Sulfur reduces arsenic accumulation in rice shoot by enhancing root retention and altering arsenic metabolism

The rhizosphere microbiome reduces the uptake of arsenic and tungsten by Blechnum orientale by increasing nutrient cycling in historical tungsten mining area soils

The growth of pioneer plants in metal mining area soil is closely related to their minimal uptake of toxic elements. Pioneer plants can inhibit the uptake of toxic elements by increasing nutrient uptake. However, few studies have focused on the mechanisms by which the rhizosphere microbiome affect nutrient cycling and their impact on the uptake of toxic elements by pioneer plants. In this study, we selected Blechnum orientale to investigate the potential roles of the rhizosphere microbiome in nutrient cycling and plant growth in a historical tungsten (W) mining area. Our results showed that while the arsenic (As) and W contents in the soil were relatively high, the enrichment levels of As and W in the B. orientale were relatively low. Furthermore, we found that the As and W contents in plants were significantly negatively correlated with soil nutrients (S, P and Mo), suggesting that elevated levels of these soil nutrients could inhibit As and W uptake by B. orientale. Importantly, we found that these nutrients were also identified as the most important factors shaping rhizosphere microbial attributes, including microbial diversity, ecological clusters, and keystone OTUs. Moreover, the genera, keystone taxa and microbial functional genes enriched in the rhizosphere soils from mining areas played a key role in nutrient (S, P and Mo) bioavailability, which could further increase the nutrient uptake by B. orientale. Taken together, our results suggest that rhizosphere microorganisms can improve pioneer plant growth by inhibiting toxic element accumulation via the increase in nutrient cycling in former W mining areas.

Interspecific variations in growth, physiology and Cd accumulation between Populus deltoides and P. × canadensis in response to Cd pollution under two soil types

Both acid and alkaline purple soils in China are increasingly affected by Cd contamination. The selection of fast-growing trees suitable for remediating different soil types is urgent, yet there is a severe lack of relevant knowledge. In this study, we conducted a controlled pot experiment to compare the growth, physiology, and Cd accumulation efficiency of two widely recognized poplar species, namely Populus deltoides and P. × canadensis, under Cd contamination (1 mg kg-1) in acid and alkaline purple soils. The objective was to determine which poplar species is best suited for remediating different soil types. Our findings are as follows: (1) the total biomass of both poplars remained largely unaffected by Cd pollution in both soil types. Notably, under Cd pollution, the total biomass of P. deltoides in acid purple soil was 1.53 times greater than that in alkaline purple soil. (2) Cd pollution did not significantly induce oxidative damage in the leaves of either poplar species in both soil types. However, in acid purple soil, Cd contamination led to a 21% increase in NO3- concentration and a 44% increase in NH4+ concentration in P. × canadensis leaves, whereas in alkaline purple soil, it led to a 59% increase in NH4+ concentration in P. deltoides leaves. (3) Cd concentrations in all root orders of P. × canadensis were significantly higher than those in P. deltoides, especially in the first three root orders, under alkaline purple soil. The total Cd accumulation by P. × canadensis in Cd-polluted alkaline purple soil was 2.18 times higher than that in Cd-polluted acid purple soil, a difference not observed in P. deltoides. (4) redundancy analysis indicated that the sequestration effect of higher soil organic matter on Cd availability in acid purple soil was more pronounced than the release effects caused by lower pH. In conclusion, P. × canadensis is better suited for remediating alkaline purple soil due to its higher capacity for Cd uptake, while P. deltoides is more suitable for remediating Cd-contaminated acid purple soil due to its better growth conditions and greater Cd enrichment capability.

Exploring the potential: Can arsenic (As) resistant silicate-solubilizing bacteria manage the dual effects of silicon on As accumulation in rice?

