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

Synergistic effects of boron and cadmium on the metal enrichment and cell wall immobilization capacity of Cosmosbipinnatus

Cadmium (Cd), as a heavy metal pollutant, can seriously affect plant growth and development. Boron (B), as an indispensable nutrient element, plays an important role in plant growth and cell wall (CW) synthesis. However, the physiological effects of B and Cd on plant growth and the mechanism of Cd chelation by the CW remain unclear. Here, we investigate the effect of exogenous B on Cd accumulation in CW components of Cosmos bipinnatus roots and its mechanism of Cd mitigation. Under B deficiency and single Cd (30 μM) treatments, the growth of C. bipinnatus was significantly inhibited, but the addition of exogenous B significantly increased plant biomass, which increased the Cd content in the underground parts of C. bipinnatus by 20.18% and reduced the Cd translocation factor by 22.22%. Meanwhile, application of exogenous B affected the subcellular Cd content across various Cd forms and alleviated Cd-induced oxidative stress in C. bipinnatus. Additionally, exogenous B and Cd and their mixtures affected the functional groups of the root CW, the proportion of polysaccharide components, the Cd content of polysaccharides, and the polysaccharide uronic acid content of C. bipinnatus. However, B application increased 3-deoxy-oct-2-ulosonic acid content, pectin esterase activity, low esterified pectin content, and its Cd content by 149.52%, 55.69%, 206.38%, and 150.02%, respectively, compared to Cd treatment alone. Thus, our study showed that B mitigates the toxicity of Cd to plants, revealing the effect of B on the physiological aspects of Cd tolerance in plants.

Investigating the impact of paclobutrazol and tannic acid on floral development of in vitro -grown cannabis plantlets

Gibberellic acid (GA3) is inhibitory to floral development of in vitro cannabis plants and inhibiting GA3 biosynthesis promotes floral development. As such, paclobutrazol (PBZ), a potent GA3 biosynthesis inhibitor may be useful for increasing floral biomass and expediting development, but due to health concerns, its use is prohibited in cannabis production. The present study was conducted to compare the use of PBZ with tannic acid (TA), a natural compound with potential GA3 inhibiting characteristics. Results confirmed that PBZ significantly affected the number of flowers, percentage of flowering plantlet, and flower appearance time. Treatment using PBZ at a concentration of 10 μM resulted in the greatest number of flowers (7.95) compared to other treatments. Moreover, this compound at concentrations of 5 and 10 μM yielded the highest percentage of flowering plantlets, at 75 % and 70 %, respectively. Flowers also appeared 7-15 days sooner than other treatments. Additionally, the energy transfer efficiency in the photosynthetic system and chlorophyll concentration in plants treated with PBZ were considerably higher than those under other treatments. Under the PBZ treatment, the length of internode was significantly reduced. In contrast, TA generally had the opposite effect of PBZ, suggesting that it does not act as a GA3 inhibitor in this context. Furthermore, positive effects of TA at a concentration of 10 μM were observed on total leaf area (840.08 mm2) and stem length (40.09 mm). The highest number of leaves (12.5) was found in the presence of TA at a concentration of 100 μM. TA at its highest concentration (1000 μM) had an inverse effect on cannabis growth and flowering but was likely due to toxicity rather than any inhibitory effects. Consequently, the obtained results confirm the importance of growth regulators and natural compounds on plant growth and can broaden our understanding for future research and achievement of objectives.

Integrated transcriptomic and physio-molecular studies unveil the melatonin and PGPR induced protection to photosynthetic attributes in Brassica juncea L. under cadmium toxicity

Cd is highly mobile, non-essential trace element, that has become serious environmental issue due to its elevated concentration in soil. The present study was taken up to work out salutary effect of melatonin (Mlt) and PGPR ((Pseudomonas putida (Pp), Pseudomonas fluorescens (Pf) in 10 days old Cd stressed (0.3 mM) Brassica juncea L. seedlings. The present work investigated growth characteristics, photosynthetic pigments, secondary metabolites in melatonin-PGPR inoculated B. juncea seedlings. It was backed by molecular studies entailing RT-PCR and transcriptomic analyses. Our results revealed, substantial increase in photosynthetic pigments and secondary metabolites, after treatment with melatonin, P.putida, P. fluorescens in Cd stressed B. juncea seedlings, further validated with transcriptome analysis. Comparative transcriptome analyses identified 455, 5953, 3368, 2238 upregulated and 4921, 430, 137, 27 down regulated DEGs, Cn-vs-Cd, Cd-vs-Mlt, Cd-vs-Mlt-Pp-Pf, Cd-vs-Mlt-Pp-Pf-Cd comparative groups respectively. In depth exploration of genome analyses (Gene ontology, Kyoto encyclopaedia of genes), revealed that Cd modifies the expression patterns of most DEGs mainly associated to photosystem and chlorophyll synthesis. Also, gene expression studies for key photosynthetic genes (psb A, psb B, CHS, PAL, and PSY) suggested enhanced expression in melatonin-rhizobacteria treated Cd stressed B. juncea seedlings. Overall, results provide new insights into probable mechanism of Mlt-PGPR induced protection to photosynthesis in Cd stressed B. juncea plants.

