Publications

Some soil microbes can methylate arsenic (As) and produce dimethylarsenate (DMA) as a main product. Excessive accumulation of DMA by rice plants can cause the straighthead disease, a physiological disorder leading to substantial yield losses. DMA can also be demethylated in soil, but the mechanism and the microbes involved are not well understood. We investigated the dynamics of methylated As species, including monomethylarsenate (MMA), DMA and dimethyl-monothioarsenate (DMMTA), in three paddy soils that produced the straighthead disease. The soils were incubated under flooded conditions with or without the addition of 2-bromoethanesulfonate (BES), a specific inhibitor of methanogenesis. DMA and DMMTA concentrations in porewater increased initially as soil redox potential decreased, and then decreased rapidly coinciding with the production of methane. BES addition largely suppressed methanogenesis and the disappearance of DMA and DMMTA, but not of MMA. BES addition suppressed the transcript levels of archaeal 16S rRNA and, particularly, mcrA gene encoding methyl-coenzyme reductase subunit A. Among the core genera of archaea, the absolute abundances of Methanomassiliicoccus and Methanosarcina were decreased significantly by BES in the three soils. In a pot experiment with two soils, BES addition significantly increased DMA accumulation in rice husks and the incidence of the straighthead disease in rice. The results suggest that DMA and DMMTA demethylation in paddy soil is coupled to methanogenesis with Methanomassiliicoccus and Methanosarcina likely playing an important role.

No abstract is provided for this article.

No abstract is provided for this article.

Field trials were undertaken to investigate the effect of the application of metal mobilizing agents, different sowing strategies and length of growing season on the extraction of Cd and Zn from soils by Thlaspi caerulescens and Arabidopsis halleri. None of the mobilizing agents used enhanced metal accumulation by T. caerulescens. Between 1998 and 2000, on average across plots where Cd or Zn exceeded allowable limits, T. caerulescens removed 1.3 and 0.3% of the total soil Cd and Zn. In one season when T. caerulescens was grown for 14months, 21.7 and 4.4% of the total soil Cd and Zn was removed. This was larger than values found when T. caerulescens was grown for 4months. A. halleri accumulated similar concentrations of Zn, but lower Cd concentrations than T. caerulescens. The results indicate that metal phytoextraction using T. caerulescens can be used to clean up soils moderately contaminated by Cd.

Summary Arsenic (As) is an important environmental and food‐chain toxin. We investigated the key components controlling As accumulation and tolerance in Arabidopsis thaliana . We tested the effects of different combinations of gene knockout, including arsenate reductase ( HAC 1 ), γ‐glutamyl‐cysteine synthetase (γ ‐ ECS ), phytochelatin synthase ( PCS 1 ) and phosphate effluxer ( PHO 1 ), and the heterologous expression of the As‐hyperaccumulator Pteris vittata arsenite efflux ( Pv ACR 3 ), on As tolerance, accumulation, translocation and speciation in A. thaliana . Heterologous expression of Pv ACR 3 markedly increased As tolerance and root‐to‐shoot As translocation in A. thaliana , with Pv ACR 3 being localized to the plasma membrane. Combining Pv ACR 3 expression with HAC 1 mutation led to As hyperaccumulation in the shoots, whereas combining HAC 1 and PHO 1 mutation decreased As accumulation. Mutants of γ‐ ECS and PCS 1 were hypersensitive to As and had higher root‐to‐shoot As translocation. Combining γ‐ ECS or PCS 1 with HAC 1 mutation did not alter As tolerance or accumulation beyond the levels observed in the single mutants. Pv ACR 3 and HAC 1 have large effects on root‐to‐shoot As translocation. Arsenic hyperaccumulation can be engineered in A. thaliana by knocking out the HAC 1 gene and expressing Pv ACR 3 . Pv ACR 3 and HAC 1 also affect As tolerance, but not to the extent of γ‐ ECS and PCS 1.

Microbial arsenic (As) methylation and demethylation are important components of the As biogeochemical cycle. Arsenic methylation is enhanced under flooded conditions in paddy soils, producing mainly phytotoxic dimethylarsenate (DMAs) that can cause rice straighthead disease, a physiological disorder occurring widely in some rice growing regions. The key microbial groups responsible for As methylation and demethylation in paddy soils are unknown. Three paddy soils were incubated under flooded conditions. DMAs initially accumulated in the soil porewater, followed by a rapid disappearance coinciding with the production of methane. The soil from a rice straighthead disease paddy field produced a much larger amount of DMAs than the other two soils. Using metabolic inhibition, quantification of functional gene transcripts, microbial enrichment cultures and 13C-labeled DMAs, we show that sulfate-reducing bacteria (SRB) and methanogenic archaea are involved in As methylation and demethylation, respectively, controlling the dynamics of DMAs in paddy soils. We present a model of As biogeochemical cycle in paddy soils, linking the dynamics of changing soil redox potential with arsenite mobilization, arsenite methylation and subsequent demethylation driven by different microbial groups. The model provides a basis for controlling DMAs accumulation and incidence of straighthead disease in rice.

