INACTIVATION OF SEED EXUDATE STIMULANTS OF PYTHIUM-ULTIMUM SPORANGIUMGERMINATION BY BIOCONTROL STRAINS OF ENTEROBACTER-CLOACAE AND OTHER SEED-ASSOCIATED BACTERIA

Strains of Enterobacter cloacoe and other seed-associated bacteria, ef fective in suppressing Pythium seed rot and damping-off, were tested f or their ability to reduce the stimulatory activity (STA) of plant see d exudates to sporangia of the seed-rotting fungus, Pythium ultimum. O ur 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. cl oacae-treated exudate, when added to untreated exudate, did not reduce STA as compared with unamended exudate, indicating that sporangium ge rmination inhibitors are not produced by E. cloacae during growth on c otton seed exudate. Bacteria, consisting largely of Pseudomonas specie s, 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 reduc ing STA. When tested for their ability to inactivate STA of linoleic a cid (a predominant stimulant found in cotton seed exudate), only one s train was as effective as E. cloacae strain EcCT-501. A positive corre lation (correlation coefficient = 0.507) was observed between levels o f linoleic acid STA inactivation and of cotton seed exudate STA inacti vation 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 sta nds following seed treatment with these bacterial strains. However, a poor correlation (correlation coefficient = 0.270) was found between t he inactivation of exudate STA and seedling stands with seed-associate d bacteria other than EcCT-501. Our results are consistent with the hy pothesis that E. cloacae strain EcCT-501 inactivates STA of seed exuda tes by metabolizing the active stimulatory molecules, including linole ic acid, present in the exudate and that this trait is important in bi ological control of Pythium seed rot by this strain. However, other tr aits may function among other genera and species of seed-associated ba cteria (C) 1997 Elsevier Science Ltd. All rights reserved.