The role of drought response genes and plant growth promoting bacteria on plant growth promotion under sustainable agriculture: A review

Drought is a major stressor that poses significant challenges for agricultural practices. It becomes difficult to meet the global demand for food crops and fodder. Plant physiology, physico-chemistry and morphology changes in plants like decreased photosynthesis and transpiration rate, overproduction of reactive oxygen species, repressed shoot and root shoot growth and modified stress signalling pathways by drought, lead to detrimental impacts on plant development and output. Coping with drought stress requires a variety of adaptations and mitigation techniques. Crop yields could be effectively increased by employing plant growth-promoting rhizobacteria (PGPR), which operate through many mechanisms. These vital microbes colonise the rhizosphere of crops and promote drought resistance by producing exopolysaccharides (EPS), 1-aminocyclopropane-1-carboxylate (ACC) deaminase and phytohormones including volatile compounds. The upregulation or downregulation of stress-responsive genes causes changes in root architecture due to acquiring drought resistance. Further, PGPR induces osmolyte and antioxidant accumulation. Another key feature of microbial communities associated with crops includes induced systemic tolerance and the production of free radical-scavenging enzymes. This review is focused on detailing the role of PGPR in assisting plants to adapt to drought stress.

A comparative study on cadmium tolerance and applicability of two Solanum lycopersicum L. cultivars

Solanum lycopersicum L. can be classified into low Cd-accumulating and high Cd-accumulating types based on their accumulation characteristics of cadmium (Cd). There are many common S. lycopersicum varieties available in the market, but their specific Cd tolerance and enrichment abilities are not well understood. This article uses two S. lycopersicum cultivars, Yellow Cherry and Yellow Pearl, as experimental materials. The experimental method of soil pot planting was adopted, and Cd concentrations in the soil were added at 0, 0.6, 1.5, 2.5, 5, and 10 mg/kg. The changes in Cd content, biomass, photosynthetic pigment content, and photosynthetic parameters of the two S. lycopersicum cultivars were analyzed to screen for low-accumulation S. lycopersicum cultivars. The results showed that S. lycopersicum are Cd-sensitive plants. The Cd accumulation, photosynthetic parameters, and other basic indicators of Yellow Cherry basically showed significant differences when the soil Cd concentration was 0.6 mg/kg, and the biomass showed significant differences when the soil Cd concentration was 1.5 mg/kg. Except for the Cd accumulation in the roots and leaves of Yellow Pearl, which showed significant differences at a soil Cd concentration of 0.6 mg/kg, the other indicators basically showed significant differences when the soil Cd concentration was 1.5 mg/kg. When the soil Cd concentration was 0.6 mg/kg, the Cd accumulation in the fruit of Yellow Pearl was 0.04 mg/kg, making it a low-accumulation S. lycopersicum variety suitable for promoting cultivation in Cd-contaminated soil at 0.6 mg/kg. In conclusion, the Cd accumulation in the fruit of Yellow Pearl is significantly lower than that of Yellow Cherry and even below the Cd limit value for fresh vegetables specified in GB2762-2017. Therefore, Yellow Pearl can be grown as edible crops in soils with Cd concentrations ≤0.6 mg/kg. Furthermore, Yellow Cherry demonstrate strong Cd tolerance and can be used for the remediation of Cd-contaminated soils.