Rice accumulates both inorganic arsenic (iAs) and organic As species such as dimethylarsenate (DMA). Although DMA is less toxic to humans, it has been shown in hydroponic studies to induce rice straighthead disease, a physiological disorder prevalent in some rice growing regions causing large yield losses. We investigated the effects of different amendments on As species dynamics in soil porewater, accumulation of As species in rice husks and grains, and the incidence of straighthead disease in five field experiments conducted over 2 years at three sites where straighthead disease was observed in previous seasons. The amendments included silicon (Si) fertilizer, micronized zero valent iron (μZVI), sulfate, nitrate, Si-rich biochar, and a mixture of trace and major elements. Straighthead disease was observed in all five experiments. Rice panicles showing the straighthead disease symptoms contained much higher DMA concentrations in husks and grains than normal panicles. Silicon fertilizer was highly effective at decreasing the disease incidence rate and increasing seed setting rate, resulting in 14.9–58.1% increase in grain yield. Silicon fertilizer increased soil porewater iAs and DMA concentrations, but decreased iAs and DMA accumulation in husks and grains, suggesting that Si suppressed the uptake of iAs and DMA by rice plants. Other amendments alleviated straighthead disease to smaller extents than Si fertilizer, with the effect of biochar and the mixture of trace and major elements likely also being attributed to the addition of Si. Results from this field-based study demonstrate that excessive accumulation of DMA is the main cause of rice straighthead disease and Si fertilizer is highly effective at mitigating this disease by suppressing DMA accumulation.

In this study, we investigated the accumulation of phytochelatins (PCs) and other low molecular weight (LMW) thiols in response to Cd exposure in two contrasting ecotypes differing in Cd accumulation. Using a root elongation test, we found that the highly accumulating ecotype Ganges was more tolerant to Cd than the low Cd‐accumulation ecotype Prayon. l ‐buthionine‐(S,R)‐sulphoximine (BSO), a potent inhibitor of the γ ‐glutamylcysteine synthetase ( γ ‐ECS) (an enzyme involved in the PC biosynthetic pathway), increased the Cd sensitivity of Prayon, but had no effect on Ganges. Although PC accumulation increased in response to Cd exposure, no significant differences were observed between the two ecotypes. Cd exposure induced a dose‐dependent accumulation of both Cys and a still unidentified LMW thiol in roots of both ecotypes. Root accumulation of Cys and this thiol was higher in Ganges than in Prayon; the ecotypic differences were more pronounced when the plants were treated with BSO. These findings suggest that PCs do not contribute to the Cd hypertolerance displayed by the Ganges ecotype of Thlaspi caerulescens , whereas Cys and other LMW thiols might be involved.

• Assessing environmental risks of metal contamination in soils is a complex task because the biologically effective concentrations of metals in soils vary widely with soil properties. • The factors influencing the toxic effect of nickel (Ni) on root growth of barley (Hordeum vulgare) were re-evaluated using published data from both soil and hydroponic cultures. The electrical potential (ψ0o) and ion activities ({Iz}0o) at the outer surfaces of root-cell plasma membranes (PMs) were computed as the basis of the re-evaluation. • The reanalyses demonstrated that root growth was related to: the Ni2+ activity at the PM surface, ({Ni2+}0o); calcium (Ca) deficiency (related to {Ca2+}0o); osmotic effects; and modification of intrinsic Ni2+ toxicity by magnesium (Mg2+; this appeared to exert an intrinsic (specific) ameliorating effect on intrinsic Ni2+ toxicity). Electrostatic toxicity models (ETM) were developed to relate root growth to these factors (R2 > 0.751). • Based on the ETM developed in soil culture and a Ni2+ solid–solution partitioning model, critical metal concentrations in soils linked to a biological effect were well predicted for 16 European soils with a wide range of properties, indicating the potential utility of ETM in risk assessment of metals in terrestrial ecosystems.

The preharvest drainage of rice paddy fields during the grain filling stage can result in a substantial mobilization of Cd in soil and, consequently, elevated grain Cd concentration. However, the processes controlling the mobilization of Cd remains poorly understood. Using 12 field-contaminated paddy soils, we investigated the factors controlling the temporal changes in Cd solubility in paddy soils that were incubated anaerobically for 40 d followed by a 20 d oxidation period. Soluble and extractable Cd concentrations decreased rapidly upon flooding but increased during the oxidation phase, with Cd solubility (aqueous Cd/soil Cd) largely depending upon porewater pH. Furthermore, inhibiting sulfate reduction or inhibiting oxidation dissolution of Cd-sulfides had little or no effect on the mobilization of Cd in the subsequent oxidation phase. Both sequential extraction and X-ray absorption spectroscopy (XAS) analyses revealed that changes in Cd solubility were largely dependent upon the transformation of Cd between the Fe–Mn (oxyhydro)oxide fraction and exchangeable fraction. Mobilization of Cd upon soil drainage was caused by a decrease in soil pH resulting in the release of Cd from Fe–Mn (oxyhydro)oxides. Taken together, Fe–Mn (oxyhydro)oxides play a critical (and prevalent) role in controlling the mobilization of Cd upon soil drainage in paddy systems.

