Crop Improvement Through Plant Tissue Culture - Crop Improvement: New Approaches and Modern Techniques - page 128

pre-existing genetic differences in the cells or variations induced by tissue culture.
The variation may be created through several types of mutations like inversion,
deletion, duplication, gene amplification or de-amplification, by the activation of
transposable element, point mutations, or re-activation of silent genes in multigene
families (Karp 1994 ). Many aspects of the mechanism of somaclonal variation re-
main undefined, however in rice transposition of retrotransposons is one of the main
causes of somaclonal variation (Pistelli et al. 2012 ). Somaclonal variation may also
be induced in cultures via tissue culture process as reported in Saintpaulia sp. (Sato
et al. 2011 ). A number of somaclonal variations observed in in vitro raised regener-
ants have been found to be of agricultural and horticultural significance. Some of
such important alterations include chloroplast and chromoplast physiology, growth
and development of the plant, seed yield, morphology of the flower and leaf, pro-
duction of essential oils, fruit solids and disease resistance. Many of the important
crops including wheat, rice, oats, maize, sugarcane, alfalfa, tobacco, tomato, potato,
oil seed rape and celery have been observed with such variations (Karp 1994 ). This
variation can also be obtained from gametic tissue.
One of the most important advantages of somaclonal variation is the induction
of more genetic variability in economically important crops (Schellenbaum et al.
2008 ). In vitro selection of such somaclonal variants or rapid plant screening meth-
ods will be valuable. Enhancement in some somaclonal variants has been reported
under in vitro conditions that include resistance to diseases, pathotoxins and herbi-
cides or tolerance to different stress conditions (Zebrowska 2010 ).
Micropropagation
Propagation of Plants
Almost all types of plants can now be regenerated into plantlets from explants or
callus. Thus, majority of the plant species have now well established micropropaga-
tion protocols and at present among the different techniques of plant tissue culture
technology micropropagation is of widest use (Loyola-Vargas and Ochoa Alejo
2012 ). At present there are a number of tissue culture firms involved in in vitro mul-
tiplication, elimination of pathogens, storage of germplasm, genetic manipulation
and plant-breeding programs (Ding et al. 2008 ). Micropropagation plays a major
role in crop improvement. However, there are several limitations to the use of this
technique. Up to 70 % of the production costs of micropropagation are required to
fulfill the cost of labour needed to transfer tissue repeatedly between vessels and for
asepsis. Tissue culture laboratory is greatly affected by the problems of vitrification,
acclimatization and contamination (Doran 2009 ). A large number of desirable eco-
nomic traits are lost in the tissue-cultured products due to genetic variation in cul-
tured lines, such as polyploidy, aneuploidy and mutations. Enhancing axillary-bud
breaking, production of adventitious buds and somatic embryogenesis are the three