Beanz defences meanz bacteria get evolutionary helping hand

Plants' attempts to defend themsleves against pathogens may have surprising side effect<em> - News Release</em>

Imperial College London press release

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Thursday 10 September 2009

Bean plants' natural defences against bacterial infections could be unwittingly driving the evolution of more highly pathogenic bacteria, according to new research published today (10 September) in Current Biology.

The study sheds new light on how bacterial pathogens evolve and adapt to stresses from host plants. This information could help researchers develop new ways of tackling pathogens that cause extensive and costly damage to beans and other food crops.

The scientists from Imperial College London and the University of the West of England (UWE) focused on a bacterial pathogen called Pseudamonas syringae, which causes a disease called halo blight in bean plants. Symptoms include brown spots on the leaves, surrounded by a yellow halo. The disease can cause bean plants to lose their leaves, wilt and die, and is a serious problem for farmers worldwide.

The research team observed that a French bean plant's defensive moves against infection caused P. syringae bacterial cells to 'swap' bits of DNA with each other. When one bacterial cell takes up DNA released by another like this, it is known as genetic transformation. This process, occurring within infected plant tissue, could speed up the evolution of more virulent forms of disease-causing microbes, say the researchers.

Professor John Mansfield from Imperial College London's Department of Life Sciences, one of the authors of the new paper, explains: "In the course of fighting off infection, and putting the invading bacteria under stress, it seems that the plants inadvertently do them a big favour. By causing the bacteria to throw out selected fragments of their DNA, which can then be taken up by other bacteria cells, the plants are effectively stimulating the bacteria to evolve. For disease-causing bacteria, this means that mechanisms meant to disable them could actually contribute to their continued survival."

Halo blight, the disease caused by P. syringae bacteria, is a serious problem for bean farmers around the world.

When a bean plant is infected by P. syringae it defends itself by sending a suicide signal to the plant cells surrounding the bacteria. When the affected plant cells die they release antimicrobial compounds that are toxic to the microbes. The toxic environment places the bacterial cells under enormous stress.

The new study shows that along with restricting bacterial multiplication, the release of these toxins seems to stimulate P. syringae cells to cut out small sections of their own DNA containing genes linked to pathogenicity. These gene 'islands' are then thrown out of the bacterial cell, and absorbed and incorporated into the DNA of other bacteria within the plant.

Professor Mansfield and colleagues are not yet sure exactly how the suicide of nearby plant cells brings about this DNA separation and removal, but say their results could have a much wider implication for how scientists understand the relationship between pathogen, host and pathogen evolution.

Dr Dawn Arnold, co author of the study from UWE's School of Life Sciences, concluded: "Although this work involves plant-bacteria interactions it also has a wider significance in that it could lead to a greater understanding of how bacteria evade the immune system of different hosts including humans."

The research was funded by the Biotechnology and Biological Sciences Research Council (BBSRC).

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For more information please contact:

Danielle Reeves, Imperial College London press office
Tel: +44 (0)20 7594 2198
Out-of-hours duty press office: +44 (0)7803 886248
Email: Danielle.reeves@imperial.ac.uk

Jane Kelly or Mary Price, University of the West of England press office
Tel: 0117 32 82208
E-mail: jane.kelly@uwe.ac.uk or mary.price@uwe.ac.uk

Notes to Editors:

1. 'Bacterial evolution by genomic island transfer occurs via DNA transformation in plants', Current Biology, online publication, Thursday 10 September 2009.

2. About Imperial College London

Consistently rated amongst the world's best universities, Imperial College London is a science-based institution with a reputation for excellence in teaching and research that attracts 13,000 students and 6,000 staff of the highest international quality.

Innovative research at the College explores the interface between science, medicine, engineering and business, delivering practical solutions that improve quality of life and the environment - underpinned by a dynamic enterprise culture.

Since its foundation in 1907, Imperial's contributions to society have included the discovery of penicillin, the development of holography and the foundations of fibre optics. This commitment to the application of research for the benefit of all continues today, with current focuses including interdisciplinary collaborations to improve health in the UK and globally, tackle climate change and develop clean and sustainable sources of energy.

www.imperial.ac.uk

3. About UWE

The University of the West of England (UWE) is one of the largest and most popular universities in the UK, with nearly 30,000 students registered on taught and research programmes. UWE prides itself on excellence in knowledge exchange and the development of professional practice. This is characterised by a vast range of research, teaching, business expertise and advice to individuals and organisations in the local, national and international community. UWE's overall research income has more than doubled in recent years, much of this research activity taking place in more than 30 research centres, and conducted in liaison with external agencies, companies, organisations and other academic institutions. Several Knowledge Exchange research institutes are also being developed to provide user-led research solutions for business and community. The University is one of the largest employers in Bristol.

www.uwe.ac.uk