Agrobacterium tumefaciens (crown gall)

Agrobacterium tumefaciens is a form of bacterium that causes the disease known as crown gall in plants and trees. Crown gall may be identified by the presence of large growths or tumors (called galls) that form at the bases of plant stems. Besides being unattractive, these galls can have a highly negative effect on plant health, as they restrict the flow of water and nutrients from the roots to the rest of the plant. As a result, the disease causes millions of dollars in damages annually. In Australia, measures used to control A. tumefaciens and related bacteria were estimated to have saved the Australian economy $100 million in 2015. Australian researchers at the University of Adelaide discovered a way to biologically control crown gall disease, which is caused by A. tumefaciens, in 2024.

In the twenty-first century, there is some debate about how to categorize Agrobacterium species within the broader taxonomic system used to classify all living creatures. Traditionally, all Agrobacterium species have been placed into the Agrobacterium genus of bacteria; however, some scientists have speculated that they should instead be placed into the Rhizobium genus, in which case the bacteria causing crown gall disease would be more properly known as Rhizobium tumefaciens.

Background

Bacteria are differentiated into different classes by a method called gram-staining. When bacteria are stained with crystal violet dye, some retain the dye’s coloring while others do not due to the presence or absence of certain chemicals in cell walls. A. tumefaciens bacteria are classified as being gram-negative, meaning that they lack these chemicals. A. tumefaciens are rod-shaped organisms with whisker-like flagella that extend around the perimeter of the bacterium. When these bacteria sense the presence of chemicals emitted by plants, certain genes are activated that cause it to form filaments called T pilus that it uses to infect the host plant.

A. tumefaciens enters plants near the soil line through a recent, unhealed wound in the roots or stems. These wounds can be the result of agricultural practices, propagation tactics like grafting, or damage from insects or frost. However, not all examples of A. tumefaciens are capable of causing crown gall disease. Only A. tumefaciens bacteria containing large enough amounts of tumor-inducing plasmids (Ti plasmids) are able to create the tumors associated with crown gall. Plasmids are circular-shaped packets of DNA that are able to reproduce independently of the rest of the cell. Once the A. tumefaciens bacteria has entered a plant, it uses these plasmids to quickly multiply and form neoplastic growths that can be seen with the naked eye eight to fourteen days after infection. These growths take the form of tumors that are the result of abnormal and excessive development of bacteria-infected cells. On a basic level, these growths are formed from a similar process that leads to the development of tumors in humans.

A plant infected with the resulting crown gall disease will have numerous galls that encircle roots and stems. These galls inhibit the plant’s ability to pass nutrients from the roots to the rest of the plant. As a result, plants especially susceptible to crown gall disease often grow sickly, with characteristically withered, small yellow leaves. Poor environmental conditions, such as heat, cold, or limited water, will intensify the symptoms.

Agriculturists recommend preventing the spread of crown gall disease by carefully monitoring all plants for signs of the disease. Once the disease is evident, it is best to remove and destroy all infected plants or trees. Dispose of the remains properly and do not reuse them for mulch because the bacteria remain in the soil even after destruction of the host plant. If the disease has been detected, carefully sterilize all tools and all surfaces with which the plant has come in contact, as they can transport bacteria between plants. The implementation of good sanitation techniques that limit the ability of A. tumefaciens to reproduce in soil has also been identified as a way to limit its spread. Botanists further recommend not using cuttings from infected plants or those that have been in close proximity to diseased plants when propagating plants. Infected soil may be cleared of the bacteria through the application of heat above 140 degrees Fahrenheit for one hour. A few treatments, such as the chemical Gallex, have shown some promise in treating infected plants. In addition, the K-84 strain of A. tumefaciens has been demonstrated to prevent infection by other gall-causing forms of the bacteria. Deliberate infection using weakened bacteria operates similarly to human inoculations. Researchers at the University of Adelaide developed a way to biologically control the disease in 2024. They built on work conducted by researchers at Cornell University, who discovered that Agrobacterium vitis F 2/5 controlled the crown gall that caused necrosis, or tissue death. The researchers at the University of Adelaide used gene editing to alter F 2/5 so it controlled the crown gall without causing necrosis.

Overview

Bacteria use a biological process called horizontal gene transfer that allows unicellular (that is, single-celled) microorganisms to infect multicellular organisms with their genetic material. By contrast, gene transfers between parents and offspring, the other primary process for transmitting genetic material, is called vertical gene transfer. A. tumefaciens has been demonstrated as being able to poison more than 650 known species from ninety-three families of plants. In most species, the resulting galls have only a moderate impact on the health of the infected plant. However, in some species, galls can stunt growth or kill the host plant. A. tumefaciens most seriously affects fruit and nut trees, grape vines, and ornamental plants like chrysanthemums in temperate regions around the world. Conifer species such as pines and fir trees seem to be largely resistant.

A. tumefaciens is the most studied member of the Agrobacterium bacteria family because of its commonness and unusual biology. As part of its process for infecting host plants, it parasitizes plant tissue by integrating its DNA with that of the plant. It then uses this altered DNA to cause the growth of tumors and promote changes to the plant’s metabolism. The ability of bacteria to easily transfer its genetic material into host plants led researchers to use it as a model species in the study of genetic engineering. In 1983, scientist Mary-Dell Chilton identified the otherwise problematic ability of bacteria to insert their genetic material into multicellular organisms as being a potentially valuable tool for genetic manipulation. Chilton showed that disease-carrying bacteria could be used to insert potentially valuable new genes into plants.

A. tumefaciens has been demonstrated to be an effective way to transport new genes between species. The plasmids of A. tumefaciens are known to be especially skilled at transmitting its genes into host bodies. As a result, this species has become one of the most preferred tools of genetic manipulation. The tumor-causing genes are removed and replaced with new DNA with more desirable traits, such as drought resistance or the ability to transport nutrients more easily. When the altered form of A. tumefaciens is inserted into a plant, the plasmids transmit the new, more advantageous DNA into the plant’s DNA.

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