The growth and uptake of Ga and In of rice (Oryza sative L.) seedlings as affected by Ga and In concentrations in hydroponic cultures

Mechanism of flavonols on detoxification, migration and transformation of indium in rhizosphere system

Soil contamination by toxic heavy metal induces serious environmental hazards. In recent years, the use of indium (In) in semiconductor products has increased considerably and the release of In is inevitable, which will pose great risk to the ecosystem. The interaction between metal and plants which are the fundamental components of all ecosystems are an indispensable aspect of indium assessment and remediation. The role of flavonols, which is essential to plant resistance to In stress, remains largely unknown. FLS1 related lines of A. thaliana (Col, fls1-3 and OE) were exposed to In stress in soil and flavonols as root exudates were analyzed in exogenous application test. The accumulation and release of flavonols could be induced by In stress. However, flavonols exhibited different function in vivo and in vitro of plant. The basic function of flavonols was to affect root morphology via regulating auxin, but being intervened by In stress. The synthesis and accumulation of flavonols in vivo could activate the antioxidant system and the metal detoxification system to alleviate the toxic effects of In on plant. In addition, plants could make phone calls to rhizosphere microbes for help when exposed to In. Flavonols in vitro might act as the information transmission. Combination of endogenous and exogenous flavonols could affect the migration and transformation of In in soil-plant system via metal complexation and transportation pathway.

Boron deficiency decreased the root activity of Ga-exposed rice seedlings by reducing iron accumulation and increasing Ga in iron plaque

Gallium (Ga) is an emerging chemical pollutant chiefly associated with high-tech industries. Boron (B) alleviates the negative effects of toxic elements on plant growth. Thereby, the effects of B fertilization on Ga toxicity in rice seedlings was studied to clarify the role of iron plaque in the distribution of Ga, Fe, and B in Ga-treated rice seedlings in the presence or absence of B. Gallium exposure significantly reduced the biomass of rice seedlings. Boron deficiency induced a significant change in the distribution of B in Ga-treated rice seedlings compared with "Ga+B" treatments. Accumulation of Ga in roots, dithionite-citrate-bicarbonate (DCB) extracts, and shoots showed a dose-dependent manner from both +B and -B rice seedlings. Boron nutrition levels affect the distribution of Fe in roots, DCB extracts, and shoots, in which DCB-extractable Fe was significantly decreased from "Ga-B" treatments compared with "Ga+B" treatments. Root activity was significantly decreased in both Ga-exposed rice seedlings; however, B-deficient seedlings showed a severe reduction than +B rice seedlings. These results reveal that Fe plaque might be a temporary sink for B accumulation when plants are grown with proper B, wherein the re-utilization of DCB-extractable B stored in Fe plaque is mandatory for plant growth under B deficiency. Correlation analysis revealed that B deficiency decreased the root activity of Ga-exposed rice seedlings by reducing DCB-extractable Fe and increasing DCB-extractable Ga in Fe plaque. This study enhances our understanding of how B nutritional levels affect Ga toxicity in rice plants.

Response to gallium (Ga) exposure and its distribution in rice plants

Root architecture is the temporal and spatial configuration of root system in the heterogeneous matrix of soil that is prone to chemical stresses. Gallium (Ga) is among the emerging chemical pollutants that are mostly associated with high-tech industries, specifically associated with semiconductors. In view of its potential risk and increasing distribution in the environment, this study was designed to evaluate the inhibition rate, Ga distribution in different tissues, and root architecture of rice seedlings under different concentrations of Ga. We observed that 2.59, 46.7, and 168.2 mg Ga/L were minimum (EC20), medium (EC50), and maximum (EC75) effective concentrations for rice plants that corresponded to the 20, 50, and 75% inhibition on the relative growth rate, respectively. Distribution of Ga in rice tissues showed that accumulation of Ga was much higher in roots than shoots of rice seedlings, and it increased with an increase in Ga doses. Evan blue staining technique reveals that the number of damaged/dead cell was dose-dependent on Ga. Moreover, several traits associated with root system architecture demonstrating that rice root system architecture altered in response to Ga stress. Collectively, the results reveal that Ga exposure inhibited the growth and development of rice plants. This study will enhance our understanding that how different concentrations of Ga in the environment can affect plants; however, more comprehensive studies are essential to further determine plant response against Ga stress.

