The role of auxins and auxin-producing bacteria in the tolerance and accumulation of cadmium by plants

Enhancing cauliflower growth under cadmium stress: synergistic effects of Cd-tolerant Klebsiella strains and jasmonic acid foliar application

The pollution of heavy metals (HMs) is a major environmental concern for agricultural farming communities due to water scarcity, which forces farmers to use wastewater for irrigation purposes in Pakistan. Vegetables grown around the cities are irrigated with domestic and industrial wastewater from areas near mining, paint, and ceramic industries that pollute edible parts of crops with various HMs. Cadmium (Cd) is an extremely toxic metal in arable soil that enters the food chain and damages the native biota, ultimately causing a reduction in plant growth and development. However, the use of microbes and growth regulators enhances plant growth and development as well as HM immobilization into the cell wall and hinders their entry into the food chain. Thus, the integrated use of bacterial consortium along with exogenously applied jasmonic acid (JA) mitigates the adverse effect of metal stress, ultimately reducing the metal mobility into roots by soil. Therefore, the current study was conducted to check the impact of Cd-tolerant bacteria and JA on the growth, nutrient status, and uptake of Cd in the cauliflower (Brassica oleracea). Our results demonstrated that increasing concentrations of Cd negatively affect growth, physiological, and biochemical attributes, while the use of a bacterial consortium (SS7 + SS8) with JA (40 μmol L-1) significantly improved chlorophyll contents, stem fresh and dry biomass (19.7, 12.7, and 17.3%), root length and root fresh and dry weights (28.8, 15.2, and 23.0%), and curd fresh and dry weights and curd diameter (18.7, 12.6, and 15.1%). However, the maximum reduction in soil Cd, roots, and curd uptake was observed by 8, 11, and 9.3%, respectively, under integrated treatment as compared to the control. Moreover, integrating bacterial consortium and JA improves superoxide dismutase (SOD) (16.79%), peroxidase dismutase (POD) (26.96%), peroxidase (POX) (26.13%), and catalase (CAT) (26.86%). The plant nitrogen, phosphorus, and potassium contents were significantly increased in soil, roots, and curd up to 8, 11, and 9.3%, respectively. Hence, a consortium of Klebsiella strains in combination with JA is a potential phytostabilizer and it reduces the uptake of Cd from soil to roots to alleviate the adverse impact on cauliflower's growth and productivity.

Effect of Auxin on Cadmium Toxicity-Induced Growth Inhibition in Solanum lycopersicum

Auxins play crucial regulatory roles in plants coping with cadmium (Cd) stress. However, the regulatory mechanism by which auxins alleviate Cd toxicity in tomato seedlings remains unclear. Here, we demonstrate that exposure to Cd stress leads to dynamic changes in the auxin response in tomato roots, characterized by an initial increase followed by a subsequent weakening. Under Cd stress, tomato seedlings show primary root- and hypocotyl-growth inhibition, accompanied by the accumulation of Cd and reactive oxygen species (ROS) in the roots. The exogenous application of 1-naphthylacetic acid (NAA) does not mitigate the inhibitory effect of Cd toxicity on primary root growth, but it does significantly enhance lateral root development under Cd stress. Auxin transport inhibitors, such as 1-N-naphthylphthalamic acid (NPA) and 2,3,5-triiodobenoic acid (TIBA), aggravate the growth inhibition of primary roots caused by Cd stress. Additionally, lateral root development was inhibited by NPA. However, applying auxin synthesis inhibitors L-kynurenine (kyn) and yucasin alleviated the tomato root growth inhibition caused by Cd stress; between them, the effect of yucasin was more pronounced. Yucasin mitigates Cd toxicity in tomato seedlings by reducing Cd2+ absorption and auxin accumulation, strengthening ROS scavenging, and reducing cell death in roots. These observations suggest that yucasin potentially mitigates Cd toxicity and improves the tolerance of tomato seedlings to Cd stress.

