Search

The hypersensitive response elicitor harpin (HrpN) of soft rot pathogen Erwinia chrysanthemi strains 3937 and EC16 is secreted via the type III secretion system and remains cell surface bound. Strain 3937 HrpN is essential for cell aggregation, but the C-terminal one-third of the protein is not required for aggregative activity.

Enterobacterial animal pathogens exhibit aggregative multicellular behavior, which is manifested as pellicles on the culture surface and biofilms at the surface-liquid-air interface. Pellicle formation behavior requires production of extracellular polysaccharide, cellulose, and protein filaments, known as curli. Protein filaments analogous to curli are formed by many protein secretion systems, including the type III secretion system (TTSS). Here, we demonstrate that Erwinia chrysanthemi, which does not carry curli genes, requires the TTSS for pellicle formation. These data support a model where cellulose and generic protein filaments, which consist of either curli or TTSS-secreted proteins, are required for enterobacterial aggregative multicellular behavior. Using this assay, we found that hrpY, which encodes a two-component system response regulator homolog, is required for activity of hrpS, which encodes a [[sigma].sup.54]-dependent enhancer-binding protein homolog. In turn, hrpS is required for activity of the sigma factor homolog hrpL, which activates genes encoding TTSS structural and secreted proteins. Pellicle formation was temperature dependent and pellicles did not form at 36[degrees]C, even though TTSS genes were expressed at this temperature. We found that cellulose is a component of the E. chrysanthemi pellicle but that pellicle formation still occurs in a strain with an insertion in a cellulose synthase subunit homolog. Since the TTSS, but not the cellulose synthase subunit, is required for E. chrysanthemi pellicle formation, this inexpensive assay can be used as a high throughput screen for TTSS mutants or inhibitors.

The host-specific plant pathogen Pseudomonas syringae causes the hypersensitive response (HR) in nonhost plants and secretes the HrpZ harpin in culture through the Hrp secretion system. The hrpW gene was observed in a region adjacent to the hrp cluster in P. syringae pv. tomato DC3000. hrpW encodes a 42.9-kDa protein with domains similar to harpins and pectate lyases, respectively. HrpW has the main characteristics of harpins.

The modified distribution of the HrpZ hairpin protein in Pseudomonas syringae pv. syringae 61 hrp mutants was found to allow assignment of these four operons to two experimentally identifiable stages in type III protein secretion. The hrpJ and hrpU operons encode an alternative to the Sec system for translocation across the inner membrane, while the hrpC operon is needed for translocation across the outer membrane.

Three pre-defined DNA-based methods were applied to examine the isolates from diseased stems and tubers and understand most reliable classification of isolates through the multilocus sequence analysis (MLSA) method. Hypersensitive response and DNA hybridization assays revealed that strains of both Pectobacterium carotovorum and Pectobacterium wasabiae lack a type III secretion system (T3SS) and the virulence of most of the T3SS-deficient strains are similar to that of T3SS-encoding strains in stems and tubers.

The construction of null knockout indole-3-acetic acid (iaaM) mutant strain Erwinia chrysanthemi (Ech138) is presented. Compared with wild-type Ech3937, Ech138 exhibited reduced ability to produce local maceration, but its multiplication in Saintpaulia ionantha was unaffected.

A screen is developed to identify transposon mutants that are defective in attachment to alfalfa sprouts in order to understand the physiology of Salmonella enterica serovar Newport on plants. Results reveal that Salmonella enterica genes important for virulence in animal systems are also required for colonization of plants, a secondary host that could serve as a vector of Salmonella enterica from animal to animal.

Genetic and biochemical methods are used to examine the genomic density of the enterobacterial plant pathogen Erwina carotovora subsp. Carotovora. The results obtained with each method showed that E. carotovora subsp. Carotovora strains isolated from one ecological niche, potato plants, are surprisingly diverse compared to related pathogens.

Research has been conducted on plant-pathogenic bacterium Pseudomonas syringae B728a. The virulence factor of P. syringae pv. syringae B728a exchangeable effector locus has been investigated via construction of deletion mutation, and the avirulence activity and secretion of the putative effector protein have been tested and discussed.