Positron-emitting Tracer Imaging System Research Articles

Visualization of the initial radiocesium dynamics after penetration in living apple trees with bark removal using a positron-emitting 127Cs tracer

Following the Fukushima nuclear accident in March 2011, radiocesium (rCs) contamination in deciduous trees remains over 10 years later even though the trees were leafless at the time of the accident. This phenomenon is considered to be the result of repeated retranslocation of rCs that initially penetrated the bark into the internal tissues. To implement effective measures after a possible accident in the future, it is necessary to clarify how rCs is translocated in the tree after penetration. In this study, rCs translocation was dynamically visualized using a positron-emitting tracer imaging system (PETIS) and autoradiography after the bark of apple branches was removed. The PETIS results showed the translocation of 127Cs from the branch to young shoots and the main stem in apple trees under controlled spring growing conditions. The transport velocity of rCs in the branch was faster than that in the main stem. The transport of rCs, which was either acropetal or basipetal, in the main stem through the branch junction favored basipetal movement. Autoradiography of transverse sections of the main stem indicated that basipetal translocation was due to transport in the phloem. This study demonstrated the initial translocation responses of rCs similar to previous field research, which indicates that rCs transport to the young shoots tends to be higher under controlled conditions. Our laboratory-based experimental system may be useful to gain an improved understanding of rCs dynamics in deciduous trees.

Rice immediately adapts the dynamics of photosynthates translocation to roots in response to changes in soil water environment.

Rice is susceptible to abiotic stresses such as drought stress. To enhance drought resistance, elucidating the mechanisms by which rice plants adapt to intermittent drought stress that may occur in the field is an important requirement. Roots are directly exposed to changes in the soil water condition, and their responses to these environmental changes are driven by photosynthates. To visualize the distribution of photosynthates in the root system of rice plants under drought stress and recovery from drought stress, we combined X-ray computed tomography (CT) with open type positron emission tomography (OpenPET) and positron-emitting tracer imaging system (PETIS) with 11C tracer. The short half-life of 11C (20.39min) allowed us to perform multiple experiments using the same plant, and thus photosynthate translocation was visualized as the same plant was subjected to drought stress and then re-irrigation for recovery. The results revealed that when soil is drier, 11C-photosynthates mainly translocated to the seminal roots, likely to promote elongation of the root with the aim of accessing water stored in the lower soil layers. The photosynthates translocation to seminal roots immediately stopped after rewatering then increased significantly in crown roots. We suggest that when rice plant experiencing drought is re-irrigated from the bottom of pot, the destination of 11C-photosynthates translocation immediately switches from seminal root to crown roots. We reveal that rice roots are responsive to changes in soil water conditions and that rice plants differentially adapts the dynamics of photosynthates translocation to crown roots and seminal roots depending on soil conditions.

Characterization of zinc uptake and translocation visualized with positron-emitting 65Zn tracer and analysis of transport-related gene expression in two Lotus japonicus accessions.

Zinc (Zn) is an essential element for humans and plants. However, Zn deficiency is widespread and 25 % of the world's population is at risk of Zn deficiency. To overcome the deficiency of Zn intake, crops with high Zn content are required. However, most crop-producing areas have Zn-deficient soils, therefore crops with excellent Zn uptake/transport characteristics (i.e. high Zn efficiency) are needed. Our objective was to identify the crucial factors responsible for high Zn efficiency in the legume Lotus japonicus. We evaluated Zn efficiency by static and real-time visualization of radioactive Zn (65Zn) uptake/transport in two L. japonicus accessions, MG-20 and B-129, that differ in Zn efficiency. The combination of visualization methods verified the dynamics of Zn accumulation and transport within the plant. We compared gene expression under a normal Zn concentration (control) and Zn deficiency to evaluate genetic factors that may determine the differential Zn efficiency of the accessions. The accession B-129 accumulated almost twice the amount of Zn as MG-20. In the static 65Zn images, 65Zn accumulated in meristematic tissues, such as root tips and the shoot apex, in both accessions. The positron-emitting tracer imaging system (PETIS), which follows the transport process in real time, revealed that 65Zn transport to the shoot was more rapid in B-129 than in MG-20. Many genes associated with Zn uptake and transport were more highly expressed in B-129 than in MG-20 under the control condition. These gene expression patterns under Zn deficiency differed from those under the control Zn condition. PETIS confirmed that the real-time transport of 65Zn to the shoot was faster in B-129 than in MG-20. The high Zn efficiency of B-129 may be due to the elevated expression of a suite of Zn uptake- and transport-related genes.