Rice (Oryza sativa L.) cultivation in regions marked by elevated arsenic (As) concentrations poses significant health concerns due to As uptake by the plant and its subsequent entry into the human food chain. With rice serving as a staple crop for a substantial share of the global population, addressing this issue is critical for food security. In flooded paddy soils, where As availability is pronounced, innovative strategies to reduce As uptake and enhance agricultural sustainability are mandatory. Silicon (Si) and Si nanoparticles have emerged as potential candidates to mitigate As accumulation in rice. However, their effects on As uptake exhibit complexity, influenced by initial Si levels in the soil and the amount of Si introduced through fertilization. While low Si additions may inadvertently increase As uptake, higher Si concentrations may alleviate As uptake and toxicity. The interplay among existing Si and As availability, Si supplementation, and soil biogeochemistry collectively shapes the outcome. Adding water-soluble Si fertilizers (e.g., Na2SiO3 and K2SiO3) has demonstrated efficacy in mitigating As toxicity stress in rice. Nonetheless, the expense associated with these fertilizers underscores the necessity for low cost innovative solutions. Silicate-solubilizing bacteria (SSB) resilient to As hold promise by enhancing Si availability by accelerating mineral dissolution within the rhizosphere, thereby regulating the Si biogeochemical cycle in paddy soils. Promoting SSB could make cost-effective Si sources more soluble and, consequently, managing the intricate interplay of Si's dual effects on As accumulation in rice. This review paper offers a comprehensive exploration of Si's nuanced role in modulating As uptake by rice, emphasizing the potential synergy between As-resistant SSB and Si availability enhancement. By shedding light on this interplay, we aspire to shed light on an innovative attempt for reducing As accumulation in rice while advancing agricultural sustainability.

Key process for reducing grains arsenic by applying sulfate varies with irrigation mode: Dual effects of microbe-mediated arsenic transformation and iron plaque

Sulfate affects the transformation of arsenic (As) in soil and its absorption by plant roots. However, the influence of sulfate and irrigation interactions on the mobility of As in the soil-rice system remains poorly understood. To address this gap, we conducted a pot experiment with varying sulfate levels and irrigation modes to examine their effects on rice As translocation, soil As forms, iron plaque formation, and microorganisms involved in As transformation. The addition of exogenous sulfate significantly reduced grain As levels by a maximum of 60.1%, 46.7%, and 70.5% under flooding (F), flooding-moist alternate (FM), moist (M) conditions, respectively. However, the changes in soil available As did not fully correspond to grains As content. Soil available As was only reduced by sulfate under the FM treatment, which limited grains As accumulation under this condition. The reduction in grains As content under F and M conditions was mainly attributed to sulfate-induced increases in soil pH, which in turn inhibited As translocation and promoted iron plaque formation. Additionally, both irrigation mode and sulfate fertilization independently or interactively influenced the abundance of Sulfuritalea, Koribacter, Geobacter, and Sulfuriferula, thereby affecting the As forms in soil through the Fe/S redox process. Specifically, under F and FM conditions, SO42--S inhibited Geobacter but stimulated Fe-oxidizing bacteria, possibly resulting in increased As bound to Fe/Mn oxides (As-F3). Under M condition, SO42--S levels regulated As adsorption and release through the participation of Fe/S cycle bacteria, specifically influencing the adsorbed As fraction (As-F2). Therefore, the addition of SO42--S hindered As translocation to grains by promoting As sequestration in the iron plaque and facilitating microbe-mediated As immobilization through the Fe/S cycle, which was dependent on soil moisture. These results can be used as a guide for sulfur fertilizer application under different soil moisture with the goal of minimizing rice grain As.

Metal(loid) Analysis of Commercial Rice from Malaysia using ICP-MS: Potential Health Risk Evaluation

