Comparative Germination of Culture-Produced and Plant-Produced Sporangia of<i>Pythium ultimum</i>in Response to Soluble Seed Exudates and Exudate Components

2003

Self Cite

101

Leaf composts were studied for their suppressive effects on Pythium ultimum sporangium germination, cottonseed colonization, and the severity of Pythium damping-off of cotton. A focus of the work was to assess the role of fatty-acid-metabolizing microbial communities in disease suppression. Suppressiveness was expressed within the first few hours of seed germination as revealed by reduced P. ultimum sporangium germination, reduced seed colonization, and reduced damping-off in transplant experiments. These reductions were not observed when cottonseeds were sown in a conducive leaf compost. Microbial consortia recovered from the surface of cottonseeds during the first few hours of germination in suppressive compost (suppressive consortia) induced significant levels of damping-off suppression, whereas no suppression was induced by microbial consortia recovered from cottonseeds germinated in conducive compost (conducive consortia). Suppressive consortia rapidly metabolized linoleic acid, whereas conducive consortia did not. Furthermore, populations of fatty-acidmetabolizing bacteria and actinobacteria were higher in suppressive consortia than in conducive consortia. Individual bacterial isolates varied in their ability to metabolize linoleic acid and protect seedlings from damping-off. Results indicate that communities of compost-inhabiting microorganisms colonizing cottonseeds within the first few hours after sowing in a Pythium-suppressive compost play a major role in the suppression of P. ultimum sporangium germination, seed colonization, and damping-off. Results further indicate that fatty acid metabolism by these seed-colonizing bacterial consortia can explain the Pythium suppression observed.

Section: Methodsmentioning

confidence: 99%

Compost-Induced Suppression of Pythium Damping-Off Is Mediated by Fatty-Acid-Metabolizing Seed-Colonizing Microbial Communities

McKellar

2003

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101

“…Pythium ultimum P4 was routinely grown on a mineral salts medium containing 0.1% ␣-phosphatidylcholine (SMϩL). Sporangia produced on this medium mimic the behavior of sporangia produced on live plant tissue (13). Agar discs (4-mm diameter) were cut from 5-day-old cultures kept at 27°C.…”

Section: Methodsmentioning

confidence: 99%

“…Although the specific exudate elicitors from most plants are unknown, the elicitors from cottonseed exudates have been identified as long-chain unsaturated fatty acids (LCUFA), comprised largely of oleic and linoleic acids (9,10,18). Other exudate molecules such as saturated fatty acids, sugars, amino acids, and other organic acids do not elicit sporangial germination (13,14,18). Once sporangia have germinated, high levels of seed colonization and subsequent infection are observed within 24 to 48 h, leading to rapid seed and seedling death (1,2,8,9,24).…”

mentioning

confidence: 99%

Differential Interference with Pythium ultimum Sporangial Activation and Germination by Enterobacter cloacae in the Corn and Cucumber Spermospheres

Windstam

2008

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Differential protection of plants by Enterobacter cloacae was studied by investigating early sensing and response behavior of Pythium ultimum sporangia toward seeds in the presence or absence of E. cloacae. Ten percent of P. ultimum sporangia were activated within the first 30 min of exposure to cucumber seeds. In contrast, 44% of the sporangia were activated as early as 15 min after exposure to corn seeds with over 80% sporangial activation by 30 min. Germ tubes emerged from sporangia after 2.5 and 1.0 h in the cucumber and corn spermospheres, respectively. Seed application of the wild-type strain of E. cloacae (EcCT-501R3) reduced sporangial activation by 45% in the cucumber spermosphere, whereas no reduction was observed in the corn spermosphere. Fatty acid transport and degradation mutants of E. cloacae (strains EcL1 and Ec31, respectively) did not reduce sporangial activation in either of the spermospheres. Although wild-type or mutant strains of E. cloacae failed to reduce seed colonization incidence, pathogen biomass on cucumber seeds was reduced in the presence of E. cloacae strains EcCT-501R3 and Ec31 by 4 and 8 h after sowing, respectively. By 12 h, levels of P. ultimum on cucumber seeds treated with E. cloacae EcCT-501R3 did not differ from levels on noninoculated seeds. On corn seeds, P. ultimum biomass was not affected by the presence of any E. cloacae strain. When introduced after sporangial activation had occurred, E. cloacae failed to reduce P. ultimum biomass on cucumber seeds compared with that on nontreated seeds. Also, increasing numbers of sporangia used to inoculate seeds yielded increased pathogen biomass at each sampling time. This indicates a direct link between the level of seed-colonizing biomass of P. ultimum and the number of activated and germinated sporangia in the spermosphere, suggesting that E. cloacae suppresses P. ultimum seed infections by reducing sporangial activation and germination within the first 30 to 90 min after sowing.

“…Pythium ultimum isolate P4 was routinely grown on a mineral salts medium (SMϩL) amended with 0.1% soy lecithin (␣-phosphatidylcholine; Sigma, United States). Previous work demonstrated that sporangia produced on this media mimic those produced on living plant tissue (19). Six-millimeter-diameter agar discs were cut from 5-dayold cultures incubated at 27°C.…”

Section: Methodsmentioning

confidence: 99%

“…Negative controls consisted of 10 mM test sugar (180 g/100 l for both glucose and fructose and 360 g/100 l for sucrose) without LA to preclude the possibility of sugars eliciting germination responses. Previous studies have demonstrated that P. ultimum sporangia produced as described above do not germinate in response to glucose, fructose, or sucrose alone (19,20). Plates were incubated at 27°C, and each treatment had three replicates.…”

Section: Methodsmentioning

confidence: 99%

Temporal Release of Fatty Acids and Sugars in the Spermosphere: Impacts on Enterobacter cloacae -Induced Biological Control

Windstam

2008

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The aim of this study was to determine the temporal release of fatty acids and sugars from corn and cucumber seeds during the early stages of seed germination in order to establish whether sugars found in exudate can prevent exudate fatty acid degradation by Enterobacter cloacae. Both saturated (long-chain saturated fatty acids [LCSFA]) and unsaturated (long-chain unsaturated fatty acids [LCUFA]) fatty acids were detected in corn and cucumber seed exudates within 15 min after seed sowing. LCSFA and LCUFA were released at a rate of 26.1 and 6.44 ng/min/seed by corn and cucumber seeds, respectively. The unsaturated portion of the total fatty acid pool from both plant species contained primarily oleic and linoleic acids, and these fatty acids were released at a combined rate of 6.6 and 0.67 ng/min/seed from corn and cucumber, respectively. In the absence of seed exudate sugars, E. cloacae degraded linoleic acid at rates of 29 to 39 ng/min, exceeding the rate of total fatty acid release from seeds. Sugars constituted a significant percentage of corn seed exudate, accounting for 41% of the total dry seed weight. Only 5% of cucumber seed exudate was comprised of sugars. Glucose, fructose, and sucrose were the most abundant sugars present in seed exudate from both plant species. Corn seeds released a total of 137 g/seed of these three sugars within 30 min of sowing, whereas cucumber seeds released 0.83 g/seed within the same time frame. Levels of glucose, fructose, and sucrose found in corn seed exudate (90 to 342 g) reduced the rate of linoleic acid degradation by E. cloacae to 7.5 to 8.8 ng/min in the presence of either sugar, leaving sufficient concentrations of linoleic acid to activate Pythium ultimum sporangia Our results demonstrate that elevated levels of sugars in the corn spermosphere can prevent the degradation of LCUFA by E. cloacae, leading to its failure to suppress P. ultimum sporangial activation, germination, and subsequent disease development.