You Shall Not Pass: Root Vacuoles as a Symplastic Checkpoint for Metal Translocation to Shoots and Possible Application to Grain Nutritional Quality

You can't always get as much iron as you want: how rice plants deal with excess of an essential nutrient

Iron (Fe) is an essential nutrient for almost all organisms. However, free Fe within cells can lead to damage to macromolecules and oxidative stress, making Fe concentrations tightly controlled. In plants, Fe deficiency is a common problem, especially in well-aerated, calcareous soils. Rice (Oryza sativa L.) is commonly cultivated in waterlogged soils, which are hypoxic and can cause Fe reduction from Fe3+ to Fe2+, especially in low pH acidic soils, leading to high Fe availability and accumulation. Therefore, Fe excess decreases rice growth and productivity. Despite the widespread occurrence of Fe excess toxicity, we still know little about the genetic basis of how rice plants respond to Fe overload and what genes are involved in variation when comparing genotypes with different tolerance levels. Here, we review the current knowledge about physiological and molecular data on Fe excess in rice, providing a comprehensive summary of the field.

Efficient regulation of cadmium accumulation by carboxymethylammonium chloride in rice: Correlation analysis and expression of transporter gene OsGLR3

The mechanism of carboxymethylammonium chloride (CC) regulating cadmium (Cd) accumulation in rice was studied in field and hydroponic experiments. Field experiments showed that 0.2-1.2 mmol L-1 CC spraying effectively reduced Cd accumulation by 44 %-77 % in early rice grains and 39 %-78 % in late rice grains, significantly increased calcium (Ca) content and amino acids content in grains, as well as alleviated Cd-induced oxidative damage in leaves. Hydroponic experiments further verified the inhibition effect of CC on Cd accumulation. 1.2 mmol L-1 CC made the highest decrease of Cd content in shoots and roots of hydroponic seedlings by 45 % and 53 %, respectively. Exogenous CC significantly increased glutamate (Glu), glycine (Gly) and glutathione (GSH) content, and improved the activities of catalase (CAT) and superoxide dismutase (SOD) by 41-131 % and 11-121 % in shoots of hydroponic seedlings, respectively. Exogenous CC also increased the relative expression of OsGLR3.1-3.5 in the shoots and roots of hydroponic seedlings. The quantum computational chemistry was used to clarify that the Gly radical provided by CC could form various complexes with Cd through carboxyl oxygen atoms. These results showed that exogenous application of CC improved the tolerance to Cd by enhancing the antioxidant capacity; inhibited the absorption, transport and accumulation of Cd in rice by (1) promoting chelation, (2) increasing the GLRs activity through upregulating the content of Glu, Gly, as well as the expression of OsGLR3.1-3.5.

Cadmium and zinc accumulation and tolerance in two Egyptian cultivars (S53 and V120) of Helianthus annuus L. as potential phytoremediator

One of the most important oil crops in the world, sunflower (Helianthus annuus L.), is recognized to help in soil phytoremediation. Heavy metal (HM) contamination is one of the most abiotic challenges that may affect the growth and productivity of such an important crop plant. We studied the influence of HM-contaminated soils on metal homeostasis and the potential hypertolerance mechanisms in two sunflower Egyptian cultivars (V120 and S53). Both cultivars accumulated significantly higher cadmium concentrations in their roots compared to their shoots during Cd and Zn/Cd treatments. Higher root concentrations of 121 mg g-1 dry weight (DW) and 125 mg g-1 DW were measured in V120 plants compared to relatively lower values of 111 mg g-1 DW and 105 mg g-1 DW in the roots of S53 plants, respectively. Cadmium contamination significantly upregulated the expression of heavy metal ATPases (HaHMA4) in the shoots of V120 plants. On the other hand, their roots displayed a notable expression of HaHMA3. This study indicates that V120 plants accumulated and sequestered Cd in their roots. Therefore, it is advised to cultivate the V120 cultivar in areas contaminated with heavy metals as it is a promising Cd phytoremediator.

Coupling of different antioxidative systems in rice under the simultaneous influence of selenium and cadmium

Selenium (Se) elevates the antioxidant ability of rice against cadmium (Cd) stress, but previous studies only focused on the variation in antioxidant enzymes or nonenzymatic substances induced by Se under Cd stress and ignored the relationships between different antioxidant parameters during the interaction. Here, hydroponic experiments with rice were performed by adding both Cd and Se at doses in the range of 0-50 μM to explore the physiological responses of rice and their relationships in the presence of different levels of Se and Cd. Exogenous Cd markedly promoted the activity of antioxidant enzymes with the exception of catalase (CAT) and the concentration of nonenzymatic substances in aerial parts. Se enhanced the antioxidant capacity by improving the activities of all the enzymes tested in this study and increasing the concentrations of nonenzymatic compounds. The couplings among different antioxidant substances within paddy rice were then determined based on cluster and linear fitting results and their metabolic process and physiological functions. The findings specifically highlight that couplings among the ascorbic acid (AsA)-glutathione (GSH) cycle, glutathione synthase (GS)-phytochelatin synthetase (PCS) coupling system and glutathione peroxidase (GPX)-superoxide dismutase (SOD) coupling system in aerial parts helps protect plants from Cd stress. These coupling systems form likely due to the fact that one enzyme generated a product that could be the substrate for another enzyme. Noticeably, such coupling systems do not emerge in roots because the stronger damage to roots than other organs activates the ascorbate peroxidase (APX)-GPX-CAT and PCS-GS-SOD systems with distinct functions and structures. This study provides new insights into the detoxification mechanisms of rice caused by the combined effect of Se and Cd.

