Evaluation of in vivo detection properties of 22Na, 65Zn, 86Rb, 109Cd and 137Cs in plant tissues using real-time radioisotope imaging system

From the Outside in: An Overview of Positron Imaging of Plant and Soil Processes

Positron-emitting nuclides have long been used as imaging agents in medical science to spatially trace processes non-invasively, allowing for real-time molecular imaging using low tracer concentrations. This ability to non-destructively visualize processes in real time also makes positron imaging uniquely suitable for probing various processes in plants and porous environmental media, such as soils and sediments. Here, we provide an overview of historical and current applications of positron imaging in environmental research. We highlight plant physiological research, where positron imaging has been used extensively to image dynamics of macronutrients, signalling molecules, trace elements, and contaminant metals under various conditions and perturbations. We describe how positron imaging is used in porous soils and sediments to visualize transport, flow, and microbial metabolic processes. We also address the interface between positron imaging and other imaging approaches, and present accompanying chemical analysis of labelled compounds for reviewed topics, highlighting the bridge between positron imaging and complementary techniques across scales. Finally, we discuss possible future applications of positron imaging and its potential as a nexus of interdisciplinary biogeochemical research.

Recent Advances in Radioisotope Imaging Technology for Plant Science Research in Japan

Soil provides most of the essential elements required for the growth of plants. These elements are absorbed by the roots and then transported to the leaves via the xylem. Photoassimilates and other nutrients are translocated from the leaves to the maturing organs via the phloem. Non-essential elements are also transported via the same route. Therefore, an accurate understanding of the movement of these elements across the plant body is of paramount importance in plant science research. Radioisotope imaging is often utilized to understand element kinetics in the plant body. Live plant imaging is one of the recent advancements in this field. In this article, we recapitulate the developments in radioisotope imaging technology for plant science research in Japanese research groups. This collation provides useful insights into the application of radioisotope imaging technology in wide domains including plant science.

What you can see by developing real-time radioisotope imaging system for plants: from water to element and CO2 gas imaging

Since plants live on inorganic elements, absorbing ions from roots and transferring them to each tissue in a plant is an essential activity. However, little is known about the movement of the elements or water in plant tissue. Though fluorescent imaging is now overwhelmingly used at the microscopic level in biology, especially to visualize chemicals or organelles in a cell, radioisotