Abstract
Zinc is an important micronutrient needed for the optimum growth and development of plants. Contrary to chemical zinc fertilizers, the use of zinc-solubilizing bacteria is an environmentally friendly option for zinc enrichment in edible parts of crops. This study was conducted with the objective of selecting potential zinc-solubilizing rhizobacteria from the rhizosphere of chickpea grown in soils of eastern Uttar Pradesh and further assessing their impact on the magnitude of zinc assimilation in wheat crops. Among 15 isolates, CRS-9, CRS-17, CRS-30, and CRS-38 produced net soluble zinc in broth to the tune of 6.1, 5.9, 5.63, and 5.6 μg ml–1, respectively, in zinc phosphate with the corresponding pH of 4.48, 5.31, 5.2, and 4.76. However, the bacterial strains CRS-17, CRS-30, CRS-38, and CRS-9 showed maximum zinc phosphate solubilization efficiency of 427.79, 317.39, 253.57, and 237.04%, respectively. The four bacterial isolates were identified as Bacillus glycinifermentans CRS-9, Microbacterium oxydans CRS-17, Paenarthrobacter nicotinovorans CRS-30, and Bacillus tequilensis CRS-38 on the basis of morphological and biochemical studies and 16S rRNA gene sequencing. Bacterial inoculants significantly colonized the roots of wheat plants and formed a biofilm in the root matrix. These strains significantly increased seed germination (%) and vigor indices in wheat grown under glasshouse conditions. After 30 days of sowing of wheat under microcosm conditions, eight zinc transporter (TaZIP) genes were expressed maximally in roots, with concomitant accumulation of higher zinc content in the bacterially treated plant compared to the absolute control. Out of the four strains tested, two bacteria, B. tequilensis CRS-38 and P. nicotinovorans CRS-30, improved seed germination (%), vigor indices (2–2.5 folds), plant biomass, grain yield (2.39 g plant–1), and biofortificated grains (54.25 μg g–1Zn) of wheat. To the best of our knowledge, this may be the first report on the presence of zinc solubilization trait in B. glycinifermentans CRS-9, M. oxydans CRS-17, and P. nicotinovorans CRS-30.
Highlights
The most important challenge to researchers, scientists, and policymakers is to ensure food and nutritional security for the burgeoning population, which may reach 9.8 billion by the year 2050
Out of the 11 zinc-solubilizing bacteria, four isolates, i.e., CRS-9 (21.33 mm), CRS-17 (25.67 mm), CRS-30 (24.33 mm), and CRS-38 (23.67 mm) were selected as potential zinc solubilizing rhizobacteria, and these isolates were chosen for further study (Supplementary Figure 1)
Maximum release of soluble zinc occurred in the liquid medium supplemented with zinc phosphate at the lowest mean pH value of 4.5, suggesting that solubilization is dependent on reduction of pH either by organic acid production or proton extrusion by bacteria (Table 1)
Summary
The most important challenge to researchers, scientists, and policymakers is to ensure food and nutritional security for the burgeoning population, which may reach 9.8 billion by the year 2050. Zn deficiency in plants causes a reduction in photosynthesis, flowering and fruit development, synthesis of carbohydrate and phytohormones, shoot and root development, and leaf size; it induces chlorosis and susceptibility to heat, light, and fungal infections; it affects water uptake and delays crop maturity, leading to decrease in crop yield and nutritional quality of grains (Alloway, 2004; Tavallali et al, 2010). Zinc deficiency in humans is due to consumption of zinc-deficient food including wheat grown in Zn-deficient soils. Cereal crops grown on these soils are zinc-deficient. Urgent corrective measures are required to eliminate Zn deficiency in edible crops to ensure proper Zn nutrition
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