Changes in the chemical properties and swelling coefficient of alfalfa root cell walls in the presence of toluene as a toxic agent

More negative charges on roots enhanced manganese(II) uptake in leguminous and non-leguminous poaceae crops

BACKGROUND

Manganese (Mn) is an essential micronutrient for plants, whereas excess Mn(II) in soils leads to its toxicity to crops. Mn(II) is adsorbed onto plant roots from soil solution and then absorbed by plants. Root charge characteristics should affect Mn(II) toxicity to crops and Mn(II) uptake by the roots of the crops. However, the differences in the effects of root surface charge on the uptake of Mn(II) among various crop species are not well understood.

RESULTS

The roots of nine legumes and six non-legume poaceae were obtained by hydroponics and the streaming potential method and spectroscopic analysis were used to measure the zeta potentials and functional groups on the roots, respectively. The results indicate that the exchangeable Mn(II) adsorbed by plant roots was significantly positively correlated with the Mn(II) accumulated in plant shoots. Legume roots carried more negative charges and functional groups than non-legume poaceae roots, which was responsible for the larger amounts of exchangeable Mn(II) on legume roots in 2 h and the Mn(II) accumulated in their shoots in 48 h. Coexisting cations, such as Ca²⁺ and Mg²⁺ , were most effective in decreasing Mn(II) taken up by roots and accumulated in shoots than K⁺ and Na⁺ . This was because Ca²⁺ and Mg²⁺ could compete with Mn(II) for active sites on plant roots more strongly compared to K⁺ and Na⁺ .

CONCLUSION

The root surface charge and functional groups are two important factors influencing Mn(II) uptake by roots and accumulation in plant shoots. © 2023 Society of Chemical Industry.

Hydrogen peroxide mediates cadmium accumulation in the root of a high cadmium-accumulating rice (Oryza sativa L.) line

Hydrogen peroxide (H₂O₂) is a vital signaling molecule in response to cadmium (Cd) stress in plants. However, the role of H₂O₂ on Cd accumulation in root of different Cd-accumulating rice lines remains unclear. Exogenous H₂O₂ and 4-hydroxy-TEMPO (H₂O₂ scavenger) were applied to investigate the physiological and molecular mechanisms of H₂O₂ on Cd accumulation in the root of a high Cd-accumulating rice line Lu527-8 through hydroponic experiments. Interestingly, it was found Cd concentration in the root of Lu527-8 increased significantly when exposed to exogenous H₂O₂, while reduced significantly when exposed to 4-hydroxy-TEMPO under Cd stress, proving the role of H₂O₂ in regulating Cd accumulation in Lu527-8. Lu527-8 showed more Cd and H₂O₂ accumulation in the roots, along with more Cd accumulation in cell wall and soluble fraction, than the normal rice line Lu527-4. In particular, more pectin accumulation, especially low demethylated pectin, was observed in the root of Lu527-8 when exposed to exogenous H₂O₂ under Cd stress, resulting in more negative functional groups with greater capacity to binding Cd in the root cell wall of Lu527-8. It indicated that H₂O₂-induced cell wall modification and vacuolar compartmentalization contributes greatly to more Cd accumulation in the root of the high Cd-accumulating rice line.

Hydrogen peroxide contributes to cadmium binding on root cell wall pectin of cadmium-safe rice line (Oryza sativa L.)

