The role of plant growth promoting rhizobacteria in strengthening plant resistance to fluoride toxicity: a review

A wide variety of bacteria are present in soil but in rhizospheric area, the majority of microbes helps plant in defending diseases and facilitate nutrient uptake. These microorganisms are supported by plants and they are known as plant growth-promoting rhizobacteria (PGPR). The PGPRs have the potential to replace chemical fertilizers in a way that is more advantageous for the environment. Fluoride (F) is one of the highly escalating, naturally present contaminants that can be hazardous for PGPRs because of its antibacterial capacity. The interactions of F with different bacterial species in groundwater systems are still not well understood. However, the interaction of PGPR with plants in the rhizosphere region reduces the detrimental effects of pollutants and increases plants' ability to endure abiotic stress. Many studies reveal that PGPRs have developed F defense mechanisms, which include efflux pumps, Intracellular sequestration, enzyme modifications, enhanced DNA repair mechanism, detoxification enzymes, ion transporter/antiporters, F riboswitches, and genetic mutations. These resistance characteristics are frequently discovered by isolating PGPRs from high F-contaminated areas or by exposing cells to fluoride in laboratory conditions. Numerous studies have identified F-resistant microorganisms that possess additional F transporters and duplicates of the well-known targets of F. Plants are prone to F accumulation despite the soil's low F content, which may negatively affect their growth and development. PGPRs can be used as efficient F bioremediators for the soil environment. Environmental biotechnology focuses on creating genetically modified rhizobacteria that can degrade F contaminants over time. The present review focuses on a thorough systemic analysis of contemporary biotechnological techniques, such as gene editing and manipulation methods, for improving plant-microbe interactions for F remediation and suggests the importance of PGPRs in improving soil health and reducing the detrimental effects of F toxicity. The most recent developments in the realm of microbial assistance in the treatment of F-contaminated environments are also highlighted.

Characterizing indigenous plant growth promoting bacteria and their synergistic effects with organic and chemical fertilizers on wheat (Triticum aestivum)

The excessive use of chemical fertilizers is deteriorating both the environment and soil, making it a big challenge faced by sustainable agriculture. To assist the efforts for the solution of this burning issue, nine different potential native strains of plant growth-promoting bacteria (PGPB) namely, SA-1(Bacillus subtilis), SA-5 (Stenotrophomonas humi),SA-7(Azospirillum brasilense), BH-1(Azospirillum oryzae), BH-7(Azotobacter armeniacus), BH-8(Rhizobium pusense), BA-3(Azospirillum zeae), BA-6(Rhizobium pusense), and BA-7(Pseudomonas fragi) were isolated that were characterized morphologically, biochemically and molecularly on the basis of 16S rRNA sequencing. Furthermore, the capability of indigenous PGPB in wheat (Triticum aestivum, Chakwal-50) under control, DAP+FYM, SA-1,5,7, BH-1,7,8, BA-3,6,7, DAP+ FYM + SA-1,5,7, DAP+FYM+ BH-1,7,8 and DAP+FYM+ BA-3,6,7 treatments was assessed in a randomized complete block design (RCBD). The results of the study showed that there was a significant increase in plant growth, nutrients, quality parameters, crop yield, and soil nutrients at three depths under SA-1,5,7, BH-1,7,8, and BA-3,6,7 in combination with DAP+FYM. Out of all these treatments, DAP+ FYM + BA-3,6,7 was found to be the most efficient for wheat growth having the highest 1000-grain weight of 55.1 g. The highest values for plant height, no. of grains/spike, spike length, shoot length, root length, shoot dry weight, root dry weight, 1000 grain weight, biological yield, and economic yield were found to be 90.7 cm, 87.7 cm, 7.20 cm, 53.5 cm, 33.5 cm, 4.87 g, 1.32 g, 55.1 g, 8209 kg/h, and 4572 kg/h, respectively, in the DAP+FYM+BA treatment. The DAP+FYM+BA treatment had the highest values of TN (1.68 µg/mL), P (0.38%), and K (1.33%). Likewise, the value of mean protein (10.5%), carbohydrate (75%), lipid (2.5%), and available P (4.68 ppm) was also highest in the DAP+FYM+BA combination. C:P was found to be significantly highest (20.7) in BA alone but was significantly lowest (11.9) in DAP+FYM+BA. Hence, the integration of strains BA-3, BA-5, and BA-7 in fertilizers can be regarded as the most suitable choice for agricultural growth in the sub-mountainous lower region of AJK. This could serve as the best choice for sustainable wheat growth and improved soil fertility with lesser impacts on the environment.

