Plant Breeding Technologies: Exploring Mutagenesis and Gene Editing for Crop Improvement
New plant breeding technologies – Mutagenesis and Gene Editing
The history of plant breeding
The history of plant breeding and plant-crop improvement started thousands of years ago, almost in prehistory, when Neolithic people domesticated wild species, and they became cultivated plants. With this domestication, agriculture was born. Many years later, with the help of science (with the help of Darwin, Mendel, etc.), menkind (farmers and plant breeders) improved these crops and made them more productive and better suited to our cultivation systems and consumers’ needs. The laws of inheritance of genes and traits that Mendel studied and described allowed us to introduce gene viability by selecting varieties with interesting agronomical traits. Afterward, Watson Crick and Rosalyn Franklin discovered and described DNA structure, opening the doors to new technologies. From this time, other techniques you can see here some words that are underlined. These techniques, like Mutagenesis or in vitro culture techniques, allowed us to develop these crops better.
It is important to remember that we work at the DNA level by introducing new genes or silencing (making “inactive”) genes. This will impact the translation of the information into proteins, which are the ones that perform functions and, therefore, give us the phenotype or interesting traits.
The convention or traditional plant breeding
So far, the majority of the plant crops have been created mainly by applying conventional breeding techniques. This process mainly consists of multiple crosses of an elite variety with a donor variety carrying one (or more) genes of interest.
As an elite crop variety or elite line, we define a genotype with desirable traits that is usually commercially acceptable. In many cases, existing commercial crop varieties that perform well but need to be improved for specific characteristics, like resistance to a specific plant disease (pathogen), can be used as elite lines. In this case, the breeders cross the plants of the elite lines with a donor plant.
As a donor plant, we can use a plant of the same species, closely related to our elite line, that carries the trait of interest, in this case, disease resistance. The goal of breeders is to cross the elite line with the donor to obtain plants that are as similar to the elite line as possible but also carry the resistance (gene/trait).
This can be challenging, especially if the elite line and the donor also differ in other characteristics. This is mostly the case when wild relatives of the plants are used as donors. These wild relatives have great variability that can give us essential genes for survival and adaptability. However, since they have not been genetically improved (as our elite line), they carry many undesirable characteristics (like low yields, bad taste or appearance, antinutrient factors, etc.).
As a result, the breeder has to take the gene of interest and “get rid of” the rest of the undesirable traits of the donor plant. To do that, he/she performs multiple crosses. After the initial cross, the breeder chooses from the progenies the ones that carry the gene of interest (in this case, the gene responsible for the disease resistance) but looks more like the elite line. These plants are back-crossed with the elite line. This procedure is performed as many times as necessary (usually 5-7 cycles are needed) to obtain a satisfactory result. Nowadays, this process can be sped up with the help of DNA markers, but it still is time and labor-consuming and has relatively low precision. At the same time, it has an increased level of difficulty for some very important crops like potatoes or tree crops due to the complex genetics and very long life cycle (among others).
Mutagenesis – What is Mutagenesis and its role in plant improvement
A mutation is the source of genetic variability and is the driving force of evolution. Mutations occur naturally (in nature) but can also be employed by scientists to create new genetic variations and, as a result, new varieties. Plant mutation breeding or Mutagenesis is a method in which mutagenic agents are applied that will act on the double standard DNA and will produce some mutations that may be interesting for us in breeding. These mutagenic agents can be:
- chemical (like ethyl methane sulfonate) or
- physical (like radiation) agents
This practice has given “birth” to over 3,000 commercialized plant varieties.
After the treatment, the scientists must select the plants that contain the mutation, giving the trait of interest.
It is essential to remember that all these mutations happen randomly in the genome, and we cannot control them.
We can take the elite variety (which was susceptible to a plant disease), mutagenize it with chemical or physical agents, and, afterward, the breeder has to select the mutants that have the resistance, back-cross them with the elite variety multiple times, and eventually obtain an elite resistant variety. This process will take up 6 to 7 years.
Plants that have been created with mutagenesis are not considered GMOs (genetically modified organisms).
What are Gene editing and CRISPR-Cas, and how can they be used to create new crop varieties
Gene editing is also a biotechnological tool, and it is based on restriction. The CRISPR (or CRISPR-Cas, or CRISPR-Cas9) genetic editing system is a restriction system. Restriction in molecular biology means cutting, so it’s based on cutting DNA. That’s why this technique is also called molecular scissors. It is based on a defense mechanism of bacteria and archaea against the viruses that attack them. When a cut occurs in the double-stranded DNA chain, the cell tries to repair it immediately.
So the basics are that when a cut in the double-standard DNA occurs in a cell, the cell tries immediately to repair it. Two different techniques can be used for this reparation.
The first one is called non-homologous end joining, and it consists of repairing the DNA and adding random nucleotides. This addition of random nucleotides results in a change in the gene that results in a knockout mutant, which resembles the effect that mutagenesis can have.
Another reparation system is based on homologous recombination. When we give another gene, by recombination, this gene can be placed in the place where we have the cut, repairing it. As a consequence of this form of recombination, this donor gene is integrated into the genome, and the result is a gene insertion similar to the one that can be obtained with transgenesis.
What is the status of gene-edited crops in the market?
More than 450 transgenic crops have been approved for cultivation and field trials in different countries around the world. The situation is very different depending on the continent and the local laws.
The gene traits introduced are either for pest resistance to viruses, bacteria, fungi, or insects, herbicide tolerance, or an improved nutrition value. These 450 transgenic crops are organized from more than 30 plant species.
Gene editing has been applied to more than 40 crops in different countries, targeting genes related to agronomical traits, food and feed quality, and resistance or tolerance to biotic or abiotic stresses. This article from Natural Genetics provides a very nice review if you want to know more.
To summarize, we have been comparing the major breeding techniques available and used today: conventional breeding, mutagenesis, transgenesis, and genome editing (CRISPR). Gene-editing is a very promising tool nowadays because, as we have seen, it allows us to direct the artificial transfer of a gene and eliminate the transgene (the machinery) in the second generation.
Further reading
Sexual Propagation of a Plant – Everything around seeds
What is Plant Propagation – Types and characteristics of Sexual and Asexual propagation material
The Importance of Crop Wild Relatives for Enhanced Plant Stress Resilience
Hybrid Potatoes – A Climate-Smart Solution for Potato Farmers
How Epigenetics Can Help Farmers Grow Healthier and More Resilient Crops
