Root-to-shoot Cd translocation via the xylem is the major process determining shoot and grain cadmium accumulation in rice

Approach to engineer tomato by expression of AtHMA4 to enhance Zn in the aerial parts

The aim of this work was to assess the potential for using AtHMA4 to engineer enhanced efficiency of Zn translocation to shoots, and to increase the Zn concentration in aerial tissues of tomato. AtHMA4, a P1B-ATPase, encodes a Zn export protein known to be involved in the control of Zn root-to-shoot translocation. In this work, 35S::AtHMA4 was expressed in tomato (Lycopersicon esculentum var. Beta). Wild-type and transgenic plants were tested for Zn and Cd tolerance; Zn, Fe and Cd accumulation patterns, and for the expression of endogenous Zn/Fe-homeostasis genes. At 10μM Zn exposure, a higher Zn concentration was observed in leaves of AtHMA4-expressing lines compared to wild-type, which is promising in terms of Zn biofortification. AtHMA4 also transports Cd and at 0.25μM Cd the transgenic plants showed similar levels of this element in leaves to wild-type but lower levels in roots, therefore indicating a reduction of Cd uptake due to AtHMA4 expression. Expression of this transgene AtHMA4 also resulted in distinct changes in Fe accumulation in Zn-exposed plants, and Fe/Zn-accumulation in Cd-exposed plants, even though Fe is not a substrate for AtHMA4. Analysis of the transcript abundance of key Zn/Fe-homeostasis genes showed that the pattern was distinct for transgenic and wild-type plants. The reduction of Fe accumulation observed in AtHMA4-transformants was accompanied by up-regulation of Fe-deficiency marker genes (LeFER, LeFRO1, LeIRT1), whereas down-regulation was detected in plants with the status of Fe-sufficiency. Furthermore, results strongly suggest the importance of the up-regulation of LeCHLN in the roots of AtHMA4-expressing plants for efficient translocation of Zn to the shoots. Thus, the modifications of Zn/Fe/Cd translocation to aerial plant parts due to AtHMA4 expression are closely related to the alteration of the endogenous Zn-Fe-Cd cross-homeostasis network of tomato.

Comparative proteomic analysis provides new insights into cadmium accumulation in rice grain under cadmium stress

Rice is one of the most important staple crops. During the growth season, rice plants are inevitably subjected to numerous stresses, among which heavy metal stress represented by cadmium contamination not only hindering the yield of rice but also affecting the food safety by Cd accumulating in rice grains. The mechanism of Cd accumulation in rice grains has not been well elucidated. In this study, we compare the proteomic difference between two genotypes with different Cd accumulation ability in grains. Verification of differentially expressed protein-encoding genes was analyzing by quantitative PCR (QPCR) and reanalysis of microarray expression data. Forty-seven proteins in total were successfully identified through proteomic screening. GO and KEGG enrichment analysis showed Cd accumulation triggered stress-related pathways in the cells, and strongly affecting metabolic pathways. Many proteins associated with nutrient reservoir and starch-related enzyme were identified in this study suggesting that a considerably damage on grain quality was caused. The results also implied stress response was initiated by the abnormal cells and the transmission of signals may mediated by reactive oxygen species (ROS). Our research will provide new insights into Cd accumulation in rice grain under Cd stress.

Characterization of OsLCT1, a cadmium transporter from indica rice (Oryza sativa)

Molecular understanding of cadmium (Cd) transport in indica rice (Oryza sativa) is still insufficient, although indica rice generally has a potential to accumulate higher Cd in shoots and grains than japonica rice. We have previously demonstrated that OsLCT1 is a Cd transporter gene responsible for grain Cd accumulation in the japonica model cultivar Nipponbare. In this study, we isolated OsLCT1 cDNA from Kasalath, a model indica (aus subgroup) cultivar and conducted cation transport activity assays in yeast and mRNA expression analysis in plants. The deduced amino acid sequence of Kasalath OsLCT1 is 91.2% identical and 93.8% similar to that of Nipponbare OsLCT1. We established the yeast heterologous system expressing the Kasalath allele of OsLCT1. Elemental profiling of the yeast cells suggested an efflux activity of Kasalath OsLCT1 for Cd, K, Mg, Ca and Mn, but not for Fe, Zn, Cu and Na. This substrate specificity was identical to that of the Nipponbare version. Quantitative real time-polymerase chain reaction (RT-PCR) showed that expression of OsLCT1 in Kasalath was higher in reproductive stage than in vegetative stage. The expression level of OsLCT1 was significantly higher in Kasalath than in Nipponbare. Phylogenetic analysis found several LCT1-like genes only in grass plants. OsLCT1 is the sole copy in the rice genome and is conserved among each rice subgroup. These newly found low-affinity cation transporter (LCT) homologs will provide a basis for further understanding of LCT-mediated Cd transport.

From laboratory to field: OsNRAMP5-knockdown rice is a promising candidate for Cd phytoremediation in paddy fields

Previously, we reported that OsNRAMP5 functions as a manganese, iron, and cadmium (Cd) transporter. The shoot Cd content in OsNRAMP5 RNAi plants was higher than that in wild-type (WT) plants, whereas the total Cd content (roots plus shoots) was lower. For efficient Cd phytoremediation, we produced OsNRAMP5 RNAi plants using the natural high Cd-accumulating cultivar Anjana Dhan (A5i). Using a positron-emitting tracer imaging system, we assessed the time-course of Cd absorption and accumulation in A5i plants. Enhanced 107Cd translocation from the roots to the shoots was observed in A5i plants. To evaluate the phytoremediation capability of A5i plants, we performed a field experiment in a Cd-contaminated paddy field. The biomass of the A5i plants was unchanged by the suppression of OsNRAMP5 expression; the A5i plants accumulated twice as much Cd in their shoots as WT plants. Thus, A5i plants could be used for rapid Cd extraction and the efficient phytoremediation of Cd from paddy fields, leading to safer food production.

Cadmium treatment alters the expression of five genes at the Cda1 locus in two soybean cultivars [Glycine max (L.) Merr]

Westag 97 has larger capacity of Cd accumulation in roots which prevents Cd from translocating into stems and leaves; conversely, AC Hime has smaller capacity of Cd accumulation in roots; more Cd is transported into stems and leaves. The different capacity of Cd in roots between Westag 97 and AC Hime causes the different Cd concentration in seeds. Meanwhile, according to the different expression levels of RSTK, ISCP, and H(+)-ATPase between Westag 97 and AC Hime, RSTK may be involved in transporting Cd into stems and leaves; H(+)-ATPase may be correlated to the capacity of Cd accumulation in roots; and Cd caused some changes of fundamental life process which leaded to the different expression patterns of ISCP between Westag 97 and AC Hime.

Characterization of cadmium uptake, translocation, and distribution in young seedlings of two hot pepper cultivars that differ in fruit cadmium concentration

The reasons why some cultivars of hot pepper (Capsicum annuum L.) accumulate low levels of Cd are poorly understood. We aimed to compare the characteristics of Cd uptake and translocation in low-Cd and high-Cd hot pepper cultivars by determining the subcellular locations and chemical forms of Cd, and its distribution among different plant organs. We conducted a hydroponic experiment to investigate the subcellular distribution and chemical forms of Cd in roots, stems, and leaves of a low-Cd (Yeshengchaotianjiao, YCT) and a high-Cd cultivar (Jinfuzaohuangjiao, JFZ). The results showed that the concentrations of Cd in almost all subcellular fractions of roots, and in all chemical forms in roots, were higher in YCT than in JFZ. Compared with YCT, JFZ had higher Cd concentrations in almost all subcellular fractions of stems and leaves, and higher Cd concentrations in almost all chemical forms in stems and leaves. Additionally, YCT had significantly higher total Cd accumulation but a lower Cd translocation rate compared with JFZ. In general, the results presented in this study revealed that root-to-shoot Cd translocation via the xylem is the key physiological processes determining the Cd accumulation level in stems and leaves of hot pepper plants. Immobilization of Cd by the cell walls of different organs is important in Cd detoxification and limiting the symplastic movement of Cd.

