Agriculture Reference
In-Depth Information
Method of increasing the frequency of
mutation
Many of the mutagenic chemicals are alkylating
agents and include: sulphur mustards; nitrogen mus-
tards; epoxides; ethylene-imines; sulphates and sul-
phones; diazoalkanes and nitroso-compounds. The
most commonly used chemical mutagens have been
ethyl-methane-sulphonate (EMS) and ethylene-imine
(EI). It should be noted that all mutagenic chemicals
are highly toxic and highly carcinogenic.
The frequency of mutation can also be influenced
by oxygen level in plant material (higher oxygen related
to increased plant injury and chromosomal abnormali-
ties), water content (also related to oxygen content) and
temperature.
In general terms there are two methods that have been
used to produce an increased frequency of mutations
in plant species, radiation and chemical induction,
with the highest frequency of mutation derived cultivars
being from radiation induced mutants.
Mutations following exposure to radiation are pro-
duced by a variety of effects from physical damage
through to disturbing chemical bonds. Two main types
of radiation have been utilized to induce mutation in
crop breeding schemes. These are:
Gamma rays are the most favoured radiation source
in plant breeding and have been used to develop
64% of the radiation-induced mutant derived culti-
vars. Gamma rays represent electromagnetic radiation
with a high energy level and are produced by the dis-
integration of radioisotopes. The two main sources
of gamma radiation for induced mutation are from
cobalt 60 and cesium 137. Plant breeding programs
have used gamma ray radiation treatments applied
in a single dose or have treated whole plants to long
exposure to gamma radiation.
Types of mutation
Mutations can be conveniently classified into four
types:
Genome mutation where there are changes in chro-
mosome number due to either addition or loss of
whole chromosomes or sets of chromosomes.
Structural changes in the chromosomes involving
translocation (a chromosomal aberration involving
an interchange between different non-homologous
chromosomes), inversions (changes in the arrange-
ment of the loci, but not in their number), deficien-
cies, deletions, duplications and fusions (reduction
or increase in the number of loci borne by the
chromosome).
X-rays were the original radiation mutagen, yet
have been responsible for only 22% of the cultivars
released world wide from mutation-induced breed-
ing programs. X-rays are produced when high-speed
electrons strike a metallic target. X-rays are high
energy ionizing radiations which have wavelength
ranging from ultraviolet to gamma radiation. Muta-
tions are induced by exposing seeds, whole plants,
plant organs, or plant parts to a source X-ray radia-
tion of a required frequency for a specific time. X-rays
have to be handled by trained radiologists, and they
are not always easily accessible to plant breeders, who
often have to rely on medical facilities (i.e. hospitals)
for mutagenic treatment of plants.
Gene mutations , often termed point mutations,
where the change is in a single gene, and often the
result of a single base pair change at the DNA level.
Extra nuclear mutations where the mutational event
occurs in one of the cytoplasmic organelles. The
DNA involved includes plastids and mitochondria
and means that the mutation will usually be trans-
mitted from one generation to the next through just
one of the sexes, usually via the egg cells, i.e. mater-
nally inherited (one example of this form of mutation
is cytoplasmic male sterility, common in many crop
species).
Other forms of radiation that has been used to induce
mutation include neutrons (an electrically neutral ele-
mentary nuclear particle produced from nuclear fission
by uranium 235 in an atomic reactor), beta radia-
tion (negatively charged particles that are emitted from
radioisotopes such as phosphorus 32 and carbon 14)
and ultraviolet radiation (used primarily for induced
mutation in pollen grains).
Plant parts to be treated
Mutagenic agents can be applied to different parts of
plants and still produce effects. Seeds can be treated
 
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