Inactivation of seed exudate stimulants of Pythium ultimum sporangium germination by biocontrol strains of Enterobacter cloacae and other seed-associated bacteria

Strains of Enterobacter cloacae and other seed-associated bacteria, effective in suppressing Pythium seed rot and damping-off, were tested for their ability to reduce the stimulatory activity (STA) of plant seed exudates to sporangia of the seed-rotting fungus, Pythium ultimum. Our results have shown that E. cloacae can utilize seed exudate from a number of plant species as a sole carbon and energy source and, at the same time, rapidly reduce the STA of exudate to P. ultimum sporangia. At higher cell densities of E. cloacae, the STA was inactivated more rapidly than at lower cell densities. Concentrates prepared from E. cloacae-treated exudate, when added to untreated exudate, did not reduce STA as compared with unamended exudate, indicating that sporangium germination inhibitors are not produced by E. cloacae during growth on cotton seed exudate. Bacteria, consisting largely of Pseudomonas species, were isolated from seeds of various plant species. Many of the seed-associated bacterial strains could reduce STA within 24 h to amounts supporting germination of less than 20% of P. ultimum sporangia. Four of 21 strains were as effective as E. cloacae strain EcCT-501 in reducing STA. When tested for their ability to inactivate STA of linoleic acid (a predominant stimulant found in cotton seed exudate), only one strain was as effective as E. cloacae strain EcCT-501. A positive correlation (correlation coefficient=0.507) was observed between levels of linoleic acid STA inactivation and of cotton seed exudate STA inactivation by seed-associated bacteria other than EcCT-501. Furthermore, a negative correlation (correlation coefficient=−0.423) was observed between the amounts of linoleic acid STA inactivation and seedling stands following seed treatment with these bacterial strains. However, a poor correlation (correlation coefficient=0.270) was found between the inactivation of exudate STA and seedling stands with seed-associated bacteria other than EcCT-501. Our results are consistent with the hypothesis that E. cloacae strain EcCT-501 inactivates STA of seed exudates by metabolizing the active stimulatory molecules, including linoleic acid, present in the exudate and that this trait is important in biological control of Pythium seed rot by this strain. However, other traits may function among other genera and species of seed-associated bacteria