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Demystifying Modern Plant Breeding
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Simply put, plant breeding is the science of optimizing the genetics of plants to improve the plant’s ability to withstand climate, disease and pest pressures while also increasing its yield potential and delivering end-use characteristics such as nutritional quality. The focus of plant breeders is on leveraging genetic diversity to create new plant variations that generate higher yield potential for farmers, more nutritious food for consumers, better feed for livestock and a healthier environment.
Improving new crops and varieties requires cross-breeding plants that have abundant genetic diversity. That’s why the most critical resource we have as a starting point in the plant-breeding process is what is known as germplasm, a collection of different types of plants from the same crop species that plant breeders use as the basis for future performance improvements.
Rigorous research and innovation in plant breeding has led to tremendous gains in food output and nutritional value over the past century. In just the last decade alone, corn yields have grown by more than 30 percent while soybean yields have increased more than 25 percent. This has been accomplished despite pressures of loss of farmland, water stress and climate change.
The plant world contains hundreds of thousands of species with an extraordinary diversity of physical and chemical characteristics.
Since the beginning of life on Earth, genes have combined and evolved in plants, animals and all living organisms to adapt, survive and reproduce. This led to plants developing diverse physical attributes such as the ability to conserve water, disperse seeds and resist pests. An example is a plant developing a bitter taste so animals won’t eat it or roses that develop thorns to protect themselves. Long before significant human intervention and selection, plants were already evolving through natural selection and fitness advantages.
Over the past several thousand years, humans have joined Mother Nature in directing the evolution of plants by selectively saving, crossing and planting seeds from the wild, and gathering plants with more favorable attributes such as better flavor, larger fruits, fewer thorns, more nutrients and harvestable seeds that will germinate the next generation. Eventually, as scientists gained a greater understanding of human nutrition and learned how to select for specific nutritional characteristics, breeders began deploying selection techniques for improvement.
Plant breeding entered a new era in the 1950s and ‘60s with the advent of the Green Revolution. This robust period of agricultural advancement ushered in a series of research, development and technology initiatives that were credited with saving over a billion people from starvation. These agricultural advancements continue at an even more rapid pace today.
A pivotal figure in The Green Revolution was a plant breeding scientist, Norman Borlaug, who worked for Corteva’s predecessor company, DuPont, for a short time before being recruited by the International Maize and Wheat Improvement Center (CIMMYT) to breed wheat varieties.
Borlaug and his team pioneered the development of high-yielding varieties of cereal grains, led the expansion of irrigation infrastructure, modernized management techniques and enhanced the distribution of improved seed varieties.
As a result of these and other initiatives, today’s farmers are now able to produce record crops with less land than a century ago.
Read the personal story of Norman Borlaug
The fundamental elements of plant breeding have not changed considerably over the years. Our plant scientists and breeders still select the best plants to achieve desired characteristics, such as drought and disease resistance, insect resistance, improved nutrition, and lower input requirements.
Today’s advanced research and understanding of genetics enable our plant breeders to more precisely and efficiently develop new and improved varieties to address increasing environmental challenges and meet the ever-changing needs of farmers and consumers.
Our work is grounded in the basic biology that has been the domain of scientists for centuries. Let’s take a look at some fundamental terms and practices that are essential to the science of plant breeding, beginning with the building blocks of life, DNA.
Deoxyribonucleic acid (DNA) is a molecule that carries genetic instructions for the development, functioning, growth and reproduction of all known living organisms. DNA and ribonucleic acid (RNA) are nucleic acids. All living organisms contain DNA as the basic building block of their characteristics.
In biology, a gene is the basic physical unit of inheritance. Genes are passed from parents to offspring and contain the information needed to specify traits. Genes are arranged, one after another, on structures called chromosomes.
Phenotype is the term used for the composite traits of an organism. The term covers the organism's morphology, or physical form and structure, as well as its developmental processes, biochemical and physiological properties and behaviors. In humans, a phenotype might describe height or weight, physical strength or a simple characteristic, such as eye color.
Natural variation, similar to genetic variation, refers to the diversity in genetic material of a population or species. This diversity is due to genes naturally (or randomly) moving from one chromosome to another. Plant breeders leverage natural variation to select for desired characteristics in a breeding population.
