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Background and aims Iron (Fe) plaque on rice roots is a mixture of Fe oxide and oxyhydroxide minerals thought to protect rice from high levels of arsenic (As) in flooded paddy soils. Silicon (Si), phosphorus (P), and selenium (Se) also exist as oxyanions in rice paddies, but the impacts of Fe plaque on uptake of these nutrients are unknown. Methods We used natural variation in paddy soil chemistry to test how Si, P, As, and Se move from porewater to plaque to plant via multiple techniques. In a pot study, we monitored Fe plaque deposition and porewater chemistry in 5 different soils over time and measured plaque/plant chemistry and Fe plaque mineralogy at harvest. We normalized oxyanion concentrations by Fe to determine the preferential retention on plaque or plant uptake. Results Low phosphorus availability increased root Fe-oxidizing activity, while Fe, Si, P, As, and Se concentrations in plaque were strongly correlated with porewater. Plaque did not appreciably retain Si and Se, and the oxyanions did not compete for adsorption sites on the Fe plaque. Conclusion Root Fe plaque seems to protect rice from As uptake, does not interfere with Si and Se uptake, and roots adapt to maintain P nutrition even with retention of porewater P on plaque.

Previous work has shown that inorganic As localizes in rice bran whereas DMA localizes in the endosperm, but less is known about co-localization of As and S species and how they are affected by growing conditions. We used high-resolution synchrotron X-ray fluorescence imaging to image As and S species in rice grain from plants grown to maturity in soil (field and pot) and hydroponically (DMA or arsenite dosed) at field-relevant As concentrations. In hydroponics, arsenite was localized in the ovular vascular trace (OVT) and the bran while DMA permeated the endosperm and was absent from the OVT in all grains analyzed, and As species had no affect on S species. In pot studies, soil amended with Si-rich rice husk with higher DMA shifted grain As into the endosperm for both japonica and indica ecotypes. In field-grown rice from low-As soil, As localized in the OVT as arsenite glutathione, arsenite, and DMA. Results support a circumferential model of grain filling for arsenite and DMA and show Si-rich soil amendments alter grain As localization, potentially lessening risk to rice consumers.

Purpose Aquatic plants, including rice, develop iron (Fe) plaques on their roots due to radial oxygen loss (ROL), and these plaques accumulate both beneficial and toxic elements. Silicon is an important nutrient for rice and both accumulates in Fe plaque and can affect ROL. How these plaques form over time and how Si affects this process remains unclear. Methods Rice was grown in a pot study with 4 levels of added Si. Root Fe plaque formation was monitored weekly using vinyl films placed between the pot and soil. Plants were grown to maturity and then ratooned to also examine the formation of Fe plaque during the ratoon crop. Results Iron plaque formation increased exponentially during the vegetative phase, peaked at the booting phase, then decreased exponentially – a pattern that repeated in the ratoon crop. While the highest Si treatment led to an earlier onset of Fe plaque formation, increasing Si decreased the amount of Fe plaque at harvest, resulting in a minimal net effect. Conclusions The kinetics of Fe plaque formation are dependent on rice growth stage, which may affect whether the Fe plaque is a source or sink of elements such as phosphorous and arsenic.

Tidal marshes are valuable global carbon sinks, yet large uncertainties in coastal marsh carbon budgets and mediating mechanisms limit our ability to estimate fluxes and predict feedbacks with global change. To improve mechanistic understanding, we assess how net carbon storage is influenced by interactions between crab activity, water movement, and biogeochemistry. We show that crab burrows enhance carbon loss from tidal marsh sediments by physical and chemical feedback processes. Burrows increase near-creek sediment permeability in the summer by an order of magnitude compared to the winter crab dormancy period, promoting carbon-rich fluid exchange between the marsh and creek. Burrows also enhance vertical exchange by increasing the depth of the strongly carbon-oxidizing zone and reducing the capacity for carbon sequestration. Results reveal the mechanism through which crab burrows mediate the movement of carbon through tidal wetlands and highlight the importance of considering burrowing activity when making budget projections across temporal and spatial scales.

Concentrations of nutrients and contaminants in rice grain affect human health, specifically through the localization and chemical form of elements. Methods to spatially quantify the concentration and speciation of elements are needed to protect human health and characterize elemental homeostasis in plants. Here, an evaluation was carried out using quantitative synchrotron radiation microprobe X‐ray fluorescence (SR‐µXRF) imaging by comparing average rice grain concentrations of As, Cu, K, Mn, P, S and Zn measured with rice grain concentrations from acid digestion and ICP‐MS analysis for 50 grain samples. Better agreement was found between the two methods for high‐Z elements. Regression fits between the two methods allowed quantitative concentration maps of the measured elements. These maps revealed that most elements were concentrated in the bran, although S and Zn permeated into the endosperm. Arsenic was highest in the ovular vascular trace (OVT), with concentrations approaching 100 mg kg−1 in the OVT of a grain from a rice plant grown in As‐contaminated soil. Quantitative SR‐µXRF is a useful approach for comparison across multiple studies but requires careful consideration of sample preparation and beamline characteristics. This work demonstrates which elements are the most easily quantifiable by synchrotron radiation for microprobe X‐ray fluorescence (SR‐µXRF) imaging in order to understand elemental distributions in plant tissues, with a focus on metals and metalloids in rice grain. This work provides SR‐µXRF users with data to understand which elements can be reliably quantified and guidance on how to consider the limitations of the technique to most effectively interpret such data.

