Plant-breeding stages, evolution and tools

STAGES OF GENETIC IMPROVEMENT

Modern genetic improvement has two stages:

  • The first is to achieve genetic variability using different techniques.
  • The second consists of selecting the crops thus developed until varieties adapted to specific conditions are obtained.

Until the twentieth century there were virtually no advances in plant breeding, since the crosses that were produced were not intentional, and therefore biodiversity was not promoted.

DEVELOPMENTS IN PLANT BREEDING

The development of new plant varieties through genetic improvement is a complex activity that requires a large long-term financial investment: the choice of the parental lines that are capable of transmitting the characteristics sought, the cross-breeding, the selection and subsequent purification, etc.

These are activities which, depending on the species, take up to 10 or 12 years and involve costly investments in equipment and technical staff. So the average cost of putting a new variety on the market amounts to several million Euro.

PLANT-BREEDING TOOLS

The purpose of plant genetic improvement is to increase the yield, quality, and/or reduce the production costs of foods, decorative plants, and other industrial products that come from growing plants.

Keeping in mind the satisfaction of the population, the producer, and the environment itself.

Different genetic procedures can be used to select or improve plants and crops. The choice of one or the other depends on the characteristics of each species and the human and economic resources available, but they always have the common goal of achieving an increase in production or substantial improvement in the different varieties in order to meet the needs of the population.

Generally, and in a simplified manner, techniques for genetic improvement can be grouped into three blocks:

Hybridization is the action of sexually crossing two individuals with different genetic constitutions, that is to say, two different varieties or species, to manage to boost the parental features in their offspring. Other undesired features also arise from this combination, so after hybridization it tends to be necessary to perform a process of selection for various generations, thus eliminating the plants that have undesirable features. When hybrids are obtained with desired features that are sufficiently developed, they are usually reproduced by asexual methods (grafting, layering) so that the identical features are sustained between individuals. This is currently a dominant technology, which made its appearance with the so-called “green revolution” in the middle of the 20th century.

In the hybridisation of nucleic acids (DNA or RNA), two nucleic acid chains are combined with complementary base sequences in a single double-stranded molecule, which acquires the double-helix structure. Two complementary chains join rapidly because the speed of hybridisation is directly proportional to the genetic similarity between the two samples.

By crossing two different varieties or species, plant breeding applies the principles of genetics to produce varieties with greater resistance to disease, better nutritional values or more pleasant flavours. This phenomenon is used in large-scale production of horticultural and corn crops.

Plant breeding can be defined as the system of crossing and genetic selection of plants for the development of new stable crops of higher quality and yield and which respond to market requirements.

Plant breeding programmes can be carried out using different methods, depending on the characteristics of the species to be improved, as well as the available human and economic resources. Conventional methods of interspecific crossbreeding and hybridization, or modern techniques such as genetic engineering (biotechnology), are some of the tools commonly used in improvement programmes.

Classical genetic improvement is based on the deliberate crossbreeding (plant-plant) of two individuals of the same species to obtain new pure lines with desirable characteristics.

Without the need for prior knowledge of the genetics of the parental lines and based on phenotypic (visual) characteristics, the resulting lines contain the genes (specific DNA sequences) with information for the most desirable traits that are artificially selected and conserved in the new varieties.

Line A Early variety (dominant gene). Ribbed fruit (recessive gene).

Line B Late variety (recessive gene). Round fruit (dominant gene).

F1 Hybrid Early variety with round fruit.

The F1 hybrid descending from 2 pure lines is perfectly homogeneous.

Classic improvement is largely based on the homologous recombination that occurs during meiosis, a type of genetic recombination that allows the interbreeding of similar paired chromosomes to generate new DNA sequences, which promotes diversity in the resulting lines.

Some of the characteristics that breeders have tried to incorporate into cultivated plants over the past 100 years include:

  • Resistance to viruses, fungi, bacteria.
  • Increased quality and quantity of crops.
  • Increased tolerance to environmental phenomena (salinity, extreme temperatures, drought).
  • Increased tolerance to insects.
  • Increased tolerance to herbicides.

Instead of crossing two plants sexually and having the entire DNA chain recombine, what is done is to incorporate one or more genes of the same (cisgenesis) or another species, through different laboratory techniques. In this manner, the desired quality is modified without affecting the rest of the plant’s characteristics. Technology that is increasingly widespread in agriculture worldwide, especially in North and South America and Asia, but with little development in Europe.

Genetic engineering is the manipulation and transfer of DNA from one organism to another in order to create new species, correct genetic defects or manufacture different compounds.

DNA is a fundamental basis of information possessed by all living organisms.

In agriculture, genetic engineering has made it possible to modify the characteristics of large numbers of plants to make them more useful to humans.

The results of genetic engineering applied to agriculture are resistance to herbicides, insects and microbial diseases; increased photosynthetic yield; improvement in the quality of agricultural products, or even the creation of products of commercial interest.

Molecular markers

Molecular markers are biomolecules that can be associated with a genetic trait of an organism, since the presence of the marker necessarily implies that of the gene concerned.

There are two types: biochemical markers (proteins and isoenzymes) and DNA markers, of which there are several classes depending on whether they are based on amplification or hybridisation or both.

Applications of molecular markers

Markers offer the possibility of studying populations of organisms and selecting those of interest to humans. These markers make it possible to improve different species in agriculture.