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

Abstract

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.

Nitrate facilitates cadmium uptake, transport and accumulation in the hyperaccumulator Sedum plumbizincicola

“…2b). A similar phenomenon was observed in previous studies of Cd uptake dynamics in rice (Fujimaki et al 2010;Ishikawa et al 2011). This may be explained in terms of the complicated combination of both apoplastic binding and symplastic uptake of Cd by the roots.…”

“…This is the first reported study to visualise Cd movement non-invasively in an intact Cd hyperaccumulator, S. plumbizincicola, using 107 Cd tracer and improved PETIS which enabled direct and simultaneous observation of the radiotracer-treated culture solution and plants (Fujimaki et al 2010;Ishikawa et al 2011). The imaging data were applied for quantitative analysis of the dynamics and kinetics of Cd uptake and transport in the whole plant supplied with two different inorganic N sources, NO 3 -and NH 4 + .…”

“…The PETIS imaging experiments were conducted following the methods of Fujimaki et al (2010) and Ishikawa et al (2011) with some modifications.…”

“…Recently, a realtime imaging system for Cd using positron-emitting 107 Cd tracer and PETIS was established (Fujimaki et al 2010). Ishikawa et al (2011) improved the design of the root box and realised direct observation of Cd uptake by the roots in solution culture, and characterised clearly the differences in Cd dynamics from the culture solution to the grains between high-and low-Cd accumulating rice cultivars. So far, there has been no application of PETIS to investigate Cd uptake by any hyperaccumulator species.…”

Quantification of zinc transport via the phloem to the grain in rice plants (Oryza sativaL.) at early grain-filling by a combination of mathematical modeling and65Zn tracing

“…The ratios of radioactivity in the glumes and grains at 4 and 6 h when 65 Zn was fed through the culm cut above the flag-leaf node were about one-fifth those found at 3 and 4.5 h when 109 Cd was fed (Table 4), which indicated that more selective transport to the grains might occur in 65 Zn transport. All the nodes, especially the flagleaf nodes, accumulated intensively radioactive Cd when 107 Cd was administered to the root-bathing medium (Ishikawa et al 2011). The nodes may be the sites of xylem-to-phloem transport leading the radioactive Cd to the panicles.…”