June 2005
Double Disease Response by Plants
David Mackey and other Ohio State researchers have uncovered the link between two biochemical pathways that plants use to defend themselves against pathogens - pathways that scientists have long believed worked independently of each other. The researchers infected Arabidopsis plants with Pseudomonas syringae, a bacterium that infects tomato crops. Both of these organisms are models commonly used to conduct basic plant research. One of the immune pathways of interest detects what they call pathogen-associated molecular patterns, or PAMPs. The PAMP pathway appears to be a plant’s first line of defense against pathogenic attackers. “The PAMP path induces a fairly weak immune response,” Mackey said. “Even so, there is growing evidence that suggests these kinds of responses are extremely important in restricting the growth of many pathogens.”
The other pathway uses disease-resistant proteins, or R-proteins, which can detect certain molecules, called effectors, that are secreted by pathogens. This pathway produces a stronger immune response than the PAMP pathway, Mackey said. He and his colleagues found that the R-protein pathway steps in when PAMP is rendered useless by a pathogen. Certain types of bacteria, including P. syringae, make a hypodermic needle-like structure that pierces the outermost membrane of a healthy plant or animal cell. The pathogen uses this conduit to send infectious effector proteins into the host cell. While P. syringae injects about 40 different varieties of effector molecules into a plant cell, the researchers focused on the actions of two of these molecules –– AvrRpt2 and AvrRpm1. Both target a protein key to Arabidopsis health. The scientists found that both of these effector molecules effectively shut down the PAMP pathway, but the plant’s R-proteins detect this, and come to the rescue. “The R-proteins detect the insidious activity by which the pathogen’s effectors block the PAMP pathway,” Mackey said. “PAMP defense responses are probably often effective, but they may be blocked by the pathogen’s effector proteins. If an R-protein recognizes a pathogen’s presence, it usually induces a very strong immune response, in most cases stopping a would-be infection. This work further suggests that plants use an active, complex immune system to combat pathogens,” he said. “They have complicated surveillance systems that detect various infection-causing molecules and trigger defensive responses.” (Ohio State University, 6/2/05).
