TRIFOLITOXIN PRODUCTION INCREASES NODULATION COMPETITIVENESS OF RHIZOBIUM-ETLI CE3 UNDER AGRICULTURAL CONDITIONS

A major barrier to the use of nitrogen-fixing inoculum strains for the enhancement of legume productivity is the inability of commercially a vailable strains to compete with indigenous rhizobia for nodule format ion, Despite extensive research on nodulation competitiveness, there a re no examples of field efficacy studies of strains that have been gen etically improved for nodulation competitiveness. We have shown previo usly that production of the peptide antibiotic trifolitoxin (TFX) by R hizobium etli results in significantly increased nodule occupancy valu es in nonsterile soil in growth chamber experiments (E. A, Robleto, A. J. Scupham, and E. W, Triplett, Mel. Plant-Microbe Interact, 10:228-2 33, 1997), To determine whether TFX production by Rhizobium etli incre ases nodulation competitiveness in held-grown plants, seeds of Phaseol us vulgaris were inoculated with mixtures of Rhizobium etli strains at different ratios. The three nearly isogenic inoculum strains used inc luded TFX-producing and non-TFX-producing strains, as well as a TFX-se nsitive reference strain. Data was obtained over 2 years for nodule oc cupancy and over 3 years for assessment of the effect of the TFX produ ction phenotype on grain yield. In comparable mixtures in which the te st strain accounted for between 5 and 50% of the inoculum, the TFX-pro ducing strain exhibited at least 20% greater nodule occupancy than the non-TFX-producing strain in both years. The TFX production phenotype had no effect on grain yield over 3 years; the average yields reached 2,400 kg/ha, These results show that addition of the TFX production ph enotype significantly increases nodule occupancy under field condition s without adverse effects on grain yield. As we used common inoculatio n methods in this work, there are no practical barriers to the commerc ial adoption of the TFX system for agriculture.