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Applied and Environmental Microbiology, March 2003, p. 1817-1826, Vol. 69, No. 3
0099-2240/03/$08.00+0     DOI: 10.1128/AEM.69.3.1817-1826.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.

Inactivation of gacS Does Not Affect the Competitiveness of Pseudomonas chlororaphis in the Arabidopsis thaliana Rhizosphere

Heike Schmidt-Eisenlohr,1 Astrid Gast,1 and Christian Baron1,2*

Bereich Mikrobiologie, Department Biologie I, Ludwig-Maximilians-Universität München, D-80638 Munich, Germany,1 Department of Biology, McMaster University, Hamilton, Ontario LS8 4K1, Canada2

Received 1 July 2002/ Accepted 5 December 2002

Quorum-sensing-controlled processes are considered to be important for the competitiveness of microorganisms in the rhizosphere. They affect cell-cell communication, biofilm formation, and antibiotic production, and the GacS-GacA two-component system plays a role as a key regulator. In spite of the importance of this system for the regulation of various processes, strains with a Gac- phenotype are readily recovered from natural habitats. To analyze the influence of quorum sensing and the influence of the production of the antibiotic phenazine-1-carboxamide on rhizosphere colonization by Pseudomonas chlororaphis, a gnotobiotic system based on Arabidopsis thaliana seedlings in soil was investigated. Transposon insertion mutants of P. chlororaphis isolate SPR044 carrying insertions in different genes required for the production of N-acyl-homoserine lactones and phenazine-1-carboxamide were generated. Analysis of solitary rhizosphere colonization revealed that after prolonged growth, the population of the wild type was significantly larger than that of the homoserine lactone-negative gacS mutant and that of a phenazine-1-carboxamide-overproducing strain. In cocultivation experiments, however, the population size of the gacS mutant was similar to that of the wild type after extended growth in the rhizosphere. A detailed analysis of growth kinetics was performed to explain this phenomenon. After cells grown to the stationary phase were transferred to fresh medium, the gacS mutant had a reduced lag phase, and production of the stationary-phase-specific sigma factor RpoS was strongly reduced. This may provide a relative competitive advantage in cocultures with other bacteria, because it permits faster reinitiation of growth after a change to nutrient-rich conditions. In addition, delayed entry into the stationary phase may allow more efficient nutrient utilization. Thus, GacS-GacA-regulated processes are not absolutely required for efficient rhizosphere colonization in populations containing the wild type and Gac- mutants.

* Corresponding author. Mailing address: Department of Biology, McMaster University, 1280 Main St. West, Hamilton, ON LS8 4K1, Canada. Phone: (905) 525-9140, ext. 26692. Fax: (905) 522-6066. E-mail: baronC{at}mcmaster.ca.

Applied and Environmental Microbiology, March 2003, p. 1817-1826, Vol. 69, No. 3
0099-2240/03/$08.00+0     DOI: 10.1128/AEM.69.3.1817-1826.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.



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