AUG 17, 2015 3:51 PM PDT

Better Know a Microbe: Agrobacterium tumefaciens

WRITTEN BY: Kerry Evans
Agrobacterium tumefaciens is a plant pathogen responsible for crown gall disease.  I know what you’re thinking, plant pathogen?  Boring.  Not so!   In fact, A. tumefaciens is an incredibly clever and industrious bacterium.  It takes up residence inside an unsuspecting plant and convinces it to dish out an all-you-can-eat buffet.  Think of it as the world’s worst roommate.
Agrobacterium tumefaciens is a Gram negative, soil dwelling bacterium. 
A. tumefaciens is a Gram negative bacterium that lives in the soil where it infects plants through wounds on the roots or stems.  It is a very close relative to bacteria in the genus Rhizobium, which fix nitrogen in the roots of legumes (so close, in fact, that there is a debate to reclassify Agrobacterium as Rhizobium).  A. tumefaciens only infects dicots (flowering plants with two embryonic leaves), but these encompass more than 60 plant families.  

A. tumefaciens is unusual because in addition to its circular chromosome, it has a linear chromosome that was likely derived from an ancestral plasmid.  The ends of the linear chromosome are capped by telomeres that consist of a covalently closed hairpin. The bacteria also carry the Ti (“tumor inducing”) plasmid that encodes genes for inducing crown gall disease.
Galls form on stems and roots of infected plants.
So, how does a plant get crown gall disease?  It all starts when a plant is injured.  Phenol compounds like acetosyringone are expelled from the wound and sensed by A. tumefaciens in the soil.  The bacteria use their flagella to move chemotactically along the phenol gradient until they reach the ill-fated plant. The bacteria then enter the wound and use their type IV secretion system to inject the plant cells with T-DNA, produced by the Ti plasmid, and other virulence proteins.

Once it enters the plant cell, the   T-DNA is incorporated into the plant’s own DNA and encodes genes that cause the plants to produce auxins and cytokinins. These chemicals regulate plant growth to produce galls (it’s a tumor!).  
 
The T-DNA also induces the plants to make opines, chemicals similar to amino acids that the bacteria use as an energy source.  As you can imagine, this isn’t a win-win situation.  The bacteria clearly benefit from this situation, while the plant does not, making the bacteria bonafide parasites.  However, A. tumefaciens does have a weak spot.  The closely related species Agrobacterium radiobacter can also utilize the opines produced in galls.  So, to outcompete A. tumefaciens, it produces a toxic compound called Agrocin 84.  This nucleotide analog is not toxic to A. radiobacter, but is taken up by A. tumefaciens where it interferes with DNA replication, ultimately killing the cell.  In fact, Agrocin 84 is widely used in industry to treat crown gall disease.
 
Researchers have taken advantage of the fact that A. tumefaciens is especially good at injecting its DNA into plant cells.  Since the 1970s, A. tumefaciens has been used to genetically modify plants.  Heard of herbicide-resistant Roundup Ready® crops?  These were developed by Monsanto beginning in 1995. What about the Flavr Savr™ tomato created by Calgene in 1994?  The idea is simple.  You take the gene you want to insert into a plant and insert it into the A. tumefaciens Ti plasmid.  This gene will then become part of the T-DNA that is injected into plant cells and incorporated into the plant genome.  Voila!  
 
To sum it all up, here’s Robert Horsch of Cold Spring Harbor discussing our new friend, Agrobacterium tumefaciens.
 
About the Author
  • Kerry received a doctorate in microbiology from the University of Arkansas for Medical Sciences.
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