Of Plants and Pathogens: A Model Relationship

Like architects studying a small-scale city block, biologists rely on models to understand complex systems. And just as office parks depend on networks of roads, in nature it's impossible to fully understand the parts without considering the relationships between them.

So when the complete genetic blueprint of a tiny plant pathogen was published in 2003, it was the fact that one of its host plants--famous for being the model organism in plant biology--had also been sequenced that most excited researchers. Taken together, the information encoded in both blueprints (known as genomes) promised to reveal the strategies pathogens use in infection and the plant defenses that fend infection off.

The pathogen, a strain of the bacterium Pseudomonas syringae, is best known as the cause of a fast-spreading plant disease called bacterial speck. There are more than 50 host-specific variants of P. syringae, and collectively they can devastate a wide variety of crops worldwide.

The genome was a product of the Pseudomonas-Plant Interaction Project, funded in part by NSF's Plant Genome Research Program and co-led by Cornell University professor Alan Collmer and Robin Buell of The Institute for Genomic Research in Rockville, Md. Though P. syringae is a microscopic organism, its genome proved to be surprisingly complex, says Collmer, with more than 6.5 million DNA base pairs. Among them are more than 300 genes related to virulence.

Serendipitously, P. syringae not only infects crops, but also a small flowering plant called Arabidopsis thaliana. Studied widely in laboratories, Arabidopsis is to plant scientists what the mouse and fruit fly are to researchers studying animals. With funding from NSF, the Arabidopsis genome was published with much fanfare in 2000, making it the first plant to be fully sequenced.

With the addition of P. syringae to the fast-growing inventory of full genome sequences, scientists now have a complete genetic host-pathogen model, says NSF program officer Jane Silverthorne. "Collmer's work provides a powerful foundation for researchers studying plant-pathogen relationships in many different contexts."

Deciphering a genome, Collmer explains, can reveal much about a given organism's biology. But organisms evolve together in interwoven communities. Plants and their pathogens often evolve in a tight cat-and-mouse genetic competition. Because the genes that allow pathogens to infect their hosts often have counterparts in the host plants--genes that help the plants resist infection--researchers are finding it increasingly difficult to study one without the other.

For this reason, having a fully sequenced plant-pathogen model is particularly useful. Many of the P. syringae genes responsible for infection have counterparts that protect Arabidopsis. And since many genes found in P. syringae are shared by other disease-causing bacteria, these similarities can be used to make inferences about related organisms.

"Pseudomonas syringae has become a model for studying plant diseases, and the genome reveals how complex the jigsaw puzzle of pathogenesis is," says Collmer. "It shows us that many parts of the puzzle are the same for plant and animal pathogens, and it enables scientists to put the pieces together more efficiently."

And knowledge of the genomes of both the host plant and the pathogen provides researchers with a gold mine of information about how bacteria infect plants. Potentially, the information could help researchers prevent other, more agriculturally relevant, infections.

Researchers chose Arabidopsis thaliana as a model species for genetics because it has a short life cycle, is small and easy to grow, and has a small genome. Pseudomonas makes a good model for bacterial pathogenesis because it is safe to work with (because it infects plants, not people) and is similar to a wide range of other infectious bacteria.

In fact, P. syringae bears a marked similarity to the bacteria that can cause fatal lung infections in cystic fibrosis patients. The pathogen-host model offered by Pseudomonas and Arabidopsis may offer insight into that disease and others, by helping researchers understand how the bacteria have adapted to their hosts. In some cases, Collmer hopes, such information could point to potential targets for new therapies.

Since the P. syringae genome was sequenced, "the whole community jumped on and created a momentum that was really special," Collmer says, adding that it is a momentum that will only grow as the genomics boom continues.

--Sarah Goforth