Tolerance of cotton to elevated levels of Pb and its potential for phytoremediation

Nitric oxide enhances copper tolerance by regulating cell wall composition and copper transporting-related transcripts in cotton roots

Little is known about nitric oxide (NO)-mediated cotton plants' response to copper (Cu) stress and the underlying tolerance mechanism. It was hypothesized that NO can alleviate Cu toxicity to cotton roots by regulating the root cell wall composition and the transcription of Cu ion transporting-related genes. Cu stress significantly increased NO synthase (EC 1.14.14.47) activity, leading to elevated endogenous NO content. Cu excess-induced growth inhibition was reversed by sodium nitroprusside (SNP, NO donor) application but exacerbated by cPTIO (NO scavenger) addition. The SNP + Cu treatment promoted more Cu ions accumulation in roots and less Cu ions transportation to leaves than Cu treatment, which also produced the largest Cu uptake amount per plant among all treatments. The concentration of cell wall pectin was significantly enhanced by 16.95% by the SNP application. Pectin methylesterase activity was up-regulated by 30.86% (p < 0.05), thus resulting in a reduction of 10.39% in pectin methylesterification degree in the Cu + SNP treatment than in Cu stress alone; additionally, Cu chaperons COX17, CCH, and ATX1, Cu chelator MT2, and Cu homeostasis regulator SPL7 exhibited higher transcriptional levels. Collectively, NO improved cotton roots' tolerance to Cu stress through the enhancement of Cu ions binding to cell wall due to increased polysaccharide biosynthesis and pectin demethylesterification degree, and via the promotion of Cu ions sequestration owing to up-regulated expressions of Cu chaperones and chelators. These findings should have significant implications for the phytoremediation of Cu-contaminated soils by using cotton plants, which needs further validation under field conditions.

Role of polyethylene glycol to alleviate lead stress in Raphanus sativus

The continuous contamination of heavy metals (HMs) in our ecosystem due to industrialization, urbanization and other anthropogenic activities has become a serious environmental constraint to successful crop production. Lead (Pb) toxicity causes ionic, oxidative and osmotic injuries which induce various morphological, physiological, metabolic and molecular abnormalities in plants. Polyethylene glycol (PEG) is widely used to elucidate drought stress induction and alleviation mechanisms in treated plants. Some recent studies have unveiled the potential of PEG in regulating plant growth and developmental procedures including seed germination, root and shoot growth and alleviating the detrimental impacts of abiotic stresses in plants. Therefore, the current study aimed to assess the effects of seed priming with various concentrations (10%, 20%, 30% and 40%) of PEG on the growth and development of radish plants growing under Pb stress (75 mg/kg soil). Lead toxicity reduced root growth (32.89%), shoot growth (32.81%), total chlorophyll (56.25%) and protein content (58.66%) in treated plants. Similarly, plants showed reduced biomass production of root (35.48%) and shoot (31.25%) under Pb stress, while 30% PEG seed priming enhanced biomass production of root (28.57%) and shoot (35.29%) under Pb contaminated regimes. On the other hand, seedlings obtained from 30% PEG priming demonstrated a notable augmentation in the concentrations of photosynthetic pigments, antioxidative activity and biomass accumulation of the plants. PEG-treated plants showed modulations in the enzymatic activities of peroxidase (PO), catalase (CAT) and superoxide dismutase (SOD). These changes collectively played a role in mitigating the adverse effects of Pb on plant physiology. Our data revealed that PEG interceded stress extenuation encompasses numerous regulatory mechanisms including scavenging of ROS through antioxidant and non-antioxidants, improved photosynthetic activity and appropriate nutrition. Hence, it becomes necessary to elucidate the beneficial role of PEG in developing approaches for improving plant growth and stress tolerance.

Genotypic difference in response to copper stress in upland cotton as revealed by physiological and molecular expression analyses

BACKGROUND

Cotton is a non-edible fiber crop with considerable potential for the remediation of copper-polluted soil. However, the Cu toxicity tolerance mechanism in cotton remains largely obscure. To address the issue, we first identified two cotton lines contrasting in response to Cu toxicity by examining 12 morphological and physiological attributes of 43 origin scattered cotton genotypes under Cu excess. Then both lines were subjected to a comprehensive comparative study, aiming to unravel the cotton Cu tolerance mechanism through integrated morphological, physio-biochemical, Cu uptake and distribution, and related molecular expression analyses.

