Plant Breeding, Biology tutorial


Plant breeding is the art and science of changing the genetics of plants in order to produce the desired features. Plant breeding can be accomplished via many various methods ranging from simply choosing plants having desirable features for propagation, to more complexes molecular methods. Plant breeding has been practiced for thousands of years, since near the starting of human civilization. It is now practiced globally by individuals like gardeners and farmers or through professional plant breeders used by organizations like government institutions, universities, crop-specific industry associations or research centers.

History and developments of Plant Breeding:

Intra-specific hybridization in a plant species was explained by Charles Darwin and Gregor Mendel and was further grown by geneticists and plant breeders. In United Kingdom, in the year 1880, it was the pioneering work of Gartons Agricultural Plant Breeders. In early 20th century, plant breeders acknowledged that Mendel's findings on the non-random nature of inheritance could be applied to seedling populations produced via deliberate pollinations to predict the frequencies of various types.

From 1904 to World War-II in Italy NazarenoStrampelli made a number of wheat hybrids. His work allowed Italy to raise extremely crop production throughout the so called 'Battle for Grain' (1925-1940) and some varieties was exported in foreign countries, as Argentina, Mexico, China and others. After the war, the work of Strampelli was rapidly forgotten; however thanks to the hybrids he created, Norman Borlaug was capable to move the very first steps of the Green Revolution.

In the year 1908, George Harrison Shull explained heterosis, as well termed as hybrid vigor. Heterosis explains the tendency of the progeny of a specific cross to outperform both parents. The detection of value of heterosis for plant breeding has led to the growth of inbred lines which reveal a heterotic yield benefit when they are crossed. Maize was the primary species where heterosis was broadly employed to produce hybrids.

By the year 1920, statistical process was developed to examine gene action and differentiate heritable variation from variation caused by environment. In the year 1933, the other significant breeding method, cytoplasmic male sterility (CMS), developed in maize, was explained by Marcus Morton Rhoades. CMS is a maternally inherited feature which makes the plant produce sterile pollen. This lets the production of hybrids devoid of the need for labor intensive detasseling.

Such early breeding methods resulted in large yield raise in the United States in the early 20th century. Alike yield rises were not produced elsewhere till after World War II, the Green Revolution raised crop production in the developing world in the 1960s.

After the World War-II, a number of methods were developed that allowed plant breeders to hybridize distantly associated species and artificially induce genetic diversity.

When distantly associated species are crossed, plant breeders utilize a number of plant tissue culture methods to produce progeny from or else fruitless mating.

Interspecific and intergeneric hybrids are generated from a cross of associated species or genera which don't generally sexually reproduce with one other. These crosses are termed to as Wide crosses. For illustration, the cereal triticale is a wheat and rye hybrid. The cells in the plants derived from the first generation made up from the cross contained an uneven number of chromosomes and as an outcome was sterile. The cell division inhibitor colchicine was employed to double the number of chromosomes in the cell and therefore let the production of a fertile line.

Failure to generate a hybrid might be due to pre or post-fertilization incompatibility. When fertilization is possible among the two species or genera, the hybrid embryo might abort before the maturation. If this does take place the embryo resultant from an Interspecific or intergeneric cross can at times be rescued and cultured to generate a whole plant.

Such a process is termed to as Embryo Rescue. This method has been employed to produce new rice for Africa (NERICA), an interspecific cross of Asian rice (or Oryzasativa) and African rice (or Oryzaglaberrima). Hybrids might as well be produced by a method termed as protoplast fusion. In this case protoplasts are fused, generally in an electric field. Viable recombinants can be regenerated in the culture.

Chemical mutagens such as Ethyl Methyl Sulphonate and Di Methyl Sulphonate, radiation and transposons are employed to produce mutants having desirable characteristics or traits to be bred by other cultivars in a procedure termed as Mutation Breeding. Classical plant breeders as well produce genetic diversity in a species through exploiting a procedure termed as somaclonal variation that takes place in plants produced from tissue culture, specifically plants derived from callus. Induced polyploidy and the addition or elimination of chromosomes by employing a method termed as chromosome engineering might as well be employed.