The Uptake of Rare Trace Elements by Perennial Ryegrass (Lolium perenne L.)

Technological development has increased the use of chemical elements that have hitherto received scant scientific attention as environmental contaminants. Successful management of these rare trace elements (RTEs) requires elucidation of their mobility in the soil-plant system. We aimed to determine the capacity of Lolium perenne (a common pasture species) to tolerate and accumulate the RTEs Be, Ga, In, La, Ce, Nd, and Gd in a fluvial recent soil. Cadmium was used as a reference as a well-studied contaminant that is relatively mobile in the soil-plant system. Soil was spiked with 2.5-283 mg kg-1 of RTE or Cd salts, representing five, 10, 20, and 40 times their background concentrations in soil. For Be, Ce, In, and La, there was no growth reduction, even at the highest soil concentrations (76, 1132, 10.2, and 874 mg kg-1, respectively), which resulted in foliar concentrations of 7.1, 12, 0.11, and 50 mg kg-1, respectively. The maximum no-biomass reduction foliar concentrations for Cd, Gd, Nd, and Ga were 0.061, 0.1, 7.1, and 11 mg kg-1, respectively. Bioaccumulation coefficients ranged from 0.0030-0.95, and increased Ce < In < Nd ≅ Gd < La ≅ Be ≅ Ga < Cd. Beryllium and La were the RTEs most at risk of entering the food chain via L. perenne, as their toxicity thresholds were not reached in the ranges tested, and the bioaccumulation coefficient (plant/soil concentration quotient) trends indicated that uptake would continue to increase at higher soil concentrations. In contrast, In and Ce were the elements least likely to enter the food chain. Further research should repeat the experiments in different soil types or with different plant species to test the robustness of the findings.

Elevated CO2 Can Improve the Tolerance of Avena sativa to Cope with Zirconium Pollution by Enhancing ROS Homeostasis

Zirconium (Zr) is one of the toxic metals that are heavily incorporated into the ecosystem due to intensive human activities. Their accumulation in the ecosystem disrupts the food chain, causing undesired alterations. Despite Zr's phytotoxicity, its impact on plant growth and redox status remains unclear, particularly if combined with elevated CO2 (eCO2). Therefore, a greenhouse pot experiment was conducted to test the hypothesis that eCO2 can alleviate the phytotoxic impact of Zr upon oat (Avena sativa) plants by enhancing their growth and redox homeostasis. A complete randomized block experimental design (CRBD) was applied to test our hypothesis. Generally, contamination with Zr strikingly diminished the biomass and photosynthetic efficiency of oat plants. Accordingly, contamination with Zr triggered remarkable oxidative damage in oat plants, with concomitant alteration in the antioxidant defense system of oat plants. Contrarily, elevated levels of CO2 (eCO2) significantly mitigated the adverse effect of Zr upon both fresh and dry weights as well as the photosynthesis of oat plants. The improved photosynthesis consequently quenched the oxidative damage caused by Zr by reducing the levels of both H2O2 and MDA. Moreover, eCO2 augmented the total antioxidant capacity with the concomitant accumulation of molecular antioxidants (e.g., polyphenols, flavonoids). In addition, eCO2 not only improved the activities of antioxidant enzymes such as peroxidase (POX), superoxide dismutase (SOD) and catalase (CAT) but also boosted the ASC/GSH metabolic pool that plays a pivotal role in regulating redox homeostasis in plant cells. In this regard, our research offers a novel perspective by delving into the previously unexplored realm of the alleviative effects of eCO2. It sheds light on how eCO2 distinctively mitigates oxidative stress induced by Zr, achieving this by orchestrating adjustments to the redox balance within oat plants.

