Physiology and molecular basis of thallium toxicity and accumulation in Arabidopsis thaliana.

Thallium (Tl) is a non-essential metal mobilized through industrial processes which can lead to it entering the environment and exerting toxic effects. Plants are fundamental components of all ecosystems. Therefore, understanding the impact of Tl on plant growth and development is of great importance for assessing the potential environmental risks of Tl. Here, the responses of Arabidopsis thaliana to Tl were elucidated using physiological, genetic, and transcriptome analyses. Thallium can be absorbed by plant roots and translocated to the aerial parts, accumulating at comparable concentrations throughout plant parts. Genetic evidence supported the regulation of Tl uptake and movement by different molecular compartments within plants. Thallium primarily caused growth inhibition, oxidative stress, leaf chlorosis, and the impairment of K homeostasis. The disturbance of redox balance toward oxidative stress was supported by significant differences in the expression of genes involved in oxidative stress and antioxidant defense under Tl exposure. Reduced GSH levels in cad2-1 mutant rendered plants highly sensitive to Tl, suggesting that GSH has a prominent role in alleviating Tl-triggered oxidative responses. Thallium down-regulation of the expression of LCHII-related genes is believed to be responsible for leaf chlorosis. These findings illuminate some of the mechanisms underlying Tl toxicity at the physiological and molecular levels in plants with an eye toward the future environment management of this heavy metal.

Arabidopsis BRUTUS-LIKE E3 ligases negatively regulate iron uptake by targeting transcription factor FIT for recycling.

Organisms need to balance sufficient uptake of iron (Fe) with possible toxicity. In plant roots, a regulon of uptake genes is transcriptionally activated under Fe deficiency, but it is unknown how this response is inactivated when Fe becomes available. Here we describe the function of 2 partially redundant E3 ubiquitin ligases, BRUTUS-LIKE1 (BTSL1) and BTSL2, in Arabidopsis thaliana and provide evidence that they target the transcription factor FIT, a key regulator of Fe uptake, for degradation. The btsl double mutant failed to effectively down-regulate the transcription of genes controlled by FIT, and accumulated toxic levels of Fe in roots and leaves. The C-terminal domains of BTSL1 and BTSL2 exhibited E3 ligase activity, and interacted with FIT but not its dimeric partner bHLH39. The BTSL proteins were able to poly-ubiquitinate FIT in vitro and promote FIT degradation in vivo. Thus, posttranslational control of FIT is critical to prevent excess Fe uptake.

Histone H3 lysine4 trimethylation-regulated GRF11 expression is essential for the iron-deficiency response in Arabidopsis thaliana.

Iron (Fe) homeostasis in plants is controlled by both transcription factors (TFs) and chromatin remodeling through histone modification. To date, few studies have reported the existence of histone modification in maintaining the Fe-deficiency response. However, the reports that do exist shed light on various histone modifications, but knowledge of the activation mark in Fe-deficiency response is lacking. By using a forward genetics approach, we identified a crucial allele for Fe-deficiency response, NON-RESPONSE TO Fe-DEFICIENCY2 (NRF2), previously described as EARLY FLOWERING8 (ELF8) associated with an activation mark on histone modification, histone H3 lysine4 trimethylation. In the nrf2-1 mutant, a point mutation at ELF8T404I , exhibits impaired expression of GENERAL REGULATORY FACTOR11 (GRF11) and downstream genes in the Fe-uptake pathway. In vivo chromatin immunoprecipitation revealed that in roots, NRF2/ELF8 is essential for the expression of GRF11 for Fe-deficiency response, whereas in shoots, NRF2/ELF8 regulates FLOWERING LOCUS C (FLC) expression for flowering time control. In summary, a key factor, NRF2/ELF8, involved in epigenetic regulation essential for both flowering time control and Fe-deficiency response is uncovered.

Divalent nutrient cations: Friend and foe during zinc stress in rice.

