ISSN- 2394-5125 VOL 7, ISSUE 12, 2020 4644 ROLE OF SOIL RHIZOBACTERIA IN UTILIZATION OF AN INDISPENSABLE MICRONUTRIENT ZINC FOR PLANT GROWTH PROMOTION Bahman Khoshru 1 *, Debasis Mitra 2 , Bhaswatimayee Mahakur 4 , Mohammad Reza Sarikhani 1 , Rittick Mondal 3 , Devvret Verma 5 , Kumud Pant 5 1 Department of Soil Science, Faculty of Agriculture, University of Tabriz, Tabriz, Iran. 2 Department of Microbiology, Raiganj University, Raiganj – 733134 Uttar Dinajpur, West Bengal, India. 3 Department of Sericulture, Raiganj University, Raiganj – 733134 Uttar Dinajpur, West Bengal, India. 4 Department of Botany and Biotechnology, Ravenshaw University, - 753003 Cuttack, India 5 Department of Biotechnology, Graphic Era (Deemed to be University), Dehradun - 248 002 Uttarakhand India. E-mail: bahmankhoshru@yahoo.com Received: 14 March 2020 Revised and Accepted: 8 May 2020 ABSTRACT: The crop yield production depends on different nutrients (macro and micronutrients), but because of the several anthropogenic and environmental factors, the plants face some micronutrient deficiencies. Among them, worldwide the zinc (Zn) deficiency are common (mainly in the arid and semi-arid regions characterized by calcareous soils). These elements have a high concentration in the soils of these areas, however, their absorbable concentration for the plant is low (because of several factors, such as high soil pH, lack of organic matter, high bicarbonate in soil solution, compaction and lack of soil aeration and high consumption of phosphorus fertilizers) and subordinate values of micronutrients would reduce crop efficiency and yield. To overcome the Zn deficiency, chemical fertilizers are used. Extreme usage of the chemical fertilizers decreases soil fertility and makes the crops more susceptible to disease. There are reports of potential for insoluble zinc dissolution in the soil by rhizospheric microbes. These microbes can increase the supplying of nutrients, including soil insoluble zinc, to the plant through mechanisms such as the proton secretions, production of siderophores and organic and mineral acids. In order to achieve sustainable agriculture and the use of environmentally friendly solutions, the microbial potentials and their metabolites, as environmentally friendly agents and their replacement by chemical compounds is of particular interest to overcome the problem of supplying zinc needed by plants in calcareous soils. This study critically evaluates the soil microbe’s effect on the insoluble zinc dissolution in the soil, which can increase the bioavailability of these elements for plants and prevent soil fertility reduction due to overuse of chemical fertilizers. Keywords: Plant rhizosphere, zinc nutrient, solubilization, plant growth promotion. I. INTRODUCTION Generally, because of inadequate dietary intake, the incidence of micronutrient deficiencies remains high (Gregory et al., 2017). More than 2 billion people in the world are exposed to Zn micronutrient deficiencies, especially in developing countries (Joy et al., 2014; Manzeke et al., 2019). Nutrient stress can lead to plants suffering when the soil nutrients availability and/or the amount of nutrients taken up in a particular growth stage is below that required to sustain metabolic processes (Rengel, 2015). This can result from an inherently low soil nutrient status, low soil nutrient mobility, poor solubility of the nutrient's given chemical type, or interactions between soil and microbes (Marschner et al., 2011). The abundance of nutrients inside the rhizosphere is regulated by combined soil properties effects, plant characteristics, and the relationships between the plant and the microorganisms and the soil. Increased availability of short-chain organic acid (OA), amino acids, and other low molecular weight organic compounds can result from solubilization and mobilization by anions. Rhizospheric soil acidification improves mobilization of micronutrients (Zn-100 times the increase in solubility for each pH-decrease unit) (Rengel, 2015). In plants, Zn requirement is 30-mg kg-1, below which its deficiency will lead. Zn conducts different biochemical and metabolic roles in a plant. It is a constituent of ribulose 1, 5-biphosphate carboxylase (RuBPC), carbonic anhydrase which are essential in photosynthesis. Zn deficiency results in a decrease in net photosynthesis by 50-70 per cent. Zn also plays a role in the synthesis of RNA, DNA and the regulations. It defends against attack by