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The Cause of Plant Tumors

September 9, 2016

Bacterial infections are more commonly thought to be associated with humans. However, bacteria just as frequently affects plant populations. Agrobacterium tumefaciens (A. tumefaciens) is one of the many bacteria typically located in soil, which is responsible for infections found in a variety of plant species (Cubero, J., Lastra, B). This bacterium is the primary cause of crown gall disease. Crown gall disease is responsible for the growth of large tumor-like galls on the roots and lower trunks of plants. This generates problems with development, root structures, and the transportation of water and minerals throughout the plant (Cubero, J., Lastra, B). The plant becomes bound by A. tumefaciens bacteria. It infects the plant by transferring DNA coding for tumor formation. Tumors then develop within the root systems, and the bacterial DNA becomes incorporated into the plants genetic makeup. Plants undergo various responses in regards to the different stages of infection.

Plant Host Response to Bacteria before Infection

Plants primary line of defense against bacterial infections is the ability to recognize the microbe and initiate a defense response. The plant being infected by the bacteria is considered to be the host. The plant host is capable of identifying parts of bacteria. The ability for plants to recognize the bacteria allows for it to fight the invader. However, A. tumefaciens has the ability to hide and be unseen by the plant host (Gohlke, J. & Deeken). Once the bacteria infects the plant, the Agrobacteria produces hormones that manipulate a plant hosts response to infection (Pacurar, D.I., Thordal-Christensen). The production of theses hormones also aids the bacteria in a more successful DNA transfer and gall development. The hormone causes the bacteria’s DNA to be incorporated into the existing plant’s DNA because the hormone causes rapid cell division. The newly made cells then change the plants traits and cause gall formation (Cubero, J., Lastra, B).

Typically, plants are able to quickly identify an invader, allowing them to kill off the infected plant cells. The ability to kill off the infected cells prevents the infection from spread to the rest of the plant. However, A. tumefaciens is difficult for plants to identify. This makes it possible for the bacteria to infect the plant with its DNA and allowing the infection to spread throughout. The plant host has little to no ability in resisting and fighting off the infection caused by A. tumefaciens (Gohlke, J. & Deeken).

Plant Host Response to Infection and Crown Gall Development

Crown gall, or tumor development, is caused by the new DNA from the bacteria to the plant. The new DNA changes the plants physical traits and metabolism. The rate at which the tumor grows and develops is dependent on the amount of hormone the bacteria releases in the plant and/or the type of DNA transferred from the bacteria to the plant. The growth and development of the crown gall tumor causes destruction in the outer most layers of the plant in the location where the tumor is forming. This disruption in the surface layer provides an exposure for other germs from the external world to migrate into the plant. This makes the plant more susceptible to diseases and uncontrolled water loss. Therefore forcing the plant to repair the damage by creating a secondary layer for protection (Gohlke, J. & Deeken).

The tumor development also plays a critical role in the way that the plant uses and produces energy. The formation of tumors causes the plant to produce energy anaerobically, or without oxygen. Due to the reduced amount of oxygen available to the infected part of the plant where the tumor is developing, the plant is forced to use a different energy source. The plant goes from using inorganic compounds to organic compounds. An inorganic compound is a non-carbon source. Whereas, an organic compound is a source solely composed of carbon. The tumor itself uses organic carbon and nitrogen absorbed through the soil. The tumor acts as a parasite to steal, minerals and other required nutrients from its plant host to assist in its growth (Cubero, J., Lastra, B).

Crown gall tumors in the plant, typically, develop in the lower trunk and/or root system. The development in the lower part of the plant is due to its ability to obtain nutrients such as, carbon and nitrogen from the soil. However, A. tumefaciens has the ability to translocate tumors from the roots into upper parts of the plant such as the crown and the stem (Gohlke, J. & Deeken).

 

Plants lack the ability to initiate a strong defense against A. tumefaciens. The bacterium is capable of releasing hormones into the plant upon infection that weakens its immune response. This also increases its chances of infection and tumor development (Gohlke, J. & Deeken). Once a plant becomes infected by A. tumefaciens it endures a long-term relationship (Pacurar, D.I., Thordal-Christensen). The plant host serves as a reservoir of minerals, water, and nutrients that assists in the tumor formation. This bacterium has the ability to create a systemic infection, or an infection throughout the entire plant (Cubero, J., Lastra, B). Bacterium have devastating effects on both the plant and human populations. The development of crown gall tumors induced by A. tumefaciens generates problems in the plants growth, root system, and difficulties conducting water and minerals to different parts of the plant. The relationship between plants and A. tumefaciens serves as an important resource in understanding the interactions among plants and other bacteria. The ability for researchers to understand the types and the ways that bacterial hormones affect plants could be used as a very beneficial tool. Information acquired from this interaction, which happens naturally in the environment, could be used in future research to manipulate how we grow, protect, or even kill off other plants and organisms.

 

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