It was Sept 4, 1939, the day after the UK declared war on Germany, when mathematician Alan Turing reported to work at the Government Code and Cypher School at Bletchley Park. Within weeks of his arrival, Turing and his colleagues were able to intercept high-level encrypted enemy communication signals and decode a vast number of these messages. The intelligence gleaned from this effort was passed on to field commanders, a process that was decisive to Allied victory.
Like the German military strategists, single-celled bacteria communicate with each other using coded messages to coordinate attacks on their targets. For bacteria these targets are plants and animals that provide the nutrients needed for growth. Until now, the diversity of codes employed by invading bacteria was thought to be extremely limited. However, our new research shows that bacteria communicate with a previously unknown signal. The research is described in two articles published December 12th in PLoS ONE and Discovery Medicine and is also reported on Scienceblogs.
In a feat worthy of the Turing cryptographers, some plants have evolved a cypher-breaking detection system, called the XA21 receptor, that intercept the bacterial code and use this information to trigger a robust immune response, preventing disease.
Over the last 20 years, researchers have shown that bacteria employ specific signals to communicate. These signaling molecules accumulate in the external environment as the cells grow. When the concentration of signal reaches a certain threshold level, the individual bacteria mobilize concerted, group actions. Professor Bonnie Bassler, an early pioneer in studies of bacterial communication, calls the signaling molecules “bacterial Esperanto”.
Until now, it was thought that two major groups of bacteria (called Gram-positive and Gram-negative bacteria) use distinctly different types of communication codes. Gram-positive bacteria use oligopeptides, whereas Gram-negative bacteria generally use acylated homoserine lactones (AHLs) or diffusible signal factors (DSF). However, the newly discovered signal, called Ax21 (Activator of Ax21-mediated immunity), from the Gram negative infectious bacterium Xanthomonas oryzae pv. oryzae), falls into neither class.
Unlike other signals in the bacterial coding repertoire, Ax21 is a small protein. It is made inside the bacterial cell, processed to generate a shorter signal and then secreted outside the bacterium. Perception of Ax21 by other bacteria of the same class, allows the bacteria to assemble into elaborate protective bunkers, called biofilms. Biofilms render the bacteria resistant to dessication and antibiotic treatment. Thus, by virtue of communication and communal living, bacteria increase their chances of survival and proliferation. Ax21 perception also regulates the production of a virulent arsenal including “effectors” that are shot directly into the host to disrupt host defenses and the initiation of motility allowing the bacteria to colonize new sites for infection.
This process transforms the bacteria from a benign organism to a fierce invader. The bacteria multiply in the main arteries of the rice water transport system, causing the plant to wither and die.
To accomplish these diverse tasks, Ax21 perception triggers a massive change in the genetic program: Nearly 500 genes (approximately 10% of Xoo genome) change their expression in response to Ax21. Bacterial mutants defective in Ax21 no longer aggregate into bunkers, move to new sites or trigger changes in gene expression.
Host cryptographer: The XA21 receptor
Most rice plants are virtually defenseless to this Ax21-mediated bacterial attack. The exceptions are those plants that carry the XA21 immune receptor that detect Ax21 produced by the invading microbe (Song/Wang and Ronald, Science magazine 1995, see also "Making Rice Disease-Resistant" in the November 1997 issue of Scientific American)
XA21 belongs to a large class of immune receptors in plants and animals that detect microbes (Ronald and Beutler, Science Magazine 2010). The importance of these receptors is reflected by the 2011 Nobel Prize in Physiology and Medicine to Professors Bruce Beutler and Jules Hoffman for their discoveries in flies and mice (you can watch Nobel Lecture by Bruce A. Beutler here). In plants and higher animals, these immune receptors detect microbial components that are conserved among diverse bacteria. These include structures that make up the bacterial cell wall or are important for motility.
[Bruce and I share more than an interest in science; my father (Robert) and Bruce's father (Ernst) were young cousins in Berlin in the 1920s. Their families fled the Nazi's and reunited in the US after the war].
