Question

What is plant breeding? Describe the breeding of crop plants for disease resistance and high yield.

Answer 1

Hint: Growing plants with desirable characteristics is called plant breeding. In order, to have high yielding plants with disease resistance genes responsible for these traits could be passed on to the desired plant by a process of hybridisation or genetic engineering techniques or by inducing and then selecting mutants with these qualities.

Complete answer:
Breeding refers to the collection of techniques applied to select for desirable characteristics such as high yields in commercially important plants including crops and ornamental plants. Essentially this involves modifying the genetic potential of the plant of interest. Humans have been engaged in plant breeding since historical times starting around 9000-11000 years.
Breeding crops for any specific traits require the following steps and this holds true when breeding for resistance to disease or to increase yields. The steps are:

I. Gathering the germplasm: Germplasm is the starting material for desired seed production and represents the sum of all alleles of a specific species of plants and its relatives. An important consideration for germplasm selection is that the seeds/ plants should be free of virus.

II. Choosing parental strains: After gathering germplasm the seeds with desired characteristics need to be selected. In this specific case this means selecting varieties of plants having disease resistance or plants with high yields when using traditional breeding techniques.
For disease resistance it is important to have a good collection of resistant variety and a well-defined test for looking into the resistance to develop disease.
In case of genetic engineering desirable characters such as genes involved in resistance or increasing yield are cloned from the desired plants and then after optimising for expression in desired plants transgenic are introduced into the host. The newly created transgenic will then produce the protein involved in resistance and this can then be propagated through generations. For example, Bt cotton and maize encode a toxin originating from a soil bacterium Bacillus thuringiensis. This toxin when ingested by caterpillars of the pest kills them.
The other modern approach is to induce mutations through mutagens and then select from the mutants those that have desirable characteristics. Chemical mutagens such as sodium azide or ethyl methane sulphonate (EMS) or physical agents such as ultraviolet and ionising radiations may be used for this purpose.

III. Creating desired hybrids: For the traditional breeding programme hybridisation is the way to introduce genetic variation and thus change the traits/characteristics desired by the breeders. Multiple types of hybridisation involved in giving rise to the final desirable progeny are:
a. intergenic: hybridisation between parents of two different genera
b. interspecies: hybridisation between two different but related species
c. intervarietal: hybridising two different varieties of the same species
d. intravarietal: hybridising between two plants of same variety
After deciding on the type of hybridisation the following steps are carried out:
Selfing of the parents: The selected parental strains are then self-crossed (selfed) for multiple generations. This is to ensure homozygosity of all the desired alleles.

Emasculation: After the desired alleles have become homozygous the male parts of a bisexual flower are removed to prevent self-pollination. This step is called emasculation.

Bagging and tagging: This follows emasculation to prevent foreign pollens of undesired/unknown genotypes fertilising the ova/eggs in the female parts. This is to maintain the dates and other details written in tags post the emasculation.

Cross pollination: This is pollinating the female flowers with the pollens of desired genotypes to create the wanted hybrids.

IV. Selecting the progeny and commercialising: This step involves selecting the hybrids and then crossing them for multiple generations to obtain the desired new varietal. This is then tested against the conditions of resistance in different settings of irrigation. For higher yield variety the new seeds are planted for at least three seasons in different farmlands having different weather and soil conditions to arrive at the optimum cultivation conditions and then finally the seeds/ plants are ready for commercialisation.
Traditional breeding for new varieties is time consuming. For example, the HUW 468 wheat, a rust resistant wheat variety suitable for cultivation without tillage and requiring surface spreading, took 12 years to be generated.

Note: An important concern for breeding high yielding plants is that often high yielding plants are more susceptible to diseases and adverse environmental conditions. Often for pest resistance, plants produce toxins, and this involves using up a lot of energy and resources to produce the toxins and consequently these plants have low yields. For resistance to draughts and other adverse conditions once again the plant must make do with low resources which affect its survival and thus energy is not diverted to producing fruits. Thus, achieving both high yield and resistance is a trade off and it is not possible to generate a plant with highest yield but perfect resistance to adverse conditions.