A cultivar, also known as a variety, is a combination of plants selected for desirable characteristics that are maintained during propagation. A cultivar is the most basic classification category of cultivated plants. For example, a single soybean variety that is distinct, uniform and stable, would be considered one cultivar.
Natural mutations occur in nature as a result of sunlight and other factors that interfere with DNA’s ability to copy accurately. Human and plant cells mutate constantly. Evolution is the result of naturally occurring ongoing mutations. For example, when a cell divides, it makes a copy of its DNA, and sometimes the copy is not quite perfect. That small difference from the original DNA sequence is a mutation. Sometimes a mutation can be a disadvantage to a plant; other times it can be an advantage.
Mutagens are physical or chemical agents that can cause changes to the genetic material. An example of a mutagen is the ultraviolet rays of sunlight that alter or cause mutations in genes of plants. Another specific example of a natural mutation is the waxy gene which results in a type of corn plant with a starch composition that makes it suitable for a wide variety of special uses, from food-grade corn starch to glossy coatings. At Corteva, we recently recreated this natural mutation using gene editing to develop a new generation of waxy corn.
Natural selection is a form of evolution. It’s a process where organisms better adapted to their environment tend to survive and produce more offspring. Naturally selected organisms tend to have some fitness advantage over others in a population. Alternatively, plant breeders practice intentional selection based on data and observations of some measured or estimated performance advantages.
Pollination is the process of a male plant or flower (or part of a flower) fertilizing the female part of that same species. In nature this happens by wind, bees, bats, insects etc, or sometimes close proximity of the male part of the flower producing pollen to the female portion containing the embryo. In all cases it results in fertilization of the embryo and ultimately in developing a seed. When plant breeders carry out pollinations, or cross pollinations, they are simply collecting the male pollen and manually placing the pollen on the receptive female portion of the flower. Plant breeders sometimes use automated methods of collecting the pollen and distributing on one or many females. For a plant breeder there are two primary purposes of making cross-pollinations: 1) to make hybrid seed for testing, evaluation and eventual commercial sale of the best performing hybrids or 2) to make a breeding cross that produces offspring for selection of higher performing off-spring.
Our plant breeders use cross-breeding, selection and other methods to create new genetic variants and leverage existing genetic diversity. The breeding and selection process takes many generations to achieve plant hybrids and varieties that look, smell, taste and yield in a more reliable and predictable manner. Although each crop breeding effort may have unique practices, their selection methods and goals are the same: improving plant productivity, quality or quantity. Society has benefited for thousands of years from the genetic changes humans have made to plants
The development of hybrids begins with inbreds -- plants that are self-crossed to create genetic uniformity. These inbred lines are selected to serve as parents for new hybrids based on complementary traits, such as yield, stress tolerance, and/or pest and disease resistance. Plant breeders combine valuable traits from each parent to generate and select offspring containing the best characteristics of both parents. For example, breeders are often trying to improve certain weaknesses that exist in one parent with complementary strengths in the other parent.
Through multiple generations and environments of selection, the breeder typically develops a few dozen elite, inbred plant lines from a population of thousands of individual plants. This process can take six or more growing seasons for typical breeding targets, with simultaneous field evaluations over multiple years and environments.
For more complicated traits, such as introducing new disease-resistant genes from a genetically distant relative, more than six plant generations are required.
A hybrid plant is created when plant breeders intentionally cross two different varieties or inbreds to produce an offspring, that is stronger and performs better than either of the parents. Cross-pollination is a process that occurs within members of the same plant species. Hybridization occurs both in nature and through human intervention.
Before any new hybrid is released to the market, it undergoes several years of development and field testing through a stringent, multi-year process designed to identify hybrids that have the best combination of performance, stability and product quality.
New hybrids are grown under standard or typical production conditions, side-by-side with industry-leading commercial hybrids. These leading hybrids are called “checks.” Yield data, as well as numerous other trait data, are recorded for the new hybrids and the checks. In addition, new hybrids with the most commercial promise are further subjected to other analytical and observational tests.
Thousands of such new hybrids are evaluated each year and only commercially released if their combination of field performance, stability and product quality will make them more competitive compared to existing commercial hybrids.