Metal species involved in long distance metal transport in plants

The mechanisms plants use to transport metals from roots to shoots are not completely understood. It has long been proposed that organic molecules participate in metal translocation within the plant. However, until recently the identity of the complexes involved in the long-distance transport of metals could only be inferred by using indirect methods, such as analyzing separately the concentrations of metals and putative ligands and then using in silico chemical speciation software to predict metal species. Molecular biology approaches also have provided a breadth of information about putative metal ligands and metal complexes occurring in plant fluids. The new advances in analytical techniques based on mass spectrometry and the increased use of synchrotron X-ray spectroscopy have allowed for the identification of some metal-ligand species in plant fluids such as the xylem and phloem saps. Also, some proteins present in plant fluids can bind metals and a few studies have explored this possibility. This study reviews the analytical challenges researchers have to face to understand long-distance metal transport in plants as well as the recent advances in the identification of the ligand and metal-ligand complexes in plant fluids.

Effects of combined amendments on heavy metal accumulation in rice (Oryza sativa L.) planted on contaminated paddy soil

Stabilization of heavy metals in situ was investigated. Two combined amendments (LS, limestone+sepiolite; HZ, hydroxyhistidine+zeolite) were applied at ratios of 0.2%, 0.4%, and 0.8% (w/w) to paddy soil with multi-metal (Pb, Cd, Cu, and Zn) contamination. The effects of these two combined amendments on heavy metal bioavailability in soil, and on uptake and accumulation of heavy metals in rice plants were investigated. Application of LS and HZ significantly increased soil pH values and cation exchange capacity contents, and resulted in a reduction in exchangeable fraction of metals and in extract metal concentrations of amended soils through toxicity characteristic leaching procedure (TCLP). LS and HZ obviously inhibited uptake and accumulation of Pb, Cd, Cu, and Zn in rice plants. Compared with the control soil, concentrations of Pb, Cd, Cu, and Zn in brown rice were decreased by 10.6-31.8%, 16.7-25.5%, 11.5-22.1%, and 11.7-16.3%, respectively, as a result of 0.2% to 0.8% addition of LS, and decreased by 5.1-40.8%, 16.7-20.0%, 8.1-16.2%, and 13.3-21.7%, respectively, as a result of 0.2-0.8% addition of HZ. Significant positive linear correlations were found between heavy metal concentrations in TCLP extracts and those in rice tissues except for Pb. Extracting heavy metals with TCLP was a more suitable method for estimating heavy metal bioavailability concentrations of amended soil than exchangeable fraction of heavy metals, because the latter underestimated heavy metal bioavailability. These results demonstrate that LS and HZ could be effective in reducing heavy metal bioavailability and accumulation in rice grown on multi-metal-contaminated soils.

Enhancing phytoextraction of Cd by combining poplar (clone "I-214") with Pseudomonas fluorescens and microbial consortia

The plant-microorganism combinations may contribute to the success of phytoextraction of heavy metal-polluted soil. The purpose of this study was to investigate the effects of cadmium (Cd) soil concentration on selected physiological parameters of the poplar clone "I-214" inoculated at root level with a strain (BT4) of Pseudomonas fluorescens and a commercial product based on microbial consortia (Micosat F Fito®). Plants were subjected to Cd treatment of 40 mg kg(-1) in greenhouse. The effects of plant-microbe interactions, plant growth, leaf physiology, and microbial activity were periodically monitored. Metal concentration and translocation factors in plant tissues proved enhanced Cd uptake in roots of plants inoculated with P. fluorescens and transfer to shoots in plants inoculated with Micosat F Fito®, suggesting a promising strategy for using microbes in support of Cd uptake. Plant-microbe integration increased total removal of Cd, without interfering with plant growth, while improving the photosynthetic capacity. Two major mechanisms of metal phytoextraction inducted by microbial inoculation may be suggested: improved Cd accumulation in roots inoculated with P. fluorescens, implying phytostabilization prospective and high Cd transfer to shoots of inoculated plants, outlining enhanced metal translocation.

Subcellular distribution and chemical forms of cadmium in two hot pepper cultivars differing in cadmium accumulation

A greenhouse experiment was conducted to compare the subcellular distribution and chemical forms of cadmium (Cd) in roots, stems, leaves, and fruits between a low-Cd cultivar (Yeshengchaotianjiao, YCT) and a high-Cd cultivar (Jinfuzaohuangjiao, JFZ) of hot pepper (Capsicum annuum L.). The Cd concentrations in the root's subcellular fractions, and in all chemical forms in roots, were 1.85-4.88- and 1.84-4.90-fold higher, respectively, in YCT than in JFZ. Compared with JFZ, YCT had significantly lower Cd concentrations in the subcellular fractions (1.10-2.42-fold) of stems and leaves and in almost all chemical forms (1.17-2.97-fold) in the stems and leaves. Also, in fruits, the concentrations of Cd in the cell wall and soluble fractions were 1.18-2.24-fold significantly lower in YCT than in JFZ, and there were lower Cd concentrations (1.36-2.08-fold) in the chemical forms in YCT than in JFZ.

Photosynthesis mediated decrease in cadmium translocation protect shoot growth of Oryza sativa seedlings up on ammonium phosphate-sulfur fertilization

Cadmium (Cd) stress responses in seedlings of two Indian rice cultivars, MTU 7029 and MO 16 were investigated under ammonium-based fertilizer amendment. Cd translocation was reduced by fertilizer treatment. An increase in the production of organic acids as well as nitrogenous compounds and maintenance of nutrient status were implicated for decrease in Cd translocation which in turn promoted shoot growth. Fertilizer treatment increased photosynthetic pigments and activity of antioxidant enzymes that ensured steady photosynthetic rate during Cd stress. MO 16 showed Cd exclusion characteristics when compared with MTU 7029. Photosynthesis performance of MO 16 was not affected by Cd treatments. These findings suggest that photosynthesis influenced decrease in Cd translocation enhanced shoot growth of seedlings during ammonium phosphate-sulfur fertilizer supplementation.

Redox-controlled changes in cadmium solubility and solid-phase speciation in a paddy soil as affected by reducible sulfate and copper

The solubility of Cd in contaminated paddy soils controls Cd uptake by rice, which is an important food safety issue. We investigated the solution and solid-phase dynamics of Cd in a paddy soil spiked with ∼20 mg kg(-1) Cd during 40 days of soil reduction followed by 28 days of soil reoxidation as a function of the amounts of sulfate available for microbial reduction and of Cu that competes with Cd for precipitation with biogenic sulfide. At an excess of sulfate over (Cd + Cu), dissolved Cd decreased during sulfate reduction and Cd was transformed into a poorly soluble phase identified as Cd-sulfide using Cd K-edge X-ray absorption spectroscopy (XAS). The extent of Cd-sulfide precipitation decreased with decreasing sulfate and increasing Cu contents, even if sulfate exceeded Cd. When both Cu and Cd exceeded sulfate, dissolved and mobilizable Cd remained elevated after 40 days of soil reduction. During soil reoxidation, Cd-sulfide was readily transformed back into more soluble species. Our data suggest that Cd-sulfide formation in flooded paddy soil may be limited when the amounts of Cd and other chalcophile metals significantly exceed reducible sulfate Therefore, in multimetal contaminated paddy soils with low sulfate contents, Cd may remain labile during soil flooding, which enhances the risk for Cd transfer into rice.

