Crop Improvement Through Plant Tissue Culture - Crop Improvement: New Approaches and Modern Techniques - page 129

methods of micropropagation. In the latter two methods, differentiated structures
arise directly or indirectly from callus. Axillary bud breaking produces very less
number of plantlets as the number of shoots cultured is affected by the number of
axillary buds cultured. However, it is the most widely used method in commercial
micropropagation as it produces the most true to type plantlets. Adventitious bud-
ding is advantageous as bud primordial may be formed on any part of the inoculums
(Brown and Thorpe 1996 ). Unfortunately, largest number of plantlets can be pro-
duced by somatic embryogenesis but only limited numbers of species respond to it.
The use of bioreactors helps in large scale production of somatic embryos and their
delivery in the form of synthetic seeds.
Synthetic Seeds
Synthetic or artificial seed is analogous to a zygotic seed and may be defined as a
somatic embryo encapsulated inside a coating (Redenbaugh 1993 ). Synthetic seeds
may be of different types: somatic embryos in a coating of water gel, dried and
coated somatic embryos, suspended in a fluid, and buds encapsulated in a water gel.
The use of synthetic seeds is advantageous over the traditional micropropagation
protocols as it may have a cost of saving, as the labour intensive step of transfer-
ring plants from in vitro to field conditions. Other applications of synthetic seeds
include the male sterile and parental line maintenance, for hybrid crop production
and the preservation and multiplication of woody plants that have long juvenile
developmental phase (Marimuthu et al. 2011 ). However, before the widespread of
this technology, somaclonal variation has to be minimized. The production of high
quality embryos at large scale must be perfected in the desired species and the mi-
cropropagation protocols will have to be cost-effective compared with existing seed
or technologies.
Pathogen Eradication
The crop plants multiplying through vegetative propagation are generally infected
with pathogens. Although in many cases the presence of the pathogen may not be
obvious, but the yield or quality may be substantially reduced due to infection. In
vitro culture has helped in increasing the yield of many crop plants. In China, virus
free potatoes, produced by in vitro culture gave higher yields than the normal field
plants with increase up to 150 % (Meiyalaghan et al. 2011 ). Seeds are responsible
for only 10 % of viral infection, therefore, careful propagation from seeds can elimi-
nate most of the viruses. The viruses are not distributed uniformly in the plant and
the apical meristems are usually free from viruses (Wang and Valkonen 2008 ). The
culture of the apical meristem coupled with chemo or thermo-therapy, have been
used to produce virus-free material for micropropagation (Cantrill et al. 2005 ).