Temporal transformation of indium speciation in rice paddy soils and spatial distribution of indium in rice rhizosphere

Indium is a potentially toxic element that could enter human food chains, including soil-rice systems. The submerged environment in rice paddy soil results in temporal and spatial variations in the chemical properties of the rice rhizosphere and bulk soils, expected to cause changes in indium's chemical speciation and consequently affect its bioavailability. Therefore, this study aimed to investigate indium speciation and fractionation in soils at different periods of rice growth under continuous submergence using X-ray absorption spectroscopy and a sequential extraction method. The predominant indium species were identified as indium-associated Fe hydroxide, and indium hydroxide and phosphate precipitates. The reductive dissolution of indium-associated Fe hydroxides led to the release of indium into the soil solution under continuous submergence of soils, and the released indium concentration decreased with time due to re-sorption and re-precipitation. Meanwhile, indium hydroxide was found to be the predominant species in rice rhizosphere using μ-X-ray absorption spectroscopy. The relative depletion of indium-associated Fe hydroxides in the rice rhizosphere was attributed to the low mobility of indium from bulk soil to rice rhizosphere and the root uptake of indium associated with Fe hydroxide around rice roots. Consequently, indium uptake by rice roots was lower during the reproductive and grain-ripening stage of rice growth. Understanding the behavior of indium will help develop a strategy to minimize uptake into crops in indium-contaminated paddy soils.

Insight into the mechanism of indium toxicity in rice

Indium is widely used in the technology industry and is an emerging form of environmental pollution. The presence of indium in soil and groundwater inhibits shoot and root growth in crops, thus reducing yields. However, the underlying mechanisms are unknown, making it difficult to design effective countermeasures. We explored the spatiotemporal effects of excess indium on the morphological, physiological and biochemical properties of rice (Oryza sativa L.). Indium accumulated mainly in the roots, severely restricting their growth and causing the acute perturbation of phosphorus, magnesium and iron homeostasis. Other effects included leaf necrosis and anatomical changes in the roots (thinned sclerenchyma and enlarged epidermal and exodermal layers). Whole-transcriptome sequencing revealed that rice immediately responded to indium stress by activating genes involved in heavy metal tolerance and phosphate starvation responses, including the expression of genes encoding phosphate-regulated transcription factors and transporters in the roots. Direct indium toxicity rather than phosphate deficiency was identified as the major factor affecting the growth of rice plants, resulting in the profound phenotypic changes we observed. The application of exogenous phosphate alleviated indium toxicity by reducing indium uptake. Our results suggest that indium immobilization could be used to prevent indium toxicity in the field.

CO2 Enrichment Differentially Upregulated Sugar, Proline, and Polyamine Metabolism in Young and Old Leaves of Wheat and Sorghum to Mitigate Indium Oxide Nanoparticles Toxicity

Soil contamination with indium oxide nanoparticles (In₂O₃-NPs) is a challenge for plant growth and productivity. Despite In₂O₃-NPs toxicity, their effects on plant growth and metabolism are largely unknown, particularly under future climate CO₂ (eCO₂). Therefore, the In₂O₃-NPs toxicity and stress mitigating impact of eCO₂ in the young and old leaves of C3 (wheat) and C4 (sorghum) plants were investigated. Overall, In₂O₃-NPs significantly retard the biomass and photosynthetic machinery of all tested crops, particularly the young leaves of C3 plants. Consequently, In₂O₃-NPs altered C and N metabolism in C3 and C4 plants. On the other hand, eCO₂ contrarily alleviated the hazardous effects of In₂O₃-NPs on growth and photosynthesis, especially in the young leaves of C4 plants. Increased photosynthesis consequently enhanced the soluble sugars' accumulation and metabolism (e.g., sucrose P synthase, cytosolic, and vacuolar invertase) in all stressed plants, but to a greater extent in C4 young leaves. High sugar availability also induced TCA organic and fatty acids' accumulation. This also provided a route for amino acids and polyamines biosynthesis, where a clear increase in proline biosynthetic enzymes [e.g., pyrroline-5-carboxylate synthetase (P5CS), ornithine aminotransferase (OAT), Pyrroline-5-carboxylate reductase (P5CR), pyrroline-5-carboxylate dehydrogenase (P5CDH), and proline dehydrogenase (PRODH)] and polyamine metabolic enzymes (e.g., spermine and spermidine synthases, ornithine decarboxylase, and adenosyl methionine decarboxylase) were mainly recorded in C4 young leaves. The observed increases in these metabolites involved in osmo- and redox-regulation to reduce In₂O₃-NPs induced oxidative damage. Overall, our study, for the first time, shed light on how eCO₂ differentially mitigated In₂O₃-NPs stress in old and young leaves of different species groups under the threat of In₂O₃-NPs contamination.