Main conclusion A natural rice rhizospheric isolate abates arsenic uptake in rice by increasing Fe plaque formation on rice roots. Rice (Oryza sativa L.) is the staple food for over half of the world's population, but its quality and yield are impacted by arsenic (As) in some regions of the world. Bacterial inoculants may be able to mitigate the negative impacts of arsenic assimilation in rice, and we identified a non-pathogenic, naturally occurring rice rhizospheric bacterium that decreases As accumulation in rice shoots in laboratory experiments. We isolated several proteobacterial strains from a rice rhizosphere that promote rice growth and enhance the oxidizing environment surrounding rice root. One Pantoea sp. strain (EA106) also demonstrated increased iron (Fe)-siderophore in culture. We evaluated EA106's ability to impact rice growth in the presence of arsenic, by inoculation of plants with EA106 (or control), subsequently grew the plants in As-supplemented medium, and quantified the resulting plant biomass, Fe and As concentrations, localization of Fe and As, and Fe plaque formation in EA106-treated and control plants. These results show that both arsenic and iron concentrations in rice can be altered by inoculation with the soil microbe EA106. The enhanced accumulation of Fe in the roots and in root plaques suggests that EA106 inoculation improves Fe uptake by the root and promotes the formation of a more oxidative environment in the rhizosphere, thereby allowing more expansive plaque formation. Therefore, this microbe may have the potential to increase food quality through a reduction in accumulation of toxic As species within the aerial portions of the plant.

Silicon (Si) is one of the beneficial plant mineral nutrients which is known to improve biotic and abiotic stress resilience and productivity in several crops. However, its beneficial role in underutilized or “orphan” crop such as tef [ Eragrostis tef (Zucc.) Trotter ] has never been studied before. In this study, we investigated the effect of Si application on tef plant performance. Plants were grown in soil with or without exogenous application of Na 2 SiO 3 (0, 1.0, 2.0, 3.0, 4.0, and 5.0 mM), and biomass and grain yield, mineral content, chlorophyll content, plant height, and expression patterns of putative Si transporter genes were studied. Silicon application significantly increased grain yield (100%) at 3.0 mM Si, and aboveground biomass yield by 45% at 5.0 mM Si, while it had no effect on plant height. The observed increase in grain yield appears to be due to enhanced stress resilience and increased total chlorophyll content. Increasing the level of Si increased shoot Si and Na content while it significantly decreased the content of other minerals including K, Ca, Mg, P, S, Fe, and Mn in the shoot, which is likely due to the use of Na containing Si amendment. A slight decrease in grain Ca, P, S, and Mn was also observed with increasing Si treatment. The increase in Si content with increasing Si levels prompted us to analyze the expression of Si transporter genes. The tef genome contains seven putative Si transporters which showed high homology with influx and efflux Lsi transporters reported in various plant species including rice. The tef Lsi homologs were deferentially expressed between tissues (roots, leaves, nodes, and inflorescences) and in response to Si, suggesting that they may play a role in Si uptake and/or translocation. Taken together, these results show that Si application improves stress resilience and yield and regulates the expression of putative Si transporter genes. However, further study is needed to determine the physiological function of the putative Si transporters, and to study the effect of field application of Si on tef productivity.

Metals and metalloids (hereafter, metal(loid)s) in plant‐based foods are a source of exposure to humans, but not all metal(loid)‐food interactions are the same. Differences exist between metal(loid)s in terms of their behavior in soils and in how they are taken up by plants and stored in the edible plant tissue/food. Thus, there cannot be one consistent solution to reducing toxic metal(loid)s exposure to humans from foods. In addition, how metal(loid)s are absorbed, distributed, metabolized, and excreted by the human body differs based on both the metal(loid), other elements and nutrients in the food, and the nutritional status of the human. Initiatives like the United States Food and Drug Administration's Closer to Zero initiative to reduce the exposure of young children to the toxic elements cadmium, lead, arsenic, and mercury from foods warrant careful consideration of each metal(loid) and plant interaction. This review explores such plant‐metal(loid) interactions using the example of spinach and the metals cadmium and lead. This review highlights differences in the magnitude of exposure, bioavailability, and the practicality of mitigation strategies while outlining research gaps and future needs. A focus on feasibility and producer needs, informed via stakeholder interviews, emphasizes the need for better analytical testing facilities and grower and consumer education. More research should focus on minimization of chloride inputs for leafy greens to lessen plant‐availability of Cd and the role of oxalate in reducing Cd bioavailability from spinach. These findings are applicable to other leafy greens (e.g., kale, lettuce), but not for other plants or metal(loid)s. Plain Language Summary Toxic metals like cadmium and lead in foods can be harmful to our health, especially for babies and young children who are more vulnerable due to their smaller size and rapid development. Leafy greens like spinach can absorb these metals from the soil but in different ways. In addition, how and where they accumulate in edible plant tissues also differs. This review uses spinach as an example to compare and contrast how cadmium and lead differ in how they move through soil and accumulate in plant foods. It also discusses practical pre‐ and post‐harvest techniques to lessen human exposure to these metals that can be adopted by producers and consumers. Finally, it highlights future needs and research directions. Key Points The toxic elements targeted in the Food and Drug Administration Closer to Zero action plan behave differently in soils and in plant uptake Mitigation strategies to reduce exposure to toxic elements must consider the drivers of soil mobility and accumulation into edible tissues Health and nutrition factors that affect metal and metalloid bioavailability upon ingestion should also be considered

