Abstract
Zinc is an imperative micronutrient required for optimum plant growth. Zinc solubilizing bacteria are potential alternatives for zinc supplementation and convert applied inorganic zinc to available forms. This study was conducted to screen zinc solubilizing rhizobacteria isolated from wheat and sugarcane, and to analyze their effect on wheat growth and development. Fourteen exo-polysaccharides producing bacterial isolates of wheat were identified and characterized biochemically as well as on the basis of 16S rRNA gene sequences. Along these, 10 identified sugarcane isolates were also screened for zinc solubilizing ability on five different insoluble zinc sources. Out of 24, five strains, i.e., EPS 1 (Pseudomonas fragi), EPS 6 (Pantoea dispersa), EPS 13 (Pantoea agglomerans), PBS 2 (E. cloacae) and LHRW1 (Rhizobium sp.) were selected (based on their zinc solubilizing and PGP activities) for pot scale plant experiments. ZnCO3 was used as zinc source and wheat seedlings were inoculated with these five strains, individually, to assess their effect on plant growth and development. The effect on plants was analyzed based on growth parameters and quantifying zinc content of shoot, root and grains using atomic absorption spectroscopy. Plant experiment was performed in two sets. For first set of plant experiments (harvested after 1 month), maximum shoot and root dry weights and shoot lengths were noted for the plants inoculated with Rhizobium sp. (LHRW1) while E. cloacae (PBS 2) increased both shoot and root lengths. Highest zinc content was found in shoots of E. cloacae (PBS 2) and in roots of P. agglomerans (EPS 13) followed by zinc supplemented control. For second set of plant experiment, when plants were harvested after three months, Pantoea dispersa (EPS 6), P. agglomerans (EPS 13) and E. cloacae (PBS 2) significantly increased shoot dry weights. However, significant increase in root dry weights and maximum zinc content was recorded for Pseudomonas fragi (EPS 1) inoculated plants, isolated from wheat rhizosphere. While maximum zinc content for roots was quantified in the control plants indicating the plant's inability to transport zinc to grains, supporting accelerated bioavailability of zinc to plant grains with zinc solubilizing rhizobacteria.
Highlights
Despite the growing industrial and technological advancement, agriculture is still the most significant sector of Pakistan, contributing one-fifth of the total GDP
Based on 16S rRNA sequence analysis, two isolates were identified as Pseudomonas fragi (EPS 1, EPS 15), five isolates as different species of Pantoea, i.e., P. dispersa (EPS 6), Pantoea sp. (EPS 4) and P. agglomerans (EPS 2, EPS 17, EPS 13), respectively; three isolates as Acinetobacter johnsonii (EPS 5, EPS 10, EPS 11), one as Kosakonia oryzae (EPS 7), one as Enterobacter cloacae (EPS 14), EPS 12 as Microbacterium sp., and one as Bacillus pumilus (EPS 16)
Plants inoculated with strains E. cloacae (PBS 2) and Rhizobium sp. (LHRW1) showed insignificant increase in grain zinc content as compared to un-inoculated controls, i.e., 2.66 mg/kg and 2.68 mg/kg in control+ZnCO3 (Figure 5A). These results indicate the efficacy of three Plant growth promoting rhizobacteria (PGPR) strains; Pseudomonas fragi (EPS 1), Pantoea dispersa (EPS 6) and Pantoea agglomerans (EPS 13) that they enhanced the bioavailability of zinc and mobilized it toward wheat grains
Summary
Despite the growing industrial and technological advancement, agriculture is still the most significant sector of Pakistan, contributing one-fifth of the total GDP. Wheat is considered to be one of the chief food crops in Pakistan, contributing 10.3% to the agricultural sector and having annual production of 25.3 million tons, calculated in 2013–2014 (Mirza et al, 2015). One of the domineering micronutrients, is required in small amount for the proper growth and development of living organisms (Hafeez et al, 2013). In plants, it is involved in carbohydrate metabolism (Alloway, 2008), auxin metabolism (Alloway, 2004) and acts as a significant anti-oxidant. Consuming zinc deficient wheat can lead to zinc deficiency in humans as well
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