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Trifolitoxin Production in Rhizobium etli Strain CE3 Increases Competitiveness for Rhizosphere Colonization and Root Nodulation of Phaseolus vulgaris in Soil

The effect of trifolitoxin (TFX) production by Rhizobium etli on rhizosphere colonization and competition for nodulation in soil conditions was determined. TFX is a potent peptide antibiotic made by Rhizobium leguminosarum bv. trifolii T24 that inhibits many α-proteobacteria (E. W. Triplett, B. T. Breil, and G. A. Splitter, Appl. Environ. Microbiol. 60:4163--4166, 1994). Seeds of Phaseolus vulgaris were inoculated with a TFX-sensitive reference strain and either of two isogenic strains that differ only in their ability to produce TFX. The pair of strains were inoculated at different ratios in sterile and nonsterile soil. The representation of the strains in the rhizosphere and nodules was determined at 96 h after inoculation and 3 weeks after planting, respectively. The TFX-producing strain was significantly more competitive for both phenotypes versus the TFX-sensitive strain, compared with the TFX-nonproducing strain versus the TFX-sensitive strain. These results show that nodule occupancy by inoculant strains, often displaced from the nodules by indigenous strains, can be increased by addition of the TFX production phenotype to R. etli in plants grown in either sterile or nonsterile soil. Also, this work shows the efficacy of the TFX system for the first time on a legume host with determinant nodules.