Flag leaf cell wall functional groups and components play a crucial role in the accumulation and translocation of Cd in rice grain via foliage application of humic acid

In-situ remediation of cadmium contamination in paddy fields: from rhizosphere soil to rice kernel

Cadmium (Cd) has become an important heavy metal pollutant because of its strong migration and high toxicity. The industrial production process aggravated the Cd pollution in rice fields. Human exposure to Cd through rice can cause kidney damage, emphysema, and various cardiovascular and metabolic diseases, posing a grave threat to health. As modern technology develops, the Cd accumulation model in rice and in-situ remediation of Cd pollution in cornfields have been extensively studied and applied, so it is necessary to sort out and summarize them systematically. Therefore, this paper reviewed the primary in-situ methods for addressing heavy metal contamination in rice paddies, including chemical remediation (inorganic-organic fertilizer remediation, nanomaterials, and composite remediation), biological remediation (phytoremediation and microbial remediation), and crop management remediation technologies. The factors that affect Cd transformation in soil and Cd migration in crops, the advantages and disadvantages of remediation techniques, remediation mechanisms, and the long-term stability of remediation were discussed. The shortcomings and future research directions of in situ remediation strategies for heavily polluted paddy fields and genetic improvement strategies for low-cadmium rice varieties were critically proposed. To sum up, this review aims to enhance understanding and serve as a reference for the appropriate selection and advancement of remediation technologies for rice fields contaminated with heavy metals.

Bioleaching of Cd from contaminated Helianthus annuus L. stalk and the safe utilization of its byproducts by Aspergillus niger

Disposal and recycling of heavy metal-enriched biomass is the key to measure the success of phytoremediation. This study employed innovative approach to use Aspergillus niger (A. niger) for the treatment of Cd-contaminated Helianthus annuus L. (sunflower) stalk after phytoremediation. Single-factor results showed that the removal of Cd at an initial pH of 3 was superior to sucrose and inoculation amount. 67.67% of Cd was removed by A. niger leaching system after 11 days based on response surface methodology optimum conditions (sucrose: 76.266 g L-1; inoculation amount: 10%; initial pH: 3), while the concentrations of nitrogen, phosphorus and potassium (N, P and K) of sunflower stalk were unaffected. While physicochemical pretreatment effectively enhanced the bioleaching efficiency, it also resulted in significant loss of P and K elements, thereby reducing the value of biomass for recycling and utilization. Therefore, the direct A. niger leaching method without pretreatment is more advantageous for the safe treatment and recycling of Cd-contaminated sunflower stalks.

Effects of different cellular and subcellular characteristics on the atmospheric Pb uptake, distribution and morphology in Tillandsia usneoides leaves

Lead (Pb) is a widespread highly toxic and persistent environmental pollutant. Plant leaves play a key role in accumulating atmospheric Pb, but its distribution in different cells and subcellular structures and the factors affecting it have been little studied. Here, Tillandsia usneoides, an indicator plant for atmospheric heavy metals, was treated with an aerosol generation device to analyze Pb contents in different cells (three types of cells in leaf surface scales, epidermal cells, mesophyll cells, vascular bundle cells), subcellular structures (cell wall, cell membrane, vacuoles, and organelles) and cell wall components (pectin, hemicellulose 1 and 2, and cellulose). Results show the different cells of T. usneoides leaves play distinct roles in the process of Pb retention. The outermost wing cells are structures that capture external pollutants, while mesophyll cells, as the aggregation site after material transport, ring cells, disc cells, epidermal cells, and vascular cells are material transporters. Pb was only detected in the cell wall and pectin, indicating the cell wall was the dominant subcellular structure for Pb retention, while pectin was the main component affecting Pb retention. FTIR analysis of cell wall components indicated the esterified carboxyl (CO) functional group in pectin may function in absorbing Pb. Pb entered leaf cells mainly in the form of low toxicity and activity to enhance its resistance.

Mutation of OsNRAMP5 reduces cadmium xylem and phloem transport in rice plants and its physiological mechanism

Natural resistance associated macrophage protein 5 (NRAMP5) is a key transporter for cadmium (Cd) uptake by rice roots; however, the effect of OsNRAMP5 on Cd translocation and redistribution in rice plants remains unknown. In this study, an extremely low Cd-accumulation mutant (lcd1) and wild type (WT) plants were utilized to investigate the effect of OsNRAMP5 mutation on Cd translocation and redistribution via the xylem and phloem and its possible physiological mechanism using field, hydroponic and isotope-labelling experiments. The results showed that OsNRAMP5 mutation reduced xylem and phloem transport of Cd, due to remarkably lower Cd translocation from roots to shoots and from the leaves Ⅰ-Ⅲ to their corresponding nodes, as well as lower Cd concentrations in xylem and phloem sap of lcd1 compared to WT plants. Mutation of OsNRAMP5 reduced Cd translocation from roots to shoots in lcd1 plants by increasing Cd deposition in cellulose of root cell walls and reducing OsHMA2-and OsCCX2-mediated xylem loading of Cd, and the citric acid- and tartaric acid-mediated long-distance xylem transport of Cd. Moreover, OsNRAMP5 mutation inhibited Cd redistribution from flag leaves to nodes and panicles in lcd1 plants by increasing Cd sequestration in cellulose and vacuoles, and decreasing OsLCT1-mediated Cd phloem transport in flag leaves.