Mechanisms of combined bioremediation by phosphate-solubilizing fungus and plants and its effects on cadmium contamination in phosphate-mining wastelands

Owing to uncontrolled mining activities and lack of ecological protection measures, phosphate-mining wastelands are contaminated with the heavy metal Cd. In this study, Penicillium oxalicum strain ZP6, a Cd-resistant phosphate-solubilizing fungus, was used in combination with the fast-growing, high-biomass plant Brassica juncea L. to enhance Cd remediation in phosphate-mining wastelands. Further, the bioremediation mechanisms were explored and elucidated. In pot experiments, strain ZP6 and Brassica juncea L. alone were significantly effective in removing Cd from phosphate-mining wastelands; however, their combination was more effective, exhibiting a high removal rate of 88.75%. The presence of phosphorite powder increases soil-enzyme activity, promotes plant growth, and reduces the bioaccumulation and translocation factors. However, Cd-inhibited plant growth and chlorophyll content increased malondialdehyde accumulation, which was alleviated by inoculation with strain ZP6. The results from the study indicate that bioremediation using a combination of strain ZP6 and plants is a restoration strategy with appreciable potential to resolve Cd contamination in phosphate-mining wastelands.

Expression of OsHARBI1-1 enhances the tolerance of Arabidopsis thaliana to cadmium

BACKGROUND

As one of the major food crops in the world, rice is vulnerable to cadmium (Cd) pollution. Understanding of the molecular mechanisms of Cd uptake, transport and detoxification in rice is essential for the breeding of low-Cd rice. However, the molecular mechanisms underlying the response of rice to Cd stress remains to be further clarified.

RESULTS

In this study, a novel Cd-responsive gene OsHARBI1-1 was identified in the rice genome and its expression pattern and function were characterized. Bioinformatics analysis showed that the promoter region of OsHARBI1-1 had multiple cis-acting elements in response to phytohormones and stress, and the expression of OsHARBI1-1 was induced by phytohormones. OsHARBI1-1 protein was targeted to the nucleus. qRT-PCR analysis results showed that the expression of OsHARBI1-1 in the roots was repressed while the expression in the shoots was increased under Cd stress. Heterologous expression of OsHARBI1-1 in yeast conferred tolerance to Cd and reduced Cd content in the cells. Meanwhile, the expression of OsHARBI1-1 in Arabidopsis thaliana (A. thaliana) enhanced the tolerance of A. thaliana to Cd stress. In addition, compared with the wild type plants, the POD activity of transgenic plants was increased, while the SOD and CAT activities were decreased. Interestingly, the accumulation of Cd in the roots of A. thaliana expressing OsHARBI1-1 was significantly increased, whereas the Cd accumulation in the shoots was slightly decreased. Compared to the WT plants, the expression of genes related to Cd absorption and chelation was upregulated in transgenic A. thaliana under Cd stress, while the expression of genes responsible for the translocation of Cd from the roots to the shoots was downregulated. Moreover, the expression of phytohormone-related genes was significantly influenced by the expression of OsHARBI1-1 with and without Cd treatment.

CONCLUSIONS

Findings of this study suggest that OsHARBI1-1 might play a role in the response of plants to Cd response by affecting antioxidant enzyme activities, Cd chelation, absorption and transport, and phytohormone homeostasis and signaling.

The Influence of Cadmium on Fountain Grass Performance Correlates Closely with Metabolite Profiles

