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United States Department of Agriculture

Agricultural Research Service

Title: Mutation in the Sdha Homologue in Enterobacter Cloacae Results in Colonization of Cucumber Roots But Does Not Affect Biocontrol of Damping-off on Cucumber Caused by Pythium Ultimum

item Lohrke, Scott
item McKenna, Laurie
item Dery, Pierre - USDA-RETIRED
item Baker, Con
item Roberts, Daniel

Submitted to: BARC Poster Day
Publication Type: Abstract Only
Publication Acceptance Date: April 18, 2002
Publication Date: N/A

Technical Abstract: Strains of Enterobacter cloacae have been shown to be effective in suppression of Pythium ultimum induced damping-off of cucumber. However, the exact mechanism(s) responsible for this disease suppression remains unclear. In addition, our knowledge of the bacterial genes involved in the colonization of plant seeds and below ground portions of plants is lacking. Identification of genes involved in seed and root colonization may allow for strategies for improved ecological fitness, survival and performance of biocontrol inoculants under production conditions. Strain M2, containing a single mini Tn5-Km insertion, was reduced in colonization of cucumber roots relative to the wild type strain 501R3 in natural soil and in soil-less mix. However, M2 was unaffected in suppression of P. ultimum induced damping-off on cucumber in experiments conducted in soil-less mix. To identify the exact site of the transposon insertion in M2, we sequenced pMB7-1, which contains a 7.0 Kb BglII fragment containing a portion of mini Tn5-Km and flanking DNA. Sequence analysis indicated that the site of insertion was in a region of the E. cloacae genome with a high degree of sequence homology to sdhA, which encodes one of four subunits of succinate dehydrogenase in Escherichia coli. Strain M2 was deficient in growth on succinate and acetate, and reduced in growth rate on oxaloacetate, malate, pyruvate, and glucose relative to strain 501R3. This growth profile is consistent with mutations in sdhA. Succinate dehydrogenase is involved in aerobic respiration and is responsible for a key metabolic step in the TCA cycle, catalyzing the conversion of succinate to fumarate. Further sequence analysis revealed the presence of open reading frames downstream of sdhA with a high degree of sequence homology to sdhB and sucA, encoding subunit B of succinate dehydrogenase and subunit E1 of 2-oxoglutarate dehydrogenase, respectively.

Last Modified: 4/22/2015
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