Rice is a predominant staple food in many countries. It is a great source of energy but can also accumulate toxic and trace metal(loid)s from the environment and pose serious health hazards to consumers if overdosed. This study aims to determine the concentration of toxic metal(loid)s [arsenic (As), cadmium (Cd), nickel (Ni)] and essential metal(loid)s [iron (Fe), selenium (Se), copper (Cu), chromium (Cr), cobalt (Co)] in various types of commercially available rice (basmati, glutinous, brown, local whites, and fragrant rice) in Malaysia, and to assess the potential human health risk. Rice samples were digested following the USEPA 3050B acid digestion method and the concentrations of metal(loid)s were analyzed using an inductively coupled plasma mass spectrometry (ICP-MS). Mean concentrations (mg/kg as dry weight) of metal(loid)s (n=45) across all rice types were found in the order of Fe (41.37)>Cu (6.51)>Cr (1.91)>Ni (0.38)>As (0.35)>Se (0.07)>Cd (0.03)>Co (0.02). Thirty-three percent and none of the rice samples surpassed, respectively, the FAO/WHO recommended limits of As and Cd. This study revealed that rice could be a primary exposure pathway to toxic metal(loid)s, leading to either noncarcinogenic or carcinogenic health problems. The non-carcinogenic health risk was mainly associated with As which contributed 63% to the hazard index followed by Cr (34%), Cd (2%), and Ni (1%). The carcinogenic risk to adults was high (>10-4) for As, Cr, Cd, and Ni. The cancer risk (CR) for each element was 5 to 8 times higher than the upper limit of cancer risk for an environmental carcinogen (<10-4). The findings from this study could provide the metal(loid)s pollution status of various types of rice which are beneficial to relevant authorities in addressing food safety and security-related issues.

Low pe+pH inhibits Cd transfer from paddy soil to rice tissues driven by S addition

Both soil irrigation and sulfur (S) are associated with the precipitation of cadmium (Cd)-sulfide in paddy soil, their interaction affecting on Cd solubility and extractability is still unknown. This study primarily discusses the effect of exogenous S addition on the bioavailability of Cd in paddy soil under unsteady pe + pH conditions. The experiment was treated with three different water strategies: continuous dryness (CD), continuous flooding (CF), and alternating dry-wet cycles for one cycle (DW). These strategies were combined with three different S concentrations. The results indicate that the CF treatment, particularly when combined with S addition, had the most significant effect on reducing pe + pH and Cd bioavailability in the soil. The reduction of pe + pH from 10.2 to 5.5 resulted in a decrease in soil Cd availability by 58.3%, and Cd accumulation in rice grain by 52.8%, compared to the other treatments. While it was more conducive to the formation of iron plaque on the root surface in DW treatment with S addition at rice maturing stage and enhanced the gathering of Fe/S/Cd. Structural equation model (SEM) analysis further confirmed a significant negative correlation (r = -0.916) between the abundance of soil Fer-reducing bacteria (FeRB) and sulfate-reducing bacteria (SRB) like Desulfuromonas, Pseudomonas, Geobacter, and the Cd content in rice grains. This study provides a basic mechanistic understanding of how soil redox status (pe + pH), S addition, and FeRB/SRB interacted with Cd transfer in paddy soil-rice tissues.

Application of glutamic acid improved As tolerance in aromatic rice at early growth stage

To alleviate the arsenic (As) toxicity in aromatic rice, a hydroponic experiment of two As concentrations (0 and 100 μM sodium arsenite: A0, A1), three glutamic acid (Glu) concentrations (0, 100, and 500 μM l-glutamic acid: G0, G1, and G2) with Xiangyaxiangzhan and Meixiangzhan 2 was conducted. Results showed that the root As content were increased under A1G2 but reduced under A1G1 for Xiangyaxiangzhan as compared with A1G0. A decrement of As was transported from root to shoot caused by up-regulated OsABCC1 relative expression in Meixiangzhan 2. Likewise, As stress enhanced the H₂O₂ and malondialdehyde content, resulting in the impaired cell wall observed by transmission electron microscopy. However, compared with A1G0, the superoxide dismutase activity, ascorbic acid, glutathione, proline, and soluble sugar content were increased under A1G1. Additionally, arsenate reductase, monodehydroascorbate reductase activity, Glu, proline, and soluble sugar content were found positively associated with the As accumulation. Further, the metabolome analysis indicated that the pathway of amino acid and arginine biosynthesis were notably enriched after Glu application. Generally, 100 μM Glu application was the better treatment to enhance As tolerance in aromatic rice through up-regulating amino acid biosynthesis with increasing antioxidants and osmolytes to scavenge excessive reactive oxygen species.