The relationship between metabolite changes and biological endpoints in response to cadmium (Cd) stress remains unclear. Fountain grass has good Cd enrichment and tolerance abilities and is widely used in agriculture and landscaping. We analyzed the metabolic responses by detecting the metabolites through UPLC-MS and examined the relationships between metabolite changes and the characteristics of morphology and physiology to different Cd stress in fountain grass. Our results showed that under Cd stress, 102 differential metabolites in roots and 48 differential metabolites in leaves were detected, with 20 shared metabolites. Under Cd stress, most of the carbohydrates in leaves and roots decreased, which contributed to the lowered leaf/root length and fresh weight. In comparison, most of the differential amino acids and lipids decreased in the leaves but increased in the roots. Almost all the differential amino acids in the roots were negatively correlated with root length and root fresh weight, while they were positively correlated with malondialdehyde content. However, most of the differential amino acids in the leaves were positively correlated with leaf length and leaf fresh weight but negatively correlated with malondialdehyde content. Metabolic pathway analysis showed that Cd significantly affects seven and eight metabolic pathways in the leaves and roots, respectively, with only purine metabolism co-existing in the roots and leaves. Our study is the first statement on metabolic responses to Cd stress and the relationships between differential metabolites and biological endpoints in fountain grass. The coordination between various metabolic pathways in fountain grass enables plants to adapt to Cd stress. This study provides a comprehensive framework by explaining the metabolic plasticity and Cd tolerance mechanisms of plants.

One-pot fabrication of magnetic adsorbent based on polymeric ionic liquid/aminated carbon nanotubes composite for efficient capture of synthetic auxins in complex samples prior to chromatographic analysis

Efficient enrichment is a challenging and indispensable step in the quantification of polar synthetic auxins in complex samples. In the current study, a new magnetic adsorbent based on polymeric ionic liquid/aminated carbon nanotube composite was fabricated with a one-pot precipitation copolymerization strategy and employed as the extraction phase of magnetic solid phase extraction of synthetic auxins. Various characterization techniques were utilized to inspect the morphology, structure, magnetic property, and functional groups of the prepared adsorbent. Under the optimal conditions, the obtained adsorbent displayed satisfactory capture performance towards studied auxins through multiple interactions. Adsorption studies evidenced that the adsorption procedure of the developed method towards analytes was fit for the Freundlich adsorption model and pseudo-second-order kinetics. Combining with high-performance liquid chromatography, sensitive and reliable method for the identification and quantification of trace synthetic auxins in environmental water and fruit juice samples was developed. The obtained limits of detection for water and fruit juice samples located in the ranges of 0.0059-0.013 and 0.018-0.031 μg/L, respectively. Recoveries in actual samples with different fortified contents varied from 82.2% to 117%, with satisfactory reproducibility. The results will evidence that the introduced extraction technique is a useful alternative for the entrapment of trace synthetic auxins from complex samples.

Metabolically versatile psychrotolerant Antarctic bacterium Pseudomonas sp. ANT_H12B is an efficient producer of siderophores and accompanying metabolites (SAM) useful for agricultural purposes

BACKGROUND

Bacterial siderophores are chelating compounds with the potential of application in agriculture, due to their plant growth-promoting (PGP) properties, however, high production and purification costs are limiting factors for their wider application. Cost-efficiency of the production could be increased by omitting purification processes, especially since siderophores accompanying metabolites (SAM) often also possess PGP traits. In this study, the metabolism versatility of Pseudomonas sp. ANT_H12B was used for the optimization of siderophores production and the potential of these metabolites and SAM was characterized in the context of PGP properties.

RESULTS

The metabolic diversity of ANT_H12B was examined through genomic analysis and phenotype microarrays. The strain was found to be able to use numerous C, N, P, and S sources, which allowed for the design of novel media suitable for efficient production of siderophores in the form of pyoverdine (223.50-512.60 μM). Moreover, depending on the culture medium, the pH of the siderophores and SAM solutions varied from acidic (pH < 5) to alkaline (pH > 8). In a germination test, siderophores and SAM were shown to have a positive effect on plants, with a significant increase in germination percentage observed in beetroot, pea, and tobacco. The PGP potential of SAM was further elucidated through GC/MS analysis, which revealed other compounds with PGP potential, such as indolic acetic acids, organic acids, fatty acids, sugars and alcohols. These compounds not only improved seed germination but could also potentially be beneficial for plant fitness and soil quality.