The Effect of Exogenous Cadmium and Zinc Applications on Cadmium, Zinc and Essential Mineral Bioaccessibility in Three Lines of Rice That Differ in Grain Cadmium Accumulation

Millions of people around the world rely on rice (Oryza sativa) for a significant portion of daily calories, but rice is a relatively poor source of essential micronutrients like iron and zinc. Rice has been shown to accumulate alarmingly high concentrations of toxic elements, such as cadmium. Cadmium in foods can lead to renal failure, bone mineral density loss, cancer, and significant neurotoxicological effects. Several strategies to limit cadmium and increase micronutrient density in staple food crops like rice have been explored, but even when cadmium concentrations are reduced by a management strategy, total cadmium levels in rice grain are an unreliable means of estimating human health risk because only a fraction of the minerals in grains are bioaccessible. The goal of this work was to assess the influence of cadmium and zinc supplied to plant roots on the bioaccessibility of cadmium and essential minerals from grains of three rice lines (GSOR 310546/low grain Cd, GSOR 311667/medium grain Cd, and GSOR 310428/high grain Cd) that differed in grain cadmium accumulation. Treatments consisted of 0 μM Cd + 2 μM Zn (c0z2), 1 μM Cd + 2 μM Zn (c1z2), or 1 μM Cd + 10 μM Zn (c1z10). Our results revealed that an increased grain cadmium concentration does not always correlate with increased cadmium bioaccessibility. Among the three rice lines tested, Cd bioaccessibility increased from 2.5% in grains from the c1z2 treatment to 17.7% in grains from the c1z10 treatment. Furthermore, Cd bioccessibility in the low-Cd-accumulating line was significantly higher than the high line in c1z10 treatment. Zinc bioaccessibility increased in the high-cadmium-accumulating line when cadmium was elevated in grains, and in the low-cadmium line when both cadmium and zinc were increased in the rice grains. Our results showed that both exogenous cadmium and elevated zinc treatments increased the bioaccessibility of other minerals from grains of the low- or high-grain cadmium lines of rice. Differences in mineral bioaccessibility were dependent on rice line. Calculations also showed that increased cadmium bioaccessibility correlated with increased risk of dietary exposure to consumers. Furthermore, our results suggest that zinc fertilization increased dietary exposure to cadmium in both high and low lines. This information can inform future experiments to analyze genotypic effects of mineral bioavailability from rice, with the goal of reducing cadmium absorption while simultaneously increasing zinc absorption from rice grains.

Natural variation of nutrient homeostasis among laboratory and field strains of Chlamydomonas reinhardtii

Natural variation among individuals and populations exists in all species, playing key roles in response to environmental stress and adaptation. Micro- and macronutrients have a wide range of functions in photosynthetic organisms, and mineral nutrition thus plays a sizable role in biomass production. To maintain nutrient concentrations inside the cell within physiological limits and prevent the detrimental effects of deficiency or excess, complex homeostatic networks have evolved in photosynthetic cells. The microalga Chlamydomonas reinhardtii (Chlamydomonas) is a unicellular eukaryotic model for studying such mechanisms. In this work, 24 Chlamydomonas strains, comprising field isolates and laboratory strains, were examined for intraspecific differences in nutrient homeostasis. Growth and mineral content were quantified in mixotrophy, as full nutrition control, and compared with autotrophy and nine deficiency conditions for macronutrients (-Ca, -Mg, -N, -P, and -S) and micronutrients (-Cu, -Fe, -Mn, and -Zn). Growth differences among strains were relatively limited. However, similar growth was accompanied by highly divergent mineral accumulation among strains. The expression of nutrient status marker genes and photosynthesis were scored in pairs of contrasting field strains, revealing distinct transcriptional regulation and nutrient requirements. Leveraging this natural variation should enable a better understanding of nutrient homeostasis in Chlamydomonas.

Nutrient carriers at the heart of plant nutrition and sensing

Plants require water and several essential nutrients for their development. The radial transport of nutrients from the soil to the root vasculature is achieved through a combination of three different pathways: apoplastic, symplastic, and transcellular. A common feature for these pathways is the requirement of carriers to transport nutrients across the plasma membrane. An efficient transport of nutrients across the root cell layers relies on a large number of carriers, each of them having their own substrate specificity, tissular and subcellular localization. Polarity is also emerging as a major feature allowing their function. Recent advances on radial transport of nutrients, especially carrier mediated nutrient transport will be discussed in this review, as well as the role of transporters as nutrient sensors.

Tobacco as an efficient metal accumulator

Tobacco (Nicotiana tabacum L.) is an important industrial crop plant. However, it efficiently accumulates metals, primarily cadmium (Cd) and also zinc (Zn), in its leaves. Therefore, it could be a source of cadmium intake by smokers. On the other hand, as a high leaf metal accumulator, it is widely used for phytoremediation of metal-contaminated soil. Both issues provide an important rationale for investigating the processes regulating metal homeostasis in tobacco. This work summarizes the results of research to date on the understanding of the molecular mechanisms determining the effective uptake of Zn and Cd, their translocation into shoots and accumulation in leaves. It also discusses the current state of research to improve the phytoremediation properties of tobacco through genetic modification and to limit leaf Cd content for the tobacco industry.