Cell wall pectin is essential for cadmium (Cd) accumulation in rice roots and hydrogen peroxide (H₂O₂) plays an important role as a signaling molecule in cell wall modification. The role of H₂O₂ in Cd binding in cell wall pectin is unclear. D62B, a Cd-safe rice line, was found to show a greater Cd binding capacity in the root cell wall than a high Cd-accumulating rice line of Wujin4B. In this study, we further investigated the mechanism of the role of H₂O₂ in Cd binding in root cell wall pectin of D62B compared with Wujin4B. Cd treatment significantly increased the H₂O₂ concentration and pectin methyl esterase (PME) activity in the roots of D62B and Wujin4B by 22.45-42.44% and 12.15-15.07%, respectively. The H₂O₂ concentration and PME activity significantly decreased in the roots of both rice lines when H₂O₂ was scavenged by 4-hydroxy-Tempo. The PME activity of D62B was higher than that of Wujin4B. The concentrations of high and low methyl-esterified pectin in the roots of D62B significantly increased when exposed to Cd alone but significantly decreased when exposed to Cd and exogenous 4-hydroxy-Tempo. No significant difference was detected in Wujin4B. Exogenous 4-hydroxy-Tempo significantly decreased the Cd concentration in the cell wall pectin in both rice lines. The modification of H₂O₂ in Cd binding was further explored by adding H₂O₂. The maximum Cd adsorption amounts on the root cell walls of both rice lines were improved by exogenous H₂O₂·H₂O₂ treatment significantly influenced the relative peak area of the main functional groups (hydroxyl, carboxyl), and the groups intensely shifted after Cd adsorption in the root cell wall of D62B, while there was no significant difference in Wujin4B. In conclusion, Cd stress stimulated the production of H₂O₂, thus promoting pectin biosynthesis and demethylation and releasing relative functional groups involved in Cd binding on cell wall pectin, which is beneficial for Cd retention in the roots of Cd-safe rice line.

The role of polysaccharides functional groups in cadmium binding in root cell wall of a cadmium-safe rice line

Exploring the mechanism of cadmium (Cd) accumulation in Cd-safe rice lines is beneficial for ensuring rice safety. D62B, a Cd-safe rice line, accumulates less than 0.2 mg Cd kg^-1 in the brown rice due to strong capacity of Cd retention in the roots, and the root cell wall (RCW) polysaccharides play important roles. However, specific underlying mechanism of Cd binding on the polysaccharides is little known. In this study, the role of polysaccharides, especially pectin and hemicellulose 1 (HC1), in RCW of D62B was investigated by adsorption experiments and Fourier Transform Infrared Spectroscopy (FTIR) analysis compared with a common rice line (Luhui17). Cadmium was adsorbed on RCW of two rice lines by a multilayer and inhomogeneous chemisorption way with the force of ion transfer or exchange. Cadmium was adsorbed on RCW rapidly at first stage with the limit of internal and external diffusion, and gradually reached saturation. With the removal of pectin, the Cd adsorption rate, maximum Cd adsorption amount and the shift degree of carboxyl groups in the RCW of D62B sharply decreased, which showed advantages compared with Luhui17. Sequential removal of HC1 further decreased the maximum Cd adsorption amount and the shift degree of hydroxyl groups. The results showed that more available functional groups, especially carboxyl groups in pectin and hydroxyl groups in HC1, contributed to Cd immobilization within the RCW of Cd-safe rice line, thus limiting Cd translocation to the shoot and reducing Cd accumulation in the brown rice.

Estimation of hazardous concentration of toluene in the terrestrial ecosystem through the species sensitivity distribution approach

Toluene is a highly flammable and commonly used industrial chemical with severe health consequences on humans upon exposure and ingestion. In this study, multispecies bioassays were conducted using a species sensitivity distribution approach to determine acute and chronic hazardous concentrations of toluene in soil. Acute and chronic toluene toxicity tests were conducted with seven soil species from four taxonomic groups. The results from the toxicity tests were used to estimate the acute and chronic HC₅ (hazardous concentration for 5 % of species) of toluene in the terrestrial environment at 58.9 (5.4-639.6) mg kg^-1 and 2.2 (0.2-19.8) mg kg^-1, respectively. To the best of our knowledge, this is the first study to estimate the hazardous concentration of toluene in soil by conducting a battery of bioassays. These values can be used as references for the environmental risk assessment of chemical accidents involving toluene and estimating its impact on soil to protect the terrestrial environment.

The regulatory role of root in cadmium accumulation in a high cadmium-accumulating rice line (Oryza sativa L.)