CONCLUSIONS

Pseudomonas sp. ANT_H12B was presented as an efficient producer of siderophores and SAM which exhibit PGP potential. It was also shown that omitting downstream processes could not only limit the costs of siderophores production but also improve their agricultural potential.

The Effect of Cadmium Tolerant Plant Growth Promoting Rhizobacteria on Plant Growth Promotion and Phytoremediation: A Review

Cadmium (Cd) is a heavy metal of considerable toxicity with destructive impacts on plants, microbes and environments. Its toxicity is due to mishandling and manual hazards in plants and is primarily observed within the soil to cause decline of plants and microbial activity inside the rhizosphere. Cadmium accumulation in crops and the probability of Cd entering the food chain are grave for public health in the worldwide. Cadmium toxicity leads to depletion in seed germination, initial seedling growth, plant biomass, chlorosis, necrosis, hindrance of photosynthetic machinery and other physiological and biological activities in plants. Cadmium triggers the reactive oxygen species (ROS) that influences gene mutation and DNA damage that affects the cell cycle and cell division. Cd toxicity altered the levels of phenolic compounds, carbohydrates, glycine betaine, proline and organic acids in crops. Under stress conditions, the plant growth promoting rhizobacteria (PGPR) have various properties such as enzymatic activities, plant growth hormones production, phosphate solubilization, siderophores production and chelating agents that help the plants tolerate against Cd stress and also increase phenolic compound levels and osmolytes. Hence, this review highlights the crucial role of cadmium tolerant PGPR for crop production, declining metal phytoavailability and enhancing morphological and physiological boundaries of plants under stress conditions. It could be an environment friendly and cost effective technology under sustainable crop production.

Auxin alleviates cadmium toxicity by increasing vacuolar compartmentalization and decreasing long-distance translocation of cadmium in Poa pratensis

Kentucky bluegrass (Poa pratensis L.) hyperaccumulates cadmium (Cd) and exhibits a hypertolerance. Thus, it has potential for the phytoremediation of Cd-containing soil. Auxin signaling is involved in the response to Cd stress. However, the mechanisms of auxin-mediated detoxification and Cd translocation in plants remain unclear. This study aimed to investigate the effects of exogenous application of indole-3-acetic acid (IAA) on the Cd translocation, subcellular Cd distribution, chemical forms of Cd, and transcriptional regulation of Kentucky bluegrass. The results showed that the exogenous application of IAA increased the amount of organelle-bound Cd and vacuole-compartmentalized Cd in root cells, reduced the Cd concentration in the leaf tissues (epidermis, mesophyll, and vascular bundle) and root tissues (rhizodermis and cortex) but increased in the stele, and alleviate Cd-induced leaf chlorosis and growth inhibition. The expression of genes associated with Cd transporters (ABCs, ZIPs, NASs, OPTs, and YSLs), phosphatases, oxalate decarboxylases and lignin biosynthesis were significantly regulated by exogenous IAA under Cd stress. A positive regulation of phosphatases and oxalate decarboxylases genes related to an increase in phosphate- and oxalate-bound Cd, as well as a decrease in pectate- and protein-bound Cd and inorganic Cd, thereby contributing to a decrease in Cd phytotoxicity. The significant regulation of Cd transporters associated with decreasing the long-distance translocation of Cd, and the activation of lignin biosynthesis may contribute to the development of root endodermal barriers and increase the deposition of undissolved Cd phosphates and oxalate-bound Cd in the stele. These results revealed the important role of auxin in Cd detoxification and translocation in Kentucky bluegrass and they provide a theoretical basis for the phytoremediation of Cd-containing soil.

