Identification and Characterization of Auxin Producing Rhizobacteria Associated with Wheat Biomass.

Some bacteria have the ability to impact plant growth by producing auxin, a phytohormone synthesized by both plants and bacteria. Our objective was to characterize the association between auxin production by rhizobacteria and biomass productivity. Here, we have isolated 1019 rhizobacteria from high, medium and low shoot biomass plants. All isolates were cultured in the presence or absence of L-tryptophan (0.1%) and the supernatants tested for auxin using a microplate adapted PC assay. DNA was extracted using beadbeating and purified by phenol/chloroform/isoamyl alcohol. The 16S rRNA genes were PCR amplified, sequenced, and classified via the RDP II Bayesian Classifier. We found that 99% of all isolates produced varying levels of auxin. Auxin production ranged from 0.34 to 266 ?g/ml. Adding L-tryptophan to the growth media significantly increased auxin production by an average of 2.9-fold. High biomass rhizospheres tended to have larger numbers of low or no auxin producing isolates (+ L-tryptophan) while low biomass plants had larger numbers of high auxin producing isolates (> 40 ug/ml). There was a highly significant negative nonlinear correlation showing a power relationship between biomass production and numbers of high level auxin producers (R2= 0.999). Inclusion of L-tryptophan in the growth media induced auxin production up to 244-fold. Thirty nine percent of all isolates were induced to produce at least 4-fold more auxin by L-tryptophan. Thirty seven isolates showed reduced auxin production in the presence of L-tryptophan, some as high as 40-fold. In conclusion, high levels of auxin production appear to be associated with deleterious effects on plant growth. The association of specific auxin producing rhizobacteria with biomass productivity will be verified using a plant growth promotion assay.