Mutagenesis—A Potential Approach for Crop Improvement - Crop Improvement: New Approaches and Modern Techniques - page 170

(DNA amounts correspond to genome length) than A. thaliana . This indicates the
amount of work that should be done to fill in the 'phenotype gap' in higher plants,
especially the major crop species. Because of the small rice genome size, large scale
mutational work should be initiated with this crop. The synteny of cereal genomes
can help in the use of mutated genes in other cereals.
To narrow the 'phenotype gap' in crop plant species, it is necessary to expand
mutant resources in breadth and depth (Brown and Peters 1996 ) by recovering mu-
tations at new loci and recovering further mutations at known mutated loci. Closing
the 'phenotype gap' requires efficient mutagenesis protocols and sensitive screen-
ing methods. As current mutagenesis has a great number of mutagenic agents such
as various types of radiation, chemical mutagenesis, in vitro conditions, insertional
mutagenesis, and activation of retro-transposons, the efficient and sensitive screen-
ing method is still the most limiting factor for isolation of a particular mutation.
There are many misconceptions related to frequency of specific locus mutations.
Most probably, underestimation of the frequency of mutations induced by radia-
tion or chemical mutagens leads to a very critical assessment of their usefulness in
generating desired genetic variability and diversity in plants. It has generally been
accepted, from the last 3 decades, that the average frequency of induced mutations
is approximately on the level of 1 × 10 −6 . This figure ignores the data related to the
level of spontaneous mutations which have almost similar level for higher eukary-
otes (Drake et al. 1998 ). Consequently, too high of mutagen doses have often been
used, which induced too many mutations in the nucleus of each treated cell. The
generative progeny that develop from this cell segregate for many characters that
may negatively influence agronomically important characters, such as adaptability
and yield potential. As a result, due to the use of high doses, many mutants were se-
lected in mutated populations but most frequently with significant modifications in
parental genetic background that made their usefulness in breeding programs high-
ly questionable. The effectiveness of mutational strategies was also compromised
by improper handling of successive mutated generations due to misunderstanding
of the genetic consequences of chimeric structure of first mutant generation (M 1 )
plants and the adoption of ' diplontic selection ' concept. In reality, the frequency
of mutations at numerous loci is much higher, as is indicated by the frequency of
mutants in the second mutant generation (M 2 ) of some crop species.
Recent developments of gene transfer technology have enormous promise for
improvement of plant productivity; however, there is a lack of available new genes
which can be transferred to current high-yielding varieties and further significantly
increase yield. In other words, there are no genes that have been identified which
can contribute to world crop production. Borlaug ( 1997 ) referred to these genes as
'master genes' and concluded that Biotechnology may be a new window through
which to search for new 'master genes' for high yield potential by eliminating the
confounding effects of other genes. Therefore until new master genes are discov-
ered, alternative solutions for crop improvement must be pursued. Further increases
in crop yield may involve breeding for improved root systems. Breeding programs
for high yield and adaptability have only indirectly selected favorable root systems.