New evidence of arsenic translocation and accumulation in Pteris vittata from real-time imaging using positron-emitting 74As tracer

Pteris vittata is an arsenic (As) hyperaccumulator plant that accumulates a large amount of As into fronds and rhizomes (around 16,000 mg/kg in both after 16 weeks hydroponic cultivation with 30 mg/L arsenate). However, the sequence of long-distance transport of As in this hyperaccumulator plant is unclear. In this study, we used a positron-emitting tracer imaging system (PETIS) for the first time to obtain noninvasive serial images of As behavior in living plants with positron-emitting 74As-labeled tracer. We found that As kept accumulating in rhizomes as in fronds of P. vittata, whereas As was retained in roots of a non-accumulator plant Arabidopsis thaliana. Autoradiograph results of As distribution in P. vittata showed that with low As exposure, As was predominantly accumulated in young fronds and the midrib and rachis of mature fronds. Under high As exposure, As accumulation shifted from young fronds to mature fronds, especially in the margin of pinna, which resulted in necrotic symptoms, turning the marginal color to gray and then brown. Our results indicated that the function of rhizomes in P. vittata was As accumulation and the regulation of As translocation to the mature fronds to protect the young fronds under high As exposure.

Shoot base responds to root-applied glutathione and functions as a critical region to inhibit cadmium translocation from the roots to shoots in oilseed rape (Brassica napus)

Glutathione (GSH) is a tripeptide involved in controlling heavy metal movement in plants. Our previous study showed that GSH, when site-specifically applied to plant roots, inhibits Cd translocation from the roots to shoots in hydroponically cultured oilseed rape (Brassica napus) plants. A factor that led to this inhibitory effect was the activation of Cd efflux from root cells. To further investigate the molecular mechanism triggered by root-applied GSH, Cd movement was non-invasively monitored using a positron-emitting tracer imaging system. The Cd absorption and efflux process in the roots were visualized successfully. The effects of GSH on Cd efflux from root cells were estimated by analyzing imaging data. Reanalysis of image data suggested that GSH applied to roots, at the shoot base, activated Cd return. Cutting the shoot base significantly inhibited Cd efflux from root cells. These experimental results demonstrate that the shoot base plays an important role in distributing Cd throughout the plant body. Furthermore, microarray analysis revealed that about 400 genes in the roots responded to root-applied GSH. Among these, there were genes for transporter proteins related to heavy metal movement in plants and proteins involved in the structure modification of cell walls.

Visualising spatio-temporal distributions of assimilated carbon translocation and release in root systems of leguminous plants

The release of rhizodeposits differs depending on the root position and is closely related to the assimilated carbon (C) supply. Therefore, quantifying the C partitioning over a short period may provide crucial information for clarifying root–soil carbon metabolism. A non-invasive method for visualising the translocation of recently assimilated C into the root system inside the rhizobox was established using 11CO2 labelling and the positron-emitting tracer imaging system. The spatial distribution of recent 11C-photoassimilates translocated and released in the root system and soil were visualised for white lupin (Lupinus albus) and soybean (Glycine max). The inputs of the recently assimilated C in the entire root that were released into the soil were approximately 0.3%–2.9% for white lupin within 90 min and 0.9%–2.3% for soybean within 65 min, with no significant differences between the two plant species; however, the recently assimilated C of lupin was released at high concentrations in specific areas (hotspots), whereas that of soybean was released uniformly in the soil. Our method enabled the quantification of the spatial C allocations in roots and soil, which may help to elucidate the relationship between C metabolism and nutrient cycling at specific locations of the root–soil system in response to environmental conditions over relatively short periods.

Dynamic Analysis of Photosynthate Translocation Into Strawberry Fruits Using Non-invasive 11C-Labeling Supported With Conventional Destructive Measurements Using 13C-Labeling.

