Agriculture Reference
In-Depth Information
with either chemical or radiation mutagens. Seed have
been preferred by many breeders because seeds are more
tolerant to a wide range of physical conditions such as
being desiccated, soaked in liquid, heated, frozen or
maintained under varying oxygen levels, which allows
their exposure to the various mutagens rather easily.
Seed treated with an induced mutagen will result in
plants which are:
chemical or radiation mutagen and screening regener-
ating plantlets. One very desirable approach is the use
of selective media, which only allows the growth of spe-
cific mutant types. This has been useful in developing
herbicide resistant cultivars where a low concentration
of the selected herbicide is added to the media.
Dose rates
•
Non-mutants, the same as the parent plant.
It is apparent that all mutagenic treatments are basi-
cally damaging to plants. When too high a dose rate
is applied all the plant cells may be killed. Conversely
if the applied mutagen dose rate is too low, then very
few mutant types will be induced. It is therefore neces-
sary to determine an appropriate dose rate to use. The
optimal dose rate will change according to crop species,
plant part exposed to the mutagen and its physiological
state. Indeed the first stage of most mutagenesis based
breeding is to determine the most appropriate dose rate
to minimize adverse effects, yet still produce sufficiently
high levels of mutation. In practice it is usual to carry
out several preliminary experiments in order to establish
a suitable dose rate and protocols, which optimize the
survival of the plant material subsequent to exposure.
In simple terms dose rate is equal to mutagenic
intensity
True mutants, where a mutation event has occurred
throughout the whole plant.
•
Chimera, where only a portion of the resulting plant
has been mutated.
•
One attractive possibility in mutation breeding is to
treat pollen grains with radiation or chemical muta-
gens. The major advantage of treating pollen grains is
that they are easily collected in large numbers and can
easily be presented to a radiation source. Pollen grains
are single cells, so induced mutation of pollen avoids the
occurrence of chimeras. Pollen grains are also haploid
in terms of their genetic composition and so this opens
the possibility, in an increasing number of species, of
tissue culture treatment leading to their direct develop-
ment into plantlets - which, with suitable treatment,
can be induced to double their chromosome number
and give true breeding, homozygous lines which will
express both recessive and dominant mutated alleles.
Treatment of whole plants is less common but can be
achieved using X-rays or gamma rays. This is often car-
ried out using small plants or plantlets but, for example
in Japan, they have built a large facility (resembling a
sports arena) with a large gamma source at its centre
and a large number of fully grown plants are exposed
over varying periods.
The treatment of cuttings and apical buds with radia-
tion or chemical mutagens can be effective in developing
mutant types in new shoots and plantlets. An impor-
tant factor is whether the meristematic region forms
mutations, since this is the region from which the new
propagules develop. Treating cuttings and apical buds
has been particularly important in developing mutant
clonally propagated cultivars.
It is now becoming more popular to combine muta-
tion with
in vitro
cell and plant growth. The idea
centres on mimicking that possible with microorgan-
isms and so often involves treating single cells with a
×
time applied. In chemical mutagenesis this
involves the concentration of mutagenic chemical and
time that plant cells were exposed to the chemical solu-
tion. Intensity of radiation can be altered by varying
the distance from the radiation source or by varying the
radiation form. The dose can be adjusted by changing
mutagenic intensity or exposure time (or both). It is
common to experiment with different dose treatments
until one is found which allows 50% of plants to survive
the treatment. These tests are called
lethal dose 50
or
LD
50
tests.
Dangers of using mutagens
Mutagenic chemical and radiation are effective because
they alter the genetic makeup of plants and create vari-
ation. They will, of course, similarly, affect the DNA of
plant breeders who are exposed to them! It is not there-
fore possible to over emphasis the importance of using
appropriate safety procedures in using any mutagen.
As already mentioned, the facilities for applying muta-
genic treatments (in this case mainly radiation) are not
always directly available to the average plant breeders,
