Xtremehorticulture

Xtremehorticulture of the Desert

Cancer like Growth on Tree Is Crown Gall

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 Q.I found this at the base of my plant. What is it? A. This looks like crown gall. Not a big problem. It is a cancer like growth that is woody caused by a bacterium which lives in the soil, Agrobacterium tumefaciens. Note the second name resembles the word “tumor” because it produces a tumor like growth.  Just clip it out (with a sanitized pruning shears) and don’t worry about it. Somehow the bacterium was transferred from the soil to the stem when it was cut. Most likely the pruning shears was laid on the soil and got dirty and was not sanitized before it was used to cut the branch.

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Science in Action: Part II. Designer Plants

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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

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Science in Action: Part I. Engineering Plants Using Crown Gall Disease

Leave a Comment / crown gall, gene gun, genetic engineering, Monstanto /

Science in Action: Part I. Engineering Plants Using Crown Gall Disease By Robert Ll. Morris 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. 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. 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. 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. 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

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