Zn deficiency is the most common micronutrient deficit in rice but Zn is also a widespread industrial pollutant. Zn deficiency responses in rice are well documented, but comparative responses to Zn deficiency and excess have not been reported. Therefore, we compared the physiological, transcriptional and biochemical properties of rice subjected to Zn starvation or excess at early and later treatment stages. Both forms of Zn stress inhibited root and shoot growth. Gene ontology analysis of differentially expressed genes highlighted the overrepresentation of Zn transport and antioxidative defense for both Zn stresses, whereas diterpene biosynthesis was solely induced by excess Zn. Divalent cations (Fe, Cu, Ca, Mn and Mg) accumulated in Zn-deficient shoots but Mg and Mn were depleted in the Zn excess shoots, mirroring the gene expression of non-specific Zn transporters and chelators. Ascorbate peroxidase activity was induced after 14 days of Zn starvation, scavenging H2 O2 more effectively to prevent leaf chlorosis via the Fe-dependent Fenton reaction. Conversely, excess Zn triggered the expression of genes encoding Mg/Mn-binding proteins (OsCPS2/4 and OsKSL4/7) required for antimicrobial diterpenoid biosynthesis. Our study reveals the potential role of divalent cations in the shoot, driving the unique responses of rice to each form of Zn stress.

The dual benefit of a dominant mutation in Arabidopsis IRON DEFICIENCY TOLERANT1 for iron biofortification and heavy metal phytoremediation.

One of the goals of biofortification is to generate iron-enriched crops to combat growth and developmental defects especially iron (Fe) deficiency anaemia. Fe-fortification of food is challenging because soluble Fe is unstable and insoluble Fe is nonbioavailable. Genetic engineering is an alternative approach for Fe-biofortification, but so far strategies to increase Fe content have only encompassed a few genes with limited success. In this study, we demonstrate that the ethyl methanesulfonate (EMS) mutant, iron deficiency tolerant1 (idt1), can accumulate 4-7 times higher amounts of Fe than the wild type in roots, shoots and seeds, and exhibits the metal tolerance and iron accumulation (Metina) phenotype in Arabidopsis. Fe-regulated protein stability and nuclear localisation of the upstream transcriptional regulator bHLH34 were uncovered. The C to T transition mutation resulting in substitution of alanine to valine at amino acid position 320 of bHLH34 (designated as IDT1A320V ) in a conserved motif among mono- and dicots was found to be responsible for a dominant phenotype that possesses constitutive activation of the Fe regulatory pathway. Overexpression of IDT1A320V in Arabidopsis and tobacco led to the Metina phenotype; a phenotype that has escalated specificity towards optimising Fe homeostasis and may be useful in Fe-biofortification. Knowledge of the high tolerance and accumulation of heavy metals of this mutant can aid the development of tools for phytoremediation of contaminants.

A HemK class glutamine-methyltransferase is involved in the termination of translation and essential for iron homeostasis in Arabidopsis.

Iron (Fe) transport and utilization are controlled by Fe-dependent transcriptional cascades. Many genes participate in these processes, transcriptionally controlled by Fe-status. Thorough knowledge of the translational check-points is lacking. We identified a non-response to Fe-deficiency1-1 (nrf1-1) mutant of Arabidopsis thaliana, which displayed a hypersensitive phenotype under Fe-deficient conditions. By mapping nrf1-1, we found that the AT3G13440 locus encoding a HemK methyltransferase is responsible for the phenotype. Analyses of ProUBQ10:NRF1CDS overexpression nrf1-1 lines and a T-DNA insertion mutant nrf1-2, confirmed that loss-of-function of NRF1 results in enhanced Fe-starvation-sensitivity. NRF1 is required for the proper expression of the majority of Fe-deficiency-inducible (FDI) genes. The nrf1 mutants accumulated more polysomes in the roots, due to stalled ribosomes on several transcripts. Ribosome-footprint (RF) mapping revealed that ribosomes are stalled at a stop codon that amplified the stalling of trailing ribosomes. We detected higher RF levels in many FDI transcripts in nrf1-2. Our study demonstrates the requirement of NRF1 for an accurate termination of protein synthesis essential not only for a precise iron homeostasis, but also cellular ion balance. NRF1 is also important for normal growth and development. A check-point that fine-tunes peptide release in plants is uncovered.

