Plant genetics is the study of genes, genetic variation, and heredity in plants. It is generally considered a field of biology and botany, but also intersects with numerous life sciences, including molecular biology, evolutionary biology, and bioinformatics. Plants are used for genetic research in a multitude of disciplines. Understanding plant genetics is used for improving crop yields, developing disease-resistant plants, advancing agricultural biotechnology and making advancements in medicine. The study of plant genetics has significant economic and agricultural implications. Thus, there are many plant models and genetic tools that have been developed to study plants. Genetic research has led to the development of high-yield, pest-resistant, and climate-adapted crops. Advances in genetic modification (GMO Crops) and selective breeding continue to enhance global food security by improving nutritional value, resistance to environmental stress, and overall crop performance.
History The earliest evidence of plant domestication found has been dated to 11,000 years before present in ancestral wheat. While initially selection may have happened unintentionally, it is very likely that by 5,000 years ago farmers had a basic understanding of heredity and inheritance. This selection over time gave rise to new crop species and varieties that are the basis of the crops we grow, eat and research today.
The field of plant genetics began with the work of Gregor Johann Mendel, who is often called the "father of genetics". He was an Augustinian priest and scientist born on 20 July 1822 in Austria-Hungary. He worked at the Abbey of St. Thomas in Brünn (now Brno, Czech Republic), where his organism of choice for studying inheritance and traits was the pea plant. Mendel's work tracked many phenotypic traits of pea plants, such as their height, flower color, and seed characteristics. Mendel showed that the inheritance of these traits follows two particular laws, which were later named after him. His seminal work on genetics, "Versuche über Pflanzen-Hybriden" (Experiments on Plant Hybrids), was published in 1866, but went almost entirely unnoticed until 1900 when prominent botanists in the UK, like Sir Gavin de Beer, recognized its importance and re-published an English translation. Its rediscovery prompted the foundation of modern genetics as his discoveries, deduction of segregation ratios, and subsequent laws have not only been used in research to gain a better understanding of plant genetics, but also play a large role in plant breeding. Mendel's works along with the works of Charles Darwin and Alfred Wallace on selection provided the basis for much of genetics as a discipline.
In the early 1900s, botanists and statisticians began to examine the segregation ratios put forth by Mendel. W.E. Castle discovered that while individual traits may segregate and change over time with selection, that when selection is stopped and environmental effects are taken into account, the genetic ratio stops changing and reach a sort of stasis, the foundation of Population Genetics. This was independently discovered by G. H. Hardy and W. Weinberg, which ultimately gave rise to the concept of Hardy–Weinberg equilibrium published in 1908.
Around this same time, genetic and plant breeding experiments in maize began. Maize that has been self-pollinated experiences a phenomenon called inbreeding depression. Researchers, like Nils Heribert-Nilsson, recognized that by crossing plants and forming hybrids, they were not only able to combine traits from two desirable parents, but the crop also experienced heterosis or hybrid vigor. This was the beginning of identifying gene interactions or epistasis. By the early 1920s, Donald Forsha Jones had invented a method that led to the first hybrid maize seed that were available commercially. The large demand for hybrid seed in the U.S. Corn Belt by the mid 1930s led to a rapid growth in the seed production industry and ultimately seed research. The strict requirements for producing hybrid seed led to the development of careful population and inbred line maintenance, keeping plants isolated and unable to out-cross, which produced plants that better allowed researchers to tease out different genetic concepts. The structure of these populations allowed scientist such a T. Dobzhansky, S. Wright, and R.A. Fisher to develop evolutionary biology concepts as well as explore speciation over time and the statistics underlying plant genetics. Their work laid the foundations for future genetic discoveries such as linkage disequilibrium in 1960.
While breeding experiments were taking place, other scientists such as Nikolai Vavilov were interested in wild progenitor species of modern crop plants. Botanists between the 1920s and 1960s often would travel to regions of high plant diversity and seek out wild species that had given rise to domesticated species after selection. Determining how crops changed over time with selection was initially based on morphological features. It developed over time to chromosomal analysis, then genetic marker analysis, and eventual genomic analysis. Identifying traits and their underlying genetics allowed for transferring useful genes and the traits they controlled from either wild or mutant plants to crop plants. Understanding and manipulating of plant genetics was in its heyday during the Green Revolution brought about by Norman Borlaug. During this time, the molecule of heredity, DNA, was also discovered, which allowed scientists to actually examine and manipulate genetic information directly.