Genome-wide identification and characterization of cadmium-responsive microRNAs and their target genes in radish (Raphanus sativus L.) roots

MicroRNAs (miRNAs) are endogenous non-coding small RNAs that play vital regulatory roles in plant growth, development, and environmental stress responses. Cadmium (Cd) is a non-essential heavy metal that is highly toxic to living organisms. To date, a number of conserved and non-conserved miRNAs have been identified to be involved in response to Cd stress in some plant species. However, the miRNA-mediated gene regulatory networks responsive to Cd stress in radish (Raphanus sativus L.) remain largely unexplored. To dissect Cd-responsive miRNAs and their targets systematically at the global level, two small RNA libraries were constructed from Cd-treated and Cd-free roots of radish seedlings. Using Solexa sequencing technology, 93 conserved and 16 non-conserved miRNAs (representing 26 miRNA families) and 28 novel miRNAs (representing 22 miRNA families) were identified. In all, 15 known and eight novel miRNA families were significantly differently regulated under Cd stress. The expression patterns of a set of Cd-responsive miRNAs were validated by quantitative real-time PCR. Based on the radish mRNA transcriptome, 18 and 71 targets for novel and known miRNA families, respectively, were identified by the degradome sequencing approach. Furthermore, a few target transcripts including phytochelatin synthase 1 (PCS1), iron transporter protein, and ABC transporter protein were involved in plant response to Cd stress. This study represents the first transcriptome-based analysis of miRNAs and their targets responsive to Cd stress in radish roots. These findings could provide valuable information for functional characterization of miRNAs and their targets in regulatory networks responsive to Cd stress in radish.

Rice DEP1, encoding a highly cysteine-rich G protein γ subunit, confers cadmium tolerance on yeast cells and plants

A rice cDNA, OsDEP1, encoding a highly cysteine (Cys)-rich G protein γ subunit, was initially identified as it conferred cadmium (Cd) tolerance on yeast cells. Of the 426 aa constituting OsDEP1, 120 are Cys residues (28.2%), of which 88 are clustered in the C-terminal half region (aa 170-426). To evaluate the independent effects of these two regions, two truncated versions of the OsDEP1-expressing plasmids pOsDEP1(1-169) and pOsDEP1(170-426) were used to examine their effects on yeast Cd tolerance. Although OsDEP1(170-426) conferred a similar level of Cd tolerance as the intact OsDEP1, OsDEP1(1-169) provided no such tolerance, indicating that the tolerance effect is localized to the aa 170-426 C-terminal peptide region. The Cd responses of transgenic Arabidopsis plants constitutively expressing OsDEP1, OsDEP1(1-169) or OsDEP1(170-426), were similar to the observations in yeast cells, with OsDEP1 and OsDEP1(170-426) transgenic plants displaying Cd tolerance but OsDEP1(1-169) plants showing no such tolerance. In addition, a positive correlation between the transcript levels of OsDEP1 or OsDEP1(170-426) in the transgenics and the Cd content of these plants upon Cd application was observed. As several Arabidopsis loss-of-function heterotrimeric G protein β and γ subunit gene mutants did not show differences in their Cd sensitivity compared with wild-type plants, we propose that the Cys-rich region of OsDEP1 may function directly as a trap for Cd ions.

Inhibition of cadmium ion uptake in rice (Oryza sativa) cells by a wall-bound form of silicon

The stresses acting on plants that are alleviated by silicon (Si) range from biotic to abiotic stresses, such as heavy metal toxicity. However, the mechanism of stress alleviation by Si at the single-cell level is poorly understood. We cultivated suspended rice (Oryza sativa) cells and protoplasts and investigated them using a combination of plant nutritional and physical techniques including inductively coupled plasma mass spectrometry (ICP-MS), the scanning ion-selective electrode technique (SIET) and X-ray photoelectron spectroscopy (XPS). We found that most Si accumulated in the cell walls in a wall-bound organosilicon compound. Total cadmium (Cd) concentrations in protoplasts from Si-accumulating (+Si) cells were significantly reduced at moderate concentrations of Cd in the culture medium compared with those from Si-limiting (-Si) cells. In situ measurement of cellular fluxes of the cadmium ion (Cd(2+) ) in suspension cells and root cells of rice exposed to Cd(2+) and/or Si treatments showed that +Si cells significantly inhibited the net Cd(2+) influx, compared with that in -Si cells. Furthermore, a net negative charge (charge density) within the +Si cell walls could be neutralized by an increase in the Cd(2+) concentration in the measuring solution. A mechanism of co-deposition of Si and Cd in the cell walls via a [Si-wall matrix]Cd co-complexation may explain the inhibition of Cd ion uptake, and may offer a plausible explanation for the in vivo detoxification of Cd in rice.

Characterization of cadmium ((108)Cd) distribution and accumulation in Tagetes erecta L. seedlings: effect of split-root and of remove-xylem/phloem

Tagetes erecta has a high potential for cadmium (Cd) phytoremediation. Through several hydroponic experiments, characteristics of (108)Cd distribution and accumulation were investigated in T. erecta with split -roots or removed xylem/phloem. The results showed that (108)Cd transport from roots to aboveground tissues showed the homolateral transport phenomenon in split-root seedlings. (108)Cd content of leaves on the +(108)Cd side and the -(108)Cd side was not significantly different, which implied that there was horizontal transport of (108)Cd from the +(108)Cd side to the -(108)Cd side in cut-root seedlings. Like (108)Cd transport, the transport of (70)Zn was homolateral. Reduction of water consumption in the removed xylem treatment significantly decreased (108)Cd accumulation; whereas, the removed phloem treatment had no significant effect on water consumption, but did decrease (108)Cd accumulation in leaves of the seedlings. The removal of phloem significantly reduced distal leaf (108)Cd content, which was significantly lower than that in the basal leaves in both the split-root and unsplit-root seedlings. Overall, the results presented in this study revealed that the root to aboveground cadmium translocation via phloem is as an important and common physiological process as xylem determination of the cadmium accumulation in stems and leaves of marigold seedlings.