In protected strawberry (Fragaria × ananassa Duch.) cultivation, environmental control based on the process of photosynthate translocation is essential for optimizing fruit quality and yield, because the process of photosynthate translocation directly affects dry matter partitioning. We visualized photosynthate translocation to strawberry fruits non-invasively with 11CO2 and a positron-emitting tracer imaging system (PETIS). We used PETIS to evaluate real-time dynamics of 11C-labeled photosynthate translocation from a 11CO2-fed leaf, which was immediately below the inflorescence, to individual fruits on an inflorescence in intact plant. Serial photosynthate translocation images and animations obtained by PETIS verified that the 11C-photosynthates from the source leaf reached the sink fruit within 1 h but did not accumulate homogeneously within a fruit. The quantity of photosynthate translocation as represented by 11C radioactivity varied among individual fruits and their positions on the inflorescence. Photosynthate translocation rates to secondary fruit were faster than those to primary or tertiary fruits, even though the translocation pathway from leaf to fruit was the longest for the secondary fruit. Moreover, the secondary fruit was 25% smaller than the primary fruit. Sink activity (11C radioactivity/dry weight [DW]) of the secondary fruit was higher than those of the primary and tertiary fruits. These relative differences in sink activity levels among the three fruit positions were also confirmed by 13C tracer measurement. Photosynthate translocation rates in the pedicels might be dependent on the sink strength of the adjoining fruits. The present study established 11C-photosynthate arrival times to the sink fruits and demonstrated that the translocated material does not uniformly accumulate within a fruit. The actual quantities of translocated photosynthates from a specific leaf differed among individual fruits on the same inflorescence. To the best of our knowledge, this is the first reported observation of real-time translocation to individual fruits in an intact strawberry plant using 11C-radioactive- and 13C-stable-isotope analyses.

Elevated glutathione synthesis in leaves contributes to zinc transport from roots to shoots in Arabidopsis

Glutathione (GSH) is a vital compound involved in several plant metabolic pathways. Our previous study indicated that foliar GSH application can increase zinc (Zn) levels in leafy vegetables. The objective of this study was to determine the mode of action of GSH as it relates to Zn transport from roots to shoots. Two types of transgenic Arabidopsis plants with genes for GSH synthesis, including StGCS-GS or AtGSH1 driven by the leaf-specific promoter of chlorophyll a/b-binding protein (pCab3) gene were generated. Both types of transgenic Arabidopsis plants showed significant increases in shoot GSH concentrations compared to the wild type (WT). Monitoring 65Zn movement by positron-emitting tracer imaging system (PETIS) analysis indicated that the 65Zn amount in the shoots of both types of transgenic Arabidopsis plants were higher than that in the WT. GSH concentration in phloem sap was increased significantly in WT with foliar applications of 10 mM GSH (WT-GSH), but not in transgenic Arabidopsis with elevated foliar GSH synthesis. Both types of transgenic Arabidopsis with elevated foliar GSH synthesis and WT-GSH exhibited increased shoot Zn concentrations and Zn translocation ratios. These results suggest that enhancement of endogenous foliar GSH synthesis and exogenous foliar GSH application affect root-to-shoot transport of Zn.

Arabidopsis thaliana FLO2 is Involved in Efficiency of Photoassimilate Translocation, Which is Associated with Leaf Growth and Aging, Yield of Seeds and Seed Quality.

