Inside the Little Green Factories
Plant breeding became a science around the turn of the twentieth century, thanks to Austrian scientist and mathematician Gregor Mendel. His studies of heredity in peas enabled him to draw conclusions about gene functioning by observing how the characteristics of parents showed up in generations of offspring. While adopting increasingly sophisticated techniques, plant breeders continued to improve crops in traditional ways, crossing the current stock with germplasm containing useful new characteristics. The success of the outcomes depended on the skill and judgment of the breeder in selecting plants to cross.
Scientists, meanwhile, sought to understand the underlying genetic mechanisms that induced plants to express inherited characteristics in certain ways. With advances in plant tissue culture techniques, biologists were able to produce novel hybrids and study them under controlled laboratory condiditons. Of particular interest were plant characteristics that might poetntially be modified in ways advantageous to humans. One scientist described plants as the "little green factories" that produce food, fibers, housing materials, and many pharmaceuticals, as well as the oxygen necessary for terrestrial life.
Knowledge about genetics grew rapidly during the 1960s and 1970s, and certain characteristics became recognized as central to all organisms: bacteria, animals, plants, and humans. For example:
Organs develop and function as they do because of the way different combinations of genes express themselves in the form of proteins produced within cells. The instructions that tell proteins to form a blood cell, a brain cell, or a flower petal are all contained within the genome, and insofar as is now known, in the chemical composition of the deoxyribonucleic acid, or DNA, in a particular gene sequence along a chromosome.
When a gene or its constituent nucleotides undergoes a sudden random change, known as mutation, the result is an abnormality in the affected cells. Mutations that render an organism better to cope with its environment are the raw material that natural selection acts on. Many of the successful mutations of an organism's ancestors, and possibly a mutation or two of its own, are reflected in the organism's genetic composition, or genome.
The genome responds to environmental forces, such as the supply of essential nutrients, to produce an organism's observable characteristics, or phenotype.
Through recombinant DNA technology, or genetic engineering, it is possible to create new strains of organisms with DNA containing the exact genes desired from different sources.
Barbara McClintock's work identifying mobile genes in corn, for which she received a Nobel Prize in 1983, provided molecular biologists with the tools necessary for the development of plant transformation. Despite the essential role that plants play in human existence, much less time and energy had gone into studying the genetic functioning of plants than of bacteria or animalsor humans. A major obstacle was the large and unwieldly mass of genetic material found in most crop plants, which were the primary subjects of scientific research.
This obstacle was a big one. A scientist who wants to find the genetic source of a mutation, such as resistance to a particular disease, has to examine the cells where the mutation was expressed and connect the genetic information there back to the DNA. The technologies for identifying and isolating genes, sequencing them, cloning them (making large numbers of exact reproductions), and determining their functions are complex, labor-intensive, and expensive. To apply these techniques to plants, molecular biologists needed a plant whose genome was of manageable size.
Increasingly, they converged on Arabidopsis thaliana, a weed of the mustard family that has one of the smallest genomes of any flowering plant. It is estimated that 20,000 to 25,000 genes are arrayed on only five chromosomes, with little of the puzzling, interminably repetitious DNA that frustrates efforts to study most plants. Arabidopsis is compact, seldom exceeding about a foot in height, and it flourishes under fluorescent lights. All of these characteristics enable scientists to raise it inexpensively in laboratories. During its short life cycle, this mustard weed produces seeds and mutants prodigiously. It can be transformed through the insertion of foreign genes and regenerated from protoplasts, plant cells stripped of their cell walls. For all of its superior properties, Arabidopsis is typical of flowering plants in its morphology, anatomy, growth, development, and environmental responses, a kind of "everyman" of the plant world. In short, Arabidopsis thaliana is a biologist's dream: a model plant.