Foliar application protected vegetable against poisonous element cadmium and mitigated human health risks

Foliar application has been reported as an effective method to facilitate plant growth and mitigate cadmium (Cd) accumulation. However, the application of foliar fertilizers on plant production, Cd uptake and health risks of Solanaceae family remains unknown. In this study, four foliar fertilizers were applied to investigate their effects on the production, Cd accumulation and human health risk assessment of two varieties of pepper (Capsicum annuum L.) and eggplant (Solanum melongena L.), respectively. Compared with CK, the foliar application increased vegetable production to 104.16 %-123.70 % in peppers, and 100.83 %-105.17 % in eggplants, accordingly. The application of foliar fertilizers largely decreased Cd TF (transportation factor) by up to 23.32 % in JY, 18.37 % in GJ of pepper varieties, and up to 14.47 % in ZL, 15.24 % in HGR of eggplant varieties. Moreover, Cd BAF (bioaccumulation factor) also declined to different extents after the application of foliar fertilizers. As for human health risk assessments, foliar application diminished the hazard index (HI) and carcinogenic risk (CR) of both pepper and eggplant varieties. The results concluded that the application of composed foliar fertilizers was most effective, and could be a promising alternative for the improvement of vegetable production and mitigation of vegetable Cd accumulation and human health risks as well. The results further highlighted the understanding of foliar fertilizer application on vegetable production and health risks, which benefited better vegetable safe production and further guaranteed human health.

Effects of cadmium and zinc interactions on the physiological biochemistry and enrichment characteristics of Iris pseudacorus

Compound pollution with cadmium (Cd) and zinc (Zn) is common in nature. The effects of compounded Cd and Zn on the growth and development of Iris pseudacorus in the environment and the plant's potential to remediate heavy metals in the environment remain unclear. In this study, the effects of single and combined Cd and Zn stress on I. pseudacorus growth and the enrichment of heavy metals in I. pseudacorus seedlings were investigated. The results showed that under Cd (160 μM) and Zn (800 μM) stress, plant growth was significantly inhibited and photosynthetic performance was affected. Cd+Zn200 (160 μM + 200 μM) reduced the levels of malondialdehyde, hydrogen peroxide, and non-protein thiols by 31.29%, 53.20%, and 13.29%, respectively, in the aboveground tissues compared with levels in the single Cd treatment. However, Cd+Zn800 (160 μM + 800 μM) had no effect. Cd and Zn800 inhibited the absorption of mineral elements, while Zn200 had little effect on plants. Compared with that for Cd treatment alone, Cd + Zn200 and Cd+Zn800 reduced the Cd content in aboveground tissues by 54.15% and 49.92%, respectively, but had no significant effect on Cd in the root system. Zn significantly reduced the Cd content in subcellular components and limited the content and proportion of Cd extracted using water and ethanol. These results suggest that a low supply of Zn reduces Cd accumulation in aboveground tissues by promoting antioxidant substances and heavy metal chelating agents, thus protecting the photosynthetic systems. The addition of Zn also reduced the mobility and bioavailability of Cd to alleviate its toxicity in I. pseudacorus.

Synergistic interactions of assorted ameliorating agents to enhance the potential of heavy metal phytoremediation

Pollution by toxic heavy metals creates a significant impact on the biotic community of the ecosystem. Nowadays, a solution to this problem is an eco-friendly approach like phytoremediation, in which plants are used to ameliorate heavy metals. In addition, various amendments are used to enhance the potential of heavy metal phytoremediation. Symbiotic microorganisms such as phosphate-solubilizing bacteria (PSB), endophytes, mycorrhiza and plant growth-promoting rhizobacteria (PGPR) play a significant role in the improvement of heavy metal phytoremediation potential along with promoting the growth of plants that are grown in contaminated environments. Various chemical chelators (Indole 3-acetic acid, ethylene diamine tetra acetic acid, ethylene glycol tetra acetic acid, ethylenediamine-N, N-disuccinic acid and nitrilotri-acetic acid) and their combined action with other agents also contribute to heavy metal phytoremediation enhancement. With modern techniques, transgenic plants and microorganisms are developed to open up an alternative strategy for phytoremediation. Genomics, proteomics, transcriptomics and metabolomics are widely used novel approaches to develop competent phytoremediators. This review accounts for the synergistic interactions of the ameliorating agent's role in enhancing heavy metal phytoremediation, intending to highlight the importance of these various approaches in reducing heavy metal pollution.