Zinc biofortification of Genovese basil: Influence on mineral profile and estimated daily intake in adults and children

Despite the well-known beneficial function of Zn in human health, its deficiency is an increasingly recognized worldwide concern. In this work, we evaluated the agronomic biofortification of two basil (Ocimum basilicum L.) cultivars ('Aroma 2' and 'Eleonora') using nutrient solutions with different Zn concentrations (0, 12.5, 25, 37.5, and 50 µM). We focused on the impact of biofortification on the mineral profile quantified by ICP OES. Compared to the control, biofortification treatments increased Zn concentration by 22.03 % (on average). Consumption of one serving of 50 µM of Zn biofortified basil 'Aroma 2' guarantees an estimated daily intake (EDI) of 275.746 and 91.915 µg day^-1 in adults and children, respectively. Furthermore, Zn biofortification positively affected the mineral profile of the leaves. Compared to the control, the B₅₀ dose of Zn (50 μM of Zn) increased the EDI of macro and microelements in adults and children. This aspect highlights how biofortified basil consumption would improve consumers' nutritional status.

A tale of two metals: Biofortification of rice grains with iron and zinc

Iron (Fe) and zinc (Zn) are essential micronutrients needed by virtually all living organisms, including plants and humans, for proper growth and development. Due to its capacity to easily exchange electrons, Fe is important for electron transport in mitochondria and chloroplasts. Fe is also necessary for chlorophyll synthesis. Zn is a cofactor for several proteins, including Zn-finger transcription factors and redox metabolism enzymes such as copper/Zn superoxide dismutases. In humans, Fe participates in oxygen transport, electron transport, and cell division whereas Zn is involved in nucleic acid metabolism, apoptosis, immunity, and reproduction. Rice (Oryza sativa L.) is one of the major staple food crops, feeding over half of the world's population. However, Fe and Zn concentrations are low in rice grains, especially in the endosperm, which is consumed as white rice. Populations relying heavily on rice and other cereals are prone to Fe and Zn deficiency. One of the most cost-effective solutions to this problem is biofortification, which increases the nutritional value of crops, mainly in their edible organs, without yield reductions. In recent years, several approaches were applied to enhance the accumulation of Fe and Zn in rice seeds, especially in the endosperm. Here, we summarize these attempts involving transgenics and mutant lines, which resulted in Fe and/or Zn biofortification in rice grains. We review rice plant manipulations using ferritin genes, metal transporters, changes in the nicotianamine/phytosiderophore pathway (including biosynthetic genes and transporters), regulators of Fe deficiency responses, and other mutants/overexpressing lines used in gene characterization that resulted in Fe/Zn concentration changes in seeds. This review also discusses research gaps and proposes possible future directions that could be important to increase the concentration and bioavailability of Fe and Zn in rice seeds without the accumulation of deleterious elements. We also emphasize the need for a better understanding of metal homeostasis in rice, the importance of evaluating yield components of plants containing transgenes/mutations under field conditions, and the potential of identifying genes that can be manipulated by gene editing and other nontransgenic approaches.

Interaction between zinc and selenium bio-fortification and toxic metals (loid) accumulation in food crops

Biofortification is the supply of micronutrients required for humans and livestock by various methods in the field, which include both farming and breeding methods and are referred to as short-term and long-term solutions, respectively. The presence of essential and non-essential elements in the atmosphere, soil, and water in large quantities can cause serious problems for living organisms. Knowledge about plant interactions with toxic metals such as cadmium (Cd), mercury (Hg), nickel (Ni), and lead (Pb), is not only important for a healthy environment, but also for reducing the risks of metals entering the food chain. Biofortification of zinc (Zn) and selenium (Se) is very significant in reducing the effects of toxic metals, especially on major food chain products such as wheat and rice. The findings show that Zn- biofortification by transgenic technique has reduced the accumulation of Cd in shoots and grains of rice, and also increased Se levels lead to the formation of insoluble complexes with Hg and Cd. We have highlighted the role of Se and Zn in the reaction to toxic metals and the importance of modifying their levels in improving dietary micronutrients. In addition, cultivar selection is an essential step that should be considered not only to maintain but also to improve the efficiency of Zn and Se use, which should be considered more climate, soil type, organic matter content, and inherent soil fertility. Also, in this review, the role of medicinal plants in the accumulation of heavy metals has been mentioned, and these plants can be considered in line with programs to improve biological enrichment, on the other hand, metallothioneins genes can be used in the program biofortification as grantors of resistance to heavy metals.

The regulatory role of abscisic acid on cadmium uptake, accumulation and translocation in plants

To date, Cd contamination of cropland and crops is receiving more and more attention around the world. As a plant hormone, abscisic acid (ABA) plays an important role in Cd stress response, but its effect on plant Cd uptake and translocation varies among plant species. In some species, such as Arabidopsis thaliana, Oryza sativa, Brassica chinensis, Populus euphratica, Lactuca sativa, and Solanum lycopersicum, ABA inhibits Cd uptake and translocation, while in other species, such as Solanum photeinocarpum and Boehmeria nivea, ABA severs the opposite effect. Interestingly, differences in the methods and concentrations of ABA addition also triggered the opposite result of Cd uptake and translocation in Sedum alfredii. The regulatory mechanism of ABA involved in Cd uptake and accumulation in plants is still not well-established. Therefore, we summarized the latest studies on the ABA synthesis pathway and comparatively analyzed the physiological and molecular mechanisms related to ABA uptake, translocation, and detoxification of Cd in plants at different ABA concentrations or among different species. We believe that the control of Cd uptake and accumulation in plant tissues can be achieved by the appropriate ABA application methods and concentrations in plants.