Determining the distribution of trace elements in biological materials with subcellular resolution is very challenging but vitally important in order to understand their mechanisms of uptake. Rice grain is efficient in the accumulation of arsenic (As) in the grain, potentially posing a severe health risk to millions of people in South-East Asia. The NanoSIMS is a state-of-the-art microscope capable of high resolution chemical imaging (down to 50 nm) and detecting very low elemental concentrations (parts-per-million levels). This makes it ideally suited for trace element localisation in biological materials. This paper shows how the NanoSIMS can be used to investigate the localisation of As in rice grain at high resolution and how different treatments result in different distributions in the grain. The precise location of parts-per-million As concentrations and its localisation with other elements at the subcellular scale in rice roots are also shown. Copyright © 2012 John Wiley & Sons, Ltd.

No abstract is provided for this article.

Recent studies have shown that higher plants are unable to methylate arsenic (As), but it is not known whether methylated As species taken up by plants can be volatilized. Rice (Oryza sativa L.) plants were grown axenically or in a nonsterile soil using a two-chamber system. Arsenic transformation and volatilization were investigated. In the axenic system, uptake of As species into rice roots was in the order of arsenate (As(V)) > monomethylarsonic acid (MMAs(V)) > dimethylarsinic acid (DMAs(V)) > trimethylarsine oxide (TMAs(V)O), but the order of the root-to-shoot transport index (Ti) was reverse. Also, volatilization of trimethylarsine (TMAs) from rice plants was detected when plants were treated with TMAs(V)O but not with As(V), DMAs(V), or MMAs(V). In the soil culture, As was volatilized mainly from the soil. Small amounts of TMAs were also volatilized from the rice plants, which took up DMAs(V), MMAs(V), and TMAs(V)O from the soil solution. The addition of dried distillers grain (DDG) to the soil enhanced As mobilization into the soil solution, As methylation and volatilization from the soil, as well as uptake of different As species and As volatilization from the rice plants. Results show that rice is able to volatilize TMAs after the uptake of TMAs(V)O but not able to convert inorganic As, MMAs(V) or DMAs(V) into TMAs and that the extent of As volatilization from rice plants was much smaller than that from the flooded soil.

Long‐term trends in iodine and selenium retention in soil, and uptake by herbage, were investigated in archived samples from the Park Grass Experiment, initiated in 1856 at Rothamsted, UK . Soil (0–23 cm) and herbage samples from plots receiving various mineral fertilizers and organic manures, with and without lime, were analysed for selenium (Se) and iodine (I) to assess the effect of soil amendments, annual rainfall, crop yield and changes in soil chemistry from 1876 to 2008. Comparing soil from limed and unlimed control (unfertilized) plots, TMAH ‐extractable Se and I concentrations both diverged, with time, with greater retention in unlimed plots; differences in concentration amounted to 92 and 1660 μ g/kg for Se and I, respectively, after 105 yr. These differences were broadly consistent with estimated additions from rainfall and dry deposition. Offtake of both elements in herbage was negligible compared to soil concentrations and annual inputs (<0.003% of total soil I and <0.006% of total soil Se). A positive correlation was observed between I and Se concentrations in herbage, suggesting some common factors controlling bioavailability. A growth‐dilution effect for I and Se was suggested by the positive correlation between growing season rainfall ( GSR ) and herbage yield together with soil‐to‐plant transfer factors decreasing with yield. Phosphate and sulphate fertilizers reduced I and Se herbage concentrations, both through ion competition and increased herbage yield. Results suggest that in intensive agriculture with soil pH control, the I requirement of grazing animals is not likely to be met by herbage alone.

Pathogen genetic diversity varies in response to environmental changes. However, it remains unclear whether plant barriers to invasion could be considered a genetic bottleneck for phytopathogen populations. Here, we implement a barcoding approach to generate a pool of 90 isogenic and individually barcoded Ralstonia solanacearum strains. We used 90 of these strains to inoculate tomato plants with different degrees of physical permeability to invasion (intact roots, wounded roots and xylem inoculation) and quantify the phytopathogen population dynamics during invasion. Our results reveal that the permeability of plant roots impacts the degree of population bottleneck, genetic diversity, and composition of Ralstonia populations. We also find that selection is the main driver structuring pathogen populations when barriers to infection are less permeable, i.e., intact roots, the removal of root physical and immune barriers results in the predominance of stochasticity in population assembly. Taken together, our study suggests that plant root permeability constitutes a bottleneck for phytopathogen invasion and genetic diversity.