Zinc biosorption, biochemical and molecular characterization of plant growth-promoting zinc-tolerant bacteria

Zinc plays a key role in plant nutrition at low levels; however, at higher concentrations Zn ions can be highly phytotoxic and plant growth-promoting rhizobacteria can be used to reduce such metal toxicity. In the present investigation we had reported the zinc biosorption and molecular characterization of plant growth-promoting zinc-tolerant bacteria. Initially, thirty bacteria having zinc solubilizing ability were screened for MIC against zinc ion and displayed high value of MIC ranging from 2.5 to 62.5 mM. Biochemically, all the 30 isolates showed significant difference in the 6 biochemical tests performed. The molecular diversity studies based on the repetitive DNA PCR viz, REP, ERIC and BOX elements showed significant genetic diversity among these 30 zinc-tolerant bacteria. These ZTB strains also showed multiple PGP activities and all ZTB strains were found positive for production of IAA, GA(3) and ammonia, whereas 24 were found positive for ACC deaminase activity, 8 showed siderophore production and 9 ZTB isolates were positive for HCN production. Out of 30 isolates, 24 showed phosphorus solubilization activity, 30 showed potash solubilization, 15 showed silica solubilization and 27 showed phytase production activities. All the 30 ZTB stains showed zinc solubilization up to 0.25% insoluble ZnO in the medium, whereas at 2% ZnO in MSM only 12 isolates showed solubilization which were further selected for zinc biosorption and pot studies. The heavy metal removal studies revealed that ZTB stains were able to remove zinc ions effectively from the medium efficiently and the highest zinc biosorption (< 90%) was recorded with the bacterial strain Z-15. Further, the inoculation of ZTB strains under zinc stress conditions (pot containing 1000 mg/kg Zn) resulted in significant increase of shoot length, root length and total chlorophyll content in maize seedlings compared with the uninoculated control. The partial 16S rDNA sequence of the potential ZTB isolates viz. Z-15, Z-24, Z-28 and Z-29 revealed their identity as Serratia sp. The ability of these zinc-tolerant bacteria to tolerate the toxic level of zinc may serve as suitable candidates for developing microbial formulations for the growth of crop plants in Zn-contaminated areas.