From Engineering Plants to Monster Plants Bob Morris http://Xtremehorticulture.blogspot.com Old-Fashioned Plant Breeding Ever since Mendel discovered that characteristics in pea plants could be inherited, scientists have been improving plants through hybridization; two related plants were crossed and the resulting offspring had characteristics of both parent plants. Breeders then selected and reproduced the offspring that had the desired traits. These conventional plant breeding techniques were relatively imprecise because they shuffled thousands of genes around and distributed them to the offspring just to get one important change in a plant that was economically worth pursuing. One challenge encountered in Mendelian breeding is that generally only closely related species of plants could be crossed. If no closely related species with desirable traits existed, breeders had no way of passing on these traits to the other plant. Another problem was that some of the genes were linked to each other. This is seen today in tomatoes that have been bred so that they can be shipped long distances but with a substantial loss in flavor. Emergence of Bioengineering Since the early 1980’s scientists have been using the tools of modern biotechnology to insert a single gene, or just two or three genes, into a plant giving it new, advantageous characteristics. With this technology a single gene could be inserted into a plant giving it a desired characteristic instead of the mixing all the genes from two plants through traditional plant breeding and hoping for the best. This technique could develop a new plant with much more control and precision and at a rate much faster than ever before. Mother Nature Made Us Do It The bioengineering of plants emerged from discoveries by researchers in previous years on how bacteria caused plant tumors, how viruses protected plants from other viruses and what enabled some bacteria to kill insects. Some first major step toward biotechnology occurred early in the twentieth century with a plant disease called crown gall. Crown galls are tumor-like plant growths that occur on many woody plants including fruit trees, grape vines, and ornamentals. In 1907 researchers at the USDA discovered that the cause of crown galls was a soil bacterium, Agrobacterium tumefaciens. Other bacteria were known to cause plant diseases but A. tumefaciens had the unusual ability to cause the plant that was hosting it to grow a disfiguring tumor. Forty years later in 1947 researchers at the Rockefeller Institute for Medical Research (now named Rockefeller University), curious about the crown gall bacterium and using it for insight into how tumors developed, grew crown gall tissue culturally free of any associated bacterium AND free of the plant host as well. They found that these uninfected crown galls could continue to grow, as crown gall tissue, independently of the crown gall bacterium and of the plant host for many years. It was concluded that normal
plant growth in some unknown way had been permanently and irretrievably transformed by A. tumefaciens. Understanding how would have to wait nearly another thirty years. Gene Splicing During the 1950s and 1960s, scientists discovered DNA’s role in transmitting information from plant to plant and ultimately controlling plant growth. Armed with this new information, scientists began looking more closely at DNA’s role in the formation of crown gall. The crown gall mystery was attacked again when several investigators began, logically, looking for the tumor-inducing factor in the bacterium's DNA. Bacterial DNA is relatively simple compared to other types of organisms since it can be normally found on a single chromosome. It wasn’t found there. Instead it was found by Flemish researchers on a smaller, mobile DNA unit called a plasmid that was not part of the bacterium's single chromosome. In a series of experiments at the University of Washington ending in 1977 researchers found that this bacterial plasmid was spliced into the chromosomes of plant cells when the bacteria infected the plants. This was at the same time that researchers in other fields were just beginning to understand how to manipulate genetic information by a technique called, in lay terms, gene splicing – to cut and splice foreign DNA into the genetic code of an organism. It became clear now that A. tumefaciens transmitted the genetic information needed to cause a plant to produce tumor-like growth through the transfer of a “packet” of information called a plasmid. This plasmid altered the genetic makeup of the plant so that the infected cells of the plant were induced to divide continually, developing galls containing the genetic information from A. tumefaciens. What if scientists could manipulate A. tumefaciens so that it no longer transferred the genetic information for creating crown gall but instead transferred genetic information into plants that produced desirable traits such as resistance to insects or disease? To convert the A. tumefaciens plasmid into a beneficial plasmid (now called a vector) researchers first had to locate and then replace the tumor-inducing genes. By 1983, plant molecular biologists had developed the first plasmid vector for plants susceptible to crown gall from A. tumefaciens. The crown gall disease had changed plant breeding forever. Enter Monsanto A tool for introducing genes into plants is useful only if scientists have found genes that they want to transfer. Enter Monsanto. In the late 1960’s researchers at Monsanto wanted to know what made the nonselective herbicide glyphosate (RoundupTM) a potent killer of so many different kinds of plants; weeds and crop plants as well. It seemed reasonable that if you could alter crop plants so that they were resistant to glyphosate, then spraying an herbicide like glyphosate “over the top” of a mixture of emerging combination of resistant crops and weeds would kill the weeds but not the crop. Through combined research starting early in the 1970’s glyphosate’s genetic “mode of action’, destruction of an enzyme vital to all plants, was specifically identified by research performed at Monsanto and by German researchers. In 1983 researchers at Calgene and Monsanto identified the gene, and Monsanto modified the gene, so that the enzyme it produced was no longer sensitive to glyphosate.
