Biochemical, Genetic, and Zoosporicidal Properties of Cyclic Lipopeptide Surfactants Produced by <i>Pseudomonas fluorescens</i>

TLDR

Zoospores are key agents in oomycete and zoosporic fungal infections of plants and animals. The study isolated six fluorescent Pseudomonas strains from wheat rhizosphere that exhibit zoosporicidal activity. Transposon mutagenesis and HPLC fractionation revealed that surfactant production by strain SS101, mediated by peptide‑synthetase condensation domains, is essential for zoosporicidal activity. The cyclic lipopeptide surfactant from Pseudomonas fluorescens SS101 rapidly immobilizes and lyses oomycete zoospores, reduces water surface tension to 30 mN m⁻¹, protects plants from Pythium root rot, and has a critical micelle concentration of 25 µg ml⁻¹.

Abstract

ABSTRACT Zoospores play an important role in the infection of plant and animal hosts by oomycetes and other zoosporic fungi. In this study, six fluorescent Pseudomonas isolates with zoosporicidal activities were obtained from the wheat rhizosphere. Zoospores of multiple oomycetes, including Pythium species, Albugo candida , and Phytophthora infestans , were rendered immotile within 30 s of exposure to cell suspensions or cell culture supernatants of the six isolates, and subsequent lysis occurred within 60 s. The representative strain SS101, identified as Pseudomonas fluorescens biovar II, reduced the surface tension of water from 73 to 30 mN m −1 . The application of cell suspensions of strain SS101 to soil or hyacinth bulbs provided significant protection against root rot caused by Pythium intermedium . Five Tn 5 mutants of strain SS101lacked the abilities to reduce the surface tension of water and to cause lysis of zoospores. Genetic characterization of two surfactant-deficient mutants showed that the transposons had integrated into condensation domains of peptide synthetases. A partially purified extract from strain SS101 reduced the surface tension of water to 30 mN m −1 and reached the critical micelle concentration at 25 μg ml −1 . Reverse-phase high-performance liquid chromatography yielded eight different fractions, five of which had surface activity and caused lysis of zoospores. Mass spectrometry and nuclear magnetic resonance analyses allowed the identification of the main constituent as a cyclic lipopeptide (1,139 Da) containing nine amino acids and a 10-carbon hydroxy fatty acid. The other four zoosporicidal fractions were closely related to the main constituent, with molecular massesranging from 1,111 to 1,169 Da.