Mutagenesis—A Potential Approach for Crop Improvement - Crop Improvement: New Approaches and Modern Techniques - page 152

Time to time, the spectrum of available mutation techniques has also signifi-
cantly increased. Important practical results have been achieved through the use of
chronic irradiation from gamma-field or by irradiation with heavy ion beam or by
chemical mutagenesis. In vitro cultured somaclonal variations have been proved to
be useful for creating variation in many characters, especially the ones that can be
selected under in vitro cultural environments. Classical insertional mutagenesis and
more recently retro-transposons have become almost irreplaceable tools in generat-
ing and tagging of new mutations for crop improvement. Potent mutagenic agents
('mutagens' that cause mutation) can be used to produce the 'morphological mu-
tants' that are essential for dissecting plant structure, functions and their regulation
(Maluszynski 1999 ). The traditional mission of mutation breeding technology deals
with the development of new and desired variation(s) through breeding programs
for overall crop improvement that has recently been significantly spread widely. In-
duced mutations can play an important role in the conservation and preservation of
crop biodiversity. Induced mutations and related advance technologies are impor-
tant not only for extending genetic diversity of major crops but also are an important
additional source of biodiversity enhancement of neglected and local crops.
Mutation and Mutagens
A mutation is a sudden heritable change in the DNA of a living cell, not caused
by the common phenomena of genetic segregation or genetic recombination. Mu-
tations may occur in nature without intentional human intervention, and are said
to be spontaneous. Spontaneous mutations may result from the activity of mobile
genetic elements (transposons) that can move around to different positions within
the genome of a single cell and affect the activity of the gene in which they are
inserted (Wessler 2006 ). Mobile genetic elements affect the gene function through
various mechanisms. Retro-transposons, for example, move in the genome by be-
ing transcribed to RNA and then back to DNA by reverse transcriptase, while DNA
transposons move directly from one position to another within the genome using
a transposase enzyme to 'cut and paste' them within the genome, causing sponta-
neous mutations (Kidwell 2005 ). Most spontaneous mutations occur in very low
frequencies (10 −6 ) of an individual gene. Moreover, not all phenotypically observed
variation refers to genetic changes. At the same time, not all the spontaneous chang-
es in the DNA ultimately result in permanent changes of the DNA. Even if such
changes would be permanent, they may not always result in visible or detectable ef-
fects (Ranel 1989 ). For example, there may be latent adaptive mutations in African
nightshade that help the plants to survive in the wild, but these are not known so far.
Besides, spontaneous mutations depend on chance and make breeding programmes
considerably slow. Although selection for economically useful spontaneous mu-
tants still takes place with some level of success (Ahloowalia et al. 2004 ; Wilde
et al. 2012 ), the purposeful induction of a specifically desired mutation at a specific