There are some key processes that regulate cadmium (Cd) accumulation in rice. Understanding the characteristics and mechanisms of Cd accumulation in high Cd-accumulating rice lines benefits for excavating relevant genes. Cd accumulation and distribution in roots of Lu527-8, a high Cd-accumulating rice line, were investigated by a hydroponic experiment, with a control of a normal rice line (Lu527-4). Lu527-8 showed significantly higher Cd concentrations in roots than Lu527-4. More than 81% of Cd in roots of two rice lines is distributed in soluble fraction and cell wall. In soluble fraction, there were more organic acids, amino acids, and phytochelatins in Lu527-8, benefiting Cd accumulation. Pectin and hemicellulose 1 (HC1), especially pectin, were main polysaccharides in cell wall. Lu527-8 showed more pectin and HC1 along with higher pectin methylesterase (PME) activity compared with Lu527-4, promoting Cd accumulation. Besides, Lu527-8 showed higher Cd translocation from root to shoot due to more amounts of ethanol-extractable Cd in roots than Lu527-4. In conclusion, specific characteristics of Cd chemical forms and subcellular distribution in roots of high Cd-accumulating rice line are important for Cd accumulation and translocation.

Effects of surface charge and chemical forms of manganese(II) on rice roots on manganese absorption by different rice varieties

The roots of 4 japonica, 4 indica, and 7 hybrid rice varieties were obtained by hydroponic experiment and used to explore the relationship between charge characteristics and exchangeable manganese(II) (Mn(II)) on rice roots and Mn(II) absorption in roots and shoots of the rice. Results indicated Mn(II) adsorbed on rice roots mainly existed as exchangeable Mn(II) after 2 h. The roots of indica and hybrid rice carried more negative charges than the roots of japonica rice. Accordingly, this led to more exchangeable Mn(II) to be adsorbed on roots of indica and hybrid rice after 2 h and more Mn(II) absorbed in the roots of the same varieties after 48 h. However, this was contrary to the result of Mn(II) absorption in rice shoots after 48 h. Coexisting cations of K⁺, Na⁺, Ca²⁺, and Mg²⁺ reduced the exchangeable Mn(II) on rice roots through their competition with Mn(II) for sorption sites on rice roots, which led to the decrease in Mn(II) absorption in rice roots and shoots. Ca²⁺ and Mg²⁺ showed a greater decrease in the Mn(II) absorbed in roots and shoots than K⁺ and Na⁺. The reduction of Mn(II) absorption in the roots of indica rice and hybrid rice induced by Ca²⁺ and Mg²⁺ was more than that of japonica rice. This was attributed to more negative charges on the roots of the former than the latter. Therefore, the absorption of Mn(II) by rice roots was determined by surface charge properties and exchangeable Mn(II) on the rice roots. The results suggested that Ca²⁺ and Mg²⁺ have potential to alleviate Mn(II) toxicity to rice.

Plants alter surface charge and functional groups of their roots to adapt to acidic soil conditions

The rapid increase in soil acidification rate has led to a decrease in global agricultural productivity owing to the debilitating effects of Al and Mn toxicities. In this study, we investigated the adaptation of plants to acidic conditions by examining the behavior of plant roots grown in hydroponic solution and pot experiments at different pHs. The Mn(II) sorption by the roots was investigated and the mechanisms involved were deduced by analyzing the changes in the zeta potential and functional groups on the root surface. The exchangeable, complexed, and precipitated Mn(II) on plant roots were extracted sequentially with 1 M KNO₃, 0.05 M EDTA-2Na, and 0.01 M HCl. The results of hydroponic experiment indicated that plant roots subjected to NH₄⁺ treatment carried lower negative charge and fewer functional groups owing to acidic pH condition induced by NH₄⁺ uptake of roots, when compared with plant roots treated with NO₃^-. Similarly, in pot experiments, the surface negative charge and functional groups of plant roots cultured in soils with lower pH were fewer than those on plant roots cultured in soils with higher pH, with the former presenting less exchangeable and complexed Mn(II) sorption than the latter. Thus, alterations in the charge properties and number of functional groups on the surface of plant roots are some of the mechanisms used by plants to adapt to acidic soil condition.

Morphological effect of dichloromethane on alfalfa (Medicago sativa) cultivated in soil amended with fertilizer manures

In this work, we investigated the morphological effect of dichloromethane (DCM) on alfalfa (Medicago sativa) plant. We studied in vitro the influence of its concentration on alfalfa germination. The plants were placed in pots for 15 weeks, and exposed to increasing concentrations of DCM (50 µg L^-1 and 84 mg L^-1). In addition, we examined the effect of two manures (cow and sheep), which were applied to a contaminated soil, on alfalfa plant growth. The effect of the presence of dichloromethane is obvious even in plant-soil manure system. In fact, in the event of contamination, the soil-cow manure mixture represents the best setting medium for the Alfalfa plant compared to other environments, regardless of the contamination level. Indeed, the presence of two types of manure does not allow the suppression of the inhibitory effect of dichloromethane on the mass of the dry matter of the aerial part which is 18.38% for the cow manure-amended soil and 13.96% for the sheep manure-amended soil.