Mitigation of arsenic toxicity in rice by the co-inoculation of arsenate reducer yeast with multifunctional arsenite oxidizing bacteria

The study aimed to explicate the role of microbial co-inoculants for the mitigation of arsenic (As) toxicity in rice. Arsenate (AsV) reducer yeast Debaryomyces hansenii NBRI-Sh2.11 (Sh2.11) with bacterial strains of different biotransformation potential was attempted to develop microbial co-inoculants. An experiment to test their efficacy (yeast and bacterial strains) on plant growth and As uptake was conducted under a stressed condition of 20 mg kg-1 of arsenite (AsIII). A combination of Sh2.11 with an As(III)-oxidizer, Citrobacter sp. NBRI-B5.12 (B5.12), resulted in ∼90% decrease in grain As content as compared to Sh2.11 alone (∼40%). Reduced As accumulation in rice roots under co-treated condition was validated with SEM-EDS analysis. Enhanced As expulsion in the selected combination under in vitro conditions was found to be correlated with higher As content in the soil during their interaction with plants. Selected co-inoculant mediated enhanced nutrient uptake in association with better production of indole acetic acid (IAA) and gibberellic acid (GA) in shoot, support microbial co-inoculant mediated better biomass under stressful condition. Boosted defense response in association with enhanced glutathione-S-transferase (GST) and glutathione reductase (GR), activities under in vitro and in vivo conditions were observed. These results indicated that the As(III) oxidizer-B5.12 accelerated the As detoxification property of the As(V) reducer-Sh2.11. Henceforth, the results confer that the coupled reduction-oxidation process of the co-inoculant reduces the accumulation of As in rice grain. These co-inoculants can be further developed for field trials to achieve higher biomass with alleviated As toxicity in rice.

Characterization of Endophytic Bacteria Isolated from Typha latifolia and Their Effect in Plants Exposed to Either Pb or Cd

Plant-associated bacteria in heavy-metal-contaminated environments could be a biotechnological tool to improve plant growth. The present work aimed to isolate lead- and cadmium-tolerant endophytic bacteria from the roots of Typha latifolia growing in a site contaminated with these heavy metals. Endophytic bacteria were characterized according to Pb and Cd tolerance, plant-growth-promoting rhizobacteria activities, and their effect on T. latifolia seedlings exposed and non-exposed to Pb and Cd. Pb-tolerant isolates were identified as Pseudomonas azotoformans JEP3, P. fluorescens JEP8, and P. gessardii JEP33, while Cd-tolerant bacteria were identified as P. veronii JEC8, JEC9, and JEC11. They all exert biochemical activities, including indole acetic acid synthesis, siderophore production, and phosphate solubilization. Plant-bacteria interaction assays showed that P. azotoformans JEP3, P. fluorescens JEP8, P. gessardii JEP33, and P. veronii JEC8, JEC9, JEC11 promote the growth of T. latifolia seedlings by increasing the root and shoot length, while in plants exposed to either 5 mg/L of Pb or 10 mg/L of Cd, all bacterial isolates increased the shoot length and the number of roots per plant, suggesting that they are plant-growth-promoting rhizobacteria that could contribute to T. latifolia adaptation to the heavy metal polluted site.

The modified biochars influence nutrient and osmotic statuses and hormonal signaling of mint plants under fluoride and cadmium toxicities

INTRODUCTION

Chemically modified biochars are a new generation of biochars that have a great ability to absorb and stabilize environmental pollutants. In this research, the physiological performance of mint plants (Mentha crispa L.) under fluoride and cadmium toxicities and biochar treatments was evaluated.

METHODS

Four levels of soil toxicities including non-toxic, 600 mg NaF kg^-1 soil, 60 mg Cd kg^-1 soil, and 600 mg NaF kg^-1 soil + 60 mg Cd kg^-1 soil were applied. The biochar addition to the soil was 25 g kg^-1 (non-biochar, solid biochar, H₂O₂, KOH, and H₃PO₄-modified biochars).

RESULTS

The results showed that the application of biochar and especially chemically modified biochars reduced fluoride (about 15-37%) and cadmium (30-52%) contents in mint leaves, while increased soil pH and cation exchange capacity (CEC), nitrogen (12-35%), phosphorus (16-59%), potassium (17-52%), calcium (19-47%), magnesium (28-77%), iron (37-114%), zinc (45-226%), photosynthetic pigments of leaves and plant biomass (about 10-25%) under toxic conditions.

