Silicate Solubilization and Plant Growth Promoting Potential of Rhizobium Sp. Isolated from Rice Rhizosphere

Silicon is a beneficial plant nutrient with a vital role in maintaining plant growth and enhancing tolerance to both biotic and abiotic stresses. The polymeric insoluble silica present in soils is solubilised during weathering to release monosilicic acid into the soil solution which is the bioavailable form of silicon absorbed by plants. In the present study, a silicate solubilizing bacterial isolate IIRR-1 from rhizopshere soil of rice was characterized for its potential to release soluble silica from insoluble inorganic (Ca, Al, K and Mg) silicates and biogenic materials like diatomaceous earth, siliceous earth, rice husk and rice straw. Incubation studies revealed the ability of the isolate to colonise and grow on all insoluble silicates leading to increased release of silica (12.45-60.15% over control) into culture media by the process of acidolytic (pH 4.5) dissolution of silicates. The isolate, besides possessing additional mineral (P, K and Zn) solubilising capacities, also exhibited phytostimulatory behaviour by producing IAA (0.95 + 0.14 mu g IAA /mu g bacterial cell protein) and ACC deaminase activity (2.52 + 0.73 mu g alpha ketobutyrate /mu g bacterial cell protein/24 h) that modulates the ethylene levels in plant. Furthermore, the isolate successfully colonized rice seedling roots and improved seedling vigour by 29.18% as compared to uninoculated control. The isolate identified as Rhizobium sp. has the potential to be employed as a plant beneficial bacterium for accelerating the weathering process and increasing the rhizosphere available silica concentration.