The A. tumefaciens plasmid vector was used to introduce the modified gene into crop plants. The new tomatoes, petunias and other modified plants were resistant to damage from glyphosate which was reported in the research in1985. The USDA requires field testing of genetically modified plants for several seasons before its release and that testing includes potential changes in food safety, nutrient levels, development of potentially toxic substances and safety to the environment. In 1996, the first glyphosate-resistant soybean, cotton, canola, and corn seeds were made available to farmers. During the1980s and 1990s other ways were developed to introduce beneficial genes into plants. One is called a "gene gun," which literally shoots DNA-covered particles attached to metal “bullets” through plant cell walls and membranes to the cell nucleus. Inside the nucleus the foreign DNA combines with the plant's own DNA and transforms the plant. Other techniques involve electrical or chemical treatments assisting DNA molecules to pass through plant cell walls and membranes barriers and combine with a plant’s genetic information, a high-tech form of plant breeding. Wouldn’t Mendel be impressed? Genetic engineering has changed commercial plant breeding forever. In years past we always thought of obtaining new plants by simple breeding and hybridizing. But to get for instance elms resistant to elm leaf beetle or turfgrass resistant to Roundup these plants had to be permanently changed in ways that simple breeding and hybridizing had not been able to accomplish. The major limitation was that the plants had to be relatively close in their evolutionary history so that a transfer of new information from one plant to another by traditional breeding techniques could occur. The Genetically Modified Organism All that has changed with genetic or bioengineering. Over the last twenty years scientists have discovered that all living organisms have genetic information that is interchangeable, even between plants and animals. Unlike traditional breeding, bioengineering has made it possible to select exactly the traits desired from nearly any living organism and insert them into a plant and create a genetically modified organism (GMO). Scientists realized that packets of new, desirable genetic information might be inserted into plants following the same method that crown gall bacterium used. Early in the development of this technology the crown gall bacterium, modified with desirable genetic information, was used as the vehicle for transferring genetic information to plants. The crown gall model of gene insertion eventually led to the development of new more efficient technologies like “gene guns” which could “shoot” new information inside of plants. Terms like “gene splicing”, which scientists use to recombine genetic information inside plants in an attempt to bioengineer a new organism with more desirable traits, results in “transgenic organisms”. This is a term that can be daunting at first until it is realized that it just means an organism that was altered or changed as a result of new genetic information which was purposefully inserted by some method. Transgenic organisms usually have some sort of benefit passed on to it from genetic engineering resulting in an economic benefit to the horticulturist and ultimately the consumer. These might be new plant traits such as improved resistance to plant pests
like viral yellows or ringspot diseases, acquired resistance to pesticides such as the Roundup Ready® line of crops, some dwarfing characteristics in agronomic crops like wheat, the preservation of food flavors such as in Flavr Savr© tomato lines, and improved resistance to insect pests by inserting genes from biological organisms that produce toxins poisonous to insects such as the bacterium Bacillus thuriengensis (Bt). Bt and Natural Pest Control Bt pesticide sprays for controlling insects have been available to commercial applicators and homeowners as a form of “natural” or “biological” pest control since the early 1960’s under a variety of different names. The first release of a Bt spray had a very narrow range of insects that it would control. Larvae of moths and butterflies with an alkaline gut pH and that fed largely on leaf surfaces were the only targets. This narrow range in pests that it controlled was both good and bad. It was good since it was very safe for humans and other animals that weren’t larvae of moths and butterflies such as beneficial insects. It was bad since it controlled such a narrow range of insects and these only in their larval stages. We now recognize this particular strain of Bt as the variety kurstaki. Since the 1980’s there have been 50 strains of Bt developed that are specific to not only moth and butterfly larvae but larvae of other insects such as the elm leaf beetle (Bt var. tenebrionus), fungus gnats (Bt var. israelensis), and a wide range of agricultural pests including beetles. All the different Bt’s had the same basic scenario however; the susceptible juvenile insect eats plant foliage that has the bacterium on its surface, Bt spores are ingested by larvae, the spores grow and reproduce inside the insect producing toxins, Toxins paralyze the digestive tract of the larvae causing it to cease eating, insect death. Death can range anywhere from a few hours to 5 days after ingestion. This depends