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

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

Triplin, a small molecule, reveals copper ion transport in ethylene signaling from ATX1 to RAN1.

Copper ions play an important role in ethylene receptor biogenesis and proper function. The copper transporter RESPONSIVE-TO-ANTAGONIST1 (RAN1) is essential for copper ion transport in Arabidopsis thaliana. However it is still unclear how copper ions are delivered to RAN1 and how copper ions affect ethylene receptors. There is not a specific copper chelator which could be used to explore these questions. Here, by chemical genetics, we identified a novel small molecule, triplin, which could cause a triple response phenotype on dark-grown Arabidopsis seedlings through ethylene signaling pathway. ran1-1 and ran1-2 are hypersensitive to triplin. Adding copper ions in growth medium could partially restore the phenotype on plant caused by triplin. Mass spectrometry analysis showed that triplin could bind copper ion. Compared to the known chelators, triplin acts more specifically to copper ion and it suppresses the toxic effects of excess copper ions on plant root growth. We further showed that mutants of ANTIOXIDANT PROTEIN1 (ATX1) are hypersensitive to tiplin, but with less sensitivity comparing with the ones of ran1-1 and ran1-2. Our study provided genetic evidence for the first time that, copper ions necessary for ethylene receptor biogenesis and signaling are transported from ATX1 to RAN1. Considering that triplin could chelate copper ions in Arabidopsis, and copper ions are essential for plant and animal, we believe that, triplin not only could be useful for studying copper ion transport of plants, but also could be useful for copper metabolism study in animal and human.

S-Nitrosoglutathione works downstream of nitric oxide to mediate iron-deficiency signaling in Arabidopsis.

Iron (Fe) is essential for plant growth and development. Knowledge of Fe signaling, from the beginning of perception to activation of the uptake process, is critical for crop improvement. Here, by using chemical screening, we identified a small molecule 3-amino-N-(3-methylphenyl)thieno[2,3-b]pyridine-2-carboxamide named R7 ('R' denoting repressor of IRON-REGULATED TRANSPORTER 1), that modulates Fe homeostasis of Arabidopsis. R7 treatment led to reduced Fe levels in plants, thus causing severe chlorosis under Fe deficiency. Expression analysis of central transcription factors, FER-LIKE IRON DEFICIENCY INDUCED TRANSCRIPTION FACTOR (FIT) and subgroup Ib basic helix-loop-helix (Ib bHLH) genes bHLH38/39/100/101, revealed that R7 targets the FIT-dependent transcriptional pathway. Exogenously supplying S-nitrosoglutathione (GSNO), but not other nitric oxide (NO) donors sodium nitroprusside (SNP) and S-nitroso-N-acetyl-dl-penicillamine (SANP), alleviated the inhibitory effects of R7 on Fe homeostasis. R7 did not inhibit cellular levels of NO or glutathione but decreased GSNO level in roots. We demonstrate that NO is involved in regulating not only the FIT transcriptional network but also the Ib bHLH networks. In addition, GSNO, from S-nitrosylation of glutathione, specifically mediates the Fe-starvation signal to FIT, which is distinct from the NO to Ib bHLH signal. Our work dissects the molecular connection between NO and the Fe-starvation response. We present a new signaling route whereby GSNO acts downstream of NO to trigger the Fe-deficiency response in Arabidopsis.

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

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

Glutathione plays an essential role in nitric oxide-mediated iron-deficiency signaling and iron-deficiency tolerance in Arabidopsis.

