Grain Unloading of Arsenic Species in Rice

Carey, Anne-Marie; Scheckel, Kirk G.; Lombi, Enzo; Newville, Matt; Choi, Y.; Norton, Gareth J.; Charnock, John; Feldmann, Jörg; Price, Adam H.; Meharg, Andrew A.

doi:10.1104/pp.109.146126

Cited by 287 publications

(239 citation statements)

References 39 publications

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“…These results indicate that methylated As, particularly the dimethyl species, are more mobile than inorganic As in the phloem. Consistent with this finding, Carey et al (2010) showed that DMA(V) was transported to rice grain much more efficiently than inorganic As. Uptake of undissociated MMA(V) and DMA(V) by rice roots is partly mediated by the aquaporin NIP2,1 (Li et al, 2009), but membrane transporters responsible for loading and unloading of methylated As species in phloem remain unknown.…”

Section: Discussionsupporting

confidence: 78%

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Arsenic Speciation in Phloem and Xylem Exudates of Castor Bean

et al. 2010

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How arsenic (As) is transported in phloem remains unknown. To help answer this question, we quantified the chemical species of As in phloem and xylem exudates of castor bean (Ricinus communis) exposed to arsenate [As(V)], arsenite [As(III)], monomethylarsonic acid [MMA(V)], or dimethylarsinic acid. In the As(V)-and As(III)-exposed plants, As(V) was the main species in xylem exudate (55%-83%) whereas As(III) predominated in phloem exudate (70%-94%). The ratio of As concentrations in phloem to xylem exudate varied from 0.7 to 3.9. Analyses of phloem exudate using high-resolution inductively coupled plasma-mass spectrometry and accurate mass electrospray mass spectrometry coupled to high-performance liquid chromatography identified high concentrations of reduced and oxidized glutathione and some oxidized phytochelatin, but no As(III)-thiol complexes. It is thought that As(III)-thiol complexes would not be stable in the alkaline conditions of phloem sap. Small concentrations of oxidized glutathione and oxidized phytochelatin were found in xylem exudate, where there was also no evidence of As(III)-thiol complexes. MMA(V) was partially reduced to MMA(III) in roots, but only MMA(V) was found in xylem and phloem exudate. Despite the smallest uptake among the four As species supplied to plants, dimethylarsinic acid was most efficiently transported in both xylem and phloem, and its phloem concentration was 3.2 times that in xylem. Our results show that free inorganic As, mainly As(III), was transported in the phloem of castor bean exposed to either As(V) or As(III), and that methylated As species were more mobile than inorganic As in the phloem.Arsenic (As) is an environmental and food chain contaminant that has attracted much attention in recent years. Soil contamination with As may lead to phytotoxicity and reduced crop yield (Panaullah et al., 2009). Food crops are also an important source of inorganic As, a class-one carcinogen, in human dietary intake, and there is a need to decrease the exposure to this toxin (European Food Safety Authority, 2009). Paddy rice (Oryza sativa) is particularly efficient in As accumulation, which poses a potential risk to the population based on a rice diet Zhao et al., 2010a). Other terrestrial food crops generally do not accumulate as much As as paddy rice; however, where soils are contaminated, relatively high concentrations of As in wheat (Triticum aestivum) grain have been reported Zhao et al., 2010b). On the other hand, some fern species in the Pteridaceae family are able to tolerate and hyperaccumulate As in the aboveground part to .1,000 mg kg 21 dry weight (e.g. Ma et al., 2001;Zhao et al., 2002); these plants offer the possibility for remediation of Ascontaminated soil or water (Salido et al., 2003;Huang et al., 2004). A better understanding of As uptake and long-distance transport, metabolism, and detoxification is needed for developing strategies for mitigating As contamination, through either decreased As accumulation in food crops or enhanced As accumulation for phytoremedi...

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Section: Discussionsupporting

confidence: 78%

“…It has been shown that phloem transport contributes substantially to As accumulation in rice grain (Carey et al, 2010). However, little is known about how As is transported in phloem ).…”

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confidence: 99%

Arsenic Speciation in Phloem and Xylem Exudates of Castor Bean

et al. 2010

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“…In the less mature root region and in lateral root junctions, As(III) was poorly correlated with S within the (pre)vascular cylinder (Figures 1 and 3) and this likely indicates the presence of more free As(III) i there. Free As(III) i is both xylem and phloem mobile [38,39]. Free As(III) i that is transported to the stele in lateral root junctions and apical regions may access the (meta)xylem and ultimately be transported to shoots, including grain.…”

Section: Discussionmentioning

confidence: 99%

Evidence for the Root-Uptake of Arsenite at Lateral Root Junctions and Root Apices in Rice (Oryza sativa L.)

Seyfferth

Ross

Webb

2017

Soils

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The uptake of arsenite (As(III) i ) at the Casparian band via Lsi1 and Lsi2 Si transporters is responsible for~75% of shoot As(III) i uptake in rice and, therefore,~25% of shoot As(III) i is taken up by other transport pathways. We hypothesized that areas devoid of Casparian bands-lateral root junctions and root apices-can transport As(III) i into roots. We analyzed the elemental distribution and As concentration, speciation, and localization in rice roots from soil-grown and solution-grown plants. With solution-grown plants dosed with As(III) i , we sectioned roots as a function of distance from the root apex and analyzed the cross-sections using confocal microscopy coupled to synchrotron X-ray fluorescence imaging and spectroscopy. We observed elevated As(III) i associated with lateral root junctions and root apices in rice. As(III) i entered the stele at lateral root junctions and radially permeated the root interior in cross-sections 130-140 µm from the root apex that are devoid of Casparian bands. Our findings suggest that lateral root junctions and rice root apices are hot-spots for As(III) i transport into rice roots, but the contribution to shoot As requires further research.

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“…The translocation efficiency of DMA to the rice grain is over an order of magnitude greater than inorganic species in As-fed excised panicles. It is because that DMA is more mobile than As(III) in both the phloem and the xylem [100]. Organic As (DMA and MMA) was mainly distributed in reproductive tissues rather than vegetative tissues.…”

Section: Benefits Of Microbial Arsenic Methylation and Volatilizationmentioning

confidence: 94%

Effects of microbial processes on the fate of arsenic in paddy soil

2012

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Arsenic (As) is a metalloid toxic to organisms including humans. Arsenic in rice represents a significant exposure pathway for the general population, particularly for those subsisting on rice. Arsenic transformation, namely reduction, oxidation and methylation, in soil-rice systems has fundamental impacts on its mobility and toxicity. In addition to soil chemical properties (pH, Eh, metallic oxides, organic matter), microorganisms play critical roles in As transformation and mobility in paddy soil, such as through ArsM (As(III) S-adenosylmethyltransferase) and interactions with iron oxides or organic matters. Arsenic species in paddy soil directly influence As speciation in rice grain because the methylated As species in rice are mainly derived from microbial methylation in paddy soil. This paper aims to provide an overview on the status of the knowledge and gaps on the chemical aspects of As transformation in soil-rice system in conjunction with microbial ecology and functional genes. In addition, potential pathways (manipulation of microorganisms in paddy soil and genetic engineering) to decrease total As and/or inorganic As in rice grain are proposed.

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Grain Unloading of Arsenic Species in Rice

Cited by 287 publications

References 39 publications

Arsenic Speciation in Phloem and Xylem Exudates of Castor Bean

Arsenic Speciation in Phloem and Xylem Exudates of Castor Bean

Evidence for the Root-Uptake of Arsenite at Lateral Root Junctions and Root Apices in Rice (Oryza sativa L.)

Effects of microbial processes on the fate of arsenic in paddy soil

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