As a result, only a very small percentage — typically less than 1% — of new hybrids are commercially marketed. At every point during this testing process, hybrids that are not competitive are eliminated from the pool of candidates.
Breeders have created countless elite cultivars over nearly a century of corn breeding, identifying high-yielding plants that possess desirable traits. This is achieved through various processes, including one known as doubled haploid, which enables breeders to develop a true, genetically uniform, inbred more quickly and efficiently. A doubled haploid is a parent line which only has one set of chromosomes, vs the normal two sets. These are called haploid plants. The number of chromosomes is then doubled again to create a genetically uniform (homozygous) plant. Doubled Haploids are valuable in research and development because scientists can develop 100 percent genetically pure plants in just two generations, compared to the usual seven generations. This increases the speed and efficiency of breeding. Doubled Haploids can be used to study traits and as inbred lines in breeding.
Another process, “embryo rescue,” refers to a number of lab techniques whose purpose is to promote the development of an immature embryo into a viable plant. Just like say chicken eggs, seeds have an embryo as well; which is the part of the seed that eventually becomes a plant (the rest of the seed is for protection and to feed the embryo). Embryo rescue is widely used to rapidly speed up the time from one plant generation to the next. Additionally, certain plant characteristics can be selected during the embryo rescue process, significantly reducing the number of different plants that need to go to the field for evaluation.
Additionally, plant breeders use DNA-based Molecular Markers from plant tissue to genetically map the genome of a crop species Molecular markers are unique sections of DNA which are associated with particular traits. They are situated along the genome, and provide a view of the genetic makeup in that specific region. Scientists use markers to screen for the presence of traits even before the plant is fully grown. There can be thousands of molecular markers for any single crop species. In fact, the more markers available to scientists, the better they will be able to screen for traits before the plant is ever grown in the field.
Using this molecular marker information, plant breeders improve the accuracy of their selections and, ultimately, the performance of resulting hybrids and varieties. Through many cycles of breeding, breeders can also monitor the level of biodiversity
As recently as the 1970s, marker technology was limited to morphological (visual) characteristics and isozymes. Isozymes are two or more enzymes with identical function but different structure. They were used to mark locations on chromosomes to map other genes of interest to the breeder.
Other forms of DNA based marker systems followed with restriction fragment length polymorphisms (RFLPs) in the 1980s to amplified fragment length polymorphisms (AFLPs) and single stranded repeats (SSRs) in the 1990s, to single nucleotide polymorphisms (SNPs) in the 2000s.
Today’s advancements In high-throughput automation have increased the capacity of molecular markers at a greatly reduced cost. Enhancements to these series of molecular marker technologies in both plant improvement and human disease diagnosis are excellent examples of why innovation should continue to progress, agnostic of any particular method of application.
Introgression is a term in genetics used to describe the movement of one gene from a genotype into another (often a more advanced, elite line) for the purpose of improving that one specific characteristic. This is accomplished by repeated crossing with a target variety.
In agriculture, certain biotechnology products are referred to as genetically modified organisms (GMOs) or Transgenics. Most of the GMO traits on the market today are designed to help protect crops from being eaten by insects or being overrun by weeds, helping farmers produce more on each acre of land with fewer inputs while ensuring a stable and sustainable food, feed, fuel and fiber supply.
Since the discovery of DNA (the genetic code of life), scientists have been striving to understand how genes work in various living organisms, including plants. Plants contain tens of thousands of genes that specify every aspect of their form and function. With rapid improvements in DNA sequencing technology, the entire genome of a species and all its genes can now be identified in just a few weeks. This provides breeders with a detailed map to guide their plant improvement efforts.
Today, the entire genetic code of many species -- from microbes to plants to animals to humans -- is available to researchers who are finding a tremendous diversity of gene functions. c. For example, the naturally occurring soil bacteria Bacillus thuringiensis, (Bt) produces proteins that are toxic to insects when eaten but have not been found to impact humans and farm animals.
For many decades, farmers have sprayed these naturally occurring Bt soil bacteria on their crops to protect them from insect damage, and in fact they are available to use as pesticides in organic agriculture.