FLO2, FLOURY ENDOSPERM 2, is highly conserved in higher plants, and rice FLO2 has been predicted to be involved in regulation of accumulation of storage compounds. We analyzed the function of Arabidopsis thaliana FLO2 (AtFLO2) because A. thaliana set structurally different seeds from those of rice. Although the flo2 mutant of A. thaliana showed normal germination, inflorescence and morphogenesis of flowers, peculiar phenotypes on leaves and siliques were observed, suggesting that this gene played important roles during both the vegetative and reproductive stages. The mutant leaves showed a decrease in chloroplast numbers, and increased total biomass with faster growth. When grown in high light intensity conditions, it was observed that aging events were induced. The flo2 mutant showed depressed transportation of photoassimilates into the sink organs. In the reproductive stage, the flo2 mutant had significantly smaller size siliques, causing a reduced yield of seeds. These seeds were structurally weak, and the quality of seeds was significantly lowered, with reduction of accumulation of storage compounds by seeds. A positron-emitting tracer imaging system (PETIS) analysis detected a decreased amount of photoassimilate transport in the flo2 mutant. Therefore, it was presumed that the phenotypes of the flo2 mutant were caused by reduced performance of translocation or transportation of the photoassimilates. Our observation suggests that AtFLO2 is strongly involved in regulation of translocation and transport of assimilates, and contributes greatly to quality control of the various processes involving substance supply or transfer, such as photoassimilation, leaf enlargement, yield of seeds in a silique and accumulation of seed storage compounds.

Kinetic Analysis of Zinc/Cadmium Reciprocal Competitions Suggests a Possible Zn-Insensitive Pathway for Root-to-Shoot Cadmium Translocation in Rice.

BackgroundAmong cereals, rice has a genetic propensity to accumulate high levels of cadmium (Cd) in grains. Xylem-mediated root-to-shoot translocation rather than root uptake has been suggested as the main physiological factor accounting for the genotypic variation observed in Cd accumulation in shoots and grains. Several evidence indicate OsHMA2 – a putative zinc (Zn) transporter – as the main candidate protein that could be involved in mediating Cd- and Zn-xylem loading in rice. However, the specific interactions between Zn and Cd in rice often appear anomalous if compared to those observed in other staple crops, suggesting that root-to-shoot Cd translocation process could be more complex than previously thought. In this study we performed a complete set of competition experiments with Zn and Cd in order to analyze their possible interactions and reciprocal effects at the root-to-shoot translocation level.ResultsThe competition analysis revealed the lack of a full reciprocity when considering the effect of Cd on Zn accumulation, and vice versa, since the accumulation of Zn in the shoots was progressively inhibited by Cd increases, whereas that of Cd was only partially impaired by Zn. Such behaviors were probably dependent on Cd-xylem loading mechanisms, as suggested by: i) the analysis of Zn and Cd content in the xylem sap performed in relation to the concentration of the two metals in the mobile fractions of the roots; ii) the analysis of the systemic movement of 107Cd in short term experiments performed using a positron-emitting tracer imaging system (PETIS).ConclusionsOur results suggest that at least two pathways may mediate root-to-shoot Cd translocation in rice. The former could involve OsHMA2 as Zn2+/Cd2+ xylem loader, whereas the latter appears to involve a Zn-insensitive system that still needs to be identified.Electronic supplementary materialThe online version of this article (doi:10.1186/s12284-016-0088-3) contains supplementary material, which is available to authorized users.

Base to Tip and Long-Distance Transport of Sodium in the Root of Common Reed [Phragmites australis (Cav.) Trin. ex Steud.] at Steady State Under Constant High-Salt Conditions.

We analyzed the directions and rates of translocation of sodium ions (Na(+)) within tissues of a salt-tolerant plant, common reed [Phragmites australis (Cav.) Trin. ex Steud.], and a salt-sensitive plant, rice (Oryza sativa L.), under constant high-salt conditions using radioactive (22)Na tracer and a positron-emitting tracer imaging system (PETIS). First, the test plants were incubated in a nutrient solution containing 50 mM NaCl and a trace level of (22)Na for 24 h (feeding step). Then the original solution was replaced with a fresh solution containing 50 mM NaCl but no (22)Na, in which the test plants remained for >48 h (chase step). Non-invasive dynamic visualization of (22)Na distribution in the test plants was conducted during feeding and chase steps with PETIS. Our results revealed that (22)Na was absorbed in the roots of common reed, but not transported to the upper shoot beyond the shoot base. During the chase step, a basal to distal movement of (22)Na was detected within the root tissue over >5 cm with a velocity of approximately 0.5 cm h(-1). On the other hand, (22)Na that was absorbed in the roots of rice was continuously translocated to and accumulated in the whole shoot. We concluded that the basal roots and the shoot base of common reed have constitutive functions of Na(+) exclusion only in the direction of root tips, even under constant high-salt conditions. This function apparently may contribute to the low Na(+) concentration in the upper shoot and high salt tolerance of common reed.

