Submitted to: Phytopathology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/22/2007
Publication Date: 9/18/2007
Citation: Mazzola, M., Zhao, X., Cohen, M., Raaijmakers, J. 2007. Cyclic lipopeptide surfactant production by Pseudomonas fluorescens SS101 is not required for suppression of complex Pythium spp. populations. Phytopathology. 97(10):1348-1355. Interpretive Summary: Pythium spp. cause serious root diseases of numerous crop plants. In certain systems, the bacterium Pseudomonas fluorescens strain SS101 previously was shown to protect plants from infection by these pathogens and did so by production of a cyclic lipopeptide termed massetolide A. This compound has surfactant properties which cause lysis of Pythium zoospores, thus limiting the number of infective propagules. In this study, SS101 and its surfactant-deficient mutant, strain 10.24, were found to control Pythium root rot of apple and wheat in a soil artificially enhanced with Pythium spp. by application of a seed meal amendment. Pythium spp. populations indigenous to the two soils employed were composed primarily of P. irregulare, P. sylvaticum and P. ultimum var. ultimum. These Pythium species either do not or rarely produce zoospores, which could account for the observation that both SS101 and mutant 10.24 were equally effective in disease control. Disease control did not result from the induction of plant resistance responses. Growth of Pythium spp. was limited by the surfactant, but sensitivity to this compound varied among the different Pythium species. Collectively, the results showed that i) P. fluorescens SS101 is very effective in controlling diverse Pythium populations on different crops grown in different soils, and ii) production of the cyclic lipopeptide massetolide A does not play a significant role in disease suppression. Other, as yet undefined mechanisms appear to play a significant role in the interaction between P. fluorescens SS101 and soil-borne Pythium spp. communities.
Technical Abstract: Previously, zoosporicidal activity and control of Pythium intermedium by Pseudomonas fluorescens strain SS101 was attributed, in part, to the production of the cyclic lipopeptide surfactant massetolide A. In the current study, capacity of SS101 and its surfactant-deficient mutant strain 10.24 to suppress populations and root infection by complex indigenous Pythium spp. communities was assessed. Bacterial strains were applied individually to soils and planted to apple or wheat. Both strains became established in the rhizosphere at similar populations and were maintained at similar levels over the course of each study. Likewise, both strains effectively suppressed resident Pythium spp. populations to an equivalent level, and did so under soil conditions that were extremely favorable to populations of Pythium resident to the test soils. When split-root plant assays were employed, neither strain suppressed Pythium spp. infection of the component of the root system physically divided from the bacterium. SS101 did not significantly inhibit in vitro growth of any Pythium sp. recovered in this study. Pythium spp. did exhibit sensitivity to the purified surfactant in vitro, and differential levels of sensitivity were observed among the different species of Pythium examined. Pythium populations indigenous to the two soils employed were composed primarily of P. irregulare, P. sylvaticum and P. ultimum var. ultimum. These Pythium species either do not or rarely produce zoospores, which could account for the observation that both strain SS101 and its surfactant-deficient mutant were equally effective in disease control. Other, possibly novel mechanisms or metabolites appear to be responsible for the control of Pythium by SS101.