The advance reported today indicates that not only does this class of receptors recognize structural components of bacteria but that they can also detect bacterial signaling molecules. The ability of plants to intercept these messages provides them with a clear tactical advantage in the evolutionary battle. To date, only the XA21 immune receptor is known to have this capacity.
Early detection of the signal produced by the invading bacteria is critical because it allows the plant time to mobilize defenses. Thus, just as the work of Turing allowed Allied convoys to detect and evade U-boat patrol lines, and then guide Allied anti-submarine forces to destroy the U-boats, XA21 intercepts the Ax21 signal, allowing rice to mount an early and potent defense response.
Ax21 is present in other pathogens of plants and animals
We have shown that not only is Ax21 present in important plant pathogens such as Xanthomonas, which infects virtually all crop plants and in a microbe that causes Pierce’s disease on grapes, but it is also present in a human pathogen that infects hospital patients, such as those suffering from cystic fibrosis.
The conservation of Ax21 in both plant and animal pathogens suggests that Ax21 also serves as a signal in these related microbes. In support of this idea, some of the functions of Ax21 discovered in the rice pathogen have been recently extended to pathogens of peppers, tomatoes and mustards as well as to a human pathogen.
The discovery that a small protein from a group of single-cell bacteria plays a dual role in both rallying invading bacteria and triggering an immune response in the targeted plant, has not previously been demonstrated. However, exploration of bacterial genomes predicts the presence of an abundance of similar small secreted proteins in many other species. These discoveries suggest the intriguing possibility that other species of bacteria use small proteins like Ax21 to communicate and coordinate infection.
The new research also suggests that rice is not the only targeted victim that has learned to detect these abundant bacterial signals. Whereas only 10 immune receptors have been identified in humans, over 300 such receptors are predicted in rice and other important cereals (Ronald and Beutler, Science 2010). Unlike rice XA21, which has been shown to bind directly to Ax21 (Lee and Ronald, Science magazine 2009), no corresponding conserved microbial signature has yet been identified for the hundreds of other predicted plant receptors. Thus, the plant immune response remains largely unexplored. An important question for future research is to identify the microbial molecules that these “orphan” receptors detect. Because most bacteria are in constant communication, it is clear that bacterial signals will accumulate in the host vicinity prior to infection. We speculate that some of the other hundreds of predicted receptors may have also evolved to intercept these bacterial messages.
How can the discovery of Ax21 be applied to control infectious disease?
Control of Gram-negative bacterial infections of plants and animals remains a major challenge for the medical profession and for farmers because conventional approaches are often not sufficient to eradicate these infections. One major reason for persistence seems to be the capability of most bacteria to grow within biofilms that protects them from adverse environmental factors and antibiotics.
The knowledge that bacteria use Ax21 to communicate is expected to lead to new methods of controlling bacterial diseases of plants and animals. For example, as described today in the journal Discovery Medicine, researchers are developing drugs that can antagonize Ax21-mediated virulence activities and biofilm formation, a process thought to occur in 65-80% of bacterial infections.
The work was carried out by Sang-Wook Han1,* Malinee Sriariyanun1,*,#, Sang-Won Lee1,2,*, Manoj Sharma1, Ofir Bahar1, Zachary Bower1 and Pamela C. Ronald1, 2, †
1, Department of Plant Pathology and the Genome Center, University of California, Davis, One Shields Ave., Davis, CA., USA
2, The Department of Plant Molecular System Biotechnology & Crop Biotech Institute, Kyung Hee University, Yongin, South Korea
#, Present address: Department of Chemical and Process Engineering, Thai-German Graduate School of Engineering, King Mongkut’s University of Technology North Bangkok, Bangkok, Thailand
Research on Ax21 was supported by National Research Initiative grants 006-01888 and 2007-35319-18397 from the USDA Cooperative State Research, Education and Extension Service. The discovery of XA21 was supported by the National Institute of Health grant # GM59962 to PCR.