Biochemical traits of Bacillus subtilis MF497446: Its implications on the development of cowpea under cadmium stress and ensuring food safety

Strengthen sunflowers resilience to cadmium in saline-alkali soil by PGPR-augmented biochar

In the center of the Nile Delta in Egypt, the Kitchener drain as the primary drainage discharges about 1.9 billion m3 per year of water, which comprises agricultural drainage (75 %), domestic water (23 %), and industrial water (2 %), to the Mediterranean Sea. Cadmium (Cd) stands out as a significant contaminant in this drain; therefore, this study aimed to assess the integration of biochar (0, 5, and 10 ton ha-1) and three PGPRs (PGPR-1, PGPR-2, and PGPR-3) to alleviate the negative impacts of Cd on sunflowers (Helianthus annuus L.) in saline-alkali soil. The treatment of biochar (10 ton ha-1) and PGPR-3 enhanced the soil respiration, dehydrogenase, nitrogenase, and phosphatase activities by 137 %, 129 %, 326 %, and 127 %, while it declined soil electrical conductivity and available Cd content by 31.7 % and 61.3 %. Also, it decreased Cd content in root, shoot, and seed by 55.3 %, 50.7 %, and 92.5 %, and biological concentration and translocation factors by 55 % and 5 %. It also declined the proline, lipid peroxidation, H2O2, and electrolyte leakage contents by 48 %, 94 %, 80 %, and 76 %, whereas increased the catalase, peroxidase, superoxide dismutase, and polyphenol oxidase activities by 80 %, 79 %, 61 %, and 116 %. Same treatment increased seed and oil yields increased by 76.1 % and 76.2 %. The unique aspect of this research is its investigation into the utilization of biochar in saline-alkali soil conditions, coupled with the combined application of biochar and PGPR to mitigate the adverse effects of Cd contamination on sunflower cultivation in saline-alkali soil.

Differences in soil bacterial community structure during the remediation of Cd-polluted cotton fields by biochar and biofertilizer in Xinjiang, China

Introduction

Heavy metal pollution is a major worldwide environmental problem. Many remediation techniques have been developed, these techniques have different performance in different environments.

Methods

In this study, soil sampling was conducted in multiple cotton fields in Xinjiang, China, and found that cadmium (Cd) was the most abundant soil heavy metal. Then, to find the most suitable technique for the remediation of Cd pollution in cotton fields, a two-year study was conducted to explore the effects of cotton straw-derived biochar (BC, 3%) and Bacillus-based biofertilizer (BF, 1.5%) on cotton Cd uptake and transport and soil microbial community structure under Cd exposure conditions (soil Cd contents: 1, 2, and 4 mg·kg-1).

Results

The results showed that the bioaccumulation coefficients (Cd content of cotton organs / soil available Cd content) of cotton roots, stems, leaves, and buds/bolls reduced by 15.93%, 14.41%, 23.53%, and 20.68%, respectively after the application of BC, and reduced by 16.83%, 17.15%, 22.21%, and 26.25%, respectively after the application of BF, compared with the control (no BC and BF). Besides, the application of BC and BF reduced the transport of Cd from soil to root system, and enhanced the diversity of soil bacterial communities (dominant species: Alphaproteobacteria and Actinobacteria) and the metabolic functions related to amino acid synthesis. It was worth noting that the differential species for BF group vs BC group including Alphaproteobacteria, Gemmatimonadetes, Bacilli, and Vicinamibacteria were associated with the enrichment and transport of Cd, especially the transport of Cd from cotton roots to stems.

Discussion

Therefore, the application of BC and BF changed the soil bacterial diversity in Cd-polluted cotton field, and then promoted the transport of Cd in cotton, ultimately improving soil quality. This study will provide a reference for the selection of soil heavy metal pollution remediation techniques in Xinjiang, China.

Comparative study between the top six heavy metals involved in the EU RASFF notifications over the last 23 years

