Plant diseases mutate to evade detection

The research team found that the bacterial colony changes to one lacking the gene that normally triggers a defence mechanism from the plant. In effect the bacteria disguise themselves to ensure they can get through a plant’s detection system. This means the invading bacteria can then infect the plant undetected and spread throughout a crop to cause disease.

The results of the study led by Andrew Pitman and Dawn Arnold, of UWE’s Centre for Research in Plant Science, were published in a recent issue of Current Biology. The disease agent, Pseudomonas syringae, or halo blight as it is commonly known, infects bean crops with small spots surrounded by a yellow halo. The bacteria cause greasy brown lesions on pods making them unmarketable.

Dawn Arnold described how the cycle of attack and defence works: “As the plants fight back, the tissue around the infection dies, preventing further spread of the blight. But this strategy often seems to fail, and the bacteria continue to infect other plants, becoming more virulent.

“In this study, we simulated an outbreak in the laboratory, exposing healthy leaves to the disease, then re-harvesting the bacteria for another cycle in healthy plants. After repeating this five times, we found that plants could no longer defend themselves against the bacteria and experienced massive tissue damage.”

By analysing the bacterium’s genome, the team discovered that the halo blight pathogen was able to remove the gene responsible for making the protein recognised by the plant. The gene migrates to the cytoplasm of the bacteria and is lost as the bacteria replicate and this loss does not arrest the growth of the bacteria itself.

According to the researchers, this is the first example of this mechanism being demonstrated in plant pathogenic bacteria – however, a similar mechanism is used by bacteria that infect animals. The plant bacteria seem able to continue to function even without their banished genes and the researchers have yet to discover why they do not get rid of them permanently.