What is Plant Breeding?
“Plant breeding is the purposeful manipulation of qualities in plants to create new varieties with a set of desired characteristics.”
Plants with higher qualities are selected by and crossed to obtain plants with desired quality. This results in a plant population with improved and desired traits.
Let us have a detailed look at the objectives and steps involved in plant breeding.
Plant Breeding Objectives
Following are the major objectives of plant breeding:
- To increase the crop yield.
- To raise plants with desired characteristics.
- To develop a disease-resistant crop.
- To develop plants that can tolerate extreme environmental stress.
Types of Plant Breeding
There are the following different types of plant breeding:
Backcrossing
In this, a plant with desired traits is crossed with a plant that does not have the desired traits but has several other traits.
Inbreeding
In this, self-fertilization occurs. The progeny produced is the same generation after generation. This helps to preserve the original traits.
Hybrid Breeding
In this, two different breeds are crossed to produce the offspring that is more productive than the parents.
Mutation Breeding
The mutations in plant genes result in new varieties. Mutations can also be induced in plants by exposing them to chemicals and radiation
Mutation is a sudden heritable change in an individual (living organism). So mutation brings about new genetic recombinants, which never existed in a population. The arising gene recombination if useful may be selected as a new variety. Useful naturally occurring mutations are however very rare. In plant breeding therefore mutations are usually induced.
Gene mutation may be induced in plants by
various methods of radiation.
Mutations for earliness, quality and other agronomical characters have
been successfully induced by radiation.
In principle, rate of mutations can be increased by exposure of plants
or seeds to radiation. Since useful mutations can be produced in nature, it is
assumed that valuable mutation may also be produced experimentally.
Mutation rate can be increased
by:
1.
Temperature
2. Long
term seed storage
3. Tissue
culture conditions
4. Ionizing
Radiation (X-rays , Gamma Rays, Neutrons, Laser beams and Ultraviolet Radiation
etc.) ,
5. Chemical
mutagens; these include:
§ Ethyl
methane sulphonate (EMS)
§ Diethylsulphonate (DES)
§ Ethyleneimine (EI)
§ N-ethyl-N-
nitro urea (ENH)
§ N-Methyl N nitro urea (MNH)
Chemical mutagens are often preferred over radiation because they are simpler to apply and produce less damaging effects. Chemical mutagens are usually less drastic in their effects than ionizing radiations, producing more gene mutations and fewer chromosome disruptions.
Types ofmutation
1.
Genome mutation - Change in chromosome number due to addition
or loss of the entire chromosome set or parts of sets.
2. Structural
changes – Caused by changes in duplication or deficiencies in one chromosome.
3. Gene
mutation – Associated with the use of chemicals and result in a new highly
desirable character.
4. Extra nuclear mutations – Involve components
of the cytoplasm. Cytoplasmic – genetic male sterility is an example of a
Cytoplasmic mutation that is useful for Crop improvement.
i)
Recessive mutations: change of
a dominant allele to a recessive allele (A---a)
ii)
Dominant mutation: Change of a
recessive allele to a dominant allele (a----A)
Mutations that convert the wild type (the common phenotype) to the mutant form (the rare phenotype) are called forward mutations while those that change a mutant phenotype to a wild phenotype are called reverse mutations. Recessive mutations are the most common types of mutations. However recessive alleles in a diploid are expressed only when they are in the homozygous state.
Plant materials
to be treated
1.
Whole plants – Seedlings or
small plants can be treated with X-rays.
Gamma rays can be used to treat small or large plants in a gamma field
or gamma room.
2.
Seeds
3.
Pollen grains:
4.
Parts of plants used for asexual
propagation including cuttings and apical buds.
5.
Cell and tissue culture
explants. Single cells or tissue
explants are treated and screened in medium. Desired mutant types are
identified and regenerated into whole plants.
Mutagenic agents
i) Physical mutagens ii) Chemical mutagens
Induction of
mutations
The basic requirement in any breeding programme is a clear definition of the breeding objectives which is also a basis principle in the mutation breeding approach. Once the plant breeder has defined his/her breeding objectives and assessed their sources of variability, then he/she is in a position to decide whether to induce mutations.
The next step in the mutation breeding procedure is to produce the desirable variation by using various known mutagens. It is important to find the dose(s) of mutagen treatments which give the maximum mutation frequency. In rice chlorophyll mutations can be used to establish the optimal doses for treatment of seeds.
When using the mutation breeding techniques, one should pay attention to the selection of the parent variety. It should, in general, be the best adapted variety for the location in question.
Plant characters to be improved by mutation Breeding
Mutational
changes in most characters of agronomic importance have been reported in
literature.
Plant height
Changes in plant height have been one of the most common mutant types. A large number of short-statured mutants have been produced in local varieties in many countries. Most of these are caused by recessive major gene mutations.
Flowering and maturity time:
Mutants with changes in flowering or ripening time have been reported in numerous experiments and a number of early mutant cultivars have been released.
The early maturing mutants have obviously been of main practical interest. Many major genes influence the flowering and maturity time in rice and the most of the mutations in these genes towards earliness are recessive and show monogenic inheritance.
Other plant characters:
Changes in many other characters have been reported. More tillering and increased panicle length and number are important for a productive plant type. Leaf size and shape could also be altered favourably. Grain type are very important in rice and many improvements in grain size, shape and non-shattering have been found. Increased protein content and changes in amylose content have also been produced by induced mutations.
Disease and pests:
Many
attempts have been made to improve resistance to important diseases in rice and
other crops. Positive results have been
reported from several countries particularly in relation to blast (Pyricularia oryzae) and bacterial leaf
blight (Xanthomonas oryzae pv oryzae)
i)
Clear
Objectives
A
programme established to select one specific trait is more focused and easier
to conduct with higher chances of success than a programme designed to select
more than one trait
ii)
Efficient
screening method
Mutation breeding programmes examine large segregating populations to increase the chance of finding the typically rare desirable mutation event. An efficient method of screening method should be developed for a mutation breeding programme.
iii)
Proper
method of mutation and method of treatment
Mutagens
vary in various properties including source, ease of use, penetration of tissue
and safety. Some are suitable for soft tissues while others are suitable for
hard tissues
iv)
Dose
rate.
The breeder should decide on the appropriate and effective dose rate (dose and duration of application). The proper rate is obtained by experimentation for each species and genotype. Mutagenic treatments kill some cells. Overdose kills too many cells and often produces crippled plants, whereas underdose tends to produce too few mutants. It is recommended that three dose levels be used in a project- the optimal rate, plus one dose above and below this rate.
How mutants can be used in
plant breeding
1.
Directly: A good mutant (high yielding, well
adapted) can be adopted as a new variety after testing.
2.
Indirectly: A mutant not good enough to be adopted as
a new variety may be used in cross breeding.