J. Microbiol. Biotechnol. (2012), 22(3), 352–359 http://dx.doi.org/10.4014/jmb.1106.05063 First published online December 30, 2011 pISSN 1017-7825 eISSN 1738-8872 Characterization of Zinc-Solubilizing Bacillus Isolates and their Potential to Influence Zinc Assimilation in Soybean Seeds Sharma, Sushil K. * , Mahaveer P. Sharma, Aketi Ramesh, and Om P. Joshi Directorate of Soybean Research (DSR-ICAR), Khandwa Road, Indore - 452 001, Madhya Pradesh, India Received: June 13, 2011 / Revised: November 2, 2011 / Accepted: November 3, 2011 One hundred thirty-four putative Bacillus isolates were recovered from soybean rhizosphere soils of Nimar region to select effective zinc solubilizers for increased assimilation of zinc (Zn) in soybean seeds. These isolates were screened in vitro for zinc-solubilization ability on Tris-minimal agar medium supplemented separately with 0.1% zinc in the form of zinc oxide, zinc phosphate, and zinc carbonate. Of all, 9 isolates and a reference Bacillus cereus ATCC 13061 were characterized and identified as Bacillus species based on Gram-positive reaction, endospore-forming cells, and the presence of iso-C 15:0 and anteiso-C 15:0 as predominant fatty acids. On plate assay, two isolates KHBD-6 and KHBAR-1 showed a greater diameter of solubilization halo and colony diameter on all the three zinc compounds. The isolates KHBD-6, KHBAR-1, BDSD-2-2C, and KHTH-4-1 and the reference strain ATCC 13061 had higher soluble zinc concentration in liquid medium supplemented with zinc phosphate and zinc carbonate compounds as compared with the other isolates and uninoculated control. Evaluation under microcosm conditions showed that inoculation of isolates KHBD-6 (57.34 µg/g), KHBAR-1 (55.67 µg/g), and strain ATCC 13061 (53.10 µg/g) significantly increased the Zn concentration in soybean seeds as compared with the other isolates and uninoculated control (47.14 µg/g). This study suggests the occurrence of zinc-solubilizing Bacillus in soils of Nimar region and isolates KHBD-6 and KHBAR-1 were found to be promising zinc solubilizers for increased assimilation of Zn in soybean seeds. Keywords: Bacillus, FAMEs, rhizosphere, soybean, zinc solubilization Soybean has established itself as one of the major oilseed crops in India. The majority of the soybean area is in Madhya Pradesh of central India (59% of total area) and is largely grown on Vertisols and associated soils under rainfed situations [3]. Nimar region, located in central India, is characterized by low rainfall, high temperature, and nutrient- deficient shallow soils, resulting in suboptimal crop yields. There is a considerable potential to abridge the yield gap between the actual and potential yields through adoption of appropriate improved resource nutrient management strategies [1]. Zinc is an essential micronutrient that plays a vital role in various metabolic processes in plants, and its deficiency adversely affects the growth and development of crop plants [2]. The crop and soil management practices mine large amounts of zinc from the native pool of the soil. For example, a harvest of 6.5 ton grain/ ha/yr removed 416 g Zn/ha/yr in soybean-wheat cropping systems [16, 34]. More than 50% of Indian soils are deficient in zinc [32] and warrants remedial measures to increase the zinc availability in these soils and sustain the growth in agricultural production. The available zinc content in Indian soils is low; however, the total zinc content is substantially high and exists in fixed forms such as smithsonite (ZnCO 3 ), sphalerite (ZnS), zincite (ZnO), franklinite (ZnFe 2 O 4 ), wellemite (Zn 2 SiO 4 ), and hopeite (Zn 3 (PO 4 ) 2 ·4H 2 O) [13], which are sparingly soluble. Consequently, large inputs of zinc fertilizers are required to be added to the soil to meet the zinc needs of crops. The strategy of ameliorating the low zinc fertility constraints and improve availability, especially in Nimar region, requires ecologically sound, efficient, and cost-effective alternatives. It is plausible that exploitation of zinc mineralizing and solubilizing bacteria may aid in overcoming zinc deficiency. Zinc-solubilizing microorganisms can solubilize zinc from inorganic and organic pools of total soil zinc and can be utilized to increase zinc availability to plants. Fungi have been extensively studied for solubilization of insoluble zinc compounds both in vitro and in vivo [7, 38]. However, only some bacterial species of the genera Acinetobacter , Bacillus, Gluconacetobacter , and Pseudomonas have been reported [4, 5, 20, 22, 23]. Occurrence of zinc-solubilizing Pseudomonas in Vertisols of Malwa region of central India *Corresponding author Phone: +91-731-2362835; Fax: +91-731-2470520; E-mail: sks_micro@rediffmail.com