Throughout evolution - in the wild and in crops cultivated by humans - plants have developed systems for resisting the attack of microbial pathogens, while these microbes themselves have depended on their ability to alter molecular attack strategies in order to flourish. In the new work, researchers have essentially caught one step of this arms race in action, and they have shed light on the molecular mechanisms employed by a bacterial pathogen to survive in the face of its host plant's defenses. The research is reported by John Mansfield and colleagues at Imperial College London, the University of the West of England, and the University of Bath.
Studying interactions between strains of the halo-bright pathogen and bean plants, the researchers found that the pathogenic bacteria essentially kicks out a section of its genome when it senses that its presence has been detected by the plant's defense system. Excising this so-called "genomic island" eliminates production of the bacterial protein detected by the plant and allows a more stealthy - and successful - invasion.
The reason the strategy can be successful is that the plant has evolved to recognize the presence of only certain bacterial proteins as warnings of an infection. This means that the bacteria can, in principle, evade detection if it can shut down production of the offending protein or proteins. In their study, the authors identify within the halo-bright pathogen genome a special island of DNA that encodes one such offending protein. But this genomic island also encodes enzymes that, when switched on, snip the DNA on either side of the island, resulting in the excision of the entire island from the genome. The researchers found that this excision occurs in response to a so-called hypersensitive resistance reaction by the plant . The specific plant signals that are sensed by the pathogen to trigger the excision event remain to be identified.
If some bacterial proteins give away the presence of the pathogen to the plant, why are they and their surrounding genomic islands maintained by the bacteria at all? There are many genes included in the genomic island that is excised by the halo-bright strains studied here, and these other genes may well encode proteins that allow the pathogen to survive optimally in the light-intensive regions of eastern and southern Africa where these strains are found. Remarkably, bacteria that have excised the genomic island do not appear compromised in their ability to grow and cause disease within the host bean plant.
These findings offer a molecular explanation for how exposure to plant resistance mechanisms can directly drive the evolution of new virulent forms of a bacterial pathogen.
The researchers include Andrew R. Pitman and Dawn L. Arnold of the University of the West of England in Bristol, United Kingdom; Robert W. Jackson of the University of the West of England in Bristol, United Kingdom and the University of Bath in Bath, United Kingdom; John W. Mansfield and Victor Kaitell of the Imperial College London in Kent, United Kingdom; Richard Thwaites of the Imperial College London in Kent, United Kingdom and Central Science Laboratory in York, United Kingdom. This work was supported by the Biotechnology and Biological Sciences Research Council.
Pitman et al.: "Exposure to Host Resistance Mechanisms Drives Evolution of Bacterial Virulence in Plants." Publishing in Current Biology, Vol. 15, 2230-2235, December 20, 2005, DOI 10.1016/j.cub.2005.10.074, www.current-biology.com.