Will Joining Cooperative Promote Farmers to Replace Chemical Fertilizers with Organic Fertilizers?

This study examines whether cooperatives can assist more than 200 million farmers in China, who are facing tightening resources and environmental constraints, in adopting green farming practices. A framework for counterfactual analysis was established to quantify the impact of farmers joining cooperatives on the reduction in chemical fertilizer consumption and the use of organic fertilizers. The study's conclusions are based on data from 712 farmers in four counties in Shandong and Henan provinces. Joining a cooperative can have a positive impact on farmers' selection of environmentally friendly production methods, which increases the likelihood of farmers reducing chemical fertilizer application by 35.6% and organic fertilizer application by 22.0%. It can also reduce the cost of chemical fertilizer application by an average of $209.2/ha. The extent to which smallholder farmers use chemical and organic fertilizers after joining cooperatives depends on the size of their farming operations and their perception of green production.

Nitrate Quantification in Fresh Vegetables in Shanghai: Its Dietary Risks and Preventive Measures

To investigate nitrate and nitrite content in fresh vegetables, 264 samples were randomly collected in the farmers' markets in Shanghai, Southeast China. The results indicate that 25.0% of the fresh vegetables were critically or more contaminated by nitrate [>1440 mg/kg FW (Fresh weight)]. Generally, leafy vegetables were more highly enriched in nitrate than root-tuber and fruit vegetables. About 22.6% of the leafy vegetables had a nitrate content exceeding the limit for edible permission (>3000 mg/kg FW). Nitrite content in the fresh vegetables was all within the safe level (<1 mg/kg FW). It was estimated that the daily nitrate intake through eating vegetables in Shanghai exceeded the WHO/FAO allowable limit. The field experiment indicated that the hyper-accumulation of nitrate and nitrite in the vegetables was mainly attributed to the excessive application of chemical fertilizers. The maxima of nitrate and nitrite in the vegetables were attained one week after applying chemical fertilizer, and thus they cannot be picked for dietary use. Applying organic manure can effectively lower the risk of nitrate and nitrite contamination in vegetables. The old leaves and leaf petioles were more easily enriched in nitrate due to their weaker metabolic activity. Vegetables with high nitrate content had a high risk of nitrite toxicity during storage due to the biological conversion of nitrate into nitrite, which is easily triggered by suitable temperature and mechanical damage processing. Therefore, fresh vegetables should be stored by rapid cooling and in undamaged forms to prevent nitrite accumulation.

Morphological, Biochemical and Molecular Characterization of Rhizobia of Faba Bean Plants Grown in North Nile Delta Egypt

&lt;b&gt;Background and Objective:&lt;/b&gt; Rhizobia are bacteria including genes codes for enzymes involved in the fixing of the atmospheric nitrogen. A set of twenty rhizobial isolates were studied to determine their morphological, biochemical, molecular characteristics using the 16S rRNA gene in addition to assess their growth and symbiotic performance. &lt;b&gt;Materials and Methods:&lt;/b&gt; Rhizobial isolates were isolated from root nodules of &lt;i&gt;Vicia faba &lt;/i&gt;L. plants. The isolates were morphologically characterized by determining cell shapes, size, Gram stain reaction, motility, sporulation, bacterial growth performance was determined by IAA production and biomass density. Symbiotic performance was measured by evaluation of nodulation status and shoot/root dry weight. Sequencing of 16S rRNA and phylogenetic analysis were done for the five promising isolates. Statistical analysis was performed using a one-sample Student t-test. &lt;b&gt;Results:&lt;/b&gt; Only five rhizobial isolates (Rh 32, Rh 6-A, Rh 3-4, Rh RL3 and Rh 8-A) were selected according to their growth and symbiotic performance and subjected to further molecular characterizations. All isolates were found to have remarkable nodulation status, IAA production, nitrogenase activity and increasing the root and shoot dry weight. The five selected rhizobial isolates were identified by partial sequencing of 16S rRNA genes and registered in the GenBank database. The alignment and phylogenetic analyses of 16S rRNA sequences closely related in the GenBank revealed that all isolates belonging to &lt;i&gt;Rhizobium leguminosarum&lt;/i&gt; bv. viciae. &lt;b&gt;Conclusion:&lt;/b&gt; The results confirmed that the five Rhizobial strains will be promising as a source of genes for nitrogen fixation and plant growth promotion.

