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.

The phloem macromolecular transport system plays a pivotal role in plant growth and development. However, little information is available regarding whether the long-distance trafficking of macromolecules is a controlled process or passive movement. Here, we demonstrate the destination-selective long-distance trafficking of phloem proteins. Direct introduction, into rice (Oryza sativa), of phloem proteins from pumpkin (Cucurbita maxima) was used to screen for the capacity of specific proteins to move long distance in rice sieve tubes. In our system, shoot-ward translocation appeared to be passively carried by bulk flow. By contrast, root-ward movement of the phloem RNA binding proteins 16-kD C. maxima phloem protein 1 (CmPP16-1) and CmPP16-2 was selectively controlled. When CmPP16 proteins were purified, the rootward movement of CmPP16-1 became inefficient, suggesting the presence of pumpkin phloem factors that are responsible for determining protein destination. Gel-filtration chromatography and immunoprecipitation showed that CmPP16-1 formed a complex with other phloem sap proteins. These interacting proteins positively regulated the root-ward movement of CmPP16-1. The same proteins interacted with CmPP16-2 as well and did not positively regulate its root-ward movement. Our data demonstrate that, in addition to passive bulk flow transport, a destination-selective process is involved in longdistance movement control, and the selective movement is regulated by protein-protein interaction in the phloem sap.

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.

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.