Iron (Fe) deficiency is a common agricultural problem that affects both the productivity and nutritional quality of plants. Thus, identifying the key factors involved in the tolerance of Fe deficiency is important. In the present study, the zir1 mutant, which is glutathione deficient, was found to be more sensitive to Fe deficiency than the wild type, and grew poorly in alkaline soil. Other glutathione-deficient mutants also showed various degrees of sensitivity to Fe-limited conditions. Interestingly, we found that the glutathione level was increased under Fe deficiency in the wild type. By contrast, blocking glutathione biosynthesis led to increased physiological sensitivity to Fe deficiency. On the other hand, overexpressing glutathione enhanced the tolerance to Fe deficiency. Under Fe-limited conditions, glutathione-deficient mutants, zir1, pad2 and cad2 accumulated lower levels of Fe than the wild type. The key genes involved in Fe uptake, including IRT1, FRO2 and FIT, are expressed at low levels in zir1; however, a split-root experiment suggested that the systemic signals that govern the expression of Fe uptake-related genes are still active in zir1. Furthermore, we found that zir1 had a lower accumulation of nitric oxide (NO) and NO reservoir S-nitrosoglutathione (GSNO). Although NO is a signaling molecule involved in the induction of Fe uptake-related genes during Fe deficiency, the NO-mediated induction of Fe-uptake genes is dependent on glutathione supply in the zir1 mutant. These results provide direct evidence that glutathione plays an essential role in Fe-deficiency tolerance and NO-mediated Fe-deficiency signaling in Arabidopsis.

Evolutionary analysis of iron (Fe) acquisition system in Marchantia polymorpha.

To acquire appropriate iron (Fe), vascular plants have developed two unique strategies, the reduction-based strategy I of nongraminaceous plants for Fe(2+) and the chelation-based strategy II of graminaceous plants for Fe(3+) . However, the mechanism of Fe uptake in bryophytes, the earliest diverging branch of land plants and dominant in gametophyte generation is less clear. Fe isotope fractionation analysis demonstrated that the liverwort Marchantia polymorpha uses reduction-based Fe acquisition. Enhanced activities of ferric chelate reductase and proton ATPase were detected under Fe-deficient conditions. However, M. polymorpha did not show mugineic acid family phytosiderophores, the key components of strategy II, or the precursor nicotianamine. Five ZIP (ZRT/IRT-like protein) homologs were identified and speculated to be involved in Fe uptake in M. polymorpha. MpZIP3 knockdown conferred reduced growth under Fe-deficient conditions, and MpZIP3 overexpression increased Fe content under excess Fe. Thus, a nonvascular liverwort, M. polymorpha, uses strategy I for Fe acquisition. This system may have been acquired in the common ancestor of land plants and coopted from the gametophyte to sporophyte generation in the evolution of land plants.

IRT1 degradation factor1, a ring E3 ubiquitin ligase, regulates the degradation of iron-regulated transporter1 in Arabidopsis.

Fe is an essential micronutrient for plant growth and development; plants have developed sophisticated strategies to acquire ferric Fe from the soil. Nongraminaceous plants acquire Fe by a reduction-based mechanism, and graminaceous plants use a chelation-based mechanism. In Arabidopsis thaliana, which uses the reduction-based method, iron-regulated transporter1 (IRT1) functions as the most important transporter for ferrous Fe uptake. Rapid and constitutive degradation of IRT1 allows plants to quickly respond to changing conditions to maintain Fe homeostasis. IRT1 degradation involves ubiquitination. To identify the specific E3 ubiquitin ligases involved in IRT1 degradation, we screened a set of insertional mutants in RING-type E3 ligases and identified a mutant that showed delayed degradation of IRT1 and loss of IRT1-ubiquitin complexes. The corresponding gene was designated IRT1 degradation factor1 (IDF1). Evidence of direct interaction between IDF1 and IRT1 in the plasma membrane supported the role of IDF1 in IRT1 degradation. IRT1 accumulation was reduced when coexpressed with IDF1 in yeast or Xenopus laevis oocytes. IDF1 function was RING domain dependent. The idf1 mutants showed increased tolerance to Fe deficiency, resulting from increased IRT1 levels. This evidence indicates that IDF1 directly regulates IRT1 degradation through its RING-type E3 ligase activity.

Root-secreted nicotianamine from Arabidopsis halleri facilitates zinc hypertolerance by regulating zinc bioavailability.