on the amount of Bt ingested, the size and variety of the larvae and variety of Bt used for control. Bt became popular in the past because it had some distinct advantages over other pesticides: it had a low hazard to humans; there was no waiting period from time of application before re-entering the field; different strains of Bt didn’t harm beneficial or non-target insects; insects that died from Bt were not dangerous to predators; Bt was not known to cause injury to plants on which it had been applied and was not considered harmful to the environment; and, little or no insect resistance had been reported. The major problem with Bt applied as a pesticide was its lack of persistence in the environment (sunlight and rain shortened its life) and it had to be eaten by the insects to work and only the larval stages of the insect were susceptible. Multiple applications needed to be applied with just the right timing or its chances of success were limited. But what if the Bt toxin could be inserted into the plant? The toxin would always be present so timing was not a problem. Persistence was not a problem since the plant protected and even produced the toxin. To insert the Bt toxin gene (let’s call it X gene) the scientists first identify the right Bt. Next they isolate the X gene and remove it from the Bt bacterium. They then attach a second gene, a gene that provides resistance to a toxic chemical such as an antibiotic or herbicide, to the Bt gene. Let’s call the second gene the Y gene. The X gene, with the attached Y gene, is inserted into plant cells. Any plant cell that has the toxic X gene now is given resistance to an applied toxic chemical due to the presence of the Y gene.
Researchers then multiply the plant cells in the presence of the antibiotic or herbicide and kill all cells that do not have the Y gene. Because the X and Y genes are attached, the resulting cells will contain the Bt toxin. These genetically transformed plant cells are then grown into whole plants by a process called tissue culture. The modified plants produce the same lethal Bt protein produced by Bt bacteria because the plants now have the same gene. The insertion of the toxic genes from Bt into plant lines so that plant itself becomes toxic is under quite a bit of controversy. First and foremost is that growing plants that continually have the Bt toxin present increases the chance that insects feeding on these plants may become resistant to the Bt gene. There is some recent research that has demonstrated that this has already happened. Problems arise primarily because the Bt toxin is always present through the plants life cycle and that it is in all plant parts. Because susceptible insects must ingest the Bt toxin to be poisoned, genetically engineered cells could be directed to plant parts that only the target pest will eat or at certain times of the year. Scientists have been working on a Bt gene that will “switch on” in plant parts that are green (leaf tissue) or “switch off” in other plant parts that are not green (flowers, pollen and seed). Plants receive genes with a genetic “promoter switch” that results in production of the Bt toxin only in certain plant parts. They Have Been Called Monster Plants They have been called Monster plants, Frankenseeds or Frankenplants. Scientists have inserted "antifreeze" protein genes from flounder into tomatoes to protect the fruit from frost damage, chicken genes have been inserted into potatoes to increase disease resistance, firefly genes have been injected into corn plants. These are plants created in laboratories that never could have been developed by the traditional means of plant breeding. Plants that have been genetically engineered to resist herbicides and insects, resist freezing temperatures, produce pharmaceutical drugs and to convert nitrogen directly from the soil and developed by large multinational companies at tremendous cost are now being grown in the hopes of much larger profits. Biotechnologies of this type have evolved so quickly that the scientific community has split in the controversy and the rapid advancement of this science has left the general public and many scientists behind in ignorance and Universities scrambling for position. Here Come the Monster Plants Genetic engineering is an imperfect science and not enough is known about what will happen in the long run. Many times researchers who insert genes, creating new organisms, operate with a scatter-gun approach, not knowing where the gene will end up over time or what effects it will have in the long run. Science knows very little about what a gene might trigger or interrupt depending on where it is inserted into the new host plant or animal. Though often thought of as being precise by laypeople, inserting genes is a rather crude technology, lacking precision and predictability. The "new" gene can end up somewhere or doing something unexpected. For example, when genes for the color red were placed into petunias, this gene not only changed the color of the flower petals but
