Submitted to: Phytopathology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/2/2010
Publication Date: 2/1/2011
Citation: Roberts, D.P., Lohrke, S.M., Mckenna, L.F., Lakshman, D.K., Kong, H.N., Lydon, J. 2011. Mutation of a degS homologue in Enterobacter cloacae decreases colonization and biological control of damping-off caused by Pythium ultimum on cucumber. Microbiology. 101:271-280. Interpretive Summary: Soilborne plant pathogens, such as Oomycetes (Pythium ultimum) and fungi, cause diseases that result in major economic losses to farmers in the United States. Biological control measures for these diseases need to be developed due to environmental problems associated with existing chemical controls. However, little is known regarding the genetic and biochemical means by which biological control agents grow on seeds and roots of plants and suppress pathogens that infect these seeds and roots. Growth on plant parts and suppression of pathogen infection of plants are thought to be important for biological control. In this study we determined the role of the degS gene of the biocontrol bacterium Enterobacter cloacae in growth and suppression of infection of the important seed rot pathogen Pythium ultimum. This gene was important for growth on cucumber seeds and roots and played a role in suppression of cucumber seed rot caused by Pythium ultimum. This information will be useful to scientists devising strategies to improve biological control through enhancement of growth on seeds or of disease suppression.
Technical Abstract: We have been using a mutational approach to determine how plant-beneficial bacteria, such as Enterobacter cloacae, deal with complex nutritional environments found in association with subterranean plant parts during colonization and disease suppression. E. cloacae C10, a mini-Tn5 Km mutant of E. cloacae 501R3, was reduced in growth on a synthetic cucumber root exudate, in colonization of cucumber seeds and roots, and in suppression of damping-off of cucumber caused by Pythium ultimum. Characterization of C10 demonstrated that mini-Tn5 Km was inserted in a gene in C10 with a high degree of DNA sequence similarity to degS. DegS is a periplasmic serine protease in the closely related bacterium Escherichia coli that is required for RpoE-mediated gene expression in response to stress in the cell envelope (periplasm and outer membrane). Strain C10 containing wild-type degS from E. cloacae 501R3 or from E. coli on pBeloBAC11 was significantly increased in growth on the synthetic root exudate, in colonization of cucumber roots, and in suppression of P. ultimum relative to strain C10 containing pBeloBAC11 alone. Further investigation suggested that reduced growth on the synthetic root exudate was due to lower rates of growth on certain reduced carbon compounds within this medium which rely on periplasmic binding proteins or outer membrane proteins for uptake into oftlinethe cell in the closely related bacterium E. coli and/or Enterobacter sp. 638. Strain C10 was not increased in sensitivity to acidic conditions, plant-derived phenolic compounds, oxidative stress caused by hydrogen peroxide, dessication, or high osmoticum relative to 501R3; stress conditions potentially found in environments in association with seeds and roots that could damage the bacterial cell envelope. This study demonstrates a role for degS in colonization and in disease suppression by E. cloacae. This study suggests that the complex nutritional environments encountered by E. cloacae in the spermosphere and rhizosphere cause extracytoplasmic stress which is typically alleviated by DegS and, by extension, the RpoE-mediated stress response.