Elevated CO2 Differentially Mitigated Oxidative Stress Induced by Indium Oxide Nanoparticles in Young and Old Leaves of C3 and C4 Crops

Soil contamination with indium (In) oxide nanoparticles (In2O3-NPs) threatens plant growth and development. However, their toxicity in plants under ambient (aCO2) and elevated (eCO2) conditions is scarcely studied. To this end, this study was conducted to investigate In2O3-NPs toxicity in the young and old leaves of C3 (barley) and C4 (maize) plants and to understand the mechanisms underlying the stress mitigating impact of eCO2. Treatment of C3 and C4 plants with In2O3-NPs significantly reduced growth and photosynthesis, induced oxidative damage (H2O2, lipid peroxidation), and impaired P and Fe homeostasis, particularly in the young leaves of C4 plants. On the other hand, this phytotoxic hazard was mitigated by eCO2 which improved both C3 and C4 growth, decreased In accumulation and increased phosphorus (P) and iron (Fe) uptake, particularly in the young leaves of C4 plants. Moreover, the improved photosynthesis by eCO2 accordingly enhanced carbon availability under the challenge of In2O3-NPs that were directed to the elevated production of metabolites involved in antioxidant and detoxification systems. Our physiological and biochemical analyses implicated the role of the antioxidant defenses, including superoxide dismutase (SOD) in stress mitigation under eCO2. This was validated by studying the effect of In2O3-stress on a transgenic maize line (TG) constitutively overexpressing the AtFeSOD gene and its wild type (WT). Although it did not alter In accumulation, the TG plants showed improved growth and photosynthesis and reduced oxidative damage. Overall, this work demonstrated that C3 was more sensitive to In2O3-NPs stress; however, C4 plants were more responsive to eCO2. Moreover, it demonstrated the role of SOD in determining the hazardous effect of In2O3-NPs.

The growth and uptake of gallium (Ga) and indium (In) of wheat seedlings in Ga- and In-contaminated soils

The emerging contaminants gallium (Ga) and indium (In) are extensively used in advanced industries and are considered as toxic to humans. Limited information is available on the dynamics of Ga and In in soil-upland crop systems. Therefore, this study aimed to investigate the effects of Ga and In on the growth and uptake of Ga and In by wheat plants grown in Ga- and In-contaminated soils. The wheat seedlings were planted in soils of different properties spiked with various Ga and In concentrations (50, 100, 200, and 400 mg kg^-1). The plant-available Ga, In, and Al in the soils were extracted by 0.02 M CaCl₂, and their concentrations in plant tissues of wheat seedlings and plant biomass were determined after harvesting. The results indicated that the Al toxicity of wheat seedlings increased with Ga and In concentrations in acidic soils. Indium phytotoxicity was found in both neutral and acidic soils. Plant analysis results indicated that the concentration of Ga and In in roots was approximately one order of magnitude higher than that in the shoots of wheat seedlings, and the capability for Ga translocation from roots to shoots was higher than for In. The results of this study suggest that the dynamics of Ga and In in soil-upland crop systems is strongly dependent on the soil properties, such as pH and Al availability.

Indium Uptake and Accumulation by Rice and Wheat and Health Risk Associated with Their Consumption

The increasing use of indium in high-tech industries has inevitably caused its release into the environment. However, knowledge of its environmental fate has been very limited so far. This study investigates the indium uptake and accumulation by two staple crops, rice (Oryza sativa L.) and wheat (Triticum aestivum L.), and evaluates potential risks associated with their consumption. Rice and wheat were grown on three kinds of soil, including acidic soils spiked with a high indium concentration (1.0 mmol kg^-1), which is considered the worst-case scenario, because high soil acidity promotes indium bioavailability. The results revealed that a large portion of soil indium was associated with iron hydroxides, even in acidic soils. Indium precipitates in soils resulted in relatively low availability at the plant root site. Most absorbed indium accumulated at the roots, with only a tiny portion reaching the grains. The corresponding Hazard Quotient indicated no adverse effects on human health. Due to the low translocation of indium from soil to grain, the consumption of rice and wheat grains harvested from indium-contaminated soils may pose an insignificant risk to human health. Further field studies are necessary to better elucidate the risks associated with consuming crops grown in indium-contaminated soils.