If a desirable characteristic has been bred into a species, a number of crosses to the favored parent are made to form the new plant as similar to the favored parent as possible. Returning to the illustration of the mildew resistant maize being crossed by a high-yielding however susceptible maize, to form the mildew resistant progeny of the cross most similar to the high-yielding parent, the progeny will be crossed back to that parent for some generations. This procedure eliminates most of the genetic contribution of the mildew resistant parent. Conventional or classical breeding is thus a cyclical procedure.

Significance of Plant breeding:

It is believed that breeding new crops is significant for:

Making sure food security by developing new varieties which are higher-yielding, resistant to pests and diseases, drought-resistant or regionally adapted to various environments and growing conditions and that encompass uniformity in maturity time and other desirable qualities.

Plant breeding in certain conditions might lead to the domestication of wild plants. Domestication of plants is an artificial selection procedure conducted through humans to produce plants that encompass more desirable characteristics than wild plants, and which render them dependent on artificial (generally improved) environments for their continued existence. The practice is anticipated to date back to around 9,000 to 11,000 years. Most of the crops in present day cultivation are the outcome of domestication in ancient times, around 5,000 years ago. In the past, domestication took a minimum of around 1,000 years and a maximum of around 7,000 years. Today, all of our principal food crops are products of domesticated varieties. Nearly all the domesticated plants employed nowadays for food and agriculture were domesticated in centers of origin which have been recognized as centers that host a great diversity of closely associated crop wild plants or relatives, which nowadays can as well be employed for enhancing modern cultivars by plant breeding.

A plant whose origin or selection is due to the primarily to intentional human activity is termed as a cultigens and a cultivated crop species which has evolved from wild populations due to the selective pressures from  traditional farmers is termed as a landrace.

Conventional plant breeding:

Classical or Conventional plant breeding employs deliberate interbreeding (that is, crossing) of closely or distantly associated individuals to generate new crop varieties or lines having desirable properties. Plants are crossbred to introduce traits or genes from one variety or line into a new genetic background. For illustration, a mildew-resistant maize Zea mays might be crossed by a high-yielding however susceptible maize, the goal of the cross being to introduce the mildew resistance devoid of losing the high-yield features. Progeny from the cross would then be crossed having the high-yielding parent to make sure that the progeny were most similar to the high-yielding parent in a procedure termed as backcrossing. The progeny from that cross would then be tested for yield and mildew resistance and high-yielding resistant plants would be further developed. Plants might as well be crossed with themselves to produce the inbred varieties for breeding.

Classical breeding rely highly on homologous recombination among chromosomes to produce genetic diversity. The classical plant breeder might as well make use of a number of in vitro methods like protoplast fusion, embryo rescue or mutagenesis to generate diversity and produce hybrid plants which would not exist in nature.

Modern plant breeding:

Modern plant breeding might use methods of molecular biology to choose, or in the case of genetic modification, to insert, desirable characteristics or traits into plants. Modern facilities in molecular biology have transformed classical plant breeding to molecular plant breeding.

Marker assisted selection:

At times most of the different genes can affect a desirable trait in plant breeding. The make use of tools like molecular markers or DNA fingerprinting can map thousands of genes. This lets plant breeders to screen big populations of plants for those that possess the characteristic or trait of interest. The screening is based on the presence or absence of a certain gene as found out by laboratory procedures, instead of on the visual recognition of the deduced trait in the plant.