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.

Nutrient and toxic elements in soils and plants across 10 urban community gardens: Comparing pXRF and ICP-based soil measurements

Urban community gardens are becoming increasingly important to rehabilitate developed lands and combat the lack of access to fresh produce. Portable X-ray fluorescence (pXRF) offers a rapid, cost-effective method for assessing the elemental composition of soils but needs further study to determine its efficacy in urban agriculture. The objectives of this study were to evaluate if pXRF measurements of macronutrients (Ca, K, P), micronutrients (Cu, Mn, Zn), and toxic elements (As, Pb) generate results comparable with traditional soil analyses and if the soil measurements correlate with plant tissue concentrations at 10 community gardens across the eastern United States. From field-condition analyses of soils by pXRF and pseudototal digestions, we observed that both methods provide agreeable estimates of concentrations for some elements (Mn, Cu, Zn, Pb) but not for macronutrients (Ca, K, P). We hypothesize that low accuracy in pXRF measurements and macronutrients within silicates caused the poor agreement between the methods. Sieved and dried soil pXRF concentrations were in strong agreement with field-condition pXRF concentrations, suggesting rock removal and drying did not improve measurements. Our results highlight that pXRF can be an accurate and effective tool for screening for Mn, Cu, Zn, and Pb. Some elements, such as Pb in fruits; Mn, Cu, and Zn in leaves; and Zn and Pb in roots, could be estimated by soil pXRF or inductively coupled plasma-based analyses. Macronutrients were poorly estimated for fruits, leaves, and roots. Instead of soil concentrations, identifying genus-specific and garden-specific factors may be important for generating plant uptake predictive models.

Overcome the limits of multi-contaminated industrial soils bioremediation: Insights from a multi-disciplinary study

Phytoremediation can be a promising and sustainable strategy to recovery Potentially Toxic Elements (PTEs) contaminated soils. However, at the field level, this tool can be limited by many issues. Herein, we combined native plant species with different cover type (mono and poly culture) in an in-field mesocosm experiment for the remediation of multi-contaminated soils from Bagnoli brownfield site (Southern Italy). We preliminary gain insights about the physical, chemical and biological features of the soils and subsequently induced a potential variation in the soil microbiome. We found that polyculture better respond both in terms of pollutant phytostabilization efficiency and from a stress tolerance perspective. Among plant species, Festuca achieved the best performance due to the overexpression of metal transporters able in both PTEs influx and sequestration from the cytoplasm. We achieved a site-specific bio-factory, which represents a strategy for the sustainable and relatively fast recovery of large contaminated areas.

Genetic biofortification of wheat with zinc: Opportunities to fine-tune zinc uptake, transport and grain loading

Zinc (Zn) is an important micronutrient in the human body, and health complications associated with insufficient dietary intake of Zn can be overcome by increasing the bioavailable concentrations in edible parts of crops (biofortification). Wheat (Triticum aestivum L) is the most consumed cereal crop in the world; therefore, it is an excellent target for Zn biofortification programs. Knowledge of the physiological and molecular processes that regulate Zn concentration in the wheat grain is restricted, inhibiting the success of genetic Zn biofortification programs. This review helps break this nexus by advancing understanding of those processes, including speciation regulated uptake, root to shoot transport, remobilisation, grain loading and distribution of Zn in wheat grain. Furthermore, new insights to genetic Zn biofortification of wheat are discussed, and where data are limited, we draw upon information for other cereals and Fe distribution. We identify the loading and distribution of Zn in grain as major bottlenecks for biofortification, recognising anatomical barriers in the vascular region at the base of the grain, and physiological and molecular restrictions localised in the crease region as major limitations. Movement of Zn from the endosperm cavity into the modified aleurone, aleurone and then to the endosperm is mainly regulated by ZIP and YSL transporters. Zn complexation with phytic acid in the aleurone limits Zn mobility into the endosperm. These insights, together with synchrotron-X-ray-fluorescence microscopy, support the hypothesis that a focus on the mechanisms of Zn loading into the grain will provide new opportunities for Zn biofortification of wheat.

The role of silicon in cadmium alleviation by rice root cell wall retention and vacuole compartmentalization under different durations of Cd exposure

