Submitted to: American Society for Microbiology
Publication Type: Abstract Only
Publication Acceptance Date: May 15, 2003
Publication Date: N/A
Strains of Enterobacter cloacae show promise as biocontrol agents for Pythium ultimum-induced damping-off on cucumber and other crops. E. cloacae strain C10 is a mini-Tn5 Km transposon mutant of strain 501R3. Strain C10 was reduced in colonization of cucumber, sunflower, and pea seeds, but unaffected in colonization of wheat and cotton seeds relative to strain 501R3 in natural soil. Populations of strain C10 were also significantly lower than those of strain 501R3 at all sampling times in cucumber and sunflower rhizosphere in long-term colonization assays carried out in natural soil. In contrast, populations of these two strains were similar in pea, cotton and wheat rhizospheres. No significant difference was detected between strains 501R3 and C10 in suppression of damping-off of cucumber caused by Pythium ultimum. Molecular characterization of strain C10 indicated mini-Tn5 Km was inserted in a region of the E. cloacae genome with a high degree of DNA and amino acid sequence similarity to degS (hhoB), a putative periplasmic serine protease. In Escherichia coli, DegS is required for RpoE-mediated gene expression in response to extracytoplasmic stress, through cleavage of the anti-sigma E factor, RseA. Similar to what is observed in a degS (hhoB) E. coli mutant, no significant difference between 501R3 and C10 was observed with regard to a wide range of stress conditions known to occur in the environment including dessication, high osmolarity, heat (42oC), reactive oxygen intermediates, acidic pH, and various plant-derived phenolic compounds. However, C10 was significantly more sensitive to a combination of heat and starvation stress compared to 501R3. This study implicates a role for degS (hhoB), and by extension, the RpoE-mediated stress response in colonization of the spermospheres and rhizospheres of certain plant species.