11 Milestones in Plant Genetics
The science of plant breeding is crucial to one of humanity’s greatest challenges: the need to feed, clothe, and nourish a growing world population in the face of climate extremes, decreased water availability, demands for renewable energy, and our responsibility for environmental stewardship.
Here we summarise 11 of the biggest milestones and innovations in modern agrigenomics that have supported humankind in meeting this global challenge.
The principles of inheritance – 1865
Gregor Mendel, through his work on pea plants, discovered the fundamental laws of inheritance. He deduced that genes come in pairs and are inherited as distinct units, one from each parent. Mendel tracked the segregation of parental genes and their appearance in subsequent offspring as dominant or recessive traits. He recognized the mathematical patterns of inheritance from one generation to the next. Mendel's laws of heredity are:
1) The Law of Segregation: Each inherited trait is defined by a gene pair. Parental genes are randomly separated to the sex cells so that sex cells contain only one gene of the pair. Offspring, therefore, inherit one genetic allele from each parent when sex cells unite in fertilization.
2) The Law of Independent Assortment: Genes for different traits are sorted separately from one another so that the inheritance of one trait is not dependent on the inheritance of another.
3) The Law of Dominance: An organism with alternate forms of a gene will express the form that is dominant.
The genetic experiments Mendel conducted with pea plants took him eight years (1856-1863) and he published his results in 1865.1
Pure line theory – 1903
Wilhelm Johannsen first proposed the distinction between genotype and phenotype in the study of heredity while working in Denmark in 1909.2 The distinction is between the hereditary dispositions of organisms, their genotypes, and the ways in which those dispositions manifest themselves in the physical characteristics of those organisms, their phenotypes. This distinction was an outgrowth of Johannsen’s experiments concerning heritable variation in plants, and this influenced his pure line theory of heredity. The genotype–phenotype distinction is now considered by many to be one of the conceptual pillars of twentieth century genetics.
Hybrid vigor – 1908
In early 1908, George Harrison Shull, then at the Cold Spring Harbor Laboratory, published a paper with the title, “the composition of a field of maize.” 3 In this paper, Shull reported that inbred lines of maize showed general deterioration in yield and vigor, but that hybrids between two inbred lines immediately and completely recovered. In many cases their yield exceeded that of the varieties from which the inbred lines were derived. Furthermore, they had a highly desirable uniformity making them better suited to agriculture. In a subsequent paper in 1909, he outlined procedures leveraging the phenomenon of hybrid vigor that later became standard in corn-breeding programs.4
The double-cross method – 1917
In 1917 Donald Forsha Jones crossed the single cross of two strains of Chester's Learning corn, with a single cross of two strains of Burr White corn. Grown in 1918 this cross, which later came to be known as a "double cross," yielded more than either of its single-cross parents and considerably more than the best open-pollinated varieties available. Within a few years corn breeding programs including the isolation of inbred strains and testing of single and double crosses had been initiated by the U.S. Department of Agriculture. By 1933 hybrid corn was in commercial production on a substantial scale and by 1949, 78 percent of the total U.S. corn acreage was planted in hybrid corn.5
Transposable elements discovered in maize – 1940’s
Transposable elements, or transposons, are DNA sequences that can move locations within a genome, also known as “jumping genes”. Discovered in corn by Nobel Prize winning geneticist Barbara McClintock in the 1940s, they were long considered by many scientists to have little role in genetics. Others however, including McClintock, hypothesized that transposons within a genome may have important roles in cells, including regulating gene expression. We now know that transposable elements are found in most organisms, making up more than 80 percent of the maize genome and nearly 50 percent of the human genome.6
Agrobacterium-mediated plant transformation - 1977
In 1977 Marc Van Montagu and Jeff Schell discovered the gene transfer mechanism between Agrobacterium and plants, which resulted in the development of methods to alter the bacterium into an efficient delivery system for genetic engineering in plants. The plasmid transfer DNA (T-DNA), used by the bacterium to cause tumors in plants, is an ideal vehicle for genetic engineering. Directed engineering is achieved by cloning your desired gene sequence into the T-DNA that will be inserted into the host plant DNA. This process has been performed using a firefly luciferase gene to produce glowing plants. The luminescence has proven to be a useful device in the study of plant chloroplast function and as a reporter gene.7
The first biotech plant – 1982
In 1982, the first biotech plant, an antibiotic resistant tobacco, was developed. In January 1983, at a genetic research meeting in Miami, three different teams reported success in using Agrobacterium tumefaciens, to carry new genes into plant cells, heralding the dawn of modern agricultural biotechnology.8
