Mutation of a degS homologue in Enterobacter cloacae decreases colonization and biological control of damping-off on cucumber.

We have been using mutagenesis to determine how biocontrol bacteria such as Enterobacter cloacae 501R3 deal with complex nutritional environments found in association with plants. E. cloacae C10, a mutant of 501R3 with a transposon insertion in degS, was diminished in growth on synthetic cucumber root exudate (SRE), colonization of cucumber seed and roots, and control of damping-off of cucumber caused by Pythium ultimum. DegS, a periplasmic serine protease in the closely related bacterium Escherichia coli K12, is required for the RpoE-mediated stress response. C10 containing wild-type degS from 501R3 or from E. coli K12 on pBeloBAC11 was significantly increased in growth on SRE, colonization of cucumber roots, and control of P. ultimum relative to C10 containing pBeloBAC11 alone. C10 and 501R3 were similar in sensitivity to acidic conditions, plant-derived phenolic compounds, oxidative stress caused by hydrogen peroxide, dessication, and high osmoticum; stress conditions potentially associated with plants. This study demonstrates a role for degS in the spermosphere and rhizosphere during colonization and disease control by Enterobacter cloacae. This study implicates, for the first time, the involvement of DegS and, by extension, the RpoE-mediated stress response, in reducing stress on E. cloacae resulting from the complex nutritional environments in the spermosphere and rhizosphere.

Role of sdhA and pfkA and catabolism of reduced carbon during colonization of cucumber roots by Enterobacter cloacae.

We have been using a mutational approach to determine how plant-beneficial bacteria such as Enterobacter cloacae 501R3 obtain carbon and energy for colonization of subterranean portions of cucumber and other plants. Reduced carbon detected in cucumber root exudate consisted of 73.3 % amino acids, 22.2 % organic acids and 4.4 % carbohydrate. Ent. cloacae M2, a mini-Tn5 Km transposon mutant of strain 501R3, was severely reduced in in vitro growth relative to strain 501R3 on the mixture of amino acids and organic acids detected in cucumber root exudate when these compounds were supplied as the sole source of carbon and energy, but was similar in growth on the mixture of carbohydrates detected in this exudate. Molecular and biochemical characterization of Ent. cloacae M2 indicated that the transposon was inserted in sdhA, which encodes a subunit of succinate dehydrogenase. Ent. cloacae A-11, a mutant of strain 501R3 with a mini-Tn5 Km insertion in pfkA, was severely reduced in in vitro growth relative to strain 501R3 on the mixture of carbohydrates detected in cucumber root exudate, but similar in growth on the mixture of amino acids and organic acids. When strains A-11 and M2 were coapplied with strain 501R3 to cucumber seeds above carrying capacity in competitive root colonization assays, populations of strains A-11 and M2 were roughly one order of magnitude lower than those of strain 501R3 in cucumber rhizosphere, while populations of strains A-11 and M2 were similar to one other when coapplied to cucumber seeds. When Ent. cloacae strains were coapplied to cucumber seeds below carrying capacity, populations of A-11 and M2 were roughly two to three orders of magnitude lower than those of 501R3 in cucumber rhizosphere, and populations of A-11 were significantly lower than those of M2 when these two strains were coapplied to cucumber seed. The experiments reported here indicate an important role for pfkA and sdhA and the catabolism of carbohydrates, and of amino acids and organic acids, respectively, in the colonization of cucumber roots by Ent. cloacae. The results reported here also indicate that catabolism of carbohydrates by this bacterium is more important than catabolism of amino acids and organic acids at lower population densities, despite the much higher relative quantities of amino acids and organic acids detected in cucumber root exudate.

Mutation in cyaA in Enterobacter cloacae decreases cucumber root colonization.

Strains of Enterobacter cloacae show promise as biological control agents for Pythium ultimum-induced damping-off on cucumber and other crops. Enterobacter cloacae M59 is a mini-Tn5 Km transposon mutant of strain 501R3. Populations of M59 were significantly lower on cucumber roots and decreased much more rapidly than those of strain 501R3 with increasing distance from the soil line. Strain M59 was decreased or deficient in growth and chemotaxis on most individual compounds detected in cucumber root exudate and on a synthetic cucumber root exudate medium. Strain M59 was also slightly less acid resistant than strain 501R3. Molecular characterization of strain M59 demonstrated that mini-Tn5 Km was inserted in cyaA, which encodes adenylate cyclase. Adenylate cyclase catalyzes the formation of cAMP and cAMP levels in cell lysates from strain M59 were approximately 2% those of strain 501R3. Addition of exogenous, nonphysiological concentrations of cAMP to strain M59 restored growth (1 mM) and chemotaxis (5 mM) on synthetic cucumber root exudate and increased cucumber seedling colonization (5 mM) by this strain without serving as a source of reduced carbon, nitrogen, or phosphorous. These results demonstrate a role for cyaA in colonization of cucumber roots by Enterobacter cloacae.

United States Department of Agriculture-Agricultural Research Service research programs in biological control of plant diseases.

A number of USDA-ARS programs directed at overcoming impediments to the use of biocontrol agents on a commercial scale are described. These include improvements in screening techniques, taxonomic studies to identify beneficial strains more precisely, and studies on various aspects of the large-scale production of biocontrol agents. Another broad area of studies covers the ecological aspects of biocontrol agents-their interaction with the pathogen, with the plant and with other aspects of the environmental complex. Examples of these studies are given and their relevance to the further development and expansion of biocontrol agents is discussed.

Mutation of rpiA in Enterobacter cloacae decreases seed and root colonization and biocontrol of damping-off caused by Pythium ultimum on cucumber.

Strains of Enterobacter cloacae show promise as biocontrol agents for Pythium ultimum-induced damping-off on cucumber and other crops. E. cloacae A145 is a mini-Tn5 Km transposon mutant of strain 501R3 that was significantly reduced in suppression of damping-off on cucumber caused by P. ultimum. Strain A145 was deficient in colonization of cucumber, sunflower, and wheat seeds and significantly reduced in colonization of corn and cowpea seeds relative to strain 501R3. Populations of strain A145 were also significantly lower than those of strain 501R3 at all sampling times in cucumber, wheat, and sunflower rhizosphere. Populations of strain A145 were not detectable in any rhizosphere after 42 days, while populations of strain 501R3 remained at substantial levels throughout all experiments. Molecular characterization of strain A145 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 rpiA, which encodes ribose-5-phosphate isomerase. In Escherichia coli, RpiA catalyzes the interconversion of ribose-5-phosphate and ribulose-5-phosphate and is a key enzyme in the pentose phosphate pathway. Ribose-5-phosphate isomerase activity in cell lysates from strain A145 was approximately 3.5% of that from strain 501R3. In addition, strain A145 was a ribose auxotroph, as expected for an rpiA mutant. Introduction of a 1.0-kb DNA fragment containing only the rpiA homologue into strain A145 restored ribose phosphate isomerase activity, prototrophy, seedling colonization, and disease suppression to levels similar to those associated with strain 501R3. Experiments reported here indicate a key role for rpiA and possibly the pentose phosphate pathway in suppression of damping-off and colonization of subterranean portions of plants by E. cloacae.