April 27, 2007

A University of Nebraska-Lincoln plant scientist's discovery of a previously unknown component in plants' immune systems provides new clues to how plants and humans fend off diseases and how invaders stifle immunity.

Microbiologist James Alfano and co-authors reported their findings in Nature, the international weekly journal of science. Their paper was published online April 22 ahead of print publication. The work stems from Alfano's discovery of a protein toxin in a plant pathogen that's also found in several animal pathogens, including those that cause diphtheria and cholera.

"It gives us a whole new avenue to pursue in understanding plant innate immunity," said Alfano, who works on UNL's Plant Science Initiative team. As different as they are, plants and animals share some of the same molecular components to defend themselves against outside invaders, Alfano explained.

His research focuses on a method of infection found in animal and plant pathogens called a type III protein secretion system. To infect a plant, pathogens inject up to 30 proteins into plant cells using this system, which resembles a kind of microscopic syringe. Once inside, the toxic mix of proteins acts like a burglar, cutting wires to a home's alarm system, disabling the defense system from calling for reinforcements and allowing the intruders to enter unimpeded.

Alfano's team discovered one of the proteins -- HopU1 -- disrupts the plant's immune system when the disease-causing bacterium Pseudomonas syringae injects it into a plant. This disruption helps the pathogen infect its plant host. Researchers found that HopU1 is a type of enzymatic protein -- an ADP-ribosyltransferase that had never before been found in plant pathogens. This type of protein is also found in organisms that cause human diseases such as cholera and diphtheria.After identifying HopU1 as one of the injected proteins, Alfano began studying which plant components this virulence protein targets. That's key to identifying new components of plant immunity.

The Nature paper reports on the team's discovery that HopU1 modifies RNA-binding proteins. Alfano's work suggests that the pathogen disrupts plant immunity by suppressing immunity-related RNA metabolism -- part of the process that turns a plant's DNA code into proteins to help fight off infection. A plant lacking one of the HopU1 targets is more susceptible to the pathogen. These RNA-binding proteins, also found in animals, were not previously known to be part of plants' or animals' immune systems. Alfano uses Arabidopsis, a well-studied plant, as a model for this research, which is funded by the National Institutes of Health. As his research leads to a better understanding of plant immunity, scientists may be able to genetically modify crop plants to better defend themselves against disease. Because plants and animals share some components of immunity, this basic research one day also could lead to improvements in human health, Alfano said.

Paper co-authors are: Zheng Qing Fu, Ming Guo and Byeong-ryool Jeong, Plant Science Initiative and Department of Plant Pathology; Fang Tian, Plant Science Initiative and School of Biological Sciences; Thomas Elthon, Plant Science Initiative and Department of Agronomy and Horticulture; and Ronald Cerny, Department of Chemistry, all from UNL; and Dorothee Staiger, University of Bielefeld, Germany.

Gillian Klucas
IANR News Release