From the Rapid Alert System for Food and Feed (RASFF) database, a total of 4728 notifications regarding the six most frequently notified heavy metals (i.e., arsenic, cadmium, lead, mercury, chromium, and nickel) were tracked from January 1, 2000, to December 31, 2022, and analyzed based on year, notification classification, notifying countries, countries of origin, product types, product categories, risk decision, and action taken. Human risk assessment owing to consumption of mercury- and cadmium-contaminated seafood was estimated as well. Results revealed that the highest numbers of notifications were on mercury (36.6%), cadmium (25.1%), and lead (14.1%). Interestingly, the number of total notifications was at its peak between 2011 and 2014; from 2015 onward, it started to decrease considerably. Alert, border rejection, and information notifications represented 29.6%, 21.9%, and 48.5% of the total notifications, respectively. Chromium and nickel resulted in 33.8% and 23.3% of border rejection notifications, respectively. About 52.0% of the alert notifications were on mercury. Serious notifications represented 34.9% of the total notifications. Mercury and cadmium notifications accounted for 54.9% and 25.8% of serious notifications, respectively. Italy was the most notifying country, recording the highest number of notifications on cadmium (29.0%), mercury (52.6%), chromium (81.0%), and nickel (78.7%). China was the most notified origin country with regards to arsenic (18.7%), cadmium (12.8%), lead (27.6%), chromium (71.2%), and nickel (66.9%) notifications. Notifications on food, food contact materials (FCM), and feed represented 71.9%, 23.4%, and 4.7%, respectively, of the total notifications. About 91.5% of mercury notifications were on fish and fish products; 24.3% of arsenic notifications related to fruits and vegetables; and 20.1% of cadmium notifications corresponded to cephalopods and products thereof. Notified products were largely withdrawn from the markets according to arsenic (20.3%), lead (17.9%), and mercury (18.0%) notifications and re-dispatched because of cadmium (20.5%), chromium (42.1%), and nickel (49.5%) notifications. The target hazard quotient (THQ) values for mercury in swordfish, sharks, and tuna and cadmium in squid were all also below the threshold value of 1, implying that there is no significant risk for consumers. Overall, media coverage of RASFF alerts and actions may raise awareness of heavy metal contamination among the general public and industry professionals. The primary dietary advice of our study is to stay away from species with high mercury contents. Also, identifying the most dangerous heavy metals (HMs) and the most polluting products can help researchers prioritize their efforts in finding sustainable solutions for them.

Biochar amendment reduces cadmium uptake by stimulating cadmium-resistant PGPR in tomato rhizosphere

Biochar amendment in the soil can exert a positive effect in reducing heavy metal toxicity in plants. However, it remains unclear the extent to which this effect is associated with the modulation of plant growth-promoting rhizobacteria (PGPR). Here, we initially conducted a pot experiment using tomato (Solanum lycopersicum L.) as a model plant grown in soil spiked with cadmium. First, we found biochar amendment to result in reduced cadmium uptake in tomato plants and trackable changes in the tomato rhizosphere microbiome. Then, a rhizosphere transplant experiment validated the importance of this microbiome modulation for cadmium-toxicity amelioration. Sequence-based analyses targeted the isolation of representative isolates of PGPR, including Bacillus and Flavisolibacter spp. that displayed in vitro cadmium tolerance and biosorption capabilities (in addition to abilities to solubilize phosphate and produce indole acetic acid). Last, we performed a soil inoculation experiment and confirmed the effectiveness of these isolates in reducing cadmium toxicity in tomato plants. Besides, we found the inoculation of these taxa as single inoculant and in combination to result in increased activities of specific antioxidant enzymes in tomato tissues. Taken together, this study revealed the ecological and physiological mechanisms by which biochar amendment indirectly alleviate cadmium toxicity in tomato plants, in this case, via the modulation and activity of specific PGPR populations. This study provides new insights into strategies able to promote beneficial PGPR in the rhizosphere with potential application to ameliorate heavy metal toxicity in plants.

Plant growth-promoting actinobacterial inoculant assisted phytoremediation increases cadmium uptake in Sorghum bicolor under drought and heat stresses

In this study, two proficient Cadmium (Cd) resistant and plant growth-promoting actinobacterial strains were isolated from metal-polluted soils and identified as Streptomyces sp. strain RA04 and Nocardiopsis sp. strain RA07. Multiple abiotic stress tolerances were found in these two actinobacterial strains, including Cd stress (CdS), drought stress (DS) and high-temperature stress (HTS). Both actinobacterial strains exhibited multifarious plant growth-promoting (PGP) traits such as phosphate solubilization, and production of indole-3-acetic acid, siderophores and 1-aminocyclopropane-1-carboxylate deaminase under CdS, DS and HTS conditions. The inoculation of strains RA04 and RA07 significantly increased Sorghum bicolor growth and photosynthetic pigments under CdS, DS, HTS, CdS + DS and CdS + HTS conditions as compared to their respective uninoculated plants. The actinobacterial inoculants reduced malondialdehyde concentration and enhanced antioxidant enzymes in plants cultivated under various abiotic stress conditions, indicating that actinobacterial inoculants reduced oxidative damage. Furthermore, strains RA04 and RA07 enhanced the accumulation of Cd in plant tissues and the translocation of Cd from root to shoot under CdS, CdS + DS and CdS + HTS treatments as compared to their respective uninoculated plants. These findings suggest that RA04 and RA07 strains could be effective bio-inoculants to accelerate phytoremediation of Cd polluted soil even in DS and HTS conditions.

