ALFALFA YIELD RESPONSE TO INOCULATION WITH RECOMBINANT STRAINS OF RHIZOBIUM-MELILOTI WITH AN EXTRA COPY OF DCTABD AND OR MODIFIED NIFA EXPRESSION/

The construction of rhizobial strains which increase plant biomass und er controlled conditions has been previously reported. However, there is no evidence that these newly constructed strains increase legume yi eld under agricultural conditions. This work tested the hypothesis tha t carefully manipulating expression of additional copies of nifA and d ctABD in strains of Rhizobium meliloti would increase alfalfa yield in the field. The rationale for this hypothesis is based on the positive regulatory role that nifA plays in the expression of the nif regulon- and the fact that a supply of dicarboxylic acids from the plant is req uired as a carbon and energy source for nitrogen fixation by the Rhizo bium bacteroids in the nodule. These recombinant strains, as well as t he wild-type strains from which they were derived, are ideal tools to examine the effects of modifying or increasing the expression of these genes on alfalfa biomass. The experimental design comprised seven rec ombinant strains, two wild-type strains, and an uninoculated control. Each treatment was replicated eight times and was conducted at four fi eld sites in Wisconsin. Recombinant strain RMBPC-2, which has an addit ional copy of both nifA and dctABD, increased alfalfa biomass by 12.9% compared with the yield with the wild-type strain RMBPC and 17.9% ove r that in the uninoculated control plot at the site where soil nitroge n and organic matter content was lowest. These increases were statisti cally significant at the 5% confidence interval for each of the three harvests made during the growing season. Strain RMBPC-2 did increase a lfalfa biomass at the Hancock site; however, no other significant incr eases or decreases in alfalfa biomass were observed with the seven oth er recombinant strains at that site. At three sites where this experim ent was conducted, either native rhizobial populations or soil nitroge n concentrations were high. At these sites, none of the recombinant st rains affected yield. We conclude that RMBPC-2 can increase alfalfa yi elds under field conditions of nitrogen limitation, low endogenous rhi zobial competitors, and sufficient moisture.