Biotechnology in Agriculture
Neil P. Schultes
Department of Biochemistry & Genetics
The Connecticut Agricultural Experiment Station
123 Huntington St.
P.O. Box 1106
New Haven, CT 06504-1106
Voice: (203) 974-8464 Fax: (203) 974-8502
E-mail: Neil.Schultes@po.state.ct.us
Advances in basic genetic and molecular biology research are now having an impact upon applied agriculture. There is increasing coverage in the popular media of recombinant DNA and genetic modification of plants and animals as these technologies emerge from the laboratory and enter every day life.
Genetically modified organisms or GMOs refer to plants or animals which have been genetically altered using modern gene splicing technology. In reality GMO is a misnomer. Since the beginnings of agriculture mankind has been genetically sculpting or engineering plants brought under domestication. During most of this 7,000 -10,000 year period plants were selected for beneficial agronomic traits, such as large seed size. Numerous land races or varieties of crop plants were developed - the ancestors of our heirloom varieties still planted today. Corn or maize is a wonderful example. Emerging in central Mexico, maize ancestors bearing flimsy inch long cobs with few small seeds, were selected over the centuries to a robust plant, yielding huge cobs bearing numerous large seed. However during this process, maize became totally dependent upon mankind for propagation. No preconceived plan was in place nor was the application of scientific theory available for this transformation of maize and all of our crop plants.
It wasn’t until the late 1800’s when the Austrian monk Gregor Mendel laid down the first laws of genetics that a truly scientific approach to crop improvement was begun. Practical applications of the basic science of genetics came soon. In 1917, Dr. Donald Jones, a new geneticists at The Experiment Station, developed a breeding programs to generate hybrid corn. During the following decades hybrid corn was adopted by American farmers and corn yields increased several fold. In the 1920 corn yields were some 25 bushels/acre whereas today yields often are 100 - 130 bushels/acre.
With today’s gene splicing technology we are on the threshold of another revolution in agriculture. Some twenty years ago genes were first isolated and manipulated in the laboratory. Procedures are now common place which allow engineered genes to be reintroduced into plants. Today there is a large effort in biology, called genomics, to find and catalogue every gene in selected organisms. This information will be used as a guide in basic as well as applied science for improvements in human health and agriculture.
By now most everyone can identify DNA as double helical molecule which contains inherited genetic information. DNA is a long chain of four bases, G, A, C or T, placed in different sequence. Corn cells contain some 3 billion bases - collectively called the genome. Chromosomes are long tightly wound DNA molecules which are most evident during cell division. A gene is a segment of DNA averaging 1000-5000 bases long, which contains the genetic information to produce one protein. Each gene encodes for a different protein. Protein, in turn, are chains of twenty amino acids of a unique sequence. Each protein performs a specific task in the cell.
Gene are composed of three parts. The central coding region contains the encrypted information to produce one protein. The promoter region at the front of the gene contains information that tells when and where the gene is turned on. Some genes are on all the time, while others are on only in leaves, flowers, roots, or only in the light or dark. The termination region at the end of the gene signals that the gene is complete. Through gene splicing technology scientists can isolate and assemble these gene parts together from different genes and even from different organisms.
One example of such trans-species genetic engineering is popularly known as BT-corn. Here the coding region of a bacterial gene has been spliced together with corn promoters and terminators and reintroduced into plants. BT -corn produce the delta-endotoxin protein from the soil bacterium, Bacillus thuringiensis, in different parts of the plant depending upon the corn promoter used. In nature B. thuringiensis spores contain this toxin and spore preparations are used by organic farmers to control a variety of insect pests. The protein toxin is poisonous only for certain insects mainly in the caterpillars and beetles. In BT- corn the main target is the European corn borer which causes an estimated 1 billion dollars per year of damage to the nations corp. Currently in is estimated that __% of the nations maize crop is transgenic, attesting to the efficacy of this genetically altered plant. Although this product works well recent preliminary data suggest that monarch butterflies, which do not eat corn, are susceptible. Milkweeds, the Monarchs diet, are common at the border of corn fields and may become dosed with BT-containing corn pollen and act as a route for toxin entry. More research needs be continued in this ecological impact of BT-corn.
This current example of a beneficial genetically engineered plant only alters one gene. However engineering plants with increased yield through more efficient photosynthesis, altered leaf structure or augmented metabolism, will require altering many genes at once. This is the challenge for the future. At the Department of Biochemistry & Genetics we engage in basic research to unravel the genetics of these processes. A more detailed description of this research is described in the barn exhibit poster manned by Drs. Richard Peterson and Neil McHale. Knowing what genes are involved and these complicated processes and how they interact, forms the basis for future successful genetic engineering.
