Submitted to: International Plant Growth Promoting Rhizobacteria Workshop
Publication Type: Proceedings
Publication Acceptance Date: August 1, 2000
Publication Date: N/A
Interpretive Summary: Biological control bacteria and other biocontrol microorganisms provide inconsistent disease suppression under commercial application conditions. This inconsistent performance is a major impediment to widespread usage of biological controls. Research was performed to determine the role of specific carbohydrates, produced by cucumber seeds, as nutrients for the biocontrol bacterium Enterobacter cloacae during suppression of damping-off of cucumber caused by the important soilborne plant pathogen, Pythium ultimum. It was determined that these carbohydrates are not absolutely required as nutrients for suppression of damping-off of cucumber by E. cloacae. This information will be useful in devising strategies to enhance biocontrol performance by E. cloacae by adjusting nutrient availability to this bacterium since certain carbohydrates released by seeds decrease disease suppression. This information is useful to scientists developing biocontrol agents for use against plant pathogens.
Enterobacter cloacae A-11 is a prototrophic, glycolytic mutant of strain 501R3 with a single mini Tn5-Km transposon insertion in pfkA (D.P. Roberts, P.D. Dery, I. Yucel, J.S. Buyer, M.A. Holtman, and D.Y. Kobayashi. 1999. Appl. Environ. Microbiol. 65: 2513-2519). Seed treatments containing strains 501R3 and A-11 provided similar and significant levels of suppression of damping-off of cucumber caused by P. ultimum performed in soil-less potting mix. Cucumber plant stands with seeds treated with strain 501R3 or A-11 were significantly greater (P is less than or equal to 0.05) than the pathogen control but less than the healthy control. However, populations of strain A-11 were significantly lower than those of strain 501R3 at 16 days and 43 days on cucumber roots when introduced into natural soil as cucumber seed treatments. Oxygen consumption experiments were performed with these bacterial strains using carbohydrates and amino acids detected in cucumber seed exudates as the sole source of reduced carbon. Strain A-11 had significantly lower oxygen consumption rates than strain 501R3 on galactose, glucose, maltose, mannitol, mannose, and trehalose. Two of these carbohydrates, glucose and mannitol, are prominent in cucumber exudates. Data from these studies are consistent with the hypothesis that the total carbohydrate catabolic capabilities of E. cloacae are not required for suppression of damping-off of cucumber caused by P. ultimum in soil-less potting mix. In contrast, pfkA and the Embden-Meyerhof pathway appear to be important for long-term persistence on cucumber roots by E. cloacae in natural soil.