Science in Action: Part II. Designer Plants

Designer Plants
Robert Ll. Morris

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.

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).

In Part I we talked about how the bacterial disease, crown gall, played a role in bioengineering by providing a biological model for scientists to use to insert desirable genetic information permanently inside plants. It was known that the crown gall organism, a bacterium, could infect a plant and insert its own information causing the plant to do something it normally would not do. In the undesirable case of the crown gall disease, produce a tumorous swelling of plant tissue that housed and protected the disease.

More on Crown Gall Disease Organism

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.

More on Gene Splicing

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 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 paralyse 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.

More on Bacillus thuringiensis or Bt

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 (lets 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. Lets 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.

My article was previously published in Southwest Trees and Turf