The gene gun method - 1986
A gene gun is a device for delivering exogenous DNA or transgenes to cells. The payload is an elemental particle of a heavy metal coated with DNA. This device can transform almost any type of cell, including plants, and is not limited to transformation of the nucleus; it can also transform organelles, including plastids. The original gene gun was an air pistol modified to fire dense tungsten particles. It was invented by John C. Sanford, Ed Wolf and Nelson Allen at Cornell University, and Ted Klein of DuPont, between 1983 and 1986. Their target was onions, chosen for their large cell size, and the device was used to successfully deliver particles coated with a marker gene. Genetic transformation was confirmed when the onion tissue expressed the gene. This gene gun technique, also known as biolistics, has since proven to be a versatile method of genetic modification and is generally preferred to engineer transformation-resistant crops, such as cereals.9
The first flowering plant genome sequenced - 2000
The first complete genome sequence of a plant, Arabidopsis thaliana, appeared in Nature in 2000. A. thaliana is a model plant used in research to study many aspects of plant biology. Originating in Europe and Central Asia, it is a dicot, as are many important staple crops, such as the potato; commercially important food crops, such as soybean; and fiber crops, such as cotton and hardwood trees. Because germination through to senescence takes only approximately 50 days, A. thaliana offers a fast system in which to study processes that may take months or years in other flowering plants. It was also chosen as the first plant species to have its genome sequenced because of its small genome size of around 120 Mb.10
The first Golden Rice field trial – 2004
Golden Rice is a variety which has been genetically engineered to biosynthesize beta-carotene, a precursor of vitamin A, in the edible parts of rice. It was developed with the intention of producing a fortified food to be grown and consumed in areas with a shortage of dietary vitamin A. Rice is a staple food crop for over half of the world's population, making up 30–72 percent of the energy intake for people in Asian countries, making it the perfect crop for targeting vitamin deficiencies. Golden Rice differs from its parental strain by the addition of three beta-carotene biosynthesis genes. The parental strain can naturally produce beta-carotene in its leaves, where it is involved in photosynthesis. However, the plant does not normally produce the pigment in the edible endosperm, where photosynthesis does not occur. In 2005, Golden Rice 2 was announced, which produces up to 23 times more beta-carotene than the original Golden Rice.11
CRISPR first applied to plants – 2013
In August 2013, five reports were published discussing the first application of CRISPR-Cas9 genome editing in plants.12 This first group of studies demonstrated the immense versatility of the technology in the field of plant biology by embracing the model species Arabidopsis thaliana and Nicotiana benthamiana as well as crops such as rice. Since then four independent groups have shown that the CRISPR-Cas9 system can introduce homozygous mutations directly into the first generation of rice and tomato transformants, highlighting the exceptionally high efficiency of the system in these species. Researchers have also demonstrated that the genetic changes induced by CRISPR are present in the germ line and segregate normally in subsequent generations without further modifications. in Arabidopsis, rice and tomato.
References
1. Mendel, G. (1996). Experiments in plant hybridization (1865). Verhandlungen des naturforschenden Vereins Brünn.) Available online: www. mendelweb. org/Mendel. html (Accessed on 5 January 2018).
2. Churchill, F. B. (1974). William Johannsen and the genotype concept. Journal of the History of Biology, 7(1), 5-30.
3. Shull, g. h. (1908). the composition of a field of maize.
4. Shull, g. h. (1909). a pure-line method in corn breeding. journal of heredity, (1), 51-58.
5. Jones, D. F., Hayes, H. K., Slate Jr, W. L., & Southwick, B. G. (1917). Increasing the yield of corn by crossing. Connecticut Agric. Exp. Stn. Rep, 323-47.
6. Pray, L., & Zhaurova, K. (2008). Barbara McClintock and the discovery of jumping genes (transposons). Nature Education, 1(1), 169.
7. Schell, J., & Van Montagu, M. (1977). The Ti-Plasmid of Agrobacterium Tumefaciens, A Natural Vector for the Introduction of NIF Genes in Plants?. In Genetic engineering for nitrogen fixation (pp. 159-179). Springer, Boston, MA.
8. FBAE. Twenty Years of Modern Agricultural Biotechnology. Available at http://www.fbae.org/2009/FBAE/website/special topics_twenty_years_of_modern_agriculture.html (Accessed on 5 January 2018).
9. Sanford, J. C. (2000). The development of the biolistic process. In Vitro Cellular & Developmental Biology-Plant, 36(5), 303-308.
10. Arabidopsis Genome Initiative. (2000). Analysis of the genome sequence of the flowering plant Arabidopsis thaliana. nature, 408(6814), 796.
11. Golden Rice Project. Available at http://www.goldenrice.org/index.php (Accessed on 5 January 2018).
12. Liu, X., Wu, S., Xu, J., Sui, C., & Wei, J. (2017). Application of CRISPR/Cas9 in plant biology. Acta Pharmaceutica Sinica B.
