Inoculation with a bacterial consortium alleviates the effect of cadmium overdose in soybean plants

Recent progress on the microbial mitigation of heavy metal stress in soybean: overview and implications

Plants are adapted to defend themselves through programming, reprogramming, and stress tolerance against numerous environmental stresses, including heavy metal toxicity. Heavy metal stress is a kind of abiotic stress that continuously reduces various crops' productivity, including soybeans. Beneficial microbes play an essential role in improving plant productivity as well as mitigating abiotic stress. The simultaneous effect of abiotic stress from heavy metals on soybeans is rarely explored. Moreover, reducing metal contamination in soybean seeds through a sustainable approach is extremely needed. The present article describes the initiation of heavy metal tolerance mediated by plant inoculation with endophytes and plant growth-promoting rhizobacteria, the identification of plant transduction pathways via sensing annotation, and contemporary changes from molecular to genomics. The results suggest that the inoculation of beneficial microbes plays a significant role in rescuing soybeans under heavy metal stress. They create a dynamic, complex interaction with plants via a cascade called plant-microbial interaction. It enhances stress metal tolerance via the production of phytohormones, gene expression, and secondary metabolites. Overall, microbial inoculation is essential in mediating plant protection responses to heavy metal stress produced by a fluctuating climate.

Endophytic Bacillus altitudinis WR10 alleviates Cu toxicity in wheat by augmenting reactive oxygen species scavenging and phenylpropanoid biosynthesis

Soil copper (Cu) pollution severely stunts crops growth and limits sustainable agri-food production. Many microbes are widely used for remediation of polluted soil, including Cu pollution. In this study, the potential of an endophytic Bacillus altitudinis WR10 to protect wheat from Cu stress and the molecular mechanisms were investigated using hydroponic model. The Cu resistance assay showed B. altitudinis WR10 can resist up to 2 mM Cu and remove about 74% Cu in medium after 24 h of fermentation. Co-culture study demonstrated WR10 increased roots length and dry weight in wheat seedlings under 50 μM Cu. These results indicated that WR10 was a Cu-resistant strain and reduced Cu toxicity in wheat. Transcriptome data and biochemical tests of wheat roots indicated that WR10 alleviated Cu toxicity through enhancing peroxidases (PODs) gene expression and activity to remove excess hydrogen peroxide (H₂O₂) and down-regulating glutathione S-transferases (GSTs) to increase glutathione (GSH) level. Moreover, enrichment and pathway analysis indicated WR10 regulated the expression of genes involved in phenylpropanoid biosynthesis, which may improve phenolic acids accumulation for protecting plant cells from Cu toxicity. Overall, this study revealed that B. altitudinis WR10 alleviated Cu toxicity in wheat via augmenting reactive oxygen species scavenging and phenylpropanoid biosynthesis.

Combined effect of Bacillus fortis IAGS 223 and zinc oxide nanoparticles to alleviate cadmium phytotoxicity in Cucumis melo

Cadmium (Cd), prevailing in most of the agricultural lands of the world contaminates food chain, thereby causing several health implications. It has become the main heavy metal contaminant in most of the agricultural lands of Pakistan due to the widespread use of phosphate fertilizers besides application of irrigation water contaminated with industrial and mining effluents. Plant growth promoting bacteria (PGPB) are capable to enhance growth and metal stress tolerance in supplemented plants. Zinc oxide nanoparticles (ZnO-NPs) are capable to alleviate various abiotic stresses when applied to plants. During current research, the efficacy of single and combined application of Bacillus fortis IAGS 223 and ZnO-NPs was evaluated for alleviation of Cd (75 mg kg^-1) induced phytotoxicity in Cucumis melo plants. For this purpose, C. melo plants, subjected to Cd stress were treated with B. fortis IAGS 223 and ZnO-NPs (20 mg kg^-1), either alone or in combination. The growth relevant characteristics including photosynthetic pigments, hydrogen peroxide (H₂O₂), malondialdehyde (MDA), and activities of antioxidative enzymes as well as Zn and Cd contents in treated plants were examined. The individual application of ZnO-NPs and B. fortis IAGS 223 slightly enhanced all the above-mentioned growth characteristics in plants under Cd stress. However, the combined application of ZnO-NPs and B. fortis IAGS-223 considerably modulated the activity of antioxidant enzymes besides upgradation of the biochemicals and growth parameters of Cd stressed plants. The decreased amount of stress markers such as H₂O₂, and MDA in addition with reduction of Cd contents was observed in shoots of ZnO-NPs and B. fortis IAGS-223 applied plants. B. fortis IAGS-223 inoculated plants supplemented with ZnO-NPs, exhibited reduced amount of Cd as well as protein bound thiols and non-protein bound thiols under Cd stress. Subsequently, the reduced Cd uptake improved growth of ZnO-NPs and B. fortis IAGS-223 applied plants. Henceforth, field trials may be performed to formulate appropriate combination of ZnO-NPs and B. fortis IAGS-223 to acquire sustainable crop production under Cd stress.

