.3%. The
treatment enhanced the germination rate by 18.4% and
24.2%, respectively.
4 Encapsulating Materials
4.1 Embryogenic synthetic seeds
The process of somatic embryogenesis, in which the
somatic cells or tissues develop into differentiated
embryos, produces somatic embryos and each fully
developed embryo is capable of developing into a
plantlet. Embryos can be obtained either directly from
cultured explants, anther or pollen, callus and isolated
single cell in culture. For synthetic seed production,
embryos may be obtained from the following sources.
4.1.2 Somatic embryos
The quality of the artificial seed depends on the
temporal qualitative supply of growth regulators and
nutrients along with an optional physical environment.
The advantages of preparing synthetic seeds from
somatic embryos have been discussed by many
authors (Gray and Purohit, 1991; Flacinelli et al., 1993;
Ara et al., 2000). The use of somatic embryos as
artificial seeds is becoming more feasible as the
advances in tissue culture technology define the
conditions for induction and development of somatic
embryos in an increasing number of plants species
(Jain et al., 1995; Ipekci and Gozukirmizi, 2003). The
sources of somatic embryos of rice are mature seed
derived callus, root culture, immature inflorescence
culture, immature embryo culture etc.
4.2 Gametic Embryos
4.2.1 Androgenic embryos
In several plants regeneration of artificial seeds into
plantlets has been reported. However, information
about production of artificial seeds from androgenic
embryos derived from androgenic callus is scanty.
Haploid plant breeding has been found to be well
established in many crops (Aljera et al., 1995; Plamer
et al., 1996; Roy and Mandal, 2004a; Roy and Mandal,
2004b). The induction of pollen embryogenesis, which
genetically differs from zygotic embryogenesis, may
be used for synthetic seed production. In rice, Roy
(2006) developed protocol for rapid and recurrent
mass multiplication of androgenic embryos and
embryo-like-structure of IR 72 an elite indica cultivar.
These embryos and embryo-like-structures can be
used as source of synthetic seed production.
4.2.2 Microspore culture
Haploid plants can also be regenerated from
microspore culture. Anther culture may associate with
production of diploid plantlets from anther wall or
from other parts of anther other than pollen. Thus
anther derived plantlets are with various ploidy levels.
This can be avoided by culturing isolated pollen.
Microspore culture usually produces homogenous
population, whereas anther culture could constitute a
heterogenous population.
4.2.3 Ovule culture
Haploid plants have been successfully developed from
culture of female gametophytic cells, that is, the egg
nucleus or ovum. It was considered as an alternative
means of haploidy as well as the expression of
totipotency of female gametophytic cells in
angiospermic plants. The first gynogenic haploid was
obtained in barley (San Noeum, 1976) culturing
ovaries. Subsequently, Chinese scientists could
regenerate haploid plants in rice, wheat, sunflower,
sugar beet and onion by culturing female
gametophytic cells.
The appropriate time of embryo sac for its culture is
uninucleate to mature stage. But it may differ species
to species, such as nearly mature (1~2 days before
anthesis), embryo sac mother cell to megaspore tetrad
stages are also suitable for culture. The explant for
development of gametophytic haploid may be the
ovary, isolated ovule or even unhusked flower.
4.2.4 Recurrent embryogenesis
The genetic improvement of crop plants en route
biotechnological approaches largely depends upon the
maintenance of differentiated cultures of callus or
redifferentiated embryos. Success in production of
synthetic seed depends on how best callus develop