Scientists soon discovered that rather than spraying the bacteria on the plants, which requires labor and mechanization, it was possible to insert the bacteria-producing genetic code into a plant to allow the plant to produce the protein and protect itself from insects. This Bt bacteria derived trait now enables the bacteria to be transferred to crops such as corn, soybeans and cotton. For example, a common Bt or GMO trait is Herculex® Rootworm resistance, developed by Corteva.
How do scientists transfer a useful gene like a Bt protein from one species to another?
First, molecular biology tools such as recombinant DNA technology allow scientists to produce a DNA strand that contains the precise sequence of genetic code they wish to transfer from one species to another (such as the Bt example above). These tools are also commonly used to develop many products we use every day, ranging from bread to insulin.
Second, microparticles coated with the desired DNA are bombarded onto plant cells at a very-high velocity. As they pass through the plant cells, they leave behind the DNA they were carrying. Through the naturally occurring cellular repair process, this donor DNA is combined into the genome of the target cell.
Third, in a more elegant approach to gene transfer, scientists make use of a natural phenomenon involving a microbe called Agrobacterium. These microbes have the capability of transferring a set of their genes to host plants, a process which over millennia has also occurred in nature. An example of this is today’s sweet potatoes or yams in which it has been confirmed that gene transfer occurred naturally from one species (Agrobacterium) to another (potato) to form a new plant (sweet potato). Biotechnologists redirect this process by removing the Agrobacterium genes and leaving only the elements required to facilitate the transfer of DNA. As a result, these Agrobacterium microbes can be used to transfer a gene of interest from any source into a plant cell.
After the gene is transferred, the final step in the biotechnology process is plant tissue culture and regeneration. Using the newly transformed cells, scientists cultivate living plant tissues in the lab. They then provide the plant tissues with the right conditions to develop into seedlings. The seedling eventually grows into an adult plant with progeny seed that contains the transferred gene.
If the transferred gene came from a different species, the result is called transgenic.(or GMO) If it is from the same species, it is called cisgenic, and is therefore not a GMO. Cisgenic gene transfers are done when a particular variety of the species adapted to one location contains a gene that would be useful to a variety adapted to another location. For example, a plant growing in the tropics may have a disease resistance gene that would help protect plants growing in temperate climates.
Until recently, our ability to influence the genetic makeup of an individual plant or animal was limited. We could use the processes of crossing and selection over multiple generations and hope to create the right combination of genes, or we could transfer one or a few genes using biotechnology. With the advent of genome editing technology, we can make precise changes to the genetic code. Drs. Emmanuelle Charpentier and Jennifer Doudna, two of the researchers who identified a new method of targeted genome editing (CRISPR-Cas9), were awarded the Nobel Prize in chemistry in 2020.
Genome editing allows us to develop plants that have beneficial traits such as better nutrition, expanded shelf life, faster growth, higher yield, and greater tolerance to droughts, extreme weather and disease. These improvements could occur in nature or be developed through conventional breeding but genome editing makes it possible to deliver them faster and more efficiently, as long as breeders know the function and characteristics that a particular gene contributes to the phenotype of the plant.
A growing number of tools can be used to conduct genome editing. These often go by acronyms such as CRISPR or TALENs, or other scientific terms such as zinc fingers and meganucleases. These tools all share one thing in common – they can search a genome for a specific DNA sequence and precisely edit that site, much like cutting and pasting in a document on a computer.
In contrast to GMO’s, researchers can use genome editing to create an improved plant that does not include DNA from a different species. This is now a novel way to use the natural biodiversity present in plant species, by tapping into and understanding the function of native genes When we do this, we rigorously test the seeds to confirm the intended changes are made and the resulting plants perform as expected over multiple years of field trials. Beyond agriculture, genome editing is also being used to discover treatments or cures for serious genetic diseases and conditions, such as sickle cell anemia, cystic fibrosis and early-onset Alzheimer’s, which could improve the lives of millions.
CRISPR1 Advanced Plant Breeding Guiding Principles2 |
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| 1“CRISPR-Cas” is derived from naturally occurring “CRISPR” found in many bacteria that protect themselves against bacteriophage. Corteva has used CRISPR for many years to improve dairy product manufacturing and to make food safe and last longer. 2These principles refer to the Corteva use of “CRISPR” as a plant breeding technique. If CRISPR is used to more efficiently develop GMOs, Corteva will follow all applicable GMO regulations. |
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