DISCUSSION

The biochar-related treatments reduced the osmotic stress and osmolytes content (proline, soluble proteins, and carbohydrates) in plant leaves. Plant leaf water content was increased by solid and modified biochar, up to 8% in toxic conditions. Furthermore, these treatments reduced the production of stress hormones [abscisic acid (27-55%), salicylic acid (31-50%), and jasmonic acid (6-12%)], but increased indole-3-acetic acid (14-31%) in plants under fluoride and cadmium stresses. Chemically modified biochars reduced fluoride and cadmium contents of plant leaves by about 20% and 22%, respectively, compared to solid biochar.

CONCLUSION

This result clearly shows the superiority of modified biochars in protecting plants from soil pollutants.

Mining Candidate Genes Related to Heavy Metals in Mature Melon (Cucumis melo L.) Peel and Pulp Using WGCNA

The content of metal ions in fruits is inseparable from plant intake of trace elements and health effects in the human body. To understand metal ion content in the fruit and pericarp of melon (Cucumis melo L.) and the candidate genes responsible for controlling this process, we analyzed the metal ion content in distinct parts of melon fruit and pericarp and performed RNA-seq. The results showed that the content of metal ions in melon fruit tissue was significantly higher than that in the pericarp. Based on transcriptome expression profiling, we found that the fruit and pericarp contained elevated levels of DEGs. GO functional annotations included cell surface receptor signaling, signal transduction, organic substance metabolism, carbohydrate derivative binding, and hormone-mediated signaling pathways. KEGG pathways included pectate lyase, pentose and glucuronate interconversions, H⁺-transporting ATPase, oxidative phosphorylation, plant hormone signal transduction, and MAPK signaling pathways. We also analyzed the expression patterns of genes and transcription factors involved in hormone biosynthesis and signal transduction. Using weighted gene co-expression network analysis (WGCNA), a co-expression network was constructed to identify a specific module that was significantly correlated with the content of metal ions in melon, after which the gene expression in the module was measured. Connectivity and qRT–PCR identified five candidate melon genes, LOC103501427, LOC103501539, LOC103503694, LOC103504124, and LOC107990281, associated with metal ion content. This study provides a theoretical basis for further understanding the molecular mechanism of heavy metal ion content in melon fruit and peel and provides new genetic resources for the study of heavy metal ion content in plant tissues.

Enhanced Cd-Accumulation in Typha latifolia by Interaction with Pseudomonas rhodesiae GRC140 under Axenic Hydroponic Conditions

The Typha genus comprises plant species extensively studied for phytoremediation processes. Recently, Pseudomonas rhodesiae GRC140, an IAA-producing bacterium, was isolated from Typha latifolia roots. This bacterium stimulates the emergence of lateral roots of Arabidopsis thaliana in the presence and absence of cadmium. However, the bacterial influence on cadmium accumulation by the plant has not been determined. Moreover, the P. rhodesiae GRC140 effect in Cd phytoextraction by T. latifolia remains poorly understood. In this work, an axenic hydroponic culture of T. latifolia was established. The plants were used to evaluate the effects of cadmium stress in axenic plants and determine the effects of P. rhodesiae GRC140 and exogenous indole acetic acid (IAA) on Cd tolerance and Cd uptake by T. latifolia. Biomass production, total chlorophyll content, root electrolyte leakage, catalase activity, total glutathione, and Cd content were determined. The results showed that Cd reduces shoot biomass and increases total glutathione and Cd content in a dose-dependent manner in root tissues. Furthermore, P. rhodesiae GRC140 increased Cd translocation to the shoots, while IAA increased the Cd accumulation in plant roots, indicating that both treatments increase Cd removal by T. latifolia plants. These results indicate that axenic plants in hydroponic systems are adequate to evaluate the Cd effects in plants and suggest that T. latifolia phytoextraction abilities could be improved by P. rhodesiae GRC140 and exogenous IAA application.