Quantitative estimation of the contribution of the phloem in cadmium transport to grains in rice plants (<i>Oryza sativa</i>L.)

Tanaka, Kensuke; Fujimaki, Shu; Fujiwara, Toru; Yoneyama, T.; Hayashi, Hiroaki

doi:10.1111/j.1747-0765.2007.00116.x

Cited by 151 publications

(88 citation statements)

References 13 publications

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“…This explains why husk concentrations of As in the first experiment were also markedly higher for the DMA treatment than those for inorganic arsenite. It has been suggested that as a nontranspiring tissue developing grain will be fed assimilates primarily via phloem transport, and the actively transpiring husk primarily via the xylem (Tanaka et al, 2007;). This appears evident in the high concentrations of the phloem marker, Rb, compared with the comparatively low grain levels Sr.…”

Section: Discussion As Speciation and Accumulation In Grainmentioning

confidence: 99%

Grain Unloading of Arsenic Species in Rice

et al. 2009

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Rice (Oryza sativa) is the staple food for over half the world's population yet may represent a significant dietary source of inorganic arsenic (As), a nonthreshold, class 1 human carcinogen. Rice grain As is dominated by the inorganic species, and the organic species dimethylarsinic acid (DMA). To investigate how As species are unloaded into grain rice, panicles were excised during grain filling and hydroponically pulsed with arsenite, arsenate, glutathione-complexed As, or DMA. Total As concentrations in flag leaf, grain, and husk, were quantified by inductively coupled plasma mass spectroscopy and As speciation in the fresh grain was determined by x-ray absorption near-edge spectroscopy. The roles of phloem and xylem transport were investigated by applying a 6 stem-girdling treatment to a second set of panicles, limiting phloem transport to the grain in panicles pulsed with arsenite or DMA. The results demonstrate that DMA is translocated to the rice grain with over an order magnitude greater efficiency than inorganic species and is more mobile than arsenite in both the phloem and the xylem. Phloem transport accounted for 90% of arsenite, and 55% of DMA, transport to the grain. Synchrotron x-ray fluorescence mapping and fluorescence microtomography revealed marked differences in the pattern of As unloading into the grain between DMA and arsenite-challenged grain. Arsenite was retained in the ovular vascular trace and DMA dispersed throughout the external grain parts and into the endosperm. This study also demonstrates that DMA speciation is altered in planta, potentially through complexation with thiols.

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Section: Discussion As Speciation and Accumulation In Grainmentioning

confidence: 99%

Grain Unloading of Arsenic Species in Rice

et al. 2009

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“…Phloem transport has also been considered a key step of Cd translocation to the grain, because xylem transport is directed mainly to the organs of highest transpiration, such as leaves, but not to the sites of highest demand for mineral, such as grains (Marschner, 1995). Cd was detected in the phloem sap of rice collected from leaf sheaths (Tanaka et al, 2003) and from the uppermost internode at the grain-filling stage (Tanaka et al, 2007). Tanaka et al (2007) also estimated that 91% to 100% of Cd in rice grains is deposited from the phloem.…”

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confidence: 99%

“…Cd was detected in the phloem sap of rice collected from leaf sheaths (Tanaka et al, 2003) and from the uppermost internode at the grain-filling stage (Tanaka et al, 2007). Tanaka et al (2007) also estimated that 91% to 100% of Cd in rice grains is deposited from the phloem. In wheat, it has been observed in a split-root system that Cd fed to one bundle of the roots moved into the other bundle, probably via the phloem (Welch et al, 1999;Page and Feller, 2005), and steam girdling, which stops only phloem transport but not xylem flow, to the peduncle below the ear reduced Cd translocation to the grain (Riesen and Feller, 2005).…”

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Tracing Cadmium from Culture to Spikelet: Noninvasive Imaging and Quantitative Characterization of Absorption, Transport, and Accumulation of Cadmium in an Intact Rice Plant

et al. 2010

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We characterized the absorption and short-term translocation of cadmium (Cd) in rice (Oryza sativa 'Nipponbare') quantitatively using serial images observed with a positron-emitting tracer imaging system. We fed a positron-emitting 107 Cd (halflife of 6.5 h) tracer to the hydroponic culture solution and noninvasively obtained serial images of Cd distribution in intact rice plants at the vegetative stage and at the grain-filling stage every 4 min for 36 h. The rates of absorption of Cd by the root were proportional to Cd concentrations in the culture solution within the tested range of 0.05 to 100 nM. It was estimated that the radial transport from the culture to the xylem in the root tissue was completed in less than 10 min. Cd moved up through the shoot organs with velocities of a few centimeters per hour at both stages, which was obviously slower than the bulk flow in the xylem. Finally, Cd arrived at the panicles 7 h after feeding and accumulated there constantly, although no Cd was observed in the leaf blades within the initial 36 h. The nodes exhibited the most intensive Cd accumulation in the shoot at both stages, and Cd transport from the basal nodes to crown root tips was observed at the vegetative stage. We conclude that the nodes are the central organ where xylem-to-phloem transfer takes place and play a pivotal role in the half-day travel of Cd from the soil to the grains at the grain-filling stage.

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“…Phloem-mediated Cd transport to grains following xylem-mediated root-to-shoot translocation is critical for the accumulation of Cd in rice grains. Nearly 100% of the Cd in rice grains is attributable to phloem transport (22,23). Fujimaki et al (22) used a noninvasive live imaging technique to follow the transport of 107 Cd in intact rice plants.…”

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

Uraguchi

Kamiya

Sakamoto

et al. 2011

Proc. Natl. Acad. Sci. U.S.A.

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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.heavy metals | food safety

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Quantitative estimation of the contribution of the phloem in cadmium transport to grains in rice plants (Oryza sativaL.)

Cited by 151 publications

References 13 publications

Grain Unloading of Arsenic Species in Rice

Grain Unloading of Arsenic Species in Rice

Tracing Cadmium from Culture to Spikelet: Noninvasive Imaging and Quantitative Characterization of Absorption, Transport, and Accumulation of Cadmium in an Intact Rice Plant

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

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