Corn as example of genetic improvement in plants

Nowadays, genetic improvement is into the spotlight. However, it is not a new topic because we have done genetic improvement during years, in plants or in animals. In this article, I will discuss genetic improvement in plants, giving the corn as example, which is a plant domesticated by humans for 10,000 years.

WHAT IS GENETIC IMPROVEMENT?

Genetic improvement in plants is the process based on theoretical principles and methods for obtaining varieties of crop plants, which guarantee under high environmental conditions and production, high and stable yields of the products grown with the required quality.

AIMS OF GENETIC IMPROVEMENT

The aims of genetic improvement are:

  • Increase performance:
    • Improvement of productivity: increasing the potential productive capacity of individuals.
    • Resistance improvement: obtaining genotypes resistant to pests, diseases and adverse environmental conditions.
    • Improvement of agronomic characteristics: obtaining new genotypes that are better adapted to the demands and application of the mechanization of agriculture.
  • Increase the quality: improvement of the nutritive value of the vegetal products obtained.
  • Extend the area of exploitation: adapting the varieties of the species already cultivated to new geographical areas with climatic characteristics or extreme soil types.
  • Taming new species: transforming wild species into crops with utility and profitability for man.

STEPS OF THE GENETIC IMPROVEMENT PROCESS

Before starting the process, you have to define the objectives you want to achieve and, therefore, define those characters that you want to improve in order to obtain a specific phenotype.

The steps that follow in the process of genetic improvement are:

  1. First step: to find within the genetic variability of the collected species, or of the species that can hybridize, individuals that have these characters.
  2. Second step: these individuals hybridize with each other and with plants with good general agronomic characteristics. The result will be a base population that will segregate for a large character name, from which individuals will be selected that are closest to the desired variety.
  3. Third step: to verify that these individuals are better in one or more aspects than the varieties that are in the market, a fact that normally forces to carry out comparative tests.

THE CORN

The maize plant (Zea Mays) has been domesticated by man for 10,000 years. At this time it has become one of the three most grained cereals in the world and this increase of the crop is linked to the development of varieties that are better adapted to the needs of each place.

Maize is one of the most important staple foods since it makes many derived products (flours, oils). As it has a great value in the industry, it is a much studied plant and its genome has been sequenced.

EUROPEAN CORN BORER

Maize is affected by the European borer (Figure 1), Ostrinia nubilalis. It is a plague of cereals, particularly of corn. It is a native lepidopteran of Europe that infested the millet, before the arrival of the corn.

Its butterfly is about 2.5cm long. The female is yellowish brown with irregular bands on the wings, and the male is smaller and darker. The female lays eggs under the leaves.

Ostrinia_nubilalis,_European_Corn_Borer,I_ILO769
Figure 1. Picture of European corn borer female (Source: Discover Life)

The borer makes tunnels inside the corn (Figure 2) that cause the plant to break and fall to the ground. It has to be taken into account that when the maize is still immature it is not affected by the borer, thanks to the natural defenses of these plants in the growing stage.

larva-earshank-fieldcorn-2-44
Figure 2. European corn borer larva in maize (Source: Iowa State University)

GENETIC MODIFICATION OF CORN

We can find two types of modified corn:

  • Crops producing their own herbicide (Bt)
  • Herbicide-tolerant crops (Monsanto)

Bt maize is a plant genetically modified by modern biotechnology to defend itself against the attack of lepidopteran insects. Using recombinant DNA technology, maize was modified by inserting a bacterium gene of Bacillus thuringensis (Bt), such that its leaves, stem and pollen express the Bt protein of bacteria. Bt maize is the importation and the new tool for the control of damages and losses caused by insect pests.

Herbicide-tolerant maize is maize that has been improved by the use of recombinant DNA technology to tolerate the use of certain types of herbicides. With the use of these technologies for the possible state deactivate or replace the sequence of susceptibility by another that confer resistance and that allow a crop plant to tolerate the use of the herbicide.

OBTAINING THE MAIZE BT

To transform a normal plant to a transgenic plant, the gene that produces a characteristic of interest is identified and separated from the rest of the gene material of a donor organism.

A donor organism can be a bacterium, fungus or any other plant. In the case of Bt maize, the donor organism is a naturally occurring soil bacterium, Bacillus thuringiensis, and the gene of interest produces a protein that kills lepidopteran larvae. This protein is called Bt delta endotoxin.

The Bt delta endotoxin was selected for the fact that it is highly effective for controlling larvae of caterpillars. It is during the larval stage when most of the damage occurs from the European corn borer. The protein is very selective, in general, it does not harm the insects in other orders (like beetles, flies, bees and wasps). Therefore, transgenics that have the Bt gene are compatible with biological control programs, since they harm predators and parasitoids less than insecticides with a broad spectrum of insects. Bt endotoxin is considered safe for humans, other mammals, fish, birds and the environment due to its selectivity.

REFERENCES

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