Plants and Bacteria 'Talk' to Thwart Disease

In plant and animal innate immunity, like many of the dances of life, it takes two to tango. A receptor molecule in the plant pairs up with a specific molecule on the invading bacteria and, presto, the immune system swings into action to defend against the invasion of the disease-causing microbe.

Unwrapping some of the mystery from how plants and bacteria communicate in this dance of immunity, hardworking scientists in my laboratory here at the University of California, Davis, have identified the bacterial molecule that matches up with a specific receptor in rice plants to ward off a devastating disease known as bacterial blight of rice.

The publication describing these results will appear tomorrow in the journal Science.

The discovery of this bacterial molecule helps us better understand how the innate immune system operates. Because similar pairs of receptors and bacterial molecules are known to exist not only in rice but also in other plants, as well as animals and humans, we are hopeful that this work will lead to new strategies for controlling diseases in plants and people

In 1995, my lab isolated and characterized a receptor called Xa21, that recognizes Xanthomonas oryzae pv. oryzae, the causal agent of bacterial blight disease. Xoo and other species of Xanthomonas infect virtually every crop species in the world. My first stab at science writing was an article I wrote for Scientific American about the discovery of Xa21.

Subsequent discoveries revealed that receptors with striking structural similarities to the XA21 receptor protein exist in other plants, flies, mice and humans. (Of special note, the mammalian receptor TLR4, the mammalian cousin to XA21, was isolated in 1997 by my third cousin, the distinguished scientist, Bruce Beutler. -Oh, if only our great-grandparents could see us now). These receptors were later named pattern recognition receptors (PRRs) because they have the ability to recognize molecules that are conserved within a large class of disease-causing microbes and to launch a protective immune defense on behalf of the plant or animal.

The receptors (PRRs) and the microbial molecules (called pathogen-associated molecular patterns-PAMPs) are key components of a system called innate immunity. As the name suggests, this form of immunity is built into the plant or animal's genetic make-up, rather than developing over time with repeated exposure to disease-causing microbes (e.g antibodies in animals- which plants don't have).

In the new Science paper, we show that a sulfated peptide, which we named ax21, is the PAMP that binds the XA21 receptor protein. The binding triggers a defense response against the bacterial disease.

ax21 is found in all species of Xanthomonas as well as in Xylella fastidiosa, a microbe that causes Pierce's disease, a devastating infection of grape. ax21 is also found in Stenotrophomonas maltophilia, a bacterium that causes respiratory and urinary tract infections in humans.

These studies have led to a convergence in our understanding of the molecular mechanisms that govern how disease-causing microbes interact with the plants and animals they infect.

We are hopeful that these discoveries will benefit agriculture and medicine in the United States and around the world by leading to the development of treatments that will disrupt bacterial infection.

Sang-Won Lee, Sang-Wook Han, Malinee Sririyanum, Chang-Jin Park, Young-Su Seo, & Pamela C. Ronald (2009). A Type I-Secreted, Sulfated Peptide Triggers XA21-Mediated Innate Immunity
Science Magazine, 326, 850-853 : DOI: 10.1126/science.1173438

Funding for the study was provided by the U.S. Department of Agriculture and by the National Institutes of Health.