Roots Engage in Underground Chemical Warfare

In addition to providing physical support and taking in nutrients, plant roots secrete a wide variety of compounds that affect other nearby roots, as well as insects and microbes. But because it goes on unseen, bactericidal root activity has not been extensively investigated—until now. Using the model plant Arabidopsis thaliana, a relative of garden-variety cabbage, Jorge Vivanco and co-workers at Colorado State University, together with Frederick Ausubel at Harvard Medical School, demonstrated that "root exudates" contain antimicrobial agents that ward off the continual attacks by soil pathogens.

The work is published in the March 10 issue of the journal Nature.

The exudates from Arabidopsis roots kill a wide range of bacteria, confirming that roots are not always vulnerable, anchored targets. The natural production of these antimicrobial chemicals offers one explanation for why so few bacteria types actually cause disease in plants. Of the more than 50,000 plant diseases occurring in the United States, fungal pathogens are the leading cause.

"Current understanding of plant defenses does not readily explain why a pathogen can cause disease in one plant species and not another," says Vivanco. "Our findings will help researchers solve the mysteries of plant disease and immunity."

In these experiments, however, root exudates did not kill all of the tested strains of bacteria. One particular strain of Pseudomonas syringae, a bacterium that causes disease in both tomatoes and Arabidopsis, has a seemingly fail-safe mechanism to overcome the plant's defenses. The bacterium not only survives exposure to the antimicrobial substances, it also blocks the plant's ability to produce them.

Both Vivanco and Ausubel are supported by separate awards from the division of Molecular and Cellular Biosciences at the National Science Foundation (NSF).

Vivanco is a recipient of NSF's prestigious Faculty Early Career Development Award (CAREER). CAREER awards support the early career development of those researcher-educators who are deemed most likely to become the academic leaders of the 21st century. Parag Chitnis, the NSF program manager of Vivanco's award said, "This work is an exciting outcome of a bold and challenging project. The work paves the way to understand and combat crop diseases."

The program manager for Ausubel's award, Michael Mishkind said, "The puzzle of why so few bacterial species are pathogens remains a fascinating problem. The simple, yet elegant experimental approaches used by this team uncovered a critical aspect of the battle that occurs between plants and microbes.

Source : National Science Foundation