Nitrate facilitates cadmium uptake, transport and accumulation in the hyperaccumulator Sedum plumbizincicola

The aims of this study are to investigate whether and how the nitrogen form (nitrate (NO3 (-)) versus ammonium (NH4 (+))) influences cadmium (Cd) uptake and translocation and subsequent Cd phytoextraction by the hyperaccumulator species Sedum plumbizincicola. Plants were grown hydroponically with N supplied as either NO3 (-) or NH4 (+). Short-term (36 h) Cd uptake and translocation were determined innovatively and quantitatively using a positron-emitting (107)Cd tracer and positron-emitting tracer imaging system. The results show that the rates of Cd uptake by roots and transport to the shoots in the NO3 (-) treatment were more rapid than in the NH4 (+) treatment. After uptake for 36 h, 5.6 (0.056 μM) and 29.0 % (0.290 μM) of total Cd in the solution was non-absorbable in the NO3 (-) and NH4 (+) treatments, respectively. The local velocity of Cd transport was approximately 1.5-fold higher in roots (3.30 cm h(-1)) and 3.7-fold higher in shoots (10.10 cm h(-1)) of NO3 (-)- than NH4 (+)-fed plants. Autoradiographic analysis of (109)Cd reveals that NO3 (-) nutrition enhanced Cd transportation from the main stem to branches and young leaves. Moreover, NO3 (-) treatment increased Cd, Ca and K concentrations but inhibited Fe and P in the xylem sap. In a 21-day hydroponic culture, shoot biomass and Cd concentration were 1.51 and 2.63 times higher in NO3 (-)- than in NH4 (+)-fed plants. We conclude that compared with NH4 (+), NO3 (-) promoted the major steps in the transport route followed by Cd from solution to shoots in S. plumbizincicola, namely its uptake by roots, xylem loading, root-to-shoot translocation in the xylem and uploading to the leaves. S. plumbizincicola prefers NO3 (-) nutrition to NH4 (+) for Cd phytoextraction.

Cadmium-tolerant bacteria reduce the uptake of cadmium in rice: potential for microbial bioremediation

We selected 24 bacterial isolates that could tolerate up to 2500 µM CdCl2 from the soil of rice fields downstream from a zinc-mineralized area contaminated with a high level of cadmium (Cd). In the presence of 500 µM CdCl2, all isolates grew slower and with a prolonged lag-phase compared to in the absence of Cd. Cd-binding capacity was high and ranged from 6.38 to 9.38 log[Cd(atom)]/cell. The stability of Cd complexes in bacteria was affected by 1mM EDTA. In 500 µM CdCl2, all isolates produced 0.7 to 4.8-fold more inorganic sulfide and 0.6 to 2.2-fold more thio-rich compounds containing SH groups. Out of 24 Cd-tolerant bacterial isolates, KKU2500-3, -8, -9 and -20 were able to promote the growth of Thai jasmine rice (Kao Hom Mali 105) seedlings in the presence of 200 µM CdCl2, and KKU2500-3 produced the highest numbers of fibrous root. Interestingly, these 4 isolates increased Cd tolerance and decreased the accumulation of Cd in rice by 61, 9, 6, and 17% when grown in the presence of 200 µM CdCl2. Of the 4 isolates, KKU2500-3 produced more inorganic sulfide when grown in CdCl2 at 500-2000 µM. XANES analyses indicated that this isolate precipitated a detectable amount of cadmium sulfide (CdS) when grown in 500 µM CdCl2. Thus, the isolate KKU2500-3 could possibly transform toxic, soluble CdCl2 into non-toxic, insoluble CdS. These 4Cd-tolerant bacterial isolates were identified via 16S rDNA sequencing and classified as Cupriavidus taiwanensis KKU2500-3 and Pseudomonas aeruginosa KKU2500-8, -9, and -20.

Cadmium uptake and partitioning in durum wheat during grain filling

BACKGROUND

Concentrations of cadmium (Cd) in the grain of many durum wheats (Triticum turgidum subsp. durum) grown in North American prairie soils often exceed international trade standards. Genotypic differences in root-to-shoot translocation of Cd are a major determinant of intraspecific variation in the accumulation of Cd in grain. We tested the extent to which changes in whole-plant Cd accumulation and the distribution of Cd between tissues influences Cd accumulation in grain by measuring Cd accumulation throughout the grain filling period in two near-isogenic lines (NILs) of durum wheat that differ in grain Cd accumulation.

RESULTS

Roots absorbed Cd and transported it to the shoots throughout the grain filling period, but the low- and high-Cd NILs did not differ in whole-plant Cd uptake. Although the majority of Cd accumulation was retained in the roots, the low- and high-Cd NILs differed substantively in root-to-shoot translocation of Cd. At grain maturity, accumulation of Cd in the shoots was 13% (low-Cd NIL) or 37% (high-Cd NIL) of whole-plant Cd accumulation. Accumulation of Cd in all shoot tissue, including grain, was at least 2-fold greater in the high-Cd NIL at all harvests. There was no net remobilization of shoot Cd pools during grain filling. The timing of Cd accumulation in grain was positively correlated with grain biomass accumulation, and the rate of grain filling peaked between 14 and 28 days post-anthesis, when both NILs accumulated 60% of total grain biomass and 61-66% of total grain Cd content.

CONCLUSIONS

These results show that genotypic variation in root-to-shoot translocation of Cd controls accumulation of Cd in durum wheat grain. Continued uptake of Cd by roots and the absence of net remobilization of Cd from leaves during grain filling support a direct pathway of Cd transport from roots to grain via xylem-to-phloem transfer in the stem.

Cadmium interferes with maintenance of auxin homeostasis in Arabidopsis seedlings

Auxin and its homeostasis play key roles in many aspects of plant growth and development. Cadmium (Cd) is a phytotoxic heavy metal and its inhibitory effects on plant growth and development have been extensively studied. However, the underlying molecular mechanism of the effects of Cd stress on auxin homeostasis is still unclear. In the present study, we found that the root elongation, shoot weight, hypocotyl length and chlorophyll content in wild-type (WT) Arabidopsis seedlings were significantly reduced after exposure to Cd stress. However, the lateral root (LR) formation was markedly promoted by Cd stress. The level and distribution of auxin were both greatly altered in primary root tips and cotyledons of Cd-treated plants. The results also showed that after Cd treatment, the IAA content was significantly decreased, which was accompanied by increases in the activity of the IAA oxidase and alteration in the expression of several putative auxin biosynthetic and catabolic genes. Application of the auxin transport inhibitor, 1-naphthylphthalamic acid (NPA) and 1-naphthoxyacetic acid (1-NOA), reversed the effects of Cd on LR formation. Additionally, there was less promotion of LR formation by Cd treatment in aux1-7 and pin2 mutants than that in the WT. Meanwhile, Cd stress also altered the expression of PINs and AUX1 in Arabidopsis roots, implying that the auxin transport pathway is required for Cd-modulated LR development. Taken together, these findings suggest that Cd stress disturbs auxin homeostasis through affecting auxin level, distribution, metabolism, and transport in Arabidopsis seedling.

Rice breaks ground for cadmium-free cereals

It is important to reduce the risk of long-term cadmium (Cd) exposure through ingestion. Cd in plant-derived foods (especially rice grain in Japan) is a major source of human Cd intake. In rice, Cd uptake and xylem loading in roots, remobilization from leaf blades and intervascular transfer in nodes to redirect Cd transport are crucial for grain Cd accumulation. Different metal transporters mediate these processes with cell-type specific expressions. In this review, we summarize and discuss the recent rapid advance in molecular understanding of Cd transport in rice. The achievements and perspectives in developing 'low-Cd rice' by the use of various alleles of Cd transporter genes are also discussed.