Silicon (Si) plays a pivotal role in mitigating phytotoxicity caused by cadmium (Cd). However, few former reports focused on the internal mechanism how Si assisted in alleviating Cd stress in rice under different durations of Cd exposure. Herein, the effects of Si on subcellular distribution of Cd in rice roots under short-term (12 h) and long-term (20 d) Cd exposure were explored. Results showed that Si decreased shoot Cd concentration but had little impact on root Cd levels. Under short-term Cd exposure, subcellular distribution analysis showed that Si increased the ratio of Cd in root cell wall by 23.2~24.0%, and decreased the ratio of Cd in root soluble fraction by 20.6~21.5%. This suggested that Si supply improved root retention of Cd by fixing it on the cell wall and thus restricted intracellular transportation of Cd. Further analysis unraveled that pectin (especially ionic-soluble pectin) of the cell wall was the main binding component, and Si supply induced more Cd accumulation in covalent-soluble pectin and hemicellulose. Moreover, the overexpression of germin-like proteins (GLPs) proved the role of cell wall in moderating Cd toxicity. Under long-term Cd exposure, Si promoted phytochelatin 2 (PC2) and phytochelatin 3 (PC3) synthesis in cytosol, at the same time, Si down-regulated the expression of the Cd efflux-related protein multidrug resistance-associated protein-like ATP-binding cassette transporters (MRP-like ABC transporters) and limited Cd transportation from vacuole to cytosol. Taken together, Si rather predominates in limiting Cd translocation by the cell wall of root under short-term Cd exposure and promoting vacuole compartmentalization to mitigate the Cd toxicity under long-term exposure, instead of reducing the absorption of Cd in rice roots, thereby decreasing Cd delivery into shoots.

A path toward concurrent biofortification and cadmium mitigation in plant-based foods

Millions of people are anemic due to inadequate consumption of foods rich in iron and zinc. Plant-based foods provide most of our dietary nutrients but may also contain the toxic heavy metal cadmium (Cd). A low level of Cd silently enters the body through the diet. Once ingested, Cd may remain for decades. Hence, prolonged intake of Cd-containing foods endangers human health. Research that leads towards micronutrient enrichment and mitigation of Cd in foods has therefore dual significance for human health. The breeding of Cd-tolerant cultivars may enable them to grow on Cd-polluted soils; however, they may not yield Cd-free foods. Conversely, sequestration of Cd in roots can prevent its accumulation in grains, but this mechanism also retains nutrients, hence counteracting biofortification efforts. A specific restriction of the Cd absorption capacity of crops would prevent Cd entry into the plant system while maintaining micronutrient accumulation and may thus be a solution to the dilemma. After recapitulating existing strategies employed for the development of Cd-tolerant and biofortified cultivars, this Viewpoint elaborates alternative approaches based on directed evolution and genome editing strategies for excluding Cd while enriching micronutrients in plant foods, which will concurrently help to eradicate malnutrition and prevent Cd intoxication.

Different effects of foliar application of silica sol on arsenic translocation in rice under low and high arsenite stress

Foliar application of Si can generally reduce As translocation from roots to shoots in rice; however, it does not always work, particularly under high As stress. Here, the effects of foliar application of nanoscale silica sol on As accumulation in rice were investigated under low (2 μmol/L) and high (8 μmol/L) arsenite stress. The results revealed that foliar Si application significantly decreased the As concentration in shoots under low arsenite stress, but showed different effects under high arsenite stress after 7 days of incubation. The reduction in root-to-shoot As translocation under the 2As+Si treatment was related to the down-regulation of OsLsi1 and OsLsi2 expression and up-regulation of OsABCC1 expression in roots. In the 8As+Si treatment, the expressions of OsLsi1, OsLsi2, and OsABCC1 were significantly promoted, which resulted in substantially higher As accumulation in both the roots and shoots. In the roots, As predominantly accumulated in the symplasts (90.6%-98.3%), in which the majority of As was sequestered in vacuoles (79.0%-94.0%) under both levels of arsenite stress. Compared with that of the 8As treatment, the 8As+Si treatment significantly increased the As concentration in cell walls, but showed no difference in the vacuolar As concentration, which remained constant at approximately 69.1-71.7 mg/kg during days 4-7. It appeared that the capacity of root cells to sequester As in the vacuoles had a threshold, and the excess As tended to accumulate in the cell walls and transfer to the shoots via apoplasts under high arsenite stress. This study provides a better understanding of the different effects of foliar Si application on As accumulation in rice from the view of arsenite-related gene expression and As subcellular distribution in roots.

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.

The critical role of the shoot base in inhibiting cadmium transport from root to shoot in a cadmium-safe rice line (Oryza sativa L.)

Cadmium (Cd) is harmful to rice and human, thus screening and understanding the mechanism of Cd-safe rice lines, which accumulate little Cd in brown rice, is necessary. D62B was screened as a Cd-safe rice line with low Cd translocation from roots to shoots, and there must be a switch restricting Cd transport from roots to shoots. Here we found that shoot base played the role as switch. Cd concentration in the shoot base of D62B was 1.57 times higher compared with a high Cd-accumulating rice line (Wujin4B) and lower Cd translocation under Cd stress. Glutathione (GSH) and phytochelatins (PCs) were important in this process. GSH and PCs concentrations in the shoot bases of D62B were 1.01- 1.83 times higher than Wujin4B as well as the glutathione S-transferase (GST) and phytochelatin synthase (PCS) concentrations, keeping in consistent with up-regulation of the genes OsGST and OsPCS1. PCs synthesis was further promoted by exogenous GSH. Our results prove the role of shoot bases as switch for restricting Cd transport in D62B due to its great potential for GSH and PCs biosynthesis, and thereby Cd chelation. This could be considered a key mechanism for low Cd accumulation in brown rice of the Cd-safe rice line.

