Abstract
Manganese (Mn) is an essential micronutrient for plant growth that is involved in the structure of photosynthetic proteins and enzymes. Mn deficiency is widespread mainly in dry, calcareous, and sandy soil, which leads to a significant decrease in crop yield. Mn-reducing bacteria promote the solubilization of Mn minerals, thus increasing Mn availability in soil. The present study aimed to assess the Mn solubilizing ability and plant growth-promoting potential of Bacillus spp. strains for maize plants with insoluble Mn compounds. Several Mn-solubilizing bacterial (MSB) strains were isolated from the maize rhizosphere using nutrient agar media amended with 50 mM MnO2. These strains were screened based on qualitative and quantitative solubilization of Mn, phosphorus, potassium, and zinc and production of ammonia. The majority of MSB strains were positive for catalase, protease, amylase, and oxidase activity, while more than 60% of tested strains were positive for lipase activity, and the production of indole-3-acetic acid and siderophores. Forty-five percent of the tested strains also showed solubilization of potassium. All the MSB strains were evaluated for their ability to promote plant growth and Mn uptake in the presence of MnO2 under axenic sand culture conditions. The results revealed that inoculation with MSB strains under sand culture significantly improved the growth of maize seedlings except for strains ASH7, ASH10, and ASH12. Comparatively, strains ASH6, ASH11, ASH19, ASH20, and ASH22 demonstrated a better increase in plant growth, fresh and dry biomass, and Mn uptake in roots and shoots than the other strains tested. All of these strains were identified as Bacillus spp. through 16S rRNA partial gene sequencing. Maize inoculation with these selected identified MSB strains also resulted in an increase in maize growth and nutrient uptake in maize roots and shoots under soil culture conditions in the presence of native soil Mn. The current study highlights the importance of MSB strain inoculation which could be a potential bioinoculants to promote plant growth under Mn deficiency.
Highlights
Plant health is of ultimate significance to agricultural productivity and prominently depends on a series of processes affecting physiological functioning
In vitro screening of selected strains for MnO2 solubilization showed that the Mn solubilization diameter ranged from 7.5 ± 0.03 mm to 35.5 ± 1.06 mm, while the bacterial growth diameter ranged from 7.0 ± 0.01 mm to 19.0 ± 0.70 mm
The maximum Mn solubilization diameter and bacterial growth diameter were reported by strain ASH22
Summary
Plant health is of ultimate significance to agricultural productivity and prominently depends on a series of processes affecting physiological functioning. Manganese (Mn) is an essential micronutrient with many functional roles in plant physiology, especially in electron transport in photosystem II (PSII), chloroplast structure, and N metabolism (Campbell and Nable, 1988). Mn deficiency causes oxidative stress and a reduction in photosynthetic electron transport, water use efficiency, and root functioning (Schmidt et al, 2016). Its deficiency causes a reduction in the wealth of PSII light-harvesting complex II supercomplexes and D1 together with PSII membrane extrinsic proteins PsbP and PsbQ (Schmidt et al, 2015). In maize (Zea mays L.), a deficiency of Mn disturbs N metabolism by restricting NO3− uptake and transportation by the production of inhibiting enzymes, viz. In maize (Zea mays L.), a deficiency of Mn disturbs N metabolism by restricting NO3− uptake and transportation by the production of inhibiting enzymes, viz. glutamine synthase, nitrate reductase, and glutamic-oxaloacetic transaminase cause a decline in chlorophyll synthesis and protein solubility (Gong et al, 2011)
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