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.

A kinetic analysis of cadmium accumulation in a Cd hyper‐accumulator fern, Athyrium yokoscense and tobacco plants

Cadmium (Cd) accumulations in a Cd hyper-accumulator fern, Athyrium yokoscense (Ay), and tobacco, Nicotiana tabacum (Nt), were kinetically analysed using the positron-emitting tracer imaging system under two medium conditions (basal and no-nutrient). In Ay, maximumly 50% and 15% of the total Cd accumulated in the distal roots and the shoots under the basal condition, respectively. Interestingly, a portion of the Cd in the distal roots returned to the medium. In comparison with Ay, a little fewer Cd accumulations in the distal roots and clearly higher Cd migration to the shoots were observed in Nt under the basal condition (maximumly 40% and 70% of the total Cd, respectively). The no-nutrient condition down-regulated the Cd migration in both species, although the regulation was highly stricter in Ay than in Nt (almost no migration in Ay and around 20% migration in Nt). In addition, the present work enabled to estimate physical and physiological Cd accumulation capacities in the distal roots, and demonstrated condition-dependent changes especially in Ay. These results clearly suggested occurrences of species-/condition-specific regulations in each observed parts. It is probable that integration of these properties govern the specific Cd tolerance/accumulation in Ay and Nt.

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 (36h) 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 36h, 5.6 (0.056μM) and 29.0% (0.290μM) 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 (3.30cmh(-1)) and 3.7-fold higher in shoots (10.10cmh(-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.

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.

植物分子イメージングの試み(5); 植物個体内の全炭素動態を可視化する

植物分子イメージングの試み(5); 植物個体内の全炭素動態を可視化する

Real-time imaging and analysis of differences in cadmium dynamics in rice cultivars (Oryza sativa) using positron-emitting107Cd tracer

BackgroundRice 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-emitting107Cd 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.ResultsImaging 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 after107Cd 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.ConclusionsThis 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.

Real-time whole-plant imaging of 11C translocation using positron-emitting tracer imaging system

Whole-plant imaging for studying the complete carbon kinetics involved in photosynthesis and subsequent photoassimilate translocation and unloading was achieved using a positron-emitting tracer imaging system (PETIS) in combination with 11CO 2 gas tracer. In an experiment with a soybean ( Glycine max cultivar Jack) plant, it was confirmed that the dynamic PETIS image data obtained followed the conservation law for total carbon. Thus, the proposed PETIS technique is a feasible noninvasive and quantitative solution to study the carbon dynamics over an entire plant in environmental and agricultural studies.

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

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 107Cd (half-life 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.

Rice metal-nicotianamine transporter, OsYSL2, is required for the long-distance transport of iron and manganese

Rice (Oryza sativa) is indispensable in the diet of most of the world's population. Thus, it is an important target in which to alter iron (Fe) uptake and homeostasis, so as to increase Fe accumulation in the grain. We previously isolated OsYSL2, a functional iron [Fe(II)]- and manganese [Mn(II)]-nicotianamine complex transporter that is expressed in phloem cells and developing seeds. We produced RNAi (OsYSL2i) and overexpression lines (OXOsYSL2) of OsYSL2. At the vegetative stage in an OsYSL2i line, the Fe and Mn concentrations were decreased in the shoots, and the Fe concentration was increased in the roots. At the reproductive stage, positron-emitting tracer imaging system analysis revealed that Fe translocation to the shoots and seeds was suppressed in OsYSL2i. The Fe and Mn concentrations were decreased in the seeds of OsYSL2i, especially in the endosperm. Moreover, the Fe concentration in OXOsYSL2 was lower in the seeds and shoots, but higher in the roots, compared with the wild type. Furthermore, when OsYSL2 expression was driven by the sucrose transporter promoter, the Fe concentration in the polished rice was up to 4.4-fold higher compared with the wild type. These results indicate that the altered expression of OsYSL2 changes the localization of Fe, and that OsYSL2 is a critical Fe-nicotianamine transporter important for Fe translocation, especially in the shoots and endosperm.