Opinion: nickel and urease in plants: still many knowledge gaps

We propose experimental strategies to expand our understanding of the role of Ni in plants, beyond the Ni-metallocenter of urease, still the only identified Ni-containing plant enzyme. While Ni has been considered an essential mineral for plants there is a clear lack of knowledge of its involvement in metabolic steps except the urease-catalyzed conversion of urea to ammonia and bicarbonate. We argue that urease (and hence, Ni) plays an important role in optimal N-use efficiency under various N regimes by recycling urea-N, which is generated endogenously exclusively from arginase action on arginine. We further suggest that urease and arginase may connect different metabolic compartments under stress situations, and therefore may be involved in stress tolerance. To determine possible non-urease roles of Ni we call for experimental manipulation of both Ni and N availability in urease-negative mutants. Plant ureases have been shown to have defense roles, distinct from their ureolytic activity, and we call for investigation of whether Ni helps maintain a urease conformation or stability for these non-ureolytic defense roles. The beneficial effects of Ni at upper concentration limits have not been fully examined. We posit a "Ni strategy" of plants whose growth/performance is stimulated by unusual amounts of soil Ni, for defense and/or for maximal N-use efficiency. While we know little about Ni and urease roles in N metabolism and defense, virtually nothing is known about Ni roles in plant-microbial 'consortia.' And, much of what we know of Ni and urease is limited to only a few plants, e.g. soybean, potato and Arabidopsis, and we suggest studies vigorously extended to other plants.

Plant science: the key to preventing slow cadmium poisoning

Practically all human populations are environmentally exposed to cadmium (Cd), mostly through plant-derived food. A growing body of epidemiological evidence suggests that there is no margin of safety between current Cd exposure levels and the threshold for adverse health effects and, hence, there is an urgent need to lower human Cd intake. Here we review recent studies on rice (Oryza sativa) and Cd-hyperaccumulating plants that have led to important insights into the processes controlling the passage of Cd from the soil to edible plant organs. The emerging molecular understanding of Cd uptake, root retention, root-to-shoot translocation and grain loading will enable the development of low Cd-accumulating crops.

Contribution of cell walls, nonprotein thiols, and organic acids to cadmium resistance in two cabbage varieties

To study possible cadmium (Cd) resistance mechanisms in cabbage (Brassica oleracea L.), several parameters of metal uptake, distribution, and complexation were compared between two varieties Chunfeng [CF (Cd-tolerant)] and Lvfeng [LF (Cd-sensitive)]. Results showed that CF contained significantly lower Cd concentrations in leaves and higher Cd concentrations in roots than LF. Approximately 70 to 74 % and 66 to 68 % of Cd taken up by LF and CF, respectively, was transported to shoots. More Cd was bound to the cell walls of leaves, stems, and roots in CF than in LF. The higher capacity of CF to limit Cd uptake into shoots could be explained by immobilization of Cd in root cell walls. Compared with control groups, Cd treatment also significantly increased concentrations of nonprotein thiols, phytochelatins (PCs), and citric acid in the leaves and roots of the two varieties; the increases were more pronounced in CF than in LF. Taken together, the results suggest that the greater Cd resistance in CF than in LF may be attributable to the greater capacity of CF to limit Cd uptake into shoots and complex Cd in cell walls and metal binding ligands, such as PCs and citric acid. However, the contributions of PCs and citric acid to Cd detoxification might be smaller than those in cell walls.

Application of glutathione to roots selectively inhibits cadmium transport from roots to shoots in oilseed rape

Glutathione is a tripeptide involved in various aspects of plant metabolism. This study investigated the effects of the reduced form of glutathione (GSH) applied to specific organs (source leaves, sink leaves, and roots) on cadmium (Cd) distribution and behaviour in the roots of oilseed rape plants (Brassica napus) cultured hydroponically. The translocation ratio of Cd from roots to shoots was significantly lower in plants that had root treatment of GSH than in control plants. GSH applied to roots reduced the Cd concentration in the symplast sap of root cells and inhibited root-to-shoot Cd translocation via xylem vessels significantly. GSH applied to roots also activated Cd efflux from root cells to the hydroponic solution. Inhibition of root-to-shoot translocation of Cd was visualized, and the activation of Cd efflux from root cells was also shown by using a positron-emitting tracer imaging system (PETIS). This study investigated a similar inhibitory effect on root-to-shoot translocation of Cd by the oxidized form of glutathione, GSSG. Inhibition of Cd accumulation by GSH was abolished by a low-temperature treatment. Root cells of plants exposed to GSH in the root zone had less Cd available for xylem loading by actively excluding Cd from the roots. Consequently, root-to-shoot translocation of Cd was suppressed and Cd accumulation in the shoot decreased.

Genetic diversity of arsenic accumulation in rice and QTL analysis of methylated arsenic in rice grains

BACKGROUND

Rice is a major source of dietary intake of arsenic (As) for the populations that consume rice as a staple food. Therefore, it is necessary to reduce the As concentration in rice to avoid the potential risk to human health. In this study, the genetic diversity in As accumulation and As speciation in rice grains was investigated using a world rice core collection (WRC) comprising 69 accessions grown over a 3-year period. Moreover, quantitative trait locus (QTL) analysis was conducted to identify QTLs controlling the dimethylarsinic acid (DMA) content of rice grains.

RESULTS

There was a 3-fold difference in the grain As concentration of WRC. Concentrations of total-As, inorganic As, and DMA were significantly affected by genotype, year, and genotype-year interaction effects. Among the WRC accessions, Local Basmati and Tima (indica type) were identified as cultivars with the lowest stable total-As and inorganic As concentrations. Using an F2 population derived from Padi Perak (a high-DMA accession) and Koshihikari (a low-DMA cultivar), we identified two QTLs on chromosome 6 (qDMAs6.1 and qDMAs6.2) and one QTL on chromosome 8 (qDMAs8) that were responsible for variations in the grain DMA concentration. Approximately 73% of total phenotypic variance in DMA was explained by the three QTLs.

CONCLUSIONS

Based on the results provided, one strategy for developing rice cultivars with a low level of toxic As would be to change the proportion of organic As on the basis of a low level of total As content.

Characterization of rapid intervascular transport of cadmium in rice stem by radioisotope imaging

Participation of the intervascular transport system within the rice stem during cadmium (Cd) partitioning was investigated by characterizing (109)Cd behaviour in the shoot. In addition, (45)Ca, (32)P, and (35)S partitioning patterns were analysed for comparison with that of (109)Cd. Each tracer was applied to the seedling roots for 15 min, and the shoots were harvested either at 15 min (i.e. immediately after tracer application) or at 48 h. Distribution patterns of each element at 15 min were studied to identify the primary transport pathway before remobilization was initiated. (32)P was preferentially transported to completely expanded leaf blades having the highest transpiration rate. The newest leaf received minimal amounts of (32)P. In contrast, the amount of (35)S transported to the newest leaf was similar to that transported to the other mature leaf blades. Preferential movement towards the newest leaf was evident for (109)Cd and (45)Ca. These results directly indicate that elemental transport is differentially regulated in the vegetative stem as early as 15 min before the elements are transported to leaves. Cd behaviour in the stem was investigated in detail by obtaining serial section images from the bottom part of shoots after (109)Cd was applied to a single crown root. At 30 min, the maximum amount of (109)Cd was distributed in the peripheral cylinder of the longitudinal vascular bundles (PV) and, interestingly, some amount of (109)Cd was transported downwards along the PV. This transport manner of (109)Cd provides evidence that Cd can be loaded on the phloem at the stem immediately after Cd is transported from the root.