Cadmium-tolerant bacteria: current trends and applications in agriculture

Cadmium (Cd) is considered a toxic heavy metal; nevertheless, its toxicity fluctuates for different organisms. Cadmium-tolerant bacteria (CdtB) are diverse and non-phylogenetically related. Because of their ecological importance these bacteria become particularly relevant when pollution occurs and where human health is impacted. The aim of this review is to show the significance, culturable diversity, metabolic detoxification mechanisms of CdtB and their current uses in several bioremediation processes applied to agricultural soils. Further discussion addressed the technological devices and the possible advantages of genetically modified CdtB for diagnostic purposes in the future.

Bacillus subtilis Impact on Plant Growth, Soil Health and Environment: Dr. Jekyll and Mr. Hyde

The increased dependence of farmers on chemical fertilizers poses a risk to soil fertility and ecosystem stability. Plant growth-promoting rhizobacteria (PGPR) are at the forefront of sustainable agriculture, providing multiple benefits for the enhancement of crop production and soil health. Bacillus subtilis is a common PGPR in soil that plays a key role in conferring biotic and abiotic stress tolerance to plants by induced systemic resistance (ISR), biofilm formation, and lipopeptide production. As a part of bioremediating technologies, Bacillus spp. can purify metal contaminated soil. It acts as a potent denitrifying agent in agroecosystems while improving the carbon sequestration process when applied in a regulated concentration. Although it harbors several antibiotic resistance genes (ARGs), it can reduce the horizontal transfer of ARGs during manure composting by modifying the genetic makeup of existing microbiota. In some instances, it affects the beneficial microbes of the rhizosphere. External inoculation of B. subtilis has both positive and negative impacts on the endophytic and semi-synthetic microbial community. Soil texture, type, pH, and bacterial concentration play a crucial role in the regulation of all these processes. Soil amendments and microbial consortia of Bacillus produced by microbial engineering could be used to lessen the negative effect on soil microbial diversity. The complex plant-microbe interactions could be decoded using transcriptomics, proteomics, metabolomics, and epigenomics strategies which would be beneficial for both crop productivity and the well-being of soil microbiota. Bacillus subtilis has more positive attributes similar to the character of Dr. Jekyll and some negative attributes on plant growth, soil health, and the environment akin to the character of Mr. Hyde.

The Combined Effect of Pseudomonas stutzeri and Biochar on the Growth Dynamics and Tolerance of Lettuce Plants (Lactuca sativa) to Cadmium Stress

Agricultural activities lead to the accumulation of cadmium (Cd) in the soil. It is necessary to identify effective and economical ways to reduce the soil Cd bioavailability. To achieve this, three bacterial strains, Pseudomonas stutzeri, P. koreensis, and P. fluorescens, were tested for tolerance and biosorption of different concentrations of Cd (0, 5, 10, 15, 20, and 25 mg L−1). During the 2020 and 2021 seasons, a pot experiment was conducted using four different soil amendments (control, biochar, P. stutzeri, and a combination) under four levels of Cd (0, 40, 80, and 120 mg kg−1) and assessing the effect on growth parameters, physiological modifications, antioxidant enzymes, and Cd accumulation in lettuce plants (Lactuca sativa cv. Balady). In vitro, the results showed that P. stutzeri was the most tolerant of Cd. Our findings in pot trials showed that T4 (biochar + P. stutzeri) was a more efficient treatment in terms of the growth parameters, with 452.00 g plant−1 was recorded for fresh weight, 40.10 g plant−1 for dry weight, 18.89 cm plant−1 for plant height, 6.03 cm2 for leaf area, and 20.48 for the number of leaves plant−1, while in terms of physiological characteristics, we recorded 1.29 mg g−1 FW, 0.35 μg g−1 FW, and 3.69 μg g−1 FW for total chlorophyll, carotenoids, and total soluble sugar, respectively; this was also reflected in the number of antioxidant enzymes and intensity of soil biological activities in soil treated with 120 mg kg−1 Cd compared with the control and other treatments in the first season. A similar trend was observed in the second season. Additionally, significantly lower Cd was observed in both the root (67%) and shoots (78%). Therefore, a combined application of biochar and P. stutzeri could be used as an alternative to mitigate Cd toxicity.