DNA
Geneticists, including plant geneticists, use the sequencing of DNA to their advantage to better find and understand the role of different genes within a given genome. Through research and plant breeding, manipulation of different plant genes and loci encoded by the DNA sequence of the plant chromosomes by various methods can be done to produce different or desired genotypes that result in different or desired phenotypes.
Plant DNA Plants, like all other known living organisms, pass on their traits using DNA. Plants however are unique from other living organisms in the fact that they have chloroplasts. Like mitochondria, chloroplasts have their own DNA. Like animals, plants experience somatic mutations regularly, but mutations within the germ line can be passed to offspring, aiding in processes like natural selection and evolution of plant species.
Some plant species are capable of self-fertilization, and some are nearly exclusively self-fertilizers. This means that a plant can be both mother and father to its offspring. Scientists and hobbyists attempting to make crosses between different plants must take special measures to prevent the plants from self-fertilizing. In plant breeding, people create hybrids between plant species for economic and aesthetic reasons. For example, the yield of Maize has increased nearly five-fold in the past century due in part to the discovery and proliferation of hybrid varieties.
Plants are generally more capable of surviving, and indeed flourishing, as polyploids. Polyploid organisms have more than two sets of homologous chromosomes. For example, humans have two sets of homologous chromosomes, meaning that a typical human will have 2 copies each of 23 different chromosomes, for a total of 46. Wheat on the other hand, while having only 7 distinct chromosomes, is considered a hexaploid and has 6 copies of each chromosome, for a total of 42. In animals, inheritable germline polyploidy is less common, and spontaneous chromosome increases may not even survive past fertilization. In plants however this is less of a problem. Polyploid individuals are created frequently by a variety of processes; however, once created, they usually cannot cross back to the parental type. Polyploid individuals that are capable of self-fertilizing can give rise to a new, genetically distinct lineage, which can be the start of a new species. This is often called "instant speciation". Polyploids generally have larger fruit, an economically desirable trait, and many human food crops, including wheat, maize, potatoes, peanuts, strawberries and tobacco, are either accidentally or deliberately created polyploids.
The main genetic components of plants include;
Nuclear DNA (nDNA): This is linear DNA or genetic information which is found within the nucleus. This DNA holds instructions for the function and structure of the organism its self. This information is organized into chromosomes and the nucleus serves as a control center for the entire organisms behavior, growth, and reproduction. Nuclear DNA content is a very important area of study in order to determine things like plant taxonomy, evolution and its conservation over time. This information is usually referred to as the organisms C-value and this data is remarkably variable among different plant species. Overall the study of nuclear DNA among plants and plant C-values is crucial for insight into biodiversity, adaptation and ultimately evolution of organisms. Mitochondrial DNA (mtDNA): This is the DNA which is found within the mitochondria which are specialized organelles that produce ATP for important cellular functions and perform processing like metabolism, cellular signaling and apoptosis or programmed cell death. This form of DNA is exclusively inherited from maternal genetic information and is smaller and more circular than nuclear DNA. Mitochondrial DNA encodes around 13 proteins and is heavily reliant on information from the nucleus and their own genome for proper functioning. Extensive studying of mitochondrial DNA is very important for evolutionary research, migration patterns and insight into genetic disease. Chloroplast DNA (cpDNA): This is genetic information found within the chloroplasts of plant cells. These specialized organelles perform photosynthesis for plants in order to convert light or photons into chemical/useful energy for plant growth and development. These chloroplast genomes usually contain around 60-100 genes that are involved in photosynthesis and other essential metabolic functions. This form of DNA like mitochondrial DNA is also maternally inherited which is advantageous to studying phylogenetics and evolution.
Model Organisms Model Organisms in the context of plant genetics are plant species that are well understood due to extensive research. This understanding of their genome and biological processes allows them to be used as a baseline to understand fundamental genetic, developmental, physiological and disease mechanisms. Discoveries from such research using a plant model organism is often able to apply its findings to other species including humans.
Plant model organisms are selected based on experimental advantages that vary depending on research objectives. Key factors influencing their selection include short life cycles, ease of genetic manipulation, and well-annotated genomes. Below, we review a few of the many plant model organisms that are widely used and their applications in plant genetics.
Arabidopsis thaliana In 2000, Arabidopsis thaliana, also known as Thale Cress, became the first plant species to have its genome fully sequenced, solidifying its status as the most widely used plant model. Even before its genome was sequenced, Arabidopsis was a popular model organism due to its small size, short generation time, and ability to self-pollinate. These traits not only made it ideal for genetic research but also facilitated its genome sequencing. By 2000, researchers had identified 25,498 coding genes and a genome size of 125 Mb.