also, unexpectedly, decreased the plant’s fertility and changed the growth of its roots and leaves. Salmon, which were genetically engineered to produce a growth hormone, not only grew bigger than expected and too fast but also turned green. These were unpredictable, scientifically termed pleiotropic, effects. How do we know that a genetically engineered food plant will not produce new toxins and allergenic substances? How will the nutritional value of genetically modified foods change or will it? What will be the effects on the environment that comes in contact with these plants and on the wildlife in the food chain? Remember DDT? Examples of unexpected results from biotechnology: • An attempt to make potato plants resistant to sap-sucking insects has made them more vulnerable to other kinds of insect pests. • Crops such as maize and cotton have already been made resistant to chewing insects by adding a gene for Bt. But adding the Bt gene has led to speculation that there will be an increased attack by insects such as leafhoppers and aphids due to an unexpected drop in chemicals that deter their feeding. • The stems of a genetically altered, herbicide-resistant soybean were found uncharacteristically to crack open in hot climates. All these questions are important questions yet they remain unanswered until the biologically altered plant leaves the test tube and enters the real world. The argument is that biotechnology fostered by corporations tends to ignore caution in favor of profits. Genetically engineered organisms will disrupt our environment. Traditional plant breeding was limited to plants or animals that were compatible biologically which in turn limited the diversity of possible offspring. Breeding through gene-splicing techniques will create life forms that have never before existed, theoretically in billions of different possible combinations which can result in billions of different possible outcomes, some predictable and others not. As these new life forms escape or are introduced into the environment and enter different habitats they may do so with no environmental checks and balances. We can look at past scenarios where biological organisms were released into new habitats with no checks and balances to see what will happen. While many have adapted without severe problems, a small percentage of them have not. These include the Kudzu vine, gypsy moth, salt cedar, Dutch Elm disease, Chestnut blight, starlings and the Mediterranean fruit fly to name a few. Whenever a genetically engineered organism is released it must be remembered that it may cause a disruption to a complex environment with pre-existing relationships that have developed over long periods of evolutionary history. This has been characterized sometimes as a type of environmental “pollution”. But because this pollution is a “living pollution” these organisms will be more unpredictable than nonbiological pollutants. Genetically engineered products reproduce. When genetically engineered crops are grown for a specific purpose, they cannot be easily isolated both from spreading into the wild and from cross-pollinating with wild relatives. It has already been shown that cross-pollination with “normal plants” can take place almost a mile away from the genetically engineered plantings. Three mile buffers are now being recommended in some countries. If we accept the concept that the environments and habitats have their own corrective mechanisms that allow them to
“heal”, then radical changes to these environments from genetically modified organisms will require stronger corrective measures if it can be healed at all. Super Pests? Ordinary pests could become "Super-pests". Much of the current effort in profitcentered, agricultural biotechnology is centered on the creation of herbicide-tolerant, pest-resistant and virus-resistant plants. The idea is to sell farmers patented seeds in the hope of increasing a company's share of both the seed and pesticide markets. The chemical companies hope to convince farmers that the new pest-tolerant crops will allow for a more efficient eradication of pests. In the case of herbicides, farmers will be able to spray at any time during the growing season, killing weeds without killing their crops. Plants engineered to be pest resistant could become so invasive they are a weed problem themselves, or they could spread their resistance to wild weeds making them more invasive. A growing number of ecologists are concerned with "gene flow" which is the transfer of genes to weeds by way of cross-pollination. Researchers are concerned that manufactured genes for herbicide tolerance, and pest and viral resistance, might escape and create weeds that are resistant to herbicides, pests and viruses. A Danish research team documented the transfer of a gene from a genetically modified crop to a weed surrounding the crop. This was unexpected among biotechnologists since they had dismissed this possibility even though critics had warned for years prior that it could happen. Another fear is that insecticide-producing plants might create "super bugs" resistant to the effects of the new pesticide-producing genetic crops and that virusresistant transgenic crops pose the equally dangerous possibility of creating new viruses that have never before existed in nature. Putting the crowning touches on ecologists’ fears was the refusal by the insurance companies to insure against catastrophic environmental damage caused by genetically engineered organisms released into the environment. Regardless of the criticism, bioengineered plants are here to stay. The question will remain how this new technology will be handled responsibly. ©
2012, Xtremehorticulture of the Desert