Comparison of corn and lupin in respect to As mobilisation, uptake and release in an arsenic contaminated floodplain soil.

Based on the differences of lupin and corn in Si requirement and their capacity to release organic acids for P mobilisation both plant species were compared in respect to As mobilisation and plant As uptake and release which is closely linked to P and Si nutrition.Plants were grown with and without P fertilization in an As contaminated floodplain soil in compartment systems which enabled temporally resolved sampling of soil solution with increasing distance from the root surface.Although no direct increase of P or AsV concentration in soil solution could be detected, lupins showed higher P and As uptake but lower root to shoot translocation of As compared to corn plants. Higher root As concentrations in lupins corresponded to higher AsIII concentration in soil solution which increased with time and corresponded to the recently demonstrated diffusion driven efflux of As in the form of AsIII via aquaglycerol channels. It is well established that AsV, the form of As initially present in the floodplain soil, is taken up via P transporters. Hence the higher As uptake of lupin is in line with previous results showing higher P uptake capacity per unit root length for lupin compared to corn. A lower capacity for As efflux in lupin as it was expected due to lower Si requirement of lupin compared to corn could not be demonstrated

Transformation and Subcellular Distribution of Arsenic and Mechanism of its Sub-lethal Toxicity in Plants

Speciation of arsenic (As) in rice, the most As affected crop, and Ceratophyllum demersum, a good laboratorymodel for shoot, through chromatography and localization in the later through synchrotron based techniqueswas performed to explore the mechanism of As toxicity. In rice plant, exposed to inorganic arsenate (AsV) andmethylated As, most of the AsV and methylarsonate (MAV) were efficiently reduced to arsenite (AsIII) and MAIIIrespectively, but dimethylarsinate (DMAV) did not transformed. A large proportion of AsIII and MAIII were complexedwith thiols showing up to 20 and 16 As species respectively in the roots. Many of them were identifiedas new As-thiol species. Despite high complexation in root, more MA was translocated to shoot, with shoot/roottransfer factor being in order DMA>MA>inorganic As in rice.C. demersum, also displayed up to 60% accumulated As in the form of thiol complexed-AsIII. Most of which wasin epidermis of mature leaves as revealed by tissue resolved X-ray absorption near-edge spectroscopy (μ-XANES) of intact hydrated leaves. At sublethal concentration, As predominantly accumulated in the nucleus ofthe epidermal cells, as revealed by μ-X-ray fluorescence (μ-XRF), indicating replacement of P by As in DNA molecules,providing in vivo evidence for the proposed toxicity mechanism of AsV. While at lethal concentration,vacuole was the main storage site of As, yet a significant increase of unbound AsIII in mesophylls of young matureleaves occurred. This small amount of As reaching chloroplasts already caused a strong and specific inhibitionof tetrapyrrole biosynthesis and severe growth retardation.Taken together these results establish the mechanism of As toxicity and reported yield loss in paddy rice growingin As contaminated areas. Further, significant translocation of MAIII (more cytotoxic than AsIII) to rice shootscould also be an important factor inducing straighthead (spikelet sterility disorder) in rice

Speciation and Distribution of Arsenic in the Nonhyperaccumulator Macrophyte Ceratophyllum demersum

Although arsenic (As) is a common pollutant worldwide, many questions about As metabolism in nonhyperaccumulator plants remain. Concentration- and tissue-dependent speciation and distribution of As was analyzed in the aquatic plant Ceratophyllum demersum to understand As metabolism in nonhyperaccumulator plants. Speciation was analyzed chromatographically (high-performance liquid chromatography-[inductively coupled plasma-mass spectrometry]-[electrospray ionization-mass spectrometry]) in whole-plant extracts and by tissue-resolution confocal x-ray absorption near-edge spectroscopy in intact shock-frozen hydrated leaves, which were also used for analyzing cellular element distribution through x-ray fluorescence. Chromatography revealed up to 20 As-containing species binding more than 60% of accumulated As. Of these, eight were identified as thiol-bound (phytochelatins [PCs], glutathione, and cysteine) species, including three newly identified complexes: Cys-As(III)-PC2, Cys-As-(GS)2, and GS-As(III)-desgly-PC2. Confocal x-ray absorption near-edge spectroscopy showed arsenate, arsenite, As-(GS)3, and As-PCs with varying ratios in various tissues. The epidermis of mature leaves contained the highest proportion of thiol (mostly PC)-bound As, while in younger leaves, a lower proportion of As was thiol bound. At higher As concentrations, the percentage of unbound arsenite increased in the vein and mesophyll of young mature leaves. At the same time, x-ray fluorescence showed an increase of total As in the vein and mesophyll but not in the epidermis of young mature leaves, while this was reversed for zinc distribution. Thus, As toxicity was correlated with a change in As distribution pattern and As species rather than a general increase in many tissues