Hyperaccumulators tolerate and accumulate extraordinarily high concentrations of heavy metals. Content of the metal chelator nicotianamine (NA) in the root of zinc hyperaccumulator Arabidopsis halleri is elevated compared with nonhyperaccumulators, a trait that is considered to be one of the markers of a hyperaccumulator. Using metabolite-profiling analysis of root secretions, we found that excess zinc treatment induced secretion of NA in A. halleri roots compared with the nonhyperaccumulator Arabidopsis thaliana. Metal speciation analysis further revealed that the secreted NA forms a stable complex with Zn(II). Supplying NA to a nonhyperaccumulator species markedly increased plant zinc tolerance by decreasing zinc uptake. Therefore, NA secretion from A. halleri roots facilitates zinc hypertolerance through forming a Zn(II)-NA complex outside the roots to achieve a coordinated zinc uptake rate into roots. Secretion of NA was also found to be responsible for the maintenance of iron homeostasis under excess zinc. Together our results reveal root-secretion mechanisms associated with hypertolerance and hyperaccumulation.

A vicilin-like seed storage protein, PAP85, is involved in tobacco mosaic virus replication.

One striking feature of viruses with RNA genomes is the modification of the host membrane structure during early infection. This process requires both virus- and host-encoded proteins; however, the host factors involved and their role in this process remain largely unknown. On infection with Tobacco mosaic virus (TMV), a positive-strand RNA virus, the filamentous and tubular endoplasmic reticulum (ER) converts to aggregations at the early stage and returns to filamentous at the late infectious stage, termed the ER transition. Also, membrane- or vesicle-packaged viral replication complexes (VRCs) are induced early during infection. We used microarray assays to screen the Arabidopsis thaliana gene(s) responding to infection with TMV in the initial infection stage and identified an Arabidopsis gene, PAP85 (annotated as a vicilin-like seed storage protein), with upregulated expression during 0.5 to 6 h of TMV infection. TMV accumulation was reduced in pap85-RNA interference (RNAi) Arabidopsis and restored to wild-type levels when PAP85 was overexpressed in pap85-RNAi Arabidopsis. We did not observe the ER transition in TMV-infected PAP85-knockdown Arabidopsis protoplasts. In addition, TMV accumulation was reduced in PAP85-knockdown protoplasts. VRC accumulation was reduced, but not significantly (P = 0.06), in PAP85-knockdown protoplasts. Coexpression of PAP85 and the TMV main replicase (P126), but not their expression alone in Arabidopsis protoplasts, could induce ER aggregations.

Iron is involved in the maintenance of circadian period length in Arabidopsis.

The homeostasis of iron (Fe) in plants is strictly regulated to maintain an optimal level for plant growth and development but not cause oxidative stress. About 30% of arable land is considered Fe deficient because of calcareous soil that renders Fe unavailable to plants. Under Fe-deficient conditions, Arabidopsis (Arabidopsis thaliana) shows retarded growth, disordered chloroplast development, and delayed flowering time. In this study, we explored the possible connection between Fe availability and the circadian clock in growth and development. Circadian period length in Arabidopsis was longer under Fe-deficient conditions, but the lengthened period was not regulated by the canonical Fe-deficiency signaling pathway involving nitric oxide. However, plants with impaired chloroplast function showed long circadian periods. Fe deficiency and impaired chloroplast function combined did not show additive effects on the circadian period, which suggests that plastid-to-nucleus retrograde signaling is involved in the lengthening of circadian period under Fe deficiency. Expression pattern analyses of the central oscillator genes in mutants defective in CIRCADIAN CLOCK ASSOCIATED1/LATE ELONGATED HYPOCOTYL or GIGANTEA demonstrated their requirement for Fe deficiency-induced long circadian period. In conclusion, Fe is involved in maintaining the period length of circadian rhythm, possibly by acting on specific central oscillators through a retrograde signaling pathway.

ZINC TOLERANCE INDUCED BY IRON 1 reveals the importance of glutathione in the cross-homeostasis between zinc and iron in Arabidopsis thaliana.