Accumulation of gallium (Ga) and indium (In) in rice grains in Ga- and In-contaminated paddy soils

To understand the risk of two emerging contaminants, gallium (Ga) and indium (In) to humans via rice consumption, effects of soil properties and concentrations of spiked Ga/In on the accumulation of Ga and In in rice grains were investigated. A pot experiment was conducted, and paddy rice was grown in three soils with different pH values and Al availabilities (i.e., Pc, TWz and Cf), which were spiked with various Ga and In concentrations. The growth index and concentrations of Ga, In, and Al in plant tissues and soil pore water were measured. Results revealed that the concentrations of Ga and In in soil pore water increase with the spiking of Ga or In in all of the tested soils, but the biomass of roots and shoots does not significantly decrease. The accumulation of Ga in shoots and brown rice was significantly reduced in high available Al acidic soils (Pc soils), but this accumulation was significantly increased in low available Al acidic soils (TWz soils), which can be explained by the competitive uptake between Ga and Al by rice plants. The extent of competitive effects between In and Al was less than that between Ga and Al because of the lower solubility and translocation capability of In than those of Ga in soil-rice systems. However, significant differences in the concentrations of Ga and In in brown rice in neutral soils (Cf soils) among the Ga or In treatment were not observed. In addition, the iron plaque formed on the root surface can serve as a barrier to reduce the accumulation of Ga in rice plants. This study suggested that the risk of accumulation of Ga and In in rice grains should be of concern when paddy rice is grown in acidic Ga- or In-contaminated soils with low Al availability.

Assessment of indium toxicity to the model plant Arabidopsis

The use of indium in semiconductor products has increased markedly in recent years. The release of indium into the ecosystem is inevitable. Under such circumstances, effective and accurate assessment of indium risk is important. An indispensable aspect of indium risk assessment is to understand the interactions of indium with plants, which are fundamental components of all ecosystems. Physiological responses of Arabidopsis thaliana exposed to indium were investigated by monitoring toxic effects, accumulation and speciation of indium in the plant. Indium can be taken up by plants and is accumulated mainly in roots. Limited indium root-to-shoot translocation occurs because of immobilization of indium in the root intercellular space and blockage of indium by the Casparian band in the endodermis. Indium caused stunted growth, oxidative stress, anthocyanization and unbalanced phosphorus nutrition. Indium jeopardizes phosphate uptake and translocation by inhibiting the accumulation of phosphate transporters PHOSPHATE TRANSPORTER1 (PHT1;1/4), responsible for phosphate uptake, and PHOSPHATE1 (PHO1), responsible for phosphate xylem loading. Organic acid secretion is stimulated by indium exposure. Secreted citrate could function as a potential detoxifier to lower indium uptake. Our findings provide insights into the potential fate and effects of indium in plants and will aid the evaluation of risks with indium contamination.

Mobility and retention of indium and gallium in saturated porous media

Transport of indium and gallium is reported in laboratory column experiments using quartz sand as a model porous medium representative of a groundwater system. With increased use of indium and gallium in recent years, mainly in the semiconductor industry, concerns arise regarding their environmental effects. The transport and retention behavior of these two metals were quantified via batch and column experiments, and numerical modeling. The effect of natural organic matter on indium and gallium mobility was studied by addition of humic acid (HA). Measured breakthrough curves from column experiments demonstrated different binding capacities between indium and gallium, stronger for indium, with the presence of HA affecting retention dynamics. For indium, the binding capacity on quartz decreases significantly in the presence of HA, leading to enhanced mobility. In contrast, gallium exhibits slightly higher retention and lower mobility in the presence of HA. In all cases, the binding capacity of gallium to quartz is much weaker than that of indium. These results are consistent with the assumption that indium and gallium form different types of complexes with organic ligands, with gallium complexes appearing more stable than indium complexes. Quantitative modeling confirmed that metal retention is controlled by complex stability.

