Isolation and identification by 16S rRNA sequence analysis of Achromobacter, Azospirillum and Rhodococcus strains from the Rhizosphere of maize and screening for the beneficial effect on plant growth

The aim of the present study was to isolate and characterize plant growth promoting rhizobacteria from the rhizosphere of field-grown maize. Among the five bacterial isolates obtained in the present study, the isolates M1, M7 and ZN1 showed typical Azospirillum-like cell morphology, motility and high sequence similarity of 16S rDNA to the genus Azospirillum. The isolates M25 and M28 were identified on the bases of 16S rRNA sequence analysis as Achromobacter and Rhodococcus, respectively. All the five isolates obtained in the present study produced phytohormone IAA in pure culture but phosphate-solubilization activity was detected only in pure cultures of Achromobacter strain M25 and Rhodococcus strain M28. The isolates were tested as inoculants for maize seedlings grown in sterilized sand from which cotyledons were removed at an early growth stage to deprive the seedlings of internal nutrient source and harvested after 5 weeks. For comparison, seedlings with intact cotyledons were used as control. In addition inoculated plants were grown in non-sterilized soil in earthen pots and harvested at maturity. In both the short-term inoculation experiments carried out in sterilized sand as well as in long-term inoculation experiment in earthen pots all the inoculated strains improved plant growth compared to respective non-inoculated controls. However, unlike the seedlings with intact cotyledons, the pattern regarding the efficiency of growth promotion by inoculated strains (Azospirillum strains ZN1 greater than M7 greater than M1 greater than Achromobacter strain M25 greater than Rhodococcus strain M28) was the same in plants with detached cotyledons and the plants grown to maturity in non-sterilized soil. It can be concluded from the short-term experiments under controlled conditions that removal of cotyledons helps effective screening of bacterial inocula compared to seedlings with intact cotyledons.