Zinc is an essential micronutrient for plants, but it is toxic in excess concentrations. In Arabidopsis, additional iron (Fe) can increase Zn tolerance. We isolated a mutant, zinc tolerance induced by iron 1, designated zir1, with a defect in Fe-mediated Zn tolerance. Using map-based cloning and genetic complementation, we identified that zir1 has a mutation of glutamate to lysine at position 385 on γ-glutamylcysteine synthetase (GSH1), the enzyme involved in glutathione biosynthesis. The zir1 mutant contains only 15% of the wild-type glutathione level. Blocking glutathione biosynthesis in wild-type plants by a specific inhibitor of GSH1, buthionine sulfoximine, resulted in loss of Fe-mediated Zn tolerance, which provides further evidence that glutathione plays an essential role in Fe-mediated Zn tolerance. Two glutathione-deficient mutant alleles of GSH1, pad2-1 and cad2-1, which contain 22% and 39%, respectively, of the wild-type glutathione level, revealed that a minimal glutathione level between 22 and 39% of the wild-type level is required for Fe-mediated Zn tolerance. Under excess Zn and Fe, the recovery of shoot Fe contents in pad2-1 and cad2-1 was lower than that of the wild type. However, the phytochelatin-deficient mutant cad1-3 showed normal Fe-mediated Zn tolerance. These results indicate a specific role of glutathione in Fe-mediated Zn tolerance. The induced accumulation of glutathione in response to excess Zn and Fe suggests that glutathione plays a specific role in Fe-mediated Zn tolerance in Arabidopsis. We conclude that glutathione is required for the cross-homeostasis between Zn and Fe in Arabidopsis.

Copper chaperone antioxidant protein1 is essential for copper homeostasis.

Copper (Cu) is essential for plant growth but toxic in excess. Specific molecular mechanisms maintain Cu homeostasis to facilitate its use and avoid the toxicity. Cu chaperones, proteins containing a Cu-binding domain(s), are thought to assist Cu intracellular homeostasis by their Cu-chelating ability. In Arabidopsis (Arabidopsis thaliana), two Cu chaperones, Antioxidant Protein1 (ATX1) and ATX1-Like Copper Chaperone (CCH), share high sequence homology. Previously, their Cu-binding capabilities were demonstrated and interacting molecules were identified. To understand the physiological functions of these two chaperones, we characterized the phenotype of atx1 and cch mutants and the cchatx1 double mutant in Arabidopsis. The shoot and root growth of atx1 and cchatx1 but not cch was specifically hypersensitive to excess Cu but not excess iron, zinc, or cadmium. The activities of antioxidant enzymes in atx1 and cchatx1 were markedly regulated in response to excess Cu, which confirms the phenotype of Cu hypersensitivity. Interestingly, atx1 and cchatx1 were sensitive to Cu deficiency. Overexpression of ATX1 not only enhanced Cu tolerance and accumulation in excess Cu conditions but also tolerance to Cu deficiency. In addition, the Cu-binding motif MXCXXC of ATX1 was required for these physiological functions. ATX1 was previously proposed to be involved in Cu homeostasis by its Cu-binding activity and interaction with the Cu transporter Heavy metal-transporting P-type ATPase5. In this study, we demonstrate that ATX1 plays an essential role in Cu homeostasis in conferring tolerance to excess Cu and Cu deficiency. The possible mechanism is discussed.

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

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

A sensitive LC-ESI-Q-TOF-MS method reveals novel phytosiderophores and phytosiderophore-iron complexes in barley.

The direct analysis of phytosiderophores (PSs) and their metal complexes in plants is critical to understanding the biological functions of different PSs. Here we report on a rapid and highly sensitive liquid chromatography-electrospray ionization-quadrupole-time of flight-mass spectrometry (LC-ESI-Q-TOF-MS) method for the direct and simultaneous determination of free PSs and their ferric complexes in plants. In addition to previously reported PSs--deoxymugineic acid (DMA), mugineic acid (MA) and epihydroxymugineic acid (epi-HMA)--two more PSs, avenic acid (AVA) and hydroxyavenic acid (HAVA), were identified by this method in roots of Hordeum vulgare cv Himalaya and in root exudates under iron (Fe) deficiency. The two identified PSs could be responsible for Fe acquisition under Fe deficiency because of their relative abundance and ability to form ferric complexes in secreted root exudates. This LC-ESI-Q-TOF-MS method greatly facilitates the identification of free PSs and PS-Fe complexes in one plant sample.