Today the scientific community is embarking on Genomics. The plan is to identifying every gene in selected organisms and determining their function. This new era of genomics, still mainly confined to basic research, will have important and profound impact upon applied agriculture for it lays the necessary ground work for engineering complex traits. Genome sequencing projects are collaborative efforts of many laboratories throughout the world. DNA sequence information is deposited into data banks and is readily available over the internet. Computer programs on line are also available to assist researchers in analyzing large amounts of genetic data. The goal is to integrate genetic information from disparate biological systems to effectively use data derived in one organism to address questions in other organisms. Comparison of data between species provides a powerful method to determine how genes function singly and together. To data most of the genome sequencing project involve model organisms.
Model organisms are used in basic research because of the following shared characteristics. They are easy to grow and manipulate in the laboratory. They have small amounts of DNA in their genome and it is easy to perform genetic and molecular experiments. Common laboratory organism include several bacteria species, fungi and yeast, nematodes and fruit flies. In plants the member of the mustard family Arabidopsis thaliana, is commonly referred to as the white lab mouse of plant science. Of no agronomic importance itself it will revolutionize agriculture in the future. It is small, easy to grow in incubators, has a short life cycle of 10 weeks and is amenable for genetic manipulation and easy to reintroduce genes back into the organism. These characteristics make Arabidopsis a prime candidate for genome sequencing.
How much DNA and how many gene are there in organisms. The entire DNA sequence of several organisms is now complete, including several bacterial genomes containing some 4000 genes. Brewers yeast, Saccharomyces cerevisiae, contains some 6000 genes. The nematode, C. elegans, contains 100 million bases of DNA totaling to about 20,000 genes. In plants, the model organism Arabidposis thaliana, contains a similar number of genes. It is projected that the entire Arabidopsis genome will be complete by the end of the year 2000. In comparison maize contains an estimated 40,000 genes and humans some 80,000 genes.
Once genes are cloned and the sequence entered into DNA data banks, the next task is to figure out the function. This is done through two approaches. One was is to disrupt the gene by mutation. Analysis of the resulting mutant plants will yield clues as to what function the gene normally performs. This approach is employed by many researchers and is described in detail on Dr. Peterson poster in the barn exhibit.
The other approach combines comparative computer analysis with laboratory experiments in microorganisms called complementation analysis. Computer assisted comparison of DNA and protein sequences, called bio-informatics, allows scientists to link proteins of unknown function with related proteins from microbes that have well defined functions. Complementation analysis involves testing the plant protein for a specific function in a well defined microbe which lacks that specific function. Both of these processes take advantage of the fact that organisms are related by evolution. In fact most metabolic reactions are similar between plants animals, fungi and bacteria. These common reactions are catalyzed by related proteins.
In my research I study plant genes encoding proteins which transport nitrogen containing compounds into and out of the cells - an important aspect of metabolism. Here is an example of a computer assisted alignment. The top row is part of a plant protein sequence designated by letter representing different amino acids. Below are other plant and bacterial proteins. Shaded letters denote the identical amino acid at the same position between these proteins. The number of amino acid identities determines how related the two proteins are. The high level of sequence convergence between the plant and microbial proteins allows me to make educated guess as to the function of the plant proteins. Although the function of the plant proteins is unknown they look very similarly to the bacterial and fungal proteins which transport compounds xanthine and uric acid.
The next step is to verify the hypothesis through complementation studies in microbes. For example normal Aspergillus, another model genetic organism, grows well on xanthine as a sole nitrogen source since it readily transports this molecule into the cell. Mutant strains of Aspergillus which lack the xanthine transport protein cannot grow. When we supply the plant gene we suspect transports xanthine into mutant Aspergillus it is now able to grow on xanthine media. Through this complementation experiments we have verified the educated guess suggested by the computer alignment data. Many other researchers are using this approach as well as it saves time and research dollars.
What does all of this research gain? By understanding how genes interact in Arabidopsis, the same knowledge can be directly applied to all crop plants. Using the bio-informatics, mutational analysis and complementation studies most experiments need only be performed once. Rather than reinventing the wheel for each crop plant, genomics allows scientists to answer question of basic science in an accurate and timely fashion. This will speed the opportunity for beneficial applications in the future.