The flooded soil conditions under which rice is typically grown are beneficial for boosting yield and decreasing herbicide inputs but may pose a food safety and environmental health risk. Flooded soils lead to reducing conditions and anaerobic metabolisms of soil microorganisms, which mobilizes arsenic from soil into soil solution, where it can be absorbed by rice roots and transported to grain. These conditions also promote the production and emission of methane (CH4)—a potent greenhouse gas. To evaluate how water management affects metal(loid) grain concentrations and CH4 emissions, we conducted a 2‐year field study in which rice paddy water was managed under a range of soil redox conditions that spanned from flooded to non‐flooded. We observed that growing rice under less flooded conditions decreased CH4 emissions and concentrations of grain total As, grain inorganic As, grain total Hg, and grain inorganic Hg relative to flooded conditions, with more reductions observed as conditions were drier; grain organic As and Hg (MeHg) species also decreased with drier conditions particularly in Year 1. However, the driest conditions tested led to a 50%–97% increase in grain Cd concentrations that exceeded the CODEX limit and grain yield reductions as high as 25% and 40% in Year 1 and 2, respectively. While concentrations of toxic metal(loid)s could be manipulated by water management, micronutrient concentrations were similar or decreased with drier conditions, potentially increasing grain Cd bioaccessibility to humans. Because practices for rice water management are gaining momentum, more research should monitor grain Cd levels along with micronutrients. Plain Language Summary Rice is typically grown in flooded paddy fields, which benefits the rice but creates risk for human and environmental safety. Flooded paddies lead to emissions of methane, a greenhouse gas, and allow arsenic to move from the soil to rice grain, posing human health risks. We tested different water management strategies aimed to decrease arsenic uptake and accumulation in rice grain and found that less flooding decreased methane emissions and concentrations of grain arsenic and mercury but increased grain cadmium concentrations and decreased yield under the least flooded conditions. As rice producers adopt new water management strategies, our findings highlight the need for careful water management and monitoring grain cadmium concentrations to balance food safety and crop productivity. Key Points Rice is uniquely prone to high grain As concentrations because it is grown in flooded conditions, which also lead to CH4 emissions Growing rice in less flooded conditions decreased CH4 emissions and concentrations of rice grain As and Hg species, but increased Cd Because rice farmers are rapidly adopting water savings practices, efforts should ensure that grain Cd does not exceed food safety limits

Background and aims Rice is prone to Cd uptake under aerobic soil conditions primarily due to the OsNramp5 Mn transport pathway. Unlike Cd, Mn availability in rice paddies decreases as redox potential increases. We tested whether increasing Mn concentrations in solution would decrease Cd accumulation in rice through competition between Mn and Cd for uptake and/or suppression of OsNramp5 expression. Methods Rice was grown to maturity under Mn concentrations that spanned three orders of magnitude (0.30 to 37 μM) that corresponded to free Mn 2+ activities of 10 –7.9 to 10 –5.0  M while free Cd 2+ activity was held as constant as achievable (10 –10.2 to 10 –10.4  M). Plant biomass and elemental concentrations were measured in roots and shoots at each stage. Fold changes in the expression of OsNramp5 , OsCd1 , OsHMA3 , OsCCX2 , and OsYSL6 genes in vegetative and grain-filling stages of rice plants were determined. Results Competition between Mn and Cd for root uptake and accumulation in shoots was observed at the highest concentration of Mn tested. OsNramp5 expression was significantly higher in rice plants at the vegetative stage compared to the grain-filling stage, while OsCd1 and OsHMA3 showed the opposite. Solution Mn concentrations previously thought to be tolerable by rice grown to the vegetative stage led to Mn toxicity as plants matured. Conclusions Mn competes with Cd during uptake into rice with OsNramp5 expression unaffected. Different translocation paths may occur for Mn and Cd within the rice plant and over the rice life cycle, with OsCCX2 correlated with shoot Cd concentration.