RESULTS

Based on the phenotypic values and corresponding tolerance indexes of 12 parameters, A2304 and A1415 were identified as Cu-tolerant and -sensitive, respectively. Compared to A1415, A2304 exhibited significantly higher antioxidant enzyme activities and non-enzymatic antioxidant levels, producing fewer amounts of reactive oxygen species and a lower level of malonyldialdehyde. On Cu excess, A2304 accumulated lower concentrations of Cu ions in various plant parts and subcellular components, and fewer Cu ions were presented in active chemical forms. However, the total Cu uptake amount per plant did not differ between both lines due to larger plant biomass with A2304. In contrast to A1415, Cu stress activated or increased the expressions of Cu homeostasis regulator (GhSPL7) and genes responsible for Cu delivery (GhCCS, GhCOX17), chelation (GhMT2), and compartmentation into vacuoles (GhHMA5), while inactivating or decreasing the expressions of genes accounting for Cu uptake (GhCOPT1) and Cu exporting from vacuoles (GhCOPT5) in the root cell with A2304. Additionally, A2304 may impede the root cell wall from binding Cu ions by enhancing the pectin methylesterification degree by up-regulating GhPMEI3 and GhPMEI9 encoding pectin methylesterase inhibitor and stabilizing the cell wall organization by down-regulating GhPLY8 and GhPLY20 encoding pectate lyases.

CONCLUSIONS

To cope with Cu toxicity, the Cu-tolerant genotype activates its antioxidative defense system, immobilizing chemically active Cu ions, and lowering the Cu uptake, bioavailability and immigration within cells by regulating the expressions of genes related to Cu uptake, transport, delivery and cell wall metabolism. This comprehensive comparison study provides insights into breeding Cu-tolerant cotton cultivars that can be utilized for the phytoremediation of Cu-contaminated soils.

Zinc and methyl jasmonate improve sugar beet tolerance to high boron stress by enhanced leaf photochemical performance

Nutrient imbalances, such as high boron (B) stress, occur within, as well as across, agricultural systems worldwide and have become an important abiotic factor that reduces soil fertility and inhibits plant growth. Sugar beet is a B-loving crop and is better suited to be grown in high B environments, but the methods and mechanisms regarding the enhancement of high-B stress tolerance traits are not clear. The main objective of this research was to elucidate the effects of the alone and/or combined foliar spraying of zinc sulfate (ZnSO4) and methyl jasmonate (MeJA) on the growth parameters, tolerance, and photochemical performance of sugar beet under high-B stress. Results demonstrated that the photosynthetic performance was inhibited under high-B stress, with a reduction of 11.33% in the net photosynthetic rate (Pn) and an increase of 25.30% in the tolerance index. The application of ZnSO4, MeJA, and their combination enhanced sugar beet's adaptability to high-B stress, with an increase in Pn of 9.22%, 4.49%, and 2.85%, respectively, whereas the tolerance index was elevated by 15.33%, 8.21%, and 5.19%, respectively. All three ameliorative treatments resulted in increased photochemical efficiency (Fv/Fm) and the photosynthetic performance index (PIABS) of PSII. Additionally, they enhanced the light energy absorption (ABS/RC) and trapping capacity (DIO/RC), reduced the thermal energy dissipation (TRO/RC), and facilitated the QA to QB transfer in the electron transport chain (ETC) of PSII, which collectively improved the photochemical performance. Therefore, spraying both ZnSO4 and MeJA can better alleviate high-B stress and promote the growth of sugar beet, but the combined spraying effect of ZnSO4 and MeJA is lower than that of individual spraying. This study provides a reference basis for enhancing the ability of sugar beet and other plants to tolerate high-B stress and for sugar beet cultivation in high B areas.

Ecological characteristics of sugar beet plant and rhizosphere soil in response to high boron stress: A study of the remediation potential

High boron (B) stress degrades the soil environment and reduces plant productivity. Sugar beet has a high B demand and potential for remediation of B-toxic soils. However, the mechanism regarding the response of sugar beet plants and rhizosphere soil microbiome to high B stress is not clear. In the potted soil experiment, we set different soil effective B environments (0.5, 5, 10, 30, 50, and 100 mg kg-1) to study the growth status of sugar beets under different B concentrations, as well as the characteristics of soil enzyme activity and microbial community changes. The results showed that sugar beet growth was optimal at 5 mg kg-1 of B. Exceeding this concentration the tolerance index decreased. The injury threshold EC20 was reached at an available B concentration of 35.8 mg kg-1. Under the treatment of 100 mg kg-1, the B accumulation of sugar beet reached 0.22 mg plant-1, and the tolerance index was still higher than 60%, which had not yet reached the lethal concentration of sugar beet. The abundance of Acidobacteriota, Chloroflexi and Patescibacteria increased, which was beneficial to the resistance of sugar beet to high B stress. In summary, under high B stress sugar beet had strong tolerance, enhanced capacity for B uptake and enrichment, and changes in soil microbial community structure. This study provides a theoretical basis for clarifying the mechanism of sugar beet resistance to high B stress and soil remediation.

Toxic effects of lead on plants: integrating multi-omics with bioinformatics to develop Pb-tolerant crops

MAIN CONCLUSION

Lead disrupts plant metabolic homeostasis and key structural elements. Utilizing modern biotechnology tools, it's feasible to develop Pb-tolerant varieties by discovering biological players regulating plant metabolic pathways under stress. Lead (Pb) has been used for a variety of purposes since antiquity despite its toxic nature. After arsenic, lead is the most hazardous heavy metal without any known beneficial role in the biological system. It is a crucial inorganic pollutant that affects plant biochemical and morpho-physiological attributes. Lead toxicity harms plants throughout their life cycle and the extent of damage depends on the concentration and duration of exposure. Higher levels of lead exposure disrupt numerous key metabolic activities of plants including oxygen-evolving complex, organelles integrity, photosystem II connectivity, and electron transport chain. This review summarizes the detrimental effects of lead toxicity on seed germination, crop growth, and yield, oxidative and ultra-structural alterations, as well as nutrient absorption, transport, and assimilation. Further, it discusses the Pb-induced toxic modulation of stomatal conductance, photosynthesis, respiration, metabolic-enzymatic activity, osmolytes accumulation, and antioxidant activity. It is a comprehensive review that reports on omics-based studies along with morpho-physiological and biochemical modifications caused by lead stress. With advances in DNA sequencing technologies, genomics and transcriptomics are gradually becoming popular for studying Pb stress effects in plants. Proteomics and metabolomics are still underrated and there is a scarcity of published data, and this review highlights both their technical and research gaps. Besides, there is also a discussion on how the integration of omics with bioinformatics and the use of the latest biotechnological tools can aid in developing Pb-tolerant crops. The review concludes with core challenges and research directions that need to be addressed soon.

Ecologically different earthworm species are the driving force of microbial hotspots influencing Pb uptake by the leafy vegetable Brassica campestris

Food chain contamination by soil lead (Pb), beginning with Pb uptake by leafy vegetables, is a threat to food safety and poses a potential risk to human health. This study highlights the importance of two ecologically different earthworm species (the anecic species Amynthas aspergillum and the epigeic species Eisenia fetida) as the driving force of microbial hotspots to enhance Pb accumulation in the leafy vegetable Brassica campestris at different Pb contamination levels (0, 100, 500, and 1,000 mg·kg-1). The fingerprints of phospholipid fatty acids (PLFAs) were employed to reveal the microbial mechanism of Pb accumulation involving earthworm-plant interaction, as PLFAs provide a general profile of soil microbial biomass and community structure. The results showed that Gram-positive (G+) bacteria dominated the microbial community. At 0 mg·kg-1 Pb, the presence of earthworms significantly reduced the total PLFAs. The maximum total of PLFAs was found at 100 mg·kg-1 Pb with E. fetida inoculation. A significant shift in the bacterial community was observed in the treatments with E. fetida inoculation at 500 and 1,000 mg·kg-1 Pb, where the G+/G- bacteria ratio was significantly decreased compared to no earthworm inoculation. Principal component analysis (PCA) showed that E. fetida had a greater effect on soil microbial hotspots than A. aspergillum, thus having a greater effect on the Pb uptake by B. campestris. Redundancy analysis (RDA) showed that soil microbial biomass and structure explained 43.0% (R2 = 0.53) of the total variation in Pb uptake by B. campestris, compared to 9.51% of microbial activity. G- bacteria explained 23.2% of the total variation in the Pb uptake by B. campestris, significantly higher than the other microbes. The Mantel test showed that microbial properties significantly influenced Pb uptake by B. campestris under the driving force of earthworms. E. fetida inoculation was favorable for the G- bacterial community, whereas A. aspergillum inoculation was favorable for the fungal community. Both microbial communities facilitated the entry of Pb into the vegetable food chain system. This study delivers novel evidence and meaningful insights into how earthworms prime the microbial mechanism of Pb uptake by leafy vegetables by influencing soil microbial biomass and community composition. Comprehensive metagenomics analysis can be employed in future studies to identify the microbial strains promoting Pb migration and develop effective strategies to mitigate Pb contamination in food chains.

Soil Amendments and Foliar Melatonin Reduced Pb Uptake, and Oxidative Stress, and Improved Spinach Quality in Pb-Contaminated Soil

Amending Pb-affected soil with biochar (BH) and magnesium potassium phosphate cement (MKC) reduces Pb uptake in plants. Moreover, foliar applications of melatonin and proline are also known to reduce plant oxidative stress and Pb uptake. However, little is known about combining both techniques, i.e., adding a combo immobilizing dose (CIA = mixture of BH and MKC at 50:50 ratio) in Pb-polluted soil and foliar application of proline and melatonin for reducing Pb uptake and oxidative stress in spinach. Control, proline, melatonin, CIA, CIA+proline, and CIA+melatonin were the treatments utilized in this pot study to see their effects on reducing plant oxidative stress, Pb uptake, and improving spinach quality in Pb-polluted soil. Moreover, Pb bioavailability, enzymatic activities, and numbers of bacteria, fungi, and actinomycetes in the soil were also evaluated. The effect of CIA on reducing Pb in the soil-plant system and improving soil enzymes and microbial numbers was more pronounced than melatonin alone. The most effective treatment was CIA+melatonin reducing Pb availability in soil (77%), shoots (95%), and roots (84%), alleviating oxidative stress, and improving plant biomass (98%) and nutrients. Soil enzymatic activities and the number of microorganisms in the rhizosphere were also highest with CIA+melatonin. Results highlight the significance of CIA+melatonin, as an inexpensive approach, in remediating Pb-polluted soil and improving spinach quality. However, further research is needed to understand the significance of CIA+melatonin on different crops and various soil Pb concentrations before employing this technique commercially in agriculture and environment sectors.

Combining phytoremediation with bioenergy production: developing a multi-criteria decision matrix for plant species selection

The use of plants to extract metal contaminants from soils has been proposed as a cost-effective means of remediation, and utilizing energy crops for this phytoextraction process is a useful way of attaining added value from the process. To simultaneously attain both these objectives successfully, selection of an appropriate plant species is crucial to satisfy a number of imporTant criteria including translocation index, metal and drought tolerance, fast growth rate, high lignocellulosic content, good biomass production, adequate calorific value, second generation attribute, and a good rooting system. In this study, we proposed a multi-criteria decision analysis (MCDA) to aid decision-making on plant species based on information generated from a systematic review survey. Eight species Helianthus annuus (sunflower), Brassica juncea (Indian mustard), Glycine max (soybean), Salix spp. (willow), Populus spp. (poplar), Panicum virgatum (switchgrass), Typha latifolia (cattails), and Miscanthus sinensis (silvergrass) were examined based on the amount of hits on a number of scientific search databases. The data was normalized by estimating their min-max values and their suitability. These criteria/indicators were weighted based on stipulated research objectives/priorities to form the basis of a final overall utility scoring. Using the MCDA, sunflower and silvergrass emerged as the top two candidates for both phytoremediation and bioenergy production. The multi-criteria matrix scores assist the process of making decisions because they compile plant species options quantitatively for all relevant criteria and key performance indicators (KPIs) and its weighing process helps incorporate stakeholder priorities to the selection process.

Lead, zinc tolerance mechanism and phytoremediation potential of Alcea rosea (Linn.) Cavan. and Hydrangea macrophylla (Thunb.) Ser. and ethylenediaminetetraacetic acid effect

In this study, we aimed to elucidate the defense mechanism of Alcea rosea (Linn.) Cavan. and Hydrangea macrophylla (Thunb.) Ser. against the single and compound toxicity of lead (Pb) and zinc (Zn) along with the synergistic effect of ethylenediaminetetraacetic acid (EDTA) in accumulation of metals in these two species. The two plant species were subjected to single metal treatment (Pb 1000 mg kg^-1, Zn 600 mg kg^-1) and compound metal treatment (Pb 1000 mg kg^-1 + Zn 600 mg kg^-1) in a greenhouse. Besides, different levels of EDTA were applied (2.5, 5.0, and 10.0 mmol kg^-1) with compound metal treatment. Several physiological and biochemical parameters, including plant photosynthetic parameters, enzymatic antioxidant system, accumulation concentration of metals, and subcellular distribution were estimated. The results showed that the antioxidative enzymes, proline, root morphological changes, and metal localization all played important roles in resisting Pb and Zn toxicity. A notable difference was that Zn was concentrated in the roots (58.5%) of H. macrophylla to reduce the damage but in the leaves (38.5%) of A. rosea to promote photosynthesis and resist the toxicity of metals. In addition, Zn reduced the toxicity of Pb to plants by regulating photosynthesis, Pb absorption and Pb distribution in subcells. The biological concentration factors (BCF) and translocation factors (TF) for Pb in two plants were less than 1, indicating that they could be considered as phytostabilizators in Pb-contaminated soils. Moreover, EDTA could enhance the enrichment and transport capacity of Pb and Zn to promote the phytoremediation effect. In summary, both plants have a certain application potential for repairing Pb-Zn-contaminated soil.