Isolation and Characterization of the High Silicate and Phosphate Solubilizing Novel Strain Enterobacter ludwigii GAK2 that Promotes Growth in Rice Plants

Abstract

Silicon (Si) and phosphorus (P) are beneficial nutrient elements for plant growth. These elements are widely used in chemical fertilizers despite their abundance in the earth’s crust. Excessive use of chemical fertilizers is a threat to sustainable agriculture. Here, we screened different Si and P solubilizing bacterial strains from the diverse rice fields of Daegu, Korea. The strain with high Si and P solubilizing ability was selected and identified as Enterobacter ludwigii GAK2 through 16S rRNA gene sequence analysis. The isolate GAK2 produced organic acids (citric acid, acetic acid, and lactic acid), indole-3-acetic acid, and gibberellic acid (GA1, GA3) in Luria-Bertani media. In addition, GAK2 inoculation promoted seed germination in a gibberellin deficient rice mutant Waito-C and rice cultivar ‘Hwayoungbyeo’. Overall, the isolate GAK2 increased root length, shoot length, fresh biomass, and chlorophyll content of rice plants. These findings reveal that E. ludwigii GAK2 is a potential silicon and phosphate bio-fertilizer.

Silicon-Solubilizing Media and Its Implication for Characterization of Bacteria to Mitigate Biotic Stress

“…Sharing of P solubilization mechanism with Si also restrict the identification of microbes with Si solubilization property. Comparison of the designed medium with other reported silicon media (Sheng et al, 2008;Kang et al, 2017;Vasanthi et al, 2018;Lee et al, 2019) showed that NBRISSM has the best reproducibility in terms of Si solubilization and least P interference. Designing of such specific medium for screening of silicon-solubilizing microbes would be salutary and an option of using the second most available element of earth's crust in agriculture through green technologies for sustainable development.…”

“…Most of aforementioned media for screening of Sisolubilizing microbes are either based on the plate assays for their zone formation ability (Kang et al, 2017;Lee et al, 2019) or liquid media containing soluble P sources like K 2 HPO 4 and Na 2 HPO 4 (Sheng et al, 2008;Vasanthi et al, 2018). There are no prior reports about the preparation of defined medium for high throughput screening (qualitative, indicator dye based) of silicon-solubilizing bacteria with discrimination of P solubilization.…”

“…Rice exhibits higher silicon (Si) uptake varying from 0.1% to 10% dry weight of shoot, making the cell walls thick and rigid, in-turn playing beneficial role in plant growth and development (Fleck et al, 2015;Bokor et al, 2017;Głazowska et al, 2018). Improved nutrient availability, increased abiotic and biotic stress tolerance, and enhanced fertilizer use efficiency are some known beneficial effects of silicon (Agostinho et al, 2017;Rao et al, 2017;Lee et al, 2019). Repeated cropping reduces the level of plant available silicon, thus enforcing the application of silicon fertilizers in forms of fine silica, sodium/calcium/potassium silicate, along with other chemical fertilizers (Datnoff and Rodrigues, 2005).…”

“…Microorganisms are known to play major role in dissolution of minerals like silicates and phosphates (Chen et al, 2016b;Lee et al, 2019). Several beneficial microbes have been reported for their positive impacts on plant under different stress conditions through better uptake of these minerals (Rogers and Bennett, 2004;Lee et al, 2019). Solubilization of both insoluble silicon and phosphate due to organic acid production by microbes is known to enhance their availability to plants (Ameen et al, 2019).…”

“…feldspar, muscovite, biotite, and magnesium trisilicate with soluble phosphate sources (di-potassium hydrogen phosphate; K 2 HPO 4 and di-sodium hydrogen phosphate; Na 2 HPO 4 ); and agar based media containing glucose and magnesium trisilicate as sole source of nutrition (Sheng et al, 2008;Kang et al, 2017;Vasanthi et al, 2018). Number of bacterial strains of genus Bacillus, Pseudomonas, Proteus, Rhizobia, Burkholderia, and Enterobacter are known to release silicon from silicates and promote plant growth (Meena et al, 2014a;Wang et al, 2015;Kang et al, 2017;Kumawat et al, 2017;Chandrakala et al, 2019;Lee et al, 2019). However, due to the common mechanism of solubilization (organic acid production) and quantification method (molybdenum blue method) for both phosphate (P, Fiske and Subbarow, 1925) and silicon (Si, Elliott and Synder, 1991) there is unavailability of a concrete siliconsolubilizing media.…”