1,135 ionomes reveal the global pattern of leaf and seed mineral nutrient and trace element diversity in Arabidopsis thaliana

Soil is a heterogeneous reservoir of essential elements needed for plant growth and development. Plants have evolved mechanisms to balance their nutritional needs based on availability of nutrients. This has led to genetically based variation in the elemental composition, the 'ionome', of plants, both within and between species. We explore this natural variation using a panel of wild-collected, geographically widespread Arabidopsis thaliana accessions from the 1001 Genomes Project including over 1,135 accessions, and the 19 parental accessions of the Multi-parent Advanced Generation Inter-Cross (MAGIC) panel, all with full-genome sequences available. We present an experimental design pipeline for high-throughput ionomic screenings and analyses with improved normalisation procedures to account for errors and variability in conditions often encountered in large-scale, high-throughput data collection. We report quantification of the complete leaf and seed ionome of the entire collection using this pipeline and a digital tool, Ion Explorer, to interact with the dataset. We describe the pattern of natural ionomic variation across the A. thaliana species and identify several accessions with extreme ionomic profiles. It forms a valuable resource for exploratory genetic mapping studies to identify genes underlying natural variation in leaf and seed ionome and genetic adaptation of plants to soil conditions.

Lead exposure-induced defense responses result in low lead translocation from the roots to aerial tissues of two contrasting poplar species

To explore whether lead (Pb)-induced defense responses are responsible for the low root-to-shoot Pb translocation, we exposed saplings of the two contrasting poplar species, Populus × canescens with relatively high root-to-shoot Pb translocation and P. nigra with low Pb translocation, to 0 or 8 mM PbCl₂. Pb translocation from the roots to aboveground tissues was lower by 57% in P. nigra than that in P. × canescens. Lower Pb concentrations in the roots and aerial tissues, greater root biomass, and lower ROS overproduction in the roots were found in P. nigra than those in P. × canescens treated with Pb. P. nigra roots had higher proportions of cell walls (CWs)-bound Pb and water insoluble Pb compounds, and higher transcript levels of some pivotal genes related to Pb vacuolar sequestration, such as phytochelatin synthetase 1.1 (PCS1.1), ATP-binding cassette transporter C1.1 (ABCC1.1) and ABCC3.1 than P. × canescens roots. Pb exposure induced defense responses including increases in the contents of pectin and hemicellulose, and elevated oxalic acid accumulation, and the transcriptional upregulation of PCS1.1, ABCC1.1 and ABCC3.1 in the roots of P. nigra and P. × canescens. These results suggest that the stronger defense barriers in P. nigra roots are probably associated with the lower Pb translocation from the roots to aerial tissues, and that Pb exposure-induced defense responses can enhance the barriers against Pb translocation in poplar roots.

Specificity and Plasticity of the Functional Ionome of Brassica napus and Triticum aestivum Exposed to Micronutrient or Beneficial Nutrient Deprivation and Predictive Sensitivity of the Ionomic Signatures

The specific variation in the functional ionome was studied in Brassica napus and Triticum aestivum plants subjected to micronutrient or beneficial mineral nutrient deprivation. Effects of these deprivations were compared to those of macronutrient deprivation. In order to identify early events, plants were harvested after 22 days, i.e., before any significant reduction in growth relative to control plants. Root uptake, tissue concentrations and relative root nutrient contents were analyzed revealing numerous interactions with respect to the 20 elements quantified. The assessment of the functional ionome under individual mineral nutrient deficiency allows the identification of a large number of interactions between elements, although it is not totally exhaustive, and gives access to specific ionomic signatures that discriminate among deficiencies in N, P, S, K, Ca, Mn, Fe, Zn, Na, Si, and Se in both species, plus Mg, Cl, Cu, and Mo in wheat. Ionome modifications and components of ionomic signatures are discussed in relation to well-known mechanisms that may explain crosstalks between mineral nutrients, such as between Na and K, V, Se, Mo and S or Fe, Zn and Cu. More surprisingly, when deprived of beneficial nutrients such as Na, Si, Co, or Se, the plant ionome was strongly modified while these beneficial nutrients contributed greatly to the leaf ionomic signature of most mineral deficiencies.

The copper economy response is partially conserved in rice (Oryza sativa L.)

Copper (Cu) is an essential element for plants, especially in photosynthesis, as it is required for plastocyanin function in electron transfer reactions at thylakoid membranes. In Arabidopsis thaliana, Cu deficiency leads to the Cu economy response, in which plants prioritize Cu usage by plastocyanin in detriment of non-essential cupric proteins. In rice (Oryza sativa), however, this response has not been characterized. Rice OsHMA5 is a Cu xylem-loading transporter involved in Cu translocation from roots to shoots, as suggested by the analysis of oshma5 mutant plants. Aiming to understand how rice plants respond to Cu deficiency and how decreased Cu translocation to shoots can affect this response, we characterized the physiological and molecular responses of WT and oshma5 plants under control and Cu deficiency treatments. We found evidence that shoots of oshma5 plants are more prone to Cu deficiency compared to shoots of WT plants, as demonstrated by decreased chlorophyll and Cu concentrations, and electron transport rate. Gene expression analysis revealed that Cu high-affinity transporters OsCOPT1 and OsCOPT5, along with a set of miRNAs and three Cu/Zn superoxide dismutases are responsive to Cu deficiency in both WT and oshma5 plants, suggesting their involvement in the Cu economy response. However, Fe superoxide dismutase was not up-regulated in rice, indicating a difference compared to the A. thaliana Cu economy model. Therefore, we provide evidence for a partially conserved Cu economy response in rice, in comparison to A. thaliana.

