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

Abstract

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.

Cadmium uptake and translocation in durum wheat varieties differing in grain-Cd accumulation

“…long culms and high spikelet number, could have reduced Cd partitioning to grain. Indeed, it was found that Cd translocation within the xylem is slowed down by binding to cell walls and by accumulation in the nodes, so that tall varieties with long spikes are less prone to accumulate Cd in grain (Kubo et al 2008, Fujimaki et al 2010, Quinn et al 2011.…”

Development of real-time radioisotope imaging systems for plant nutrient uptake studies

“…Positron emitters, which are widely used in the medical imaging technique of positron emission tomography, have been applied to plant research [3,4]. A positron emitted from the sample couples with an electron in the tissue and converts it into two identical g-rays, a phenomenon called annihilation.…”

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

“…This system enables monitoring of the real-time movement of the tracer in living plants as a video, and also quantitative analysis of the movement of substances by freely selecting a region of interest (ROI) on the image data obtained. 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.…”

“…However, only the static distribution of Cd at a given moment can be obtained by autoradiography. In recent years, the positron-emitting tracer imaging system (PETIS) has been employed to study the uptake and translocation of metals using the positronemitting tracers 52 Mn (Tsukamoto et al 2006), 62 Zn (Suzuki et al 2008), 52 Fe (Tsukamoto et al 2009) and 107 Cd (Fujimaki et al 2010) in intact living plants. This system enables monitoring of the real-time movement of the tracer in living plants as a video, and also quantitative analysis of the movement of substances by freely selecting a region of interest (ROI) on the image data obtained.…”