ACC deaminase containing endophytic bacteria ameliorate salt stress in Pisum sativum through reduced oxidative damage and induction of antioxidative defense systems

Approximately 6% of the world's total land area and 20% of the irrigated land are affected by salt stress. Egypt is one such country affected by salt-stress problems. This paper focuses on the role of isolated bacteria, such as Bacillus subtilis and Pseudomonas fluorescens, in alleviating the harmful effects of salt stress. The results show that the irrigation of plants with different concentrations of saline water (0, 75, and 150 mM NaCl) leads to significantly decreased growth criteria, photosynthetic pigments (i.e., chl a, chl b, and carotenoids), and membrane stability index (MSI) values. Moreover, malondialdehyde (MDA), glutathione content, endogenous proline, the antioxidant defense system, 1-aminocyclopropane-1-carboxylic acid (ACC) content, ACC synthase (ACS), ACC oxidase (ACO), and Na+ content were significantly increased under NaCl-stress exposure. On the contrary, treatment with endophytic bacteria significantly increased the resistance of pea plants to salt stress by increasing the enzymatic antioxidant defenses (i.e., superoxide dismutase, catalase, peroxidase, and glutathione reductase), non-enzymatic antioxidant defenses (i.e., glutathione), osmolyte substances such as proline, and antioxidant enzyme gene expression. As a result, endophytic bacteria's use was significantly higher compared to control values for indole-3-acetic acid (IAA), gibberellic acid GA3, MSI, and photosynthetic pigments. The use of endophytic bacteria significantly decreased Na+ accumulation while, at the same time, promoting K+ uptake. In conclusion, the induction of endophytic bacterium-induced salt tolerance in pea plants depends primarily on the effect of endophytic bacteria on osmoregulation, the antioxidant capacity, and ion uptake adjustment by limiting the uptake of Na+ and, alternatively, increasing the accumulation of K+ in plant tissue.

Siderophore production by Bacillus subtilis MF497446 and Pseudomonas koreensis MG209738 and their efficacy in controlling Cephalosporium maydis in maize plant

Late wilt disease, caused by Cephalosporium maydis in maize plant, is one of the main economical diseases in Egypt. Therefore, to cope with this problem, we investigated the potentiality of plant growth promoting rhizobacteria in controlling this disease. Six strains (Bacillus subtilis, B. circulance, B. coagulanse, B. licheniformis, Pseudomonas fluroscence and P. koreensis) were screened for siderophore production, and using dual plate culture method and greenhouse experiment, antagonistic activity against C. maydis was studied. Using two superior strains, single and dual inoculation treatments in maize were applied in field experiment during the 2018 and 2019 seasons. Results indicated that B. subtilis and P. koreensis strains had shown the most qualitative and quantitative assays for siderophore production and antagonistic activities. In greenhouse, the most effective treatments on the pre- and post-emergence damping off as well as growth promotion of maize were T3 treatment (inoculated with B. subtilis), and T8 treatment (inoculated with P. koreensis). In field experiment, T5 treatment (inoculated with a mixture of B. subtilis and P. koreensis) showed significant increases in catalase (CAT), peroxidase (POX) and polyphenol oxidase (PPO) activities, as well as total chlorophyll and carotenoids than control treatments during the two growing seasons. In the same way, the highest effect in reducing infection and increasing the thickness of the sclerenchymatous sheath layer surrounding the vascular bundles in maize stem was observed and these results were a reflection of the increase in yield and yield parameters.

Alleviation of Cd phytotoxicity and enhancement of rape seedling growth by plant growth-promoting bacterium Enterobacter sp. Zm-123

The present study aims to investigate the impact of a metal-tolerant bacterium on metal detoxification and rape seedling growth promotion under Cd stress. The results showed that the isolated bacterium Enterobacter sp. Zm-123 has capability to resist Cd (200 mg/L), produce IAA (26.67 mg/L) and siderophores (82.34%), and solubilize phosphate (137.5 mg/L), etc. Zm-123 inoculation significantly enhanced the fresh weight of rape seedlings from 9.47 to 19.98% and the root length from 10.42 to 57.05% compared with non-inoculation group under different concentrations of Cd (0, 0.5, 1, 3, 5 mg/L) (p < 0.05). It also significantly increased the content of chlorophyll, soluble sugar, soluble protein, and proline (p < 0.05) in rape seedlings. Moreover, a significant elevation in catalase (CAT) and peroxidase (POD) activities and a significant reduction in malondialdehyde (MDA), electrolyte leakage (EL), and Cd content in rape seedlings were detected owing to Zm-123 inoculation (p < 0.05). The combined results imply that strain Zm-123 can alleviate the Cd phytotoxicity and promote the rape seedling growth by improving the physiological activity and antioxidant level, which can be potentially applied to protect plants from Cd toxicity.