Chemical Speciation, Plant Uptake, and Toxicity of Heavy Metals in Agricultural Soils

Heavy metals in agricultural soils exist in diverse dissolved (free cations and complexed species of positive, neutral, or negative charges), particulate (sorbed, structural, and coprecipitated), and colloidal (micro- and nanometer-sized particles) species. The fate of different heavy metal species is controlled by the master variables: pH (solubility), ionic strength (activity and charge-shielding), and dissolved organic carbon (complexation). In the rhizosphere, chemical speciation controls toxicokinetics (uptake and transport of metals by plants) while toxicodynamics (interaction between the plant and absorbed species) drives the toxicity outcome. Based on the critical review, the authors recommend omics and data mining techniques to link discrete knowledge bases from the speciation dynamics, soil microbiome, and plant transporter/gene expression relevant to homeostasis conditions of modern agriculture. Such efforts could offer a disruptive application tool to improve and sustain plant tolerance, food safety, and environmental quality.

The barrier function of plant roots: biological bases for selective uptake and avoidance of soil compounds

The root is the main organ through which water and mineral nutrients enter the plant organism. In addition, root fulfils several other functions. Here, we propose that the root also performs the barrier function, which is essential not only for plant survival but for plant acclimation and adaptation to a constantly changing and heterogeneous soil environment. This function is related to selective uptake and avoidance of some soil compounds at the whole plant level. We review the toolkit of morpho-anatomical, structural, and other components that support this view. The components of the root structure involved in selectivity, permeability or barrier at a cellular, tissue, and organ level and their properties are discussed. In consideration of the arguments supporting barrier function of plant roots, evolutionary aspects of this function are also reviewed. Additionally, natural variation in selective root permeability is discussed which suggests that the barrier function is constantly evolving and is subject of natural selection.

The influence of phylogeny and ecology on root, shoot and plant ionomes of 14 native Brazilian species

The ionome is the elemental composition of a living organism, its tissues, cells or cell compartments. The ionomes of roots, stems and leaves of 14 native Brazilian forest species were characterised to examine the relationships between plant and organ ionomes and the phylogenetic and ecological affiliations of species. The null hypothesis that ionomes of Brazilian forest species and their organs do not differ was tested. Concentrations of mineral nutrients in roots, stems and leaves were determined for 14 Brazilian forest species, representing seven angiosperm orders, grown hydroponically in a complete nutrient solution. The 14 species could be differentiated by their ionomes and the partitioning of mineral nutrients between organs. The ionomic differences between the 14 species did not reflect their phylogenetic relationships or successional ecology. Differences between shoot ionomes and root ionomes were greater than differences in the ionome of an organ when compared among genotypes. In conclusion, differences in ionomes of species and their organs reflect a combination of ancient phylogenetic and recent environmental adaptations.

Comparing the performance of four macrophytes in bacterial assisted floating treatment wetlands for the removal of trace metals (Fe, Mn, Ni, Pb, and Cr) from polluted river water

Here we compared the performance of four macrophytes namely Brachia mutica, Typha domingensis, Phragmites australis and Leptochala fusca, in bacterially assisted floating treatment wetlands (FTWs) for the clean-up of five trace metals (Fe, Mn, Ni, Pb, and Cr) from polluted river water. The river water was artificially spiked with reagent grade chemicals to increase the trace metal pollution. The macrophytes were planted in a polystyrene sheet to prepare FTWs, which were placed over the metal-contaminated river water. The consortium of five rhizospheric and endophytic bacterial strains, i.e., Aeromonas salmonicida, Pseudomonas indoloxydans, Bacillus cerus, Pseudomonas gessardii, and Rhodococcus sp., was inoculated support the natural remediation ability. We found a significant reduction in the metal content by all four macrophytes and the removal was significantly enhanced when bacterial inoculum was applied. The maximum removal was observed in FTWs planted with P. australis and inoculated with bacteria. In this treatment (T6) the Fe, Mn, Ni, Pb and Cr contents were reduced to 0.53, 0.20, 0.09, 1.04 and 0.07 mg L^-1 after five weeks retention time. The bacterial inoculation sufficiently increased the plant biomass. All macrophytes depicted potential to uptake and translocate trace metals in the roots instead of shoots. The bacterial inoculation acclimatize the plants roots followed by shoots and enhanced the uptake of metals by macrophytes. This study emphasized the usefulness of macrophytes-bacteria mutualism in FTWs system for the remediation of trace metals. The similar systems may provide practical solutions for the remediation of trace metals of polluted river water.

Transcriptional plasticity buffers genetic variation in zinc homeostasis

In roots of Arabidopsis thaliana, Zn can be either loaded into the xylem for translocation to the shoot or stored in vacuoles. Vacuolar storage is achieved through the action of the Zn/Cd transporter HMA3 (Heavy Metal Atpase 3). The Col-0 accession has an HMA3 loss-of-function allele resulting in high shoot Cd, when compared to accession CSHL-5 which has a functional allele and low shoot Cd. Interestingly, both Col-0 and CSHL-5 have similar shoot Zn concentrations. We hypothesize that plants sense changes in cytosolic Zn that are due to variation in HMA3 function, and respond by altering expression of genes related to Zn uptake, transport and compartmentalisation, in order to maintain Zn homeostasis. The expression level of genes known to be involved in Zn homeostasis were quantified in both wild-type Col-0 and Col-0::HMA3CSHL-5 plants transformed with the functional CSHL-5 allele of HMA3. We observed significant positive correlations between expression of HMA3 and of genes known to be involved in Zn homeostasis, including ZIP3, ZIP4, MTP1, and bZIP19. The results support our hypothesis that alteration in the level of function of HMA3 is counterbalanced by the fine regulation of the Zn homeostasis gene network in roots of A. thaliana.