Microbial Fortification Improved Photosynthetic Efficiency and Secondary Metabolism in Lycopersicon esculentum Plants Under Cd Stress

Environmental stress including heavy metal pollution is increasing at high speed and is polluting the cultivable land. Consequently, it results in affecting human population through entering into food chain. The current study aims that Cd stress (0.4 mM) led to toxicity and deleterious effects on 45-day-old Lycopersicon esculentum plants. The use of rhizobacterial strains underlines the main hypothesis of the present research that have been exploited in order to alleviate the Cd induced stress in plants and promoting their growth sidewise. The morphological parameters, plant pigments, and gaseous exchange parameters were estimated and found to be reduced in plants due to Cd toxicity. Along with this, the levels of phenolic compounds and osmoprotectants were stimulated in plants raised in Cd spiked soils. In addition, free amino acid content was reduced in plants under Cd treatment. It was revealed that these bacterial strains Pseudomonas aeruginosa (M1) and Burkholderia gladioli (M2) when inoculated to tomato plants improved the morphological characteristics and enhanced photosynthetic attributes. Moreover, the level of phenolic compounds and osmoprotectants were further enhanced by both the inoculating agents independently. However, in situ localization studies of phenol accumulation in root sections was found to be enhanced in Cd treated plants as revealed through higher intensity of yellowish-brown colour. The supplementation of bacterial strains further accumulated the phenols in Cd stressed root sections as evidenced through increased colour intensity. Therefore, the present study suggested that bacterial strains mitigates Cd stress from tomato plants through improving morphological, physiological and metabolite profiles. Consequently, the present research advocates the best utilization of rhizobacteria as stress alleviators for sustainable agriculture.

Supplementation with plant growth promoting rhizobacteria (PGPR) alleviates cadmium toxicity in Solanum lycopersicum by modulating the expression of secondary metabolites

The current study evaluated the synergistic role of Plant growth promoting rhizobacteria (PGPR), Pseudomonas aeruginosa and Burkholderia gladioli on different physiological, biochemical and molecular activities of 10-days old Solanum lycopersicum seedlings under Cd stress. Cd toxicity altered the levels of phenolic compounds (total phenols (30.2%), flavonoids (92.7%), anthocyanin (59.5%), polyphenols (368.7%)), osmolytes (total osmolytes (10.3%), total carbohydrates (94%), reducing sugars (64.5%), trehalose (112.5%), glycine betaine (59%), proline (54.8%), and free amino acids (63%)), and organic acids in S. lycopersicum seedlings. Inoculation of P. aeruginosa and B. gladioli alleviated Cd-induced toxicity, which was manifested through enhanced phenolic compound levels and osmolytes. Additionally, the levels of low molecular weight organic acids (fumaric acid, malic acid, succinic acid, and citric acid) were also elevated. The expression of genes encoding enzymes for phenols and organic acid metabolism were also studied to be modulated that included CHS (chalcone synthase; 138.4%), PAL (phenylalanine ammonia lyase; 206.7%), CS (citrate synthase; 61.3%), SUCLG1 (succinyl Co-A ligase; 33.6%), SDH (succinate dehydrogenase; 23.2%), FH (fumarate hydratase; 12.4%), and MS (malate synthase; 41.2%) and found to be upregulated in seedlings inoculated independently with P. aeruginosa and B. gladioli. The results provide insights into the role of micro-organisms in alleviating Cd-induced physiological damage by altering levels of different metabolites.

Enhancement of the efficiency of Cd phytoextraction using bacterial endophytes isolated from Chromolaena odorata, a Cd hyperaccumulator

Phytoextraction is a technique using a hyperaccumulator to remove heavy metals from soil. The efficiency of heavy metal uptake can be enhanced by the inoculation of endophytes. In this study, we isolated and identified 23 endophytes from Chromolaena odorata, a cadmium (Cd) hyperaccumulator that consisted of 19 bacteria, 2 actinomycetes and 2 fungi. All bacteria and fungi could produce at least 1 plant growth promoting factors. However, only 4 bacterial isolates; Paenibacillus sp. SB12, Bacillus sp. SB31, Bacillus sp. LB51, and Alcaligenes sp. RB54 showed the highest minimum inhibitory concentration (MIC) value (2.9 mM), followed by Exiguobacterium sp.RB51 (1.7 mM). Then, these 5 high-MIC bacteria and 1 low-MIC bacterium, Bacillus sp. LB15 were inoculated onto sunflower grown in soil supplemented with 250 mg/kg of Cd. After 60 days, all inoculated plants accumulated significantly higher Cd concentration than the non-inoculated counterparts, and those inoculated with strain LB51 showed the highest Cd accumulation and growth. Interestingly, strain LB15 with low MIC also enhanced Cd accumulation in plants. The results suggest that these bacteria, particularly strain LB51, could be applied to improve Cd accumulation in plants, and that bacteria with low MIC also have the potential to enhance the efficiency of phytoextraction.