Effect of Pesticides and Chemical Fertilizers on the Nitrogen Cycle and Functional Microbial Communities in Paddy Soils: Bangladesh Perspective

The concept of the Nitrogen (N) cycle has been modified over the years based on certain new pathways, including comammox, anammox, and DNRA (dissimilatory nitrate reduction to ammonium). Comammox, nitrification, anammox, denitrification, DNRA, and nitrogen fixation pathways play key roles in the N cycle in paddy soils. Pesticides and chemical fertilizers' effects on the N cycle in paddy soils together with the possible manifestation of these newly discovery pathways are the focus of this review. Both chemical fertilizers and pesticides' overuse affect nitrifying archaea/bacteria and denitrifying and anammox bacteria, while heavy metals affect the nitrification rates in paddy soils. To add extra value to this study, we quantified the comammox amoA single copy gene from the Nitrospira strain 'Nitrospira inopinata'. This review will help researchers access the latest information on the N cycle, particularly in the light of the most recent discoveries.

The Modulation of Water, Nitrogen, and Phosphorous Supply for Growth Optimization of the Evergreen Shrubs Ammopiptanthus mongolicus for Revegetation Purpose

Surface mining is a critical anthropogenic activity that significantly alters the ecosystem. Revegetation practices are largely utilized to compensate for these detrimental impacts of surface mining. In this study, we investigated the effects of five water (W) regimes [W₄₀: 40%, W₄₈: 48%, W₆₀: 60%, W₇₂: 72%, and W₈₀: 80% of field capacity (FC)], five nitrogen (N) (N₀: 0, N₂₄: 24, N₆₀: 60, N₉₆: 96, and N₁₂₀: 120 mg kg^-1 soil), and five phosphorus (P) fertilizer doses (P₀: 0, P₃₆: 36, P₉₀: 90, P₁₄₄: 144, and P₁₈₀: 180 mg kg^-1 soil) on morpho-physiological and biochemical parameters of Ammopiptanthus mongolicus plants to assess the capability of this species to be used for restoration purposes. The results showed that under low W-N resources, A. mongolicus exhibited poor growth performance (i.e., reduced plant height, stem diameter, and dry biomass) in coal-degraded spoils, indicating that A. mongolicus exhibited successful adaptive mechanisms by reducing its biomass production to survive long in environmental stress conditions. Compared with control, moderate to high W and N-P application rates greatly enhanced the net photosynthesis rates, transpiration rates, water-use efficiency, chlorophyll (Chl) a, Chl b, total Chl, and carotenoid contents. Under low-W content, the N-P fertilization enhanced the contents of proline and soluble sugar, as well as the activities of superoxide dismutase, catalase, and peroxidase in leaf tissues, reducing the oxidative stress. Changes in plant growth and metabolism in W-shortage conditions supplied with N-P fertilization may be an adaptive strategy that is essential for its conservation and restoration in the desert ecosystem. The best growth performance was observed in plants under W supplements corresponding to 70% of FC and N and P doses of 33 and 36 mg kg^-1 soil, respectively. Our results provide useful information for revegetation and ecological restoration in coal-degraded and arid-degraded lands in the world using endangered species A. mongolicus.

in vitro

Effects of chemical fertilizers (urea, super phosphate and potash) on toxicities of two carbamate insecticides, carbaryl and carbofuran, individually to the N2-fixing cyanobacterium, Cylindrospermum sp. were studied in vitro at partially lethal levels (below highest permissive concentrations) of each insecticide. The average number of vegetative cells between two polar heterocysts was 16.3 in control cultures, while the mean value of filament length increased in the presence of chemical fertilizers, individually. Urea at the 10 ppm level was growth stimulatory and at the 50 ppm level it was growth inhibitory in control cultures, while at 100 ppm it was antagonistic, i.e. toxicity-enhancing along with carbaryl, individually to the cyanobacterium, antagonism was recorded. Urea at 50 ppm had toxicity reducing effect with carbaryl or carbofuran. At 100 and 250 ppm carbofuran levels, 50 ppm urea only had a progressive growth enhancing effect, which was marked well at 250 ppm carbofuran level, a situation of synergism. Super phosphate at the 10 ppm level only was growth promoting in control cultures, but it was antagonistic at its higher levels (50 and 100 ppm) along with both insecticides, individually. Potash (100, 200, 300 and 400 ppm) reduced toxicity due to carbaryl 20 and carbofuran 250 ppm levels, but potash was antagonistic at the other insecticide levels. The data clearly showed that the chemical fertilizers used were antagonistic with both the insecticides during toxicity to Cylindrospermum sp.