Reverse Breeding and Doubled Haploids (DH):

A process for proficiently producing homozygous plants from heterozygous beginning plants that consists of all desirable traits. This beginning plant is induced to generate doubled haploid from haploid cells and later on making homozygous or doubled haploid plants from such cells. Whereas in natural offspring genetic recombination takes place and traits can be unlinked from one other, in doubled haploid cells and in the resultant DH plants recombination is no longer an issue. There, a recombination among the two corresponding chromosomes doesn't lead to un-linkage of alleles or traits, as it just leads to recombination with its similar copy. Therefore, traits on one chromosome stay linked. Choosing those offspring having the desired set of chromosomes and crossing they will outcome in a final F1 hybrid plant, having precisely the similar set of chromosomes, genes and traits as the beginning hybrid plant. The homozygous parental lines can reconstitute the original heterozygous plant through crossing, if desired even in a big quantity. An individual heterozygous plant can be transformed into a heterozygous variety (F1 hybrid) devoid of the necessity of vegetative propagation however as the outcome of the cross of two homozygous/doubled haploid lines derived from the originally chosen plant. 

Genetic modification:

Genetic modification of plants is accomplished by adding up a specific gene or genes to a plant, or by knocking down a gene having RNAi, to produce a desirable phenotype. The plants resultant from adding a gene is often termed to as transgenic plants. If for genetic modification genes of the species or of a crossable plant are employed beneath control of their native promoter, then they are termed as cisgenic plants. Genetic modification can generate a plant having the desired trait or traits faster than the classical breeding as the majority of the plant's genome is not modified.

To genetically transform a plant, a genetic construct should be designed so that the gene to be added or eliminated will be deduced by the plant. To do this, a promoter to drive transcription and a termination series to stop transcription of the new gene, and the gene or genes of interest should be introduced to the plant. A marker for the selection of transformed plants is as well comprised. In the laboratory, antibiotic resistance is a generally used marker: Plants which have been successfully converted will grow on media having antibiotics; plants which have not been converted will die. In some cases markers for selection are eliminated through backcrossing having the parent plant prior to the commercial release.

Issues and concerns on modern plant breeding:

Modern plant breeding, whether classical or via genetic engineering, comes by means of issues of concern, specifically with regard to food crops. The question of whether breeding can encompass a negative effect on the nutritional value is central in this respect. However relatively little direct research in this region has been done, there are scientific signs that, by favoring some features of a plant's development, other features might be retarded. A study published in the Journal of the American College of Nutrition in the year 2004, entitled Changes in USDA Food Composition Data for 43 Garden Crops, 1950 to 1999, compared nutritional analysis of the vegetables done in the year 1950 and in 1999, and found substantial reduces in six of 13 nutrients measured, comprising 6% of protein and 38% of riboflavin. Decrease in phosphorus, calcium, iron and ascorbic acid were as well found. The study, conducted at the Biochemical Institute, University of Texas at Austin, completed in summary - 'We propose that any real declines are usually most simply explained by changes in cultivated varieties among 1950 and 1999, in which there might be trade-offs between yield and nutrient content'.

The debate nearby genetically modified food throughout the 1990s peaked in early 2000 in terms of media coverage and risk perception and carries on nowadays - for illustration, 'Germany has thrown its weight behind the growing European mutiny over genetically modified crops through banning the planting of a broadly grown pest-resistant corn variety'. The debate includes the environmental impact of genetically modified plants, the safety of genetically modified food and concepts employed for safety evaluation such as substantial equivalence. These concerns are not new to the plant breeding. Most of the countries have regulatory methods in place to help make sure that new crop varieties entering the market-place are both safe and meet up the requirements of farmers.

Examples comprise variety registration, seed schemes, and regulatory authorizations for GM plants and so on.

Participatory Plant Breeding:

The growth of agricultural science, with phenomenon such as the Green Revolution occurring, has left millions of farmers in developing countries, most of whom work small farms under unstable and difficult growing conditions, in a shaky condition. The adoption of new plant varieties by this group has been obstructed through the constraints of poverty and the international policies promoting an industrialized model of agriculture. Their response has been the creation of a novel and promising set of research process collectively termed as participatory plant breeding. Participatory signifies that farmers are more engaged in the breeding method and breeding goals are stated by farmers rather than international seed companies having their large-scale breeding programs. The groups of farmers and NGOs, for illustration, might wish to verify local people's rights over genetic resources produce seeds themselves, build farmer's technical expertise, or build up new products for niche markets, such as organically grown food.

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