Pseudomonas fluorescens and L-tryptophan application triggered the phytoremediation potential of sunflower (Heliantus annuus L.) in lead-contaminated soil

Lead, a toxic heavy metal present in soil, hampers biological activities and affects the metabolism of plants, animals, and human beings. Its higher concentration may disturb the various physio-chemical processes, which result in stunted and poor plant growth. An interactive approach of plant growth promoting rhizobacteria (PGPR) and L-tryptophan can be used to mitigate the lethal effects of lead. A pot experiment was conducted, and two weeks before sowing, the level of lead (300 mg kg-1) was maintained by spiking the PbCl2 salt. Pseudomonas fluorescens and L-tryptophan were applied individually as well as in combination to segregate the effect of both in contaminated soil under a completely Randomized Design (CRD). Statistical analysis revealed that plant growth was significantly reduced up to 22% due to lead contamination. However, the interactive approach of PGPR and L-tryptophan significantly improved the plant growth, physiology, and yield with relative productive index (RPI) under a lead-stressed environment. Moreover, integrated use of PGPR and L-tryptophan demonstrated a considerable increase (22%) in lead removal efficiency (LRE) by improving bioconcentration factor (BCF) and translocation factor (TF) for shoot without increasing the lead concentration in achenes. The reduced lead concentration in achene was due to its immobilization in shoot and root by negatively charged particles and improved the lead sequestration in vegetative parts which abridged the translocation of lead into achenes.

Rhizospheric Lactobacillus spp. contribute to the high Cd-accumulating characteristics of Phytolacca spp. in acidic Cd-contaminated soil

Screening high Cd-accumulating plants and understanding the interactions between plants, rhizospheric microbes and Cd are important in developing microbe-assisted phytoremediation techniques for Cd-contaminated soils. In this study, the Cd tolerance and accumulation characteristics of Phytolacca americana L., P. icosandra L. and P. polyandra Batalin growing in acidic Cd-contaminated soil were compared to evaluate their phytoremediation potential. According to Cd concentrations (root: 8.26-37.09 mg kg-1, shoot: 2.80-9.26 mg kg-1), bioconcentration factors (BCFs) and translocation factors (TFs), the three Phytolacca species exhibited high Cd-accumulation capacities, ranked in the following order: P. icosandra (root BCF: 1.25, shoot BCF: 0.31, TF: 0.25) > P. polyandra (root BCF: 0.68, shoot BCF: 0.26, TF: 0.44) > P. americana (root BCF: 0.28, shoot BCF: 0.09, TF: 0.38). Phytolacca icosandra and P. polyandra can thus be considered as two new Cd accumulators for phytoremediation. Soil pH, available Cd (ACd) concentration and certain bacterial taxa (e.g. Lactobacillus, Helicobacter, Alistipes, Desulfovibrio and Mucispirillum) were differentially altered in the rhizospheres of the three Phytolacca species in comparison to unplanted soil. Correlation analysis showed that there were significant interactions between rhizospheric ACd concentration, pH and Lactobacillus bacteria (L. murinus, L. gasseri and L. reuteri), which affected Cd uptake by Phytolacca plants. The mono- and co-inoculation of L. murinus strain D51883, L. gasseri strain D51533 and L. reuteri strain D24591 in the rhizosphere of P. icosandra altered the rhizospheric pH and ACd concentrations, in addition to increasing the shoot Cd contents by 31.9%-44.6%. These results suggest that recruitment of rhizospheric Lactobacillus spp. by Phytolacca plants contributes to their high Cd-accumulating characteristics. This study provides novel insights into understanding the interactions between plants, rhizobacteria and heavy metals.

Transcriptomics and metabolomics association analysis revealed the responses of Gynostemma pentaphyllum to cadmium

Gynostemma pentaphyllum an important medicinal herb, can absorb high amounts of cadmium (Cd) which can lead to excessive Cd contamination during the production of medicines and tea. Hence, it is crucial to investigate the response mechanism of G. pentaphyllum under Cd stress to develop varieties with low Cd accumulation and high tolerance. Physiological response analysis, transcriptomics and metabolomics were performed on G. pentaphyllum seedlings exposed to Cd stress. Herein, G. pentaphyllum seedlings could significantly enhance antioxidant enzyme activities (POD, CAT and APX), proline and polysaccharide content subject to Cd stress. Transcriptomics analysis identified the secondary metabolites, carbohydrate metabolism, amino acid metabolism, lipid metabolism, and signal transduction pathways associated with Cd stress, which mainly involved the XTH, EXP and GST genes. Metabolomics analysis identified 126 differentially expressed metabolites, including citric acid, flavonoid and amino acids metabolites, which were accumulated under Cd stress. Multi-omics integrative analysis unraveled that the phenylpropanoid biosynthesis, starch, and sucrose metabolism, alpha-linolenic acid metabolism, and ABC transporter were significantly enriched at the gene and metabolic levels in response to Cd stress in G. pentaphyllum. In conclusion, the genetic regulatory network sheds light on Cd response mechanisms in G. pentaphyllum.