Reduced translocation of cadmium from roots is associated with increased production of phytochelatins and their precursors

Cadmium (Cd) is a non-essential trace element and its environmental concentrations are approaching toxic levels, especially in some agricultural soils. Understanding how and where Cd is stored in plants is important for ensuring food safety. In this study, we examined two plant species that differ in the distribution of Cd among roots and leaves. Lettuce and barley were grown in nutrient solution under two conditions: chronic (4 weeks) exposure to a low, environmentally relevant concentration (1.0 μM) of Cd and acute (1 h) exposure to a high concentration (5.0 mM) of Cd. Seedlings grown in solution containing 1.0 μM CdCl₂ did not show symptoms of toxicity and, at this concentration, 77% of the total Cd was translocated to leaves of lettuce, whereas only 24% of the total Cd was translocated to barley leaves. We tested the hypothesis that differential accumulation of Cd in roots and leaves is related to differential concentrations of phytochelatins (PCs), and its precursor peptides. The amounts of PCs and their precursor peptides in the roots and shoots were measured using HPLC. Each of PC₂₋₄ was synthesized in the barley root upon chronic exposure to Cd and did not increase further upon acute exposure. In the case of lettuce, no PCs were detected in the root given either Cd treatment. The high amounts of PCs produced in barley root could have contributed to preferential retention of Cd in barley roots.

Design of a tobacco exon array with application to investigate the differential cadmium accumulation property in two tobacco varieties

BACKGROUND

For decades the tobacco plant has served as a model organism in plant biology to answer fundamental biological questions in the areas of plant development, physiology, and genetics. Due to the lack of sufficient coverage of genomic sequences, however, none of the expressed sequence tag (EST)-based chips developed to date cover gene expression from the whole genome. The availability of Tobacco Genome Initiative (TGI) sequences provides a useful resource to build a whole genome exon array, even if the assembled sequences are highly fragmented. Here, the design of a Tobacco Exon Array is reported and an application to improve the understanding of genes regulated by cadmium (Cd) in tobacco is described.

RESULTS

From the analysis and annotation of the 1,271,256 Nicotiana tabacum fasta and quality files from methyl filtered genomic survey sequences (GSS) obtained from the TGI and ~56,000 ESTs available in public databases, an exon array with 272,342 probesets was designed (four probes per exon) and tested on two selected tobacco varieties.Two tobacco varieties out of 45 accumulating low and high cadmium in leaf were identified based on the GGE biplot analysis, which is analysis of the genotype main effect (G) plus analysis of the genotype by environment interaction (GE) of eight field trials (four fields over two years) showing reproducibility across the trials. The selected varieties were grown under greenhouse conditions in two different soils and subjected to exon array analyses using root and leaf tissues to understand the genetic make-up of the Cd accumulation.

CONCLUSIONS

An Affymetrix Exon Array was developed to cover a large (~90%) proportion of the tobacco gene space. The Tobacco Exon Array will be available for research use through Affymetrix array catalogue. As a proof of the exon array usability, we have demonstrated that the Tobacco Exon Array is a valuable tool for studying Cd accumulation in tobacco leaves. Data from field and greenhouse experiments supported by gene expression studies strongly suggested that the difference in leaf Cd accumulation between the two specific tobacco cultivars is dependent solely on genetic factors and genetic variability rather than on the environment.

Ion-beam irradiation, gene identification, and marker-assisted breeding in the development of low-cadmium rice

Rice (Oryza sativa L.) grain is a major dietary source of cadmium (Cd), which is toxic to humans, but no practical technique exists to substantially reduce Cd contamination. Carbon ion-beam irradiation produced three rice mutants with <0.05 mg Cd⋅kg(-1) in the grain compared with a mean of 1.73 mg Cd⋅kg(-1) in the parent, Koshihikari. We identified the gene responsible for reduced Cd uptake and developed a strategy for marker-assisted selection of low-Cd cultivars. Sequence analysis revealed that these mutants have different mutations of the same gene (OsNRAMP5), which encodes a natural resistance-associated macrophage protein. Functional analysis revealed that the defective transporter protein encoded by the mutant osnramp5 greatly decreases Cd uptake by roots, resulting in decreased Cd in the straw and grain. In addition, we developed DNA markers to facilitate marker-assisted selection of cultivars carrying osnramp5. When grown in Cd-contaminated paddy fields, the mutants have nearly undetectable Cd in their grains and exhibit no agriculturally or economically adverse traits. Because mutants produced by ion-beam radiation are not transgenic plants, they are likely to be accepted by consumers and thus represent a practical choice for rice production worldwide.

Cadmium impairs ion homeostasis by altering K+ and Ca2+ channel activities in rice root hair cells

Cadmium (Cd2+) interferes with the uptake, transport and utilization of several macro- and micronutrients, which accounts, at least in part, for Cd2+ toxicity in plants. However, the mechanisms underlying Cd2+ interference of ionic homeostasis is not understood. Using biophysical techniques including membrane potential measurements, scanning ion-selective electrode technique for non-invasive ion flux assays and patch clamp, we monitored the effect of Cd2+ on calcium (Ca2+) and potassium (K+) transport in root hair cells of rice. Our results showed that K+ and Ca2+ contents in both roots and shoots were significantly reduced when treated with exogenous Cd2+. Further studies revealed that three cellular processes may be affected by Cd2+, leading to changes in ionic homeostasis. First, Cd2+ -induced depolarization of the membrane potential was observed in root hair cells, attenuating the driving force for cation uptake. Second, the inward conductance of Ca2+ and K+ was partially blocked by Cd2+, decreasing uptake of K+ and Ca2+ . Third, the outward K+ conductance was Cd2+ -inducible, decreasing the net content of K+ in roots. These results provide direct evidence that Cd2+ impairs uptake of Ca2+ and K+, thereby disturbing ion homeostasis in plants.

Cadmium tolerance in Brassica juncea roots and shoots is affected by antioxidant status and phytochelatin biosynthesis

Indian mustard (Brassica juncea L. Czern.) tolerates high concentrations of heavy metals and is a promising species for the purpose of phytoextraction of cadmium (Cd) from metal-contaminated soils. This work investigates the extent to which antioxidant and metal sequestering mechanisms are responsible for this tolerance. To this end, seedlings of Indian mustard were grown for 7 days in 0, 50 or 200 μM Cd. Increasing Cd concentrations led to a progressive Cd accumulation in roots and shoots, accompanied by an organ-dependent alteration in mineral uptake, and a decrease in root/shoot length and fresh/dry weight. Cd negatively affected chlorophyll and carotenoid contents and activated the xanthophyll cycle, suggesting the need to protect the photosynthetic apparatus from photoinhibition. Shoots seemed to be less efficient than roots in ROS scavenging, as indicated by the different response to Cd stress shown by peroxidase and catalase activities and, solely with regard to the highest Cd concentration, by ascorbate level. Such a different antioxidant capacity might at least partly explain differences in the trend of lipid peroxidation observed in the two organs. Moreover, in both roots and shoots, glutathione and phytochelatin content markedly increased under Cd stress, regardless of the metal concentration involved.

How cobalt facilitates cadmium- and ethylene precursor-induced growth inhibition and radial cell expansion in barley root tips

Significant root growth inhibition was observed during the very short 5 minute exposure time of barley roots to the low 10 μM concentration of cadmium. In addition to the cadmium-induced root growth inhibition, considerable radial expansion of roots was observed as a characteristic symptom of transient short-term exposure of roots to cadmium. The cadmium-induced radial expansion of roots was observed mainly the cortical cells of elongation zone that were twice as large as in control roots. Similarly as in cadmium-treated roots, short-term treatment with ACC significantly inhibited root growth and caused a marked radial expansion of cortical cells. The ethylene synthesis inhibitor cobalt significantly alleviated both the cadmium- and ethylene precursor-induced root growth inhibition and radial root expansion. The results indicate that ethylene probably plays a crucial role in the short-term cadmium-induced inhibition of root growth and radial cell expansion of barley root tips, which are the very early symptoms of cadmium toxicity.

