Erwinia Chrysanthemi: Sequence Analysis of the Hrp/hrc Genes and Flanking Regions and the Role of Novel Virulence Factors in Pathogenesis
- ISBN-10
- 0493961801
- ISBN-13
- 9780493961804
- Pages
- 246
- Language
- English
- Published
- 2003
- Publisher
- Cornell University
- Author
- Clemencia Maria Rojas
Description
Erwinia chrysanthemi is a broad-host-range plant pathogen whose mechanism of pathogenesis is mainly dependent on the production of an arsenal of pectolytic enzymes that cause tissue maceration and cell death. However, other virulence factors contribute at different stages of disease development. Among these are the hrp/hrc genes, which encode a functional type III secretion system and play an important role in disease initiation at low levels of inoculum. The presence of hrp/hrc genes in a broad-host-range pathogen such as E. chrysanthemi is intriguing as the native effectors traveling through the type III secretion apparatus have not been identified. My work was initiated by showing that the E. chrysanthemi harpin is a substrate for the type III secretion apparatus, whose expression is regulated by HrpL and is exported to the bacterial surface in a hrp-dependent manner. I completed the sequence of the E. chrysanthemi hrp/hrc genes and continued sequencing the flanking regions with the expectation of finding other potential effectors that travel through the Hrp system. Characterization of the regions flanking the hrp/hrc genes in other plant pathogenic bacteria has shown that these regions are rich in effectors. However, sequencing the regions flanking the hrp/hrc genes in E. chrysanthemi did not reveal any easily definable type III effector, rather ORFs apparently not related to type III secretion. The function of most of those ORFs in E. chrysanthemi remains to be elucidated but at least one, ORF11, encodes a virulence factor. Finally, another gene previously found in the left flanking region, hecA, was characterized in this work and shown to be a virulence factor that is necessary for the bacterium to attach to the surface of axenically grown N. clevelandii seedlings and to promote bacterial aggregation that precedes the pectolytic attack.