The mobility and plant uptake of gallium and indium, two emerging contaminants associated with electronic waste and other sources

Gallium (Ga) and indium (In) are increasingly susceptible to soil contamination via disposal of electronic equipment. Chemically similar to aluminium (Al), these elements may be mobile and bioavailable under acidic conditions. We sought to determine extent and nature of Ga and In mobility in the soil - plant system and thus their potential to enter the food chain. Batch sorption experiments on a high fertility silt loam (pH 5.95, CEC 22 meq 100 g^-1) showed strong retention of both elements to the soil matrix, with mean distribution coefficient (K_D) values of 408 and 2021 L kg^-1 for Ga and In respectively. K_D increased with concentration, which we attributed to precipitation of excess ions as insoluble hydroxides. K_D decreased with increased pH as Ga/In(OH)²⁺ and Ga/In(OH)₂⁺ transitioned to Ga/In(OH)₄^-. Movement into the aboveground portions of perennial ryegrass (Lolium perenne L.) was low, with bioaccumulation factors of 0.0037 for Ga and 0.0002 for In; foliar concentrations peaked at 11.6 mg kg^-1 and 0.015 mg kg^-1 respectively. The mobility of Ga and In in the soil - plant system is low compared to other common trace element contaminants such as cadmium, copper, and zinc. Therefore, Ga and In are likely to accumulate in soils and soil ingestion, either directly, via inhaled dust, or dust attached to food, will be the largest pathway into the food chain. Future work should focus on the effect of redox conditions on Ga and In, as well as uptake into acidophilic plants such as Camellia spp., which accumulate Al.

Growth inhibition of rice (Oryza sativa L.) seedlings in Ga- and In-contaminated acidic soils is respectively caused by Al and Al+In toxicity

Limited information exists on the effects of emerging contaminants gallium (Ga) and indium (In) on rice plant growth. This study investigated the effects on growth and uptake of Ga and In by rice plants grown in soils with different properties. Pot experiment was conducted and the rice seedlings were grown in two soils of different pH (Pc and Cf) spiked with various Ga and In concentrations. The results showed concentrations of Ga, In, and Al in soil pore water increased with Ga- or In-spiking in acidic Pc soils, significantly decreasing growth indices. According to the dose-response curve, we observed that the EC₅₀ value for Ga and In treatments were 271 and 390mgkg^-1 in Pc soils, respectively. The context of previous hydroponic studies suggests that growth inhibition of rice seedlings in Ga-spiked Pc soils is mainly due to Al toxicity resulting from enhanced Al release through competitive adsorption of Ga, rather than from Ga toxicity. In-spiked Pc soils, both In and Al toxicity resulted in growth inhibition, while no such effect was found in Cf soils due to the low availability of Ga, In and Al under neutral pH conditions.

Effect of Gallium Exposure in Arabidopsis thaliana is Similar to Aluminum Stress

Although gallium (Ga) is a rare element, it is widely used in semiconductor devices. Ga contamination of the environment has been found in semiconductor-producing countries. Here, the physiological and molecular impacts of Ga in the model plant Arabidopsis thaliana were investigated in medium culture. The primary symptom of Ga toxicity is inhibition of root growth. The increased production of malondialdehyde (MDA) suggests that Ga stress could cause oxidative damage in plants. Roots were the main Ga accumulating sites. The distinctive Ga granules were deposited within the intercellular space in roots. The granules are Ga(OH)₃ precipitation, which indicates immobilization or limited translocation of Ga in A. thaliana. Ga stress induces root secretion of organic acids such as citrate and malate. The expression of the transporters AtALMT and AtMATE, responsible for citrate and malate secretion, respectively, were elevated under Ga stress, so the secretion may play a role in the resistance. Indeed, supplying exogenous citrate significantly enhanced Ga tolerance. The overall response to Ga exposure in A. thaliana is highly similar to that with aluminum stress. Our findings provide information for risk assessment in Ga-contaminated soil.