Role of the node in controlling traffic of cadmium, zinc, and manganese in rice

Heavy metals are transported to rice grains via the phloem. In rice nodes, the diffuse vascular bundles (DVBs), which enclose the enlarged elliptical vascular bundles (EVBs), are connected to the panicle and have a morphological feature that facilitates xylem-to-phloem transfer. To find a mechanism for restricting cadmium (Cd) transport into grains, the distribution of Cd, zinc (Zn), manganese (Mn), and sulphur (S) around the vascular bundles in node I (the node beneath the panicle) of Oryza sativa 'Koshihikari' were compared 1 week after heading. Elemental maps of Cd, Zn, Mn, and S in the vascular bundles of node I were obtained by synchrotron micro-X-ray fluorescence spectrometry and electron probe microanalysis. In addition, Cd K-edge microfocused X-ray absorption near-edge structure analyses were used to identify the elements co-ordinated with Cd. Both Cd and S were mainly distributed in the xylem of the EVB and in the parenchyma cell bridge (PCB) surrounding the EVB. Zn accumulated in the PCB, and Mn accumulated around the protoxylem of the EVB. Cd was co-ordinated mainly with S in the xylem of the EVB, but with both S and O in the phloem of the EVB and in the PCB. The EVB in the node retarded horizontal transport of Cd toward the DVB. By contrast, Zn was first stored in the PCB and then efficiently transferred toward the DVB. Our results provide evidence that transport of Cd, Zn, and Mn is differentially controlled in rice nodes, where vascular bundles are functionally interconnected.

Characterizing the role of rice NRAMP5 in Manganese, Iron and Cadmium Transport

Metals like manganese (Mn) and iron (Fe) are essential for metabolism, while cadmium (Cd) is toxic for virtually all living organisms. Understanding the transport of these metals is important for breeding better crops. We have identified that OsNRAMP5 contributes to Mn, Fe and Cd transport in rice. OsNRAMP5 expression was restricted to roots epidermis, exodermis, and outer layers of the cortex as well as in tissues around the xylem. OsNRAMP5 localized to the plasma membrane, and complemented the growth of yeast strains defective in Mn, Fe, and Cd transport. OsNRAMP5 RNAi (OsNRAMP5i) plants accumulated less Mn in the roots, and less Mn and Fe in shoots, and xylem sap. The suppression of OsNRAMP5 promoted Cd translocation to shoots, highlighting the importance of this gene for Cd phytoremediation. These data reveal that OsNRAMP5 contributes to Mn, Cd, and Fe transport in rice and is important for plant growth and development.

Selection of reference genes for normalization of qRT-PCR analysis of differentially expressed genes in soybean exposed to cadmium

Accurate normalization of gene expression with qRT-PCR depends on the use of appropriate reference genes (RGs) for the species under a given set of experimental conditions. Multiple RGs for gene expression analysis of soybean exposed to heavy metal stress treatment have not been reported in the literature. In this study, we evaluated the expression stability of ten candidate RGs in leaves, roots and stems of two soybean cultivars exposed to cadmium (Cd). Based on the geNorm and NormFinder analysis, ACT3, PP2A, ELF1B and F-box were the most stable RGs in these gene expression studies. In contrast, G6PD, UBC2, TUB, and ELF1A were the most variable ones and should not be used as RGs in these experimental conditions.

[Reducing cadmium content of rice grains by means of flooding and a few problems]

The effects of water management in rice paddies on the levels of cadmium (Cd) and arsenic (As) in Japanese rice grains were tested. In order to reduce the Cd concentration in rice grains, flooding for 3 weeks before and after heading was most effective, but this treatment increased As concentration considerably. Aerobic treatment was effective in reducing As concentration in rice grains, but increased Cd concentration markedly. In the pot experiment, flooding treatment after heading was more effective than flooding treatment before heading in reducing both Cd and As concentrations in rice grains. The concentration of dimethylarsinic acid (DMA) in rice grains was very low under aerobic conditions, but increased in the continuous-flooding treatment. In the field experiment, the grain As concentration in the case of flooding for 3 weeks before and after heading was higher than that in the case of intermittent irrigation. The ratios of DMA to the total As concentration were 3-52% in the pot experiment and 7-13% in the field experiment.

Cadmium transport and tolerance in rice: perspectives for reducing grain cadmium accumulation

Cadmium (Cd) is a toxic heavy metal which harms human health. In Japan, a major source of human Cd-intake is rice grains and contamination of paddy soils by Cd and accumulation of Cd in rice grains are the serious agricultural issues. There also exist Cd contamination of rice and its toxicity in several populations in countries including China and Thailand. Understanding the Cd transport mechanisms in rice can be a basis for regulating rice Cd transport and accumulation by molecular engineering and marker-assisted breeding. Recently, a number of studies have revealed the behavior of Cd in rice, genetic diversity of Cd accumulation, quantitative trait loci controlling Cd accumulation and transporter molecules regulating Cd accumulation and distribution in rice. In this article, we summarize recent advances in the field and discuss perspectives to reduce grain Cd contents.

The phytochelatin transporters AtABCC1 and AtABCC2 mediate tolerance to cadmium and mercury

Heavy metals such as cadmium (Cd) and mercury (Hg) are toxic pollutants that are detrimental to living organisms. Plants employ a two-step mechanism to detoxify toxic ions. First, phytochelatins bind to the toxic ion, and then the metal-phytochelatin complex is sequestered in the vacuole. Two ABCC-type transporters, AtABCC1 and AtABCC2, that play a key role in arsenic detoxification, have recently been identified in Arabidopsis thaliana. However, it is unclear whether these transporters are also implicated in phytochelatin-dependent detoxification of other heavy metals such as Cd(II) and Hg(II). Here, we show that atabcc1 single or atabcc1 atabcc2 double knockout mutants exhibit a hypersensitive phenotype in the presence of Cd(II) and Hg(II). Microscopic analysis using a Cd-sensitive probe revealed that Cd is mostly located in the cytosol of protoplasts of the double mutant, whereas it occurs mainly in the vacuole of wild-type cells. This suggests that the two ABCC transporters are important for vacuolar sequestration of Cd. Heterologous expression of the transporters in Saccharomyces cerevisiae confirmed their role in heavy metal tolerance. Over-expression of AtABCC1 in Arabidopsis resulted in enhanced Cd(II) tolerance and accumulation. Together, these results demonstrate that AtABCC1 and AtABCC2 are important vacuolar transporters that confer tolerance to cadmium and mercury, in addition to their role in arsenic detoxification. These transporters provide useful tools for genetic engineering of plants with enhanced metal tolerance and accumulation, which are desirable characteristics for phytoremediation.

Low-affinity cation transporter (OsLCT1) regulates cadmium transport into rice grains

Accumulation of cadmium (Cd) in rice (Oryza sativa L.) grains poses a potential health problem, especially in Asia. Most Cd in rice grains accumulates through phloem transport, but the molecular mechanism of this transport has not been revealed. In this study, we identified a rice Cd transporter, OsLCT1, involved in Cd transport to the grains. OsLCT1-GFP was localized at the plasma membrane in plant cells, and OsLCT1 showed Cd efflux activity in yeast. In rice plants, strong OsLCT1 expression was observed in leaf blades and nodes during the reproductive stage. In the uppermost node, OsLCT1 transcripts were detected around large vascular bundles and in diffuse vascular bundles. RNAi-mediated knockdown of OsLCT1 did not affect xylem-mediated Cd transport but reduced phloem-mediated Cd transport. The knockdown plants of OsLCT1 accumulated approximately half as much Cd in the grains as did the control plants. The content of other metals in rice grains and plant growth were not negatively affected by OsLCT1 suppression. These results suggest that OsLCT1 functions at the nodes in Cd transport into grains and that in a standard japonica cultivar, the regulation of OsLCT1 enables the generation of "low-Cd rice" without negative effects on agronomical traits. These findings identify a transporter gene for phloem Cd transport in plants.

