Type III secretion systems (TTSSs) are key to the virulence of many bacterial pathogens of plants and animals. The genes encoding these systems and the effectors they inject into host cells are often associated with horizontally transferred "pathogenicity islands." This paper reported the first complete sequence of a plant pathogen TTSS pathogenicity island. It was based on the model pathogen Pseudomonas syringae pv. tomato DC3000, whose hosts include tomato and Arabidopsis.

The paper revealed that the TTSS genes are flanked by two regions enriched in effector genes: an exchangeable effector locus (with different effector genes in various P. syringae strains) and a conserved effector locus. Deletion of the latter resulted in a strong reduction in virulence.

  How did you become involved in this research?

We have a long-standing interest in the molecular basis for bacterial plant pathogenicity. The importance of the TTSS in this model plant pathogen led us to this work.

Bacterial virulence systems often involve modular functions that are encoded by gene clusters, and these clusters may be transferred horizontally among bacteria. This paper provided insights into the structure and function of a particularly important gene cluster in the plant pathogen Pseudomonas syringae, which encodes a virulence protein injection system and several of the injected proteins.

James R. Alfano
The Plant Science Initiative and the Department of Plant Pathology
University of Nebraska
Lincoln, NE, USA