Characterization of novel plant growth promoting endophytic bacterium Achromobacter xylosoxidans from wheat plant

Nine diazotrophic bacteria were isolated from surface-sterilized roots and culms of wheat variety Malviya-234, which is grown with very low or no inputs of nitrogen fertilizer. Out of the nine bacteria, four showed indole acetic acid (IAA) production, and five were positive for P solubilization. One isolate, WM234C-3, showed appreciable level of nitrogenase activity, IAA production, and P solubilization ability, and was further characterized with a view to exploiting its plant growth promoting activity. Based on 16S rDNA sequence analysis, this isolate was identified as Achromobacter xylosoxidans. Diazotrophic nature of this particular isolate was confirmed by Western blot analysis of dinitrogenase reductase and amplification of nifH. Analysis of the nifH sequence showed close homology with typical diazotrophic bacteria. Endophytic nature and cross-infection ability of WM234C-3 were tested by molecular tagging with gusA fused to a constitutive promoter followed by inoculation onto rice seedlings in axenic conditions. At 21 days after inoculation, the roots showed blue staining, the most intense color being at the emergence of lateral roots and root tips. Microscopic observation confirmed colonization of gus-tagged WM234C-3 in the intercellular spaces of cortical as well as vascular zones of roots. Inoculation of gus-tagged WM234C-3 to rice plants resulted in significant increase in root/shoot length, fresh weight, and chlorophyll a content. Plant growth promoting features coupled with cross-infection ability suggest that this endophytic bacterium may be exploited as agricultural agent for various crops after a thorough and critical pathogenicity test.