Phytoavailability, human risk assessment and transfer characteristics of cadmium and zinc contamination from urban gardens in Kano, Nigeria

BACKGROUND

Quantitative data about phytoavailability and transfer into consumed plant parts for heavy metals in intensively managed urban vegetable production areas of sub-Saharan Africa are scarce. We therefore studied the transfer of zinc (Zn) and cadmium (Cd) from soil to the root and subsequent translocation to edible portions of four vegetables in six urban gardens.

RESULTS

While respective diethylenetriaminepentaacetic acid (DTPA)-available Zn and Cd concentrations ranged from 18 to 66 mg kg(-1) and from 0.19 to 0.35 mg kg(-1) , respectively, in soils, total Zn and Cd were 8.4-256 mg kg(-1) and 0.04-1.7 mg kg(-1) in shoot parts. Metal transfer factor (MTF) ratios were higher in Zn (0.2-0.9) than in Cd (0.1-0.6). Our data suggest that total Zn concentration in soil is a reliable indicator to assess its transfer from soil to crop in lettuce, carrot and parsley, while for Cd DTPA-extractable concentration may be used to estimate soil-crop transfer of Cd in amaranthus and carrot. Overall, Cd was more easily translocated to the aerial plant parts than Zn.

CONCLUSION

Zinc and Cd accumulation by vegetables in our soils is mainly a metabolically controlled process. Such accumulation can contaminate the ecosystem but under our conditions intake and ingestion of these metals will likely have to occur over a prolonged period to experience health hazard.

Real-time imaging and analysis of differences in cadmium dynamics in rice cultivars (Oryza sativa) using positron-emitting 107Cd tracer

BACKGROUND

Rice is a major source of dietary intake of cadmium (Cd) for populations that consume rice as a staple food. Understanding how Cd is transported into grains through the whole plant body is necessary for reducing rice Cd concentrations to the lowest levels possible, to reduce the associated health risks. In this study, we have visualized and quantitatively analysed the real-time Cd dynamics from roots to grains in typical rice cultivars that differed in grain Cd concentrations. We used positron-emitting 107Cd tracer and an innovative imaging technique, the positron-emitting tracer imaging system (PETIS). In particular, a new method for direct and real-time visualization of the Cd uptake by the roots in the culture was first realized in this work.

RESULTS

Imaging and quantitative analyses revealed the different patterns in time-varying curves of Cd amounts in the roots of rice cultivars tested. Three low-Cd accumulating cultivars (japonica type) showed rapid saturation curves, whereas three high-Cd accumulating cultivars (indica type) were characterized by curves with a peak within 30 min after 107Cd supplementation, and a subsequent steep decrease resulting in maintenance of lower Cd concentrations in their roots. This difference in Cd dynamics may be attributable to OsHMA3 transporter protein, which was recently shown to be involved in Cd storage in root vacuoles and not functional in the high-Cd accumulating cultivars. Moreover, the PETIS analyses revealed that the high-Cd accumulating cultivars were characterized by rapid and abundant Cd transfer to the shoots from the roots, a faster transport velocity of Cd to the panicles, and Cd accumulation at high levels in their panicles, passing through the nodal portions of the stems where the highest Cd intensities were observed.

CONCLUSIONS

This is the first successful visualization and quantification of the differences in whole-body Cd transport from the roots to the grains of intact plants within rice cultivars that differ in grain Cd concentrations, by using PETIS, a real-time imaging method.

The OsNRAMP1 iron transporter is involved in Cd accumulation in rice

Cadmium (Cd) is a heavy metal toxic to humans and the accumulation of Cd in the rice grain is a major agricultural problem, particularly in Asia. The role of the iron transporter OsNRAMP1 in Cd uptake and transport in rice was investigated here. An OsNRAMP1:GFP fusion protein was localized to the plasma membrane in onion epidermal cells. The growth of yeast expressing OsNRAMP1 was impaired in the presence of Cd compared with yeast transformed with an empty vector. Moreover, the Cd content of OsNRAMP1-expressing yeast exceeded that of the vector control. The expression of OsNRAMP1 in the roots was higher in a high Cd-accumulating cultivar (Habataki) than a low Cd-accumulating cultivar (Sasanishiki) regardless of the presence of Cd, and the amino acid sequence of OsNRAMP1 showed 100% identity between Sasanishiki and Habataki. Over-expression of OsNRAMP1 in rice increased Cd accumulation in the leaves. These results suggest that OsNRAMP1 participates in cellular Cd uptake and Cd transport within plants, and the higher expression of OsNRAMP1 in the roots could lead to an increase in Cd accumulation in the shoots. Our results indicated that OsNRAMP1 is an important protein in high-level Cd accumulation in rice.

Cadmium retention in rice roots is influenced by cadmium availability, chelation and translocation

Analysis of rice plants exposed to a broad range of relatively low and environmentally realistic Cd concentrations showed that the root capacity to retain Cd ions rose from 49 to 79%, corresponding to increases in the external Cd²+ concentration in the 0.01-1 µM range. Fractioning of Cd ions retained by roots revealed that different events along the metal sequestration pathway (i.e. chelation by thiols, vacuolar compartmentalization, adsorption) contributed to Cd immobilization in the roots. However, large amounts of Cd ions (around 24% of the total amount) predictable as potentially mobile were still found in all conditions, while the amount of Cd ions loaded in the xylem seemed to have already reached saturation at 0.1 µM Cd²+, suggesting that Cd translocation may also play an indirect role in determining Cd root retention, especially at the highest external concentrations. In silico search and preliminary analyses in yeast suggest OsHMA2 as a good candidate for the control of Cd xylem loading in rice. Taken as a whole, data indicate Cd chelation, compartmentalization, adsorption and translocation processes as components of a complex 'firewall system' which acts in limiting Cd translocation from the root to the shoot and which reaches different equilibrium positions depending on Cd external concentration.

Plant high tolerance to excess manganese related with root growth, manganese distribution and antioxidative enzyme activity in three grape cultivars

The cuttings of grape (Vitis vinifera Linn.) were exposed to Hoagland's solution containing five different manganese (Mn) concentrations to investigate Mn toxicity and the possible detoxifying responses. Three genotypes (i.e. cultivars Combiner, Jingshou and Shuijing) were used in present study. The results showed that grape species is highly tolerant to excess Mn. The plant growth is stimulated by as high as 15 or 30 mM Mn, and then depressed by higher Mn levels. The grape tolerance to excess Mn is related with plant capacity to keep constant or increased root growth as well as to keep high root activity. Also, the grape could employ some effective but intraspecific strategies to detoxify cellular Mn stress by excluding excess Mn out of leaf tissues or by enhancing antioxidative capacity. On the other hand, the present study showed that there existed different (or contrast) distribution pattern for excess Mn in grape. Majority of Mn was transferred and accumulated in the above-ground part in Combiner while Jingshou stored most Mn in root systems. For the first time our result showed the extreme tolerance and contrast performance at Mn translocation in an important fruit species with revealed genomic information.