New Protein Plays Espionage Role In Bacterial Attack On Plants

The plant protein, called RIN4, interacts with invading pathogen molecules and a protein from within the plant cell itself, a protein in the plant’s disease resistance strategy. The discovery adds important new knowledge to how bacterial pathogens target a host plant’s molecular machinery to make it more hospitable, even beneficial, to its plundering invasion.

The new research from the University of North Carolina at Chapel Hill was funded by the National Science Foundation and the U.S. Department of Energy and is reported in the March 22 issue of the journal Cell.

"This research will increase our understanding of how plant genes mediate resistance to pathogenic bacteria that cause disease and crop losses," says Sharman O'Neill, program director in NSF's division of integrative biology and neuroscience. "This information is likely to lead to novel approaches for pathogen control, and to the improvement of disease resistance in plants. The combination of genetics and biochemistry will allow a unique assault on a disease resistance signaling pathway," O’Neill added.

“This study is largely about how plants perceive pathogens,” said senior study author Dr. Jeff Dangl, John N. Couch professor of biology, a member of the curriculum in genetics at the university, and an adjunct professor of microbiology and immunology at the School of Medicine. “When we study the interaction between host and pathogen, we need to understand it from both sides: the pathogen side, including the targets it wants to hit and why, the weapons it uses, and, on the plant side, what guard molecules the host deploys.”

Known internationally for his work in this area of research, Dangl pointed to biologically conserved similarities between some bacterial pathogens of animals -- E. coli, salmonella, shigela, and yersinia, which causes plague -- and bacterial pathogens of plants in their tactics of attack. While growing in colonies outside the host cell, these bacteria shoot proteins through a specialized tube into the inside of the host cells. These proteins, or type III effectors, “suppress the host defenses by subverting some aspect of host cell molecular signaling and cause the host plant or animal cell to leak nutrients to the waiting bacterial colony.”

Through disease-resistance proteins, the plant’s surveillance system responds to the presence of pathogen molecules. “If the plant’s surveillance system is going to respond to the presence of the bacterial pathogen, and because the bacterial pathogen is shooting these proteins into the host cell, then it makes perfect sense that the host disease resistance protein that’s going to perceive this invader is inside the cell,” Dangl explains.

He and his biology department colleagues -- post-doctoral fellow David Mackey, graduate student Ben F. Holt III, and research analyst Aaron Wiig -- studied how the wild mustard plant Arabidopsis thaliana responds to Pseudomonas syringae, a bacterial pathogen that also causes diseases in crops like beans, peas and tomatoes. Completion of the Arabidopsis genome sequence in December 2000, the first for a higher plant, made it ideal for molecular investigation such as this. In addition, this small weed has homologues or counterparts of many important human proteins involved in disease, including cystic fibrosis and breast cancer.

The Carolina researchers focused on the Arabidopsis disease resistance protein, RPM1, which they previously showed was anchored to the inside of the plant cell membrane. “It is logical from an evolutionary perspective of defense that you put the putative receptor, in this case RPM1, in the right cellular compartment to interdict the molecules that are going to get the host in trouble,” Dangl said.

But until now, no one had found a direct interaction between the pathogen’s type III effector proteins that get shot into the cell and the RPM1 receptor. Thus, the Dangl team predicted there might be a host protein that was targeted by these invading proteins, one that might play a role in regulating plant defenses.

Indeed, the new study identified such a target, the protein RIN4. It interacts with either of two type III effector proteins “and we were able to show that RIN4 also interacts with RPM1 inside the cell,” Dangl said.

“RIN4 is a protein that bridges between the pathogen-encoded type III disease effector and the plant-encoded disease resistance protein,” Dangl said. “We also postulated that RIN4 is part of an as yet undefined cellular machinery that is protected by RPM1.”

When type III effectors are inside the cell, a phosphate is added to RIN4. Dangl and colleagues postulate that this phosphorylation of RIN4 reduces cellular defense. Thus, RIN4 “is normally a negative regulator of defense, and the type III effectors target it and lock it into a negative regulatory mode. Pathogen growth would be facilitated by slowing the host defense response,” Dangl said.

But the resistance protein RPM1 appears to serve as a “guard” for cellular proteins that are potential targets of pathogen molecules, like RIN4. Dangl and other researchers in this field are beginning to think that resistance gene products such as RPM1 might be deployed to physically associate with those targets and to interdict the pathogen molecule when it enters the cell.

“This paper is the first to solidly identify a target of pathogen disease effectors that actually also associates with a disease-resistance product,” Dangl said. “The next challenge will be to define the spectrum of type III effectors and the corresponding spectrum of plant cellular machines that they target.”