Exploring the mechanism of Cd uptake and translocation in rice: Future perspectives of rice safety

Cadmium (Cd) contamination in rice fields has been recognized as a severe global agro-environmental issue. To reach the goal of controlling Cd risk, we must pay more attention and obtain an in-depth understanding of the environmental behavior, uptake and translocation of Cd in soil-rice systems. However, to date, these aspects still lack sufficient exploration and summary. Here, we critically reviewed (i) the processes and transfer proteins of Cd uptake/transport in the soil-rice system, (ii) a series of soil and other environmental factors affecting the bioavailability of Cd in paddies, and (iii) the latest advances in regard to remediation strategies while producing rice. We propose that the correlation between the bioavailability of Cd and environmental factors must be further explored to develop low Cd accumulation and efficient remediation strategies in the future. Second, the mechanism of Cd uptake in rice mediated by elevated CO2 also needs to be given more attention. Meanwhile, more scientific planting methods (direct seeding and intercropping) and suitable rice with low Cd accumulation are important measures to ensure the safety of rice consumption. In addition, the relevant Cd efflux transporters in rice have yet to be revealed, which will promote molecular breeding techniques to address the current Cd-contaminated soil-rice system. The potential for efficient, durable, and low-cost soil remediation technologies and foliar amendments to limit Cd uptake by rice needs to be examined in the future. Conventional breeding procedures combined with molecular marker techniques for screening rice varieties with low Cd accumulation could be a more practical approach to select for desirable agronomic traits with low risk.

Cichorium intybus L. is a potential Cd-accumulator for phytoremediation of agricultural soil with strong tolerance and detoxification to Cd

Identifying suitable plants for phytoremediation of Cd (cadmium) contaminated agricultural soil is critical. In this study, whether chicory (Cichorium intybus L.) qualified as an ideal accumulator for phytoremediation was investigated. The hydroponic and pot experiments showed that Cd concentration in chicory leaves exceeded 100 mg kg-1 (BCF >1, TF >1) with 40 mg kg-1 Cd in pot; No significant effects on chicory growth, leaf protein and physiological and biochemical aspects when treated with ≤ 20 μM or 40 mg kg-1 Cd, because chicory could relieve Cd toxicity by increasing activities of photoprotection mechanisms, the reactive oxygen species scavenging system and concentrations of functional groups in plant tissues. In field experiment, 16.2 and 26.6 t ha-1 of chicory leaves was harvested in winter and summer, respectively. The highest Cd concentration in leaves was close to 25.0 mg kg-1 (BCF >1, TF >1) from the acid soil with 0.980 mg kg-1 Cd. Over 320 g ha-1 Cd was extracted from soil by harvesting chicory leaves both in winter and summer, with 9.24% and 12.9% of theoretical phytoremediation efficiency. Therefore, chicory can be as an ideal Cd-accumulator for phytoremediation of slight-to-moderate Cd-contaminated agricultural soil in any season.

Exogenous application of low and high molecular weight organic acids differentially affected the uptake of cadmium in wheat-rice cropping system in alkaline calcareous soil

Anthropogenic cadmium (Cd) in arable soils is becoming a global concern due to its harmful effects on crop yield and quality. The current study examined the role of exogenously applied low molecular weight organic acids (LMWOAs) including oxalic acid (OxA), tartaric acid (TA) and high molecular weight organic acids (HMWOAs) like citric acid (CA) and humic acid (HA) for the bioavailability of Cd in wheat-rice cropping system. Maximum increase in root dry-weight, shoot dry-weight, and grain/paddy yields was recorded with HA for both crops. The HA significantly decreased AB-DTPA Cd in contaminated soils which remained 41% for wheat and 48% for rice compared with their respective controls. The minimum concentration of Cd in roots, shoots and grain/paddy was observed in HA treatment in both crops. The organic acids significantly increased the growth parameters, photosynthetic activity, and relative leaf moisture contents for both wheat and rice crops compared to that with the contaminated control. Application of OxA and TA increased the bioavailability of Cd in soils and plant tissues while CA and HA decreased the bioavailability of Cd in soils and plants. The highest decrease in Cd uptake, bioaccumulation, translocation factor, immobilization, translocation, harvest, and health risk indices were observed with HA while maximum increase was recorded with OxA for both wheat and rice. The results concluded that use of HMWOAs is effective in soil Cd immobilization being maximum with HA. While LMWOAs can be used for the phytoextraction of Cd in contaminated soils having maximum potential with OxA.