Agrobacterium

Agrobacterium is a type of soil bacteria known for causing a tumor-like infection in plants and trees. Agrobacterium accomplishes this by actually transferring some of its genetic material to the host plant. Once the bacterium enters the host plant through a wound, it alters the plant’s genetic makeup to create a protective home for itself and a source of nutrients. Agrobacterium is common in soil throughout the world and typically travels by attaching itself to dust particles. Scientists have been aware that the bacterium is the cause of plant infections for more than a century. Its genetic-transferring ability is a more recent discovery and one that scientists have used to make significant advancements in agricultural genetic engineering.

Background

In the late nineteenth century, researchers were attempting to determine the cause of a destructive plant infection they called crown gall. Galls were first studied by scientists in the seventeenth century. They are abnormal, tumor-like growths that seem to spontaneously form on plants, similar to how warts grow on human hands. Crown gall can cause large tumors to form near the roots and lower branches of plants or trees, often just above the soil line. The tumors can damage plants by weakening their ability to transmit water and other nutrients through their stems or trunks. The infection can even prove fatal to young plants.

In 1897, Italian botanist Fridiano Cavara studied the tumors that formed at the bases of grapevines and determined that a bacterium caused crown gall. A decade later, American researchers identified the bacterium as a strain of soil bacteria they called Bacterium tumefaciens, later renamed agrobacterium tumefaciens. They determined that the bacterium traveled by attaching itself to dust particles and entering the plant through a wound on its surface. From there, the bacterium acted like a cancer cell in a mammalian body, causing uncontrollable cell growth at the site of the wound.

In the 1940s and 1950s, biologist Armin Braun noticed something unusual about the bacterium. He observed that the disease caused by the bacterium seemed to change the properties of the infected cells. Furthermore, the changes in the cells continued even when the bacterium was no longer present. Braun referred to the catalyst that caused the disease as the tumor inducing principle (TIP) and theorized that it may be caused by the bacteria’s DNA, which is a molecule that carries the genetic instructions for living organisms. However, at the time, he and other researchers did not have the necessary technology to discover the answers to the mystery.

Overview

In the late 1960s and early 1970s, scientists discovered that DNA was in fact the means by which agrobacterium caused the tumors to form. Further research showed that agrobacterium DNA has an extra genetic structure called a Ti plasmid, or tumor-inducing plasmid. A plasmid is a circular strand of genetic material that can separate from its DNA and reproduce itself. When agrobacterium encounters an open wound on the stem or root of a plant, it invades the host and attaches itself to the walls of the plant cells.

The agrobacterium releases a portion of the Ti plasmid into the host cell, where it makes its way into the nucleus and inserts itself directly into the cell’s chromosomes. Chromosomes are thread-like structures that contain an organism’s DNA. Once there, the genes of the Ti plasmid release an enzyme that hijacks the plant cells and forces the plant to create an environment where the agrobacterium can thrive. The genes cause the plant to begin producing cells that grow to form the galls, or tumors. These tumors contain numerous folds and nooks that act as the home for the bacterium and protect it from the elements. The genes also instruct the host plant to make nitrogen-rich compounds that act as nutrients, allowing bacteria colonies to grow. The agrobacterium continues to multiply and infect the plant until the host dies. From there, the bacterium returns to the soil, where it awaits a new host.

Though much scientific research has been focused on Agrobacterium tumefaciens, several different strains of agrobacterium exist. Agrobacterium rhizogenes causes hairy root disease, an infection that causes uncontrollable growth of plant roots. This strain releases a root-inducing, or Ri plasmid, that genetically alters the host’s root system. Agrobacterium rubi causes cane gall disease, which primarily affects vines and shrubs with brambles, such as blackberry plants. The infection damages parts of the plant stems, causing the plant to produce dry, seed-filled berries and stunting the growth of additional plants. One strain, Agrobacterium radiobacter, is considered avirulent, meaning it is incapable of causing infections. This strain relies on dead plant material for its nutrients.

Scientists have found that they can change the infectious properties of a strain of agrobacterium by changing its genetic makeup and replacing one plasmid with another. For example, while Agrobacterium tumefaciens causes crown gall disease, if the Ti plasmid is replaced with an Ri plasmid, that strain of agrobacterium will cause hairy root disease. The genetic-altering properties of agrobacterium have made it a useful tool for researchers who use it as a vehicle to transfer genetic material from one plant species to another. Scientists can remove the genes from the plasmids that cause diseases to form and replace them with genes that will produce the desired effects. In this way, plant tissue can be genetically altered to create more efficiently grown plants or plants that regenerate faster.

Because genetic transfer using agrobacterium does not affect all types of plants, scientists at first believed its use could be limited. However, researchers in the twenty-first century discovered a method that makes the bacterium effective on cash crops such as corn, soybeans, cotton, canola, potatoes, wheat, and barley. In addition to its use in the agricultural field, agrobacterium has also been shown to facilitate genetic transfer in certain types of fungi, including yeast. In laboratory experiments, it has also been used to transfer DNA to human cell samples.

Bibliography

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