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Flagella-Driven Chemotaxis Towards Exudate Components Is an Important Trait for Tomato Root Colonization by Pseudomonas fluorescens

November 2002 , Volume 15 , Number  11
Pages  1,173 - 1,180

Sandra de Weert , 1 Hans Vermeiren , 2 Ine H. M. Mulders , 1 Irene Kuiper , 1 Nico Hendrickx , 2 Guido V. Bloemberg , 1 Jos Vanderleyden , 2 René De Mot , 2 and Ben J. J. Lugtenberg 1

1Leiden University, Institute of Molecular Plant Sciences, Wassenaarseweg 64, 2333 AL Leiden, The Netherlands; 2Catholic University of Leuven, Centre of Microbial and Plant Genetics, Kasteelpark Arenberg 20, B-3001 Heverlee, Belgium

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Accepted 17 July 2002.

Motility is a major trait for competitive tomato root-tip colonization by Pseudomonas fluorescens. To test the hypothesis that this role of motility is based on chemotaxis toward exudate components, cheA mutants that were defective in flagella-driven chemotaxis but retained motility were constructed in four P. fluorescens strains. After inoculation of seedlings with a 1:1 mixture of wild-type and nonmotile mutants all mutants had a strongly reduced competitive root colonizing ability after 7 days of plant growth, both in a gnotobiotic sand system as well as in non-sterile potting soil. The differences were significant on all root parts and increased from root base to root tip. Significant differences at the root tip could already be detected after 2 to 3 days. These experiments show that chemotaxis is an important competitive colonization trait. The best competitive root-tip colonizer, strain WCS365, was tested for chemotaxis toward tomato root exudate and its major identified components. A chemotactic response was detected toward root exudate, some organic acids, and some amino acids from this exudate but not toward its sugars. Comparison of the minimal concentrations required for a chemotactic response with concentrations estimated for exudates suggested that malic acid and citric acid are among major chemo-attractants for P. fluorescens WCS365 cells in the tomato rhizosphere.

© 2002 The American Phytopathological Society