Enhancement in Plant Growth and Zinc Biofortification of Chickpea (Cicer arietinum L.) by Bacillus altitudinis

Zinc-solubilizing bacteria (ZSB) are potential and ideal substitutes for Zn supplement. Currently, limited reports are available on the bacteria displaying capability to transform insoluble forms of zinc (Zn) to an available form. Therefore, attempts were made to identify and characterize Zn-solubilizing strains from chickpea rhizosphere for growth promotion and Zn biofortification. Sixty-one bacteria isolated from rhizosphere of chickpea from different parts of Indo-Gangetic plains (IGP) of India were screened for Zn-solubilizing capabilities by cultivating on BTG medium supplemented with ZnO, Zn-3(PO4)(2), and ZnCO3. Two potential ZSB (BT3 and CT8) were chosen based on their zinc solubilization index and zinc solubilization efficiency and later identified by 16S rRNA gene sequence analysis. In vitro assays were performed to determine their plant growth-promoting attributes. PCR assay was performed to confirm the presence of zinc uptake regulator (zur) gene in potential strains. Pot experiments were carried out to evaluate the effect of two potential ZSB (BT3 and CT8) on growth promotion and Zn biofortification in chickpea. Atomic absorption spectroscopy was employed to determine the Zn content in the plants. Out of twenty-four ZSB, only two strains (BT3 and CT8) were able to solubilize all the three Zn sources. Isolate, BT3 released maximum concentration of Zn from ZnCO3 (150.2 +/- 1.1 mg l(-1)), while CT8 released maximum from ZnO (130.2 +/- 1.2 mg l(-1)) and Zn-3(PO4)(2) (116.5 +/- 1.2 mg l(-1)), respectively. The pH of medium was strongly acidic which ranged from 5.2 to 3.9, and the organic acid analysis revealed hexanoic acid, pentanoic acid, and mandelic acids as prime organic acids produced by isolates. The presence of zur genes indicated the involvement of zur-mediated molecular mechanisms in these strains. These isolates also showed plant growth-promoting traits such as production of indole acetic acid (IAA), ammonia, siderophores, and phosphate solubilization. The 16S rRNA sequence analysis revealed that both the isolates had close similarity with Bacillus altitudinis. Inoculation of isolates BT3 and CT8 improved the growth parameters of chickpea and increased the plant's Zn uptake by 3.9-6.0%. B. altitudinis strains (BT3 and CT8) show excellent capabilities to solubilize insoluble Zn compounds like oxides, phosphates, and carbonates of Zn, making them valuable source for improving Zn uptake and growth of chickpea.