Studies of the mode of action of Bacillus subtilis culture filtrates in the model pathosystem tomato seedling - Fusarium oxysporum f sp. Radicis-Lycopersici

Studies of Bacillus subtilis isolates (FZB Biotechnik GmbH Berlin collection) have shown a broad range of mechanisms leading to the production of bacterial metabolites which induce phytosanitary effects. These effects are two-fold, proceeding first via the host plant by promoting growth and health, and second by antibiotic effects against pathogens. Both pro- and antibiotic effects are demonstrated experimentally with sterile culture filtrates (CF) of two B. subtilis isolates (FZB C and G) from different bacterial fermentation phases in the model pathosystem chosen: axenic seedling cultures of tomato and Fusarium oxysporum f. sp. radicislycopersici. Concentrations of 0.1 and 1 % CFs were tested. First, the CFs (0.1 %) of the logarithmic- and transition- phases were added to a defined nutrient solution and were found to stimulate the root growth of tomato seedlings. The 0.1 % CFs from the stationary fermentation phase were found to have no growth-promoting effect. Furthermore, we believe that substances with phytohormone activity in these filtrates have been responsible for the observed effects. Some culture filtrates not only stimulated plant growth, but also promoted plant tolerance towards the Fusarium test pathogen. The highest plant tolerance was observed after pre-cultivation in the trans.- phase filtrates from both concentrations. On the other hand, direct contact with 1 % CFs (especially from the stationary fermentation phase) with the test pathogen F. oxysporum led to an antibiotic, inhibitory effect on the pathogen. This is caused mainly by the presence of bacterial-borne peptide antibiotics in the filtrates. Culture filtrates from the transition phase were also fractionated by high-performance liquid chromatography and tested on the same pathosystem. Seedlings pretreated with certain HPLC fractions showed significant promotion of root growth and induced tolerance towards pathogen attack. We conclude that the observed phytosanitary effects of the rhizobacteria Bacillus subtilis are based on a complex mode of actions along two different lines - one probiotic, the second antibiotic.