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Leaf me alone.
A bean leaf infected with Halo blight shows off its tissue damage.

Dawn Arnold, Andrew Pitman, and John Mansfield.

Out, Out Damn Gene!

Scientists have discovered how a common crop pest evades detection. When the invader's cover is blown, the bacterium masks itself by ditching its genetic identification, setting the stage for a quiet and deadly invasion.

Commonly known as Halo blight, Pseudomonas syringae pv. Phaseolicola infects bean crops. Leaves develop small, water-soaked spots outlined by a yellow halo. As the plants fight back, the tissue around the infection dies, preventing further spread of the blight. But this strategy often fails, and as the bacteria move from leaf to leaf, they usually grow more virulent. In some cases, a single contaminated bean seed has unleashed a severe epidemic.

To learn how Halo blight gets away with this, microbiologists Dawn Arnold and Andrew Pitman of the University of the West of England in Bristol, U.K., and colleagues simulated an outbreak. They exposed healthy looking leaves to the bugs, waited for the plant to begin fighting back, and then reharvested the bacteria for yet another cycle through a batch of healthy greenery. After five iterations, plants could no longer defend themselves against the bacteria and experienced massive tissue damage.

Genetic analysis indicated that Halo blight was pulling a molecular disappearing act. Upon sensing the bean plant's response, the bacterium kicked out the portion of its genome responsible for making proteins that could be recognized by the plant. This DNA migrated to the cytoplasm, where it formed dormant circular strands. "This is the first example of this mechanism in plant pathogenic bacteria," says Arnold, although she notes that similar dirty tricks have been observed in bacteria that infect animals. Curiously, the bacteria appear to work just fine without their banished genes, so it's unclear why they haven't dropped them for good. The team reports its findings 20 December in Current Biology.

The findings demonstrate the varied ways plants and pathogens have coevolved, says Hei-Ti Hsu, a microbiologist at the U.S. Department of Agriculture's National Arboretum in Washington, D.C. And Jonathan Jones, a biologist at the Sainsbury Laboratory in Norwich, U.K., says Halo blight's unique strategy may allow it to increase its host range and attack plants of other species.

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