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

Abstract

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.

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

“…However, not all plants are protected by E. cloacae. For example, E. cloacae is extremely effective in protecting seeds of plants such as cotton and cucumber but fails to protect seeds of plants such as pea and corn (10) because of its failure to block sporangial activation and germination (27). One possible explanation for this failure is that other exudate molecules are released into the spermosphere that regulate the degradation of LCUFA by E. cloacae.…”

“…Germination of P. ultimum sporangia in the spermosphere is initiated by molecules released from seeds within the first 15 to 30 min after sowing (27). With cotton seeds, long-chain unsaturated fatty acids (LCUFA) found in seed exudates are able to elicit this rapid sporangial germination response (24).…”

“…With cotton seeds, long-chain unsaturated fatty acids (LCUFA) found in seed exudates are able to elicit this rapid sporangial germination response (24). Rapid mycelial growth and seed colonization follow sporangial germination (27), resulting in embryo infection within 24 to 48 h (7,15,28). Both sporangial germination and seed colonization are critical stages in Pythium pathogenesis, and if either is eliminated, disease will not develop (8,12).…”

“…The mechanism by which E. cloacae achieves such control is by preventing the initiation of sporangial germination in the spermosphere within 1 h of sowing (27), thereby suppressing subsequent disease development. However, not all plants are protected by E. cloacae.…”

“…It is conceivable that exudate sugars serve as regulators of E. cloacae FA degradation in the spermosphere, explaining the differential activity of E. cloacae in corn and cucumber spermospheres. The qualitative and quantitative changes in FA and sugar exudation within the first few minutes of sowing, the most critical window for E. cloacae activity in the spermosphere (27), and the influence of exudate sugars on E. cloacae LCUFA degradation are unknown (16). Based on these observations, the current study was designed to test the following two hypotheses: (i) the rate of LCUFA release from corn seeds greatly exceeds the rate of degradation by E. cloacae, and alternatively, (ii) corn seeds release sugars at the same time as elicitor FA and at concentrations sufficient to repress E. cloacae LCUFA degradation.…”