Different strategies of cadmium detoxification in the submerged macrophyte Ceratophyllum demersum L.

The heavy metal cadmium (Cd) is highly toxic to plants. To understand the mechanisms of tolerance and resistance to Cd, we treated the rootless, submerged macrophyte Ceratophyllum demersum L. with sub-micromolar concentrations of Cd under environmentally relevant conditions. X-ray fluorescence measurements revealed changing distribution patterns of Cd and Zn at non-toxic (0.2 nM, 2 nM), moderately toxic (20 nM) and highly toxic (200 nM) levels of Cd. Increasing Cd concentrations led to enhanced sequestration of Cd into non-photosynthetic tissues like epidermis and vein. At toxic Cd concentrations, Zn was redistributed and mainly found in the vein. Cd treatment induced the synthesis of phytochelatins (PCs) in the plants, with a threshold of induction already at 20 nM Cd for PC3. In comparison, in plants treated with Cu, elevated PC levels were detected only at the highest concentrations (100–200 nM Cu). Our results show that also non-accumulators like C. demersum store toxic metals in tissues where the heavy metal interferes least with metabolic pathways, but remaining toxicity interferes with micronutrient distribution. Furthermore, we found that the induction of phytochelatins is not proportional to metal concentration, but has a distinct threshold, specific for each PC species. Finally we could show that 20 nM Cd, which was previously regarded as non-toxic to most plants, already induces detoxifying mechanisms

Poster P3: Toxicity and Detoxification of Cadmium in the Aquaticmacrophyte Ceratophyllum Demersum; session "Metal homeostasis and detoxification"

The heavy metal cadmium (Cd) is an important pollutant and poisonousto many organisms. We studied the effects of Cd on C.demersum under environmentally relevant conditions. High, moderateand low concentrations of Cd had different effects. Lethally toxicconcentrations (100–200 nM) led to growth stop and the plants’ability to perform photosynthesis (measured as Fv/Fm) decreased morethan twofold, consistent with decreased pigment content. Moderatelytoxic concentrations (10–50 nM) led to reduced growth, slightlyreduced pigment content, but hardly affected photosynthesis (measuredas O2 exchange and as Fv/Fm). Lower concentrations(0.2–5 nM) even had beneficial effects, like enhanced growth rate.When applied in low concentrations, Cd was homogeneously distributedin the whole cross-section of the leaves like a nutrient.Moderate and high Cd concentrations led to sequestration of Cd in thevascular bundle and the epidermis cells, where Cd does not affectphotosynthetic molecules. At toxic Cd concentrations, Zn was redistributedand mainly found in the vein along with Cd, indicatinginhibition of Zn transporters. Consistently, by metalloproteomics(HPLC-ICP-MS) we found that during Cd toxicity concentrations of Cdincreased in fractions of the major photosynthetic complexes while Mgdecreased, suggesting the replacement of Mg by Cd in chlorophylls.Furthermore, the induction of phytochelatins was not proportionalto metal concentration, but had distinct thresholds, specific for each PCspecies. PC3 especially was switch-like induced already at 20 nM Cd,which was previously regarded as non-toxic to most plants. Phytochelatinlevels at the lowest Cd concentrations were not detectable orbelow 0.1 % of the level at sublethally toxic concentrations, suggestingthat they do not have another function than metal detoxification