The Combined Strategy for iron uptake is not exclusive to domesticated rice (Oryza sativa)

Iron (Fe) is an essential micronutrient that is frequently inaccessible to plants. Rice (Oryza sativa L.) plants employ the Combined Strategy for Fe uptake, which is composed by all features of Strategy II, common to all Poaceae species, and some features of Strategy I, common to non-Poaceae species. To understand the evolution of Fe uptake mechanisms, we analyzed the root transcriptomic response to Fe deficiency in O. sativa and its wild progenitor O. rufipogon. We identified 622 and 2,017 differentially expressed genes in O. sativa and O. rufipogon, respectively. Among the genes up-regulated in both species, we found Fe transporters associated with Strategy I, such as IRT1, IRT2 and NRAMP1; and genes associated with Strategy II, such as YSL15 and IRO2. In order to evaluate the conservation of these Strategies among other Poaceae, we identified the orthologs of these genes in nine species from the Oryza genus, maize and sorghum, and evaluated their expression profile in response to low Fe condition. Our results indicate that the Combined Strategy is not specific to O. sativa as previously proposed, but also present in species of the Oryza genus closely related to domesticated rice, and originated around the same time the AA genome lineage within Oryza diversified. Therefore, adaptation to Fe2+ acquisition via IRT1 in flooded soils precedes O. sativa domestication.

Selection of Appropriate Reference Genes for Gene Expression Analysis under Abiotic Stresses in Salix viminalis

Salix viminalis is a fast growing willow species with potential as a plant used for biomass feedstock or for phytoremediation. However, few reference genes (RGs) for quantitative real-time polymerase chain reaction (qPCR) are available in S. viminalis, thereby limiting gene expression studies. Here, we investigated the expression stability of 14 candidate reference genes (RGs) across various organs exposed to five abiotic stresses (cold, heat, drought, salt, and poly-metals). Four RGs ranking algorithms, namely geNorm^PLUS, BestKeeper, NormFinder, and GrayNorm were applied to analyze the qPCR data and the outputs were merged into consensus lists with RankAggreg, a rank aggregation algorithm. In addition, the optimal RG combinations were determined with geNorm^PLUS and GrayNorm. The genes that were the most stable in the roots were TIP41 and CDC2. In the leaves, TIP41 was the most stable, followed by EF1b and ARI8, depending on the condition tested. Conversely, GAPDH and β-TUB, two genes commonly used for qPCR data normalization were the least stable across all organs. Nevertheless, both geNorm^PLUS and GrayNorm recommended the use of a combination of genes rather than a single one. These results are valuable for research of transcriptomic responses in different S. viminalis organs.

Genotype Variation in Rice (Oryza sativa L.) Tolerance to Fe Toxicity Might Be Linked to Root Cell Wall Lignification

Iron (Fe) is an essential element to plants, but can be harmful if accumulated to toxic concentrations. Fe toxicity can be a major nutritional disorder in rice (Oryza sativa) when cultivated under waterlogged conditions, as a result of excessive Fe solubilization of in the soil. However, little is known about the basis of Fe toxicity and tolerance at both physiological and molecular level. To identify mechanisms and potential candidate genes for Fe tolerance in rice, we comparatively analyzed the effects of excess Fe on two cultivars with distinct tolerance to Fe toxicity, EPAGRI 108 (tolerant) and BR-IRGA 409 (susceptible). After excess Fe treatment, BR-IRGA 409 plants showed reduced biomass and photosynthetic parameters, compared to EPAGRI 108. EPAGRI 108 plants accumulated lower amounts of Fe in both shoots and roots compared to BR-IRGA 409. We conducted transcriptomic analyses of roots from susceptible and tolerant plants under control and excess Fe conditions. We found 423 up-regulated and 92 down-regulated genes in the susceptible cultivar, and 42 up-regulated and 305 down-regulated genes in the tolerant one. We observed striking differences in root gene expression profiles following exposure to excess Fe: the two cultivars showed no genes regulated in the same way (up or down in both), and 264 genes were oppositely regulated in both cultivars. Plants from the susceptible cultivar showed down-regulation of known Fe uptake-related genes, indicating that plants are actively decreasing Fe acquisition. On the other hand, plants from the tolerant cultivar showed up-regulation of genes involved in root cell wall biosynthesis and lignification. We confirmed that the tolerant cultivar has increased lignification in the outer layers of the cortex and in the vascular bundle compared to the susceptible cultivar, suggesting that the capacity to avoid excessive Fe uptake could rely in root cell wall remodeling. Moreover, we showed that increased lignin concentrations in roots might be linked to Fe tolerance in other rice cultivars, suggesting that a similar mechanism might operate in multiple genotypes. Our results indicate that changes in root cell wall and Fe permeability might be related to Fe toxicity tolerance in rice natural variation.