Alginate gene expression by Pseudomonas syringae pv. tomato DC3000 in host and non-host plants

Abstract

Pseudomonas syringae produces the exopolysaccharide alginate, a copolymer of mannuronic and guluronic acid. Although alginate has been isolated from plants infected by P. syringae, the signals and timing of alginate gene expression in planta have not been described. In this study, an algD : : uidA transcriptional fusion, designated pDCalgDP, was constructed and used to monitor alginate gene expression in host and non-host plants inoculated with P. syringae pv. tomato DC3000. When leaves of susceptible collard plants were spray-inoculated with DC3000(pDCalgDP), algD was activated within 72 h post-inoculation (p.i.) and was associated with the development of water-soaked lesions. In leaves of the susceptible tomato cv. Rio Grande-PtoS, algD activity was lower than in collard and was not associated with water-soaking. The expression of algD was also monitored in leaves of tomato cv. Rio Grande-PtoR, which is resistant to P. syringae pv. tomato DC3000. Within 12 h p.i., a microscopic hypersensitive response (micro-HR) was observed in Rio Grande-PtoR leaves spray-inoculated with P. syringae pv. tomato DC3000(pDCalgDP). As the HR progressed, histochemical staining indicated that individual bacterial cells on the surface of resistant tomato leaves were expressing algD. These results indicate that algD is expressed in both susceptible (e.g. collard, tomato) and resistant (Rio Grande-PtoR) host plants. The expression of algD in an incompatible host-pathogen interaction was further explored by monitoring transcriptional activity in leaves of tobacco, which is not a host for P. syringae pv. tomato. In tobacco inoculated with DC3000(pDCalgDP), an HR was evident within 12 h p.i., and algD expression was evident within 8-12 h p.i. However, when tobacco was inoculated with an hrcC mutant of DC3000, the HR did not occur and algD expression was substantially lower. These results suggest that signals that precede the HR may stimulate alginate gene expression in P. syringae. Histochemical staining with nitro blue tetrazolium indicated that the superoxide anion (O 2 2 ) is a signal for algD activation in planta. This study indicates that algD is expressed when P. syringae attempts to colonize both susceptible and resistant plant hosts.

The Pseudomonas syringae Genome Encodes a Combined Mannuronan C-5-epimerase and O-Acetylhydrolase, Which Strongly Enhances the Predicted Gel-forming Properties of Alginates

“…Consistent with this, PG4180 produced optimal levels of the virulence factor coronatine at 18°C and negligible amounts at 28°C (27). In addition to coronatine, alginate is produced as a loosely attached capsule by many P. syringae strains, and the production seems to be correlated with virulence (28,29). Although PG4180 also produces alginate (30), temperature-dependent production of alginate has not been reported for this strain.…”

Alginate Production by Pseudomonas putida Creates a Hydrated Microenvironment and Contributes to Biofilm Architecture and Stress Tolerance under Water-Limiting Conditions

“…Interestingly, evidence of alginate gene expression by P. syringae pv. tomato DC3000 in planta (22) preceding the hypersensitive response (HR) coincides with inhibitory levels of water stress during the HR (50). It is conceivable that in addition to or coincident with reactive oxygen species-mediated induction of alginate synthesis in planta, matric (water limitation) stress occurs during the initial stages of the HR.…”

“…Despite the nearly universal ability of Pseudomonas species to produce alginate, a class of polymers comprising the uronic acids D-mannuronic and/or its epimer L-guluronic acid assembled into ␤-1,4-linked blocks that can be O acetylated (37,44), the exact benefits of alginate production have been elusive. Potential functions have been indicated by examining alginate biosynthesis structural (22,53) or regulatory (39,49) mutants or by identifying environmental factors influencing the expression of alginate genes (9,21,34,46). Pseudomonas aeruginosa and Pseudomonas syringae share conserved signals, including elevated osmolarity and elevated levels of reactive oxygen species, for activating transcription from alginate promoters, yet neither requires alginate for biofilm formation in flowthrough systems (25,49).…”

Virulence strategies of phytopathogenic bacteria and their role in plant disease pathogenesis

“…Several plant pathogenic bacteria have been considered as biofilm producer as virulence factors including X. compestris (Dow et al, 2003) and P. syringae (Keith et al, 2003).…”