Agriculture Reference
In-Depth Information
male homozygotes, when used as pollinators of homozygous diploid females, pro-
duced entire populations of male hybrids (Chase 1974 ). There have been reports of
embryogenic callus and in some cases, successful plantlet regeneration, following
androgenesis of tropical and subtropical fruit species, many of which are difficult
to micropropagate e.g. Annona squamosa (Nair et al. 1983 ), Carica papaya (Tsay
and Su 1985 ), Euphoria longan (Yang and Wei 1984 ), Litchi chinensis (Fu and Tang
1983 ), Feijoa sellowiana (Canhoto and Cruz 1994 ), Pouteria lucama (Jordan et al.
1994 ) and Psidium guajava (Babbar and Gupta 1986 ).
The culture of mature embryos is one of the easier in vitro techniques but it
becomes progressively harder however, with immature or poorly formed embryos.
Embryo culture has been used to culture recalcitrant species; to overcome problems
associated with seed dormancy and incomplete embryo development (e.g. propaga-
tion of macapuno coconut); and to develop highly regenerable cultures for gene
transfer systems (Hu and Wang 1986 ; Drew et al. 1995 ). The major application of
embryo culture is, however, in the production of interspecific hybrids, where it has
been used successfully to bypass post-zygotic incompatibility barriers. Hybrid plant
production is often prevented by embryo abortion caused by embryo-endosperm in-
compatibility (Emsweller and Uhring 1962 ) or rapid deterioration of the endosperm
before the embryo matures (Hu and Wang 1986 ). Embryo culture has been used to
produce interspecific hybrids between Carica papaya and five wild relatives (Drew
et al. 1998 ), and between commercial cucurbit species and Curbita ecuadorensis
(Herrington et al. 1988 , 1989 ).
Somatic hybridisation provides a non-sexual method of gene transfer but com-
mercial applications are limited. Somatic hybridisation involves the fusion of proto-
plasts from different genotypes or species and was the first means of crossing plants
while bypassing sexual reproduction and thus traditional methods of plant breeding.
Although wide crosses have been achieved (e.g. potato x tomato; Melchers et al.
1978 ), useful applications to plant improvement have been limited by incompat-
ibilities between species. A further limitation is that regeneration from protoplasts is
arguably the most difficult in vitro procedure and has not been applied successfully
to a wide range of species. The most noteworthy applications of this technique have
been in Citrus species and these were reviewed by Ochatt et al. ( 1992 ).
Genetically Modified Cultivars
Tissue culture methods offer a rich scope for creation, conservation, and utilization
of genetic variability for the improvement of field, fruit, vegetable, and forest crops,
and medicinal/aromatic plants (Gosal et al. 2010 ). Micropropagation technology
ensures true-to-type, rapid and mass multiplication of plants that possesses spe-
cial significance in vegetatively propagated plant species. This technology has wit-
nessed a huge expansion globally, with an estimated global market of $US 15 bil-
lion/annum for tissue-culture products.
Vast amounts of research have also been undertaken to develop plant tissue cul-
ture techniques which underpin the new technologies that allow the production of
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