Iron Plaque Prevents Partitioning of Polycyclic Aromatic Hydrocarbons on Rice (Oryza sativa) Root Surface

Adsorption of polycyclic aromatic hydrocarbons (PAHs) on root surfaces has essential impacts on PAH phytoremediation. An Fe plaque is commonly formed on the root surface of aquatic plants. Therefore, it is worth investigating the impact of the Fe plaque on PAH adsorption on rice root surfaces. Using Bayesian linear water-methanol cosolvent models, we estimated accurate water-biosorbent partition coefficient values for phenanthrene, pyrene, and benzo[a]pyrene between water and rice root biosorbent fractions, including rice root materials with Fe plaque, removed Fe plaque, and removed Fe plaque and lipids. Our results showed that Fe plaque inhibited the adsorption of PAHs on rice root surface; the inhibition impacts increased with hydrophobicity of PAHs. This result highlights the need for considering the impact of Fe plaque on PAH adsorption during phytoremediation. Integr Environ Assess Manag 2020;16:392-399. © 2020 SETAC.

Characteristics of cadmium immobilization in the cell wall of root in a cadmium-safe rice line (Oryza sativa L.)

Cultivating cadmium (Cd)-safe rice lines, which show low Cd accumulation in brown rice, is generally beneficial to ensure food safety. The Cd retention in root of Cd-safe rice line D62B plays an important role in its low Cd translocation from root to shoot. To understand the mechanism of Cd retention in root, a hydroponic experiment was conducted to investigate the subcellular distribution of Cd and the contribution of polysaccharides to Cd binding to the root cell wall of a Cd-safe rice line D62B with a common rice line Luhui17 as a control material. D62B retained more Cd in the root by sequestrated a higher proportion of Cd in the cell wall, further it transferred less Cd to shoot. Close to half of the Cd in the root cell wall of D62B was accumulated in the hemicellulose 1 (HC1), and the proportions of HC1 in it were 1.2-1.7 times higher than these of Luhui17. The proportion of Cd in the pectin showed a dose-dependent increase in two rice lines. D62B contained significantly higher uronic acid concentrations of the pectin and greater pectin methyl esterase (PME) activities than Luhui17 in the root cell wall. These results indicated that a superior Cd binding capacity of the cell wall polysaccharides in D62B played an important role in its Cd retention in root.

Effects of 1-butyl-3-methylimidazolium chloride on the photosynthetic system and metabolism of maize (Zea mays L.) seedlings

Ionic liquids (ILs) are widely used in various chemical processes. However, a growing number of studies have found that ILs are potentially toxic to different types of living organisms, including crops. The present study analysed the effects of 1-butyl-3-methylimidazolium chloride ([C₄mim]Cl) on the photosynthetic system and metabolism of maize seedlings. Results showed that [C₄mim]Cl could significantly reduce maize leaf chlorophyll level and cause extensive leaf bleaching. The activity of photosystem II (PSII) was significantly inhibited when seedlings exposed to higher concentration of [C₄mim]Cl. The maximum quantum yield of PSII and the potential efficiency of PSII were reduced by 63% and 88% under 800 mg/L [C₄mim]Cl treatment in comparison with the control treatment. The RNA sequencing analysis performed to examine gene expression profiles of maize leaves under [C₄mim]Cl treatment revealed 639 differentially expressed genes (DEGs), 115 of which were categorized into different metabolic pathways. Among these DEGs, the seven genes involved in the photosynthetic Calvin cycle were down-regulated by [C₄mim]Cl exposure. For carbohydrates and amino acids metabolism, the genes for starch synthesis were down-regulated, while the genes for amino acids and protein degradation were up-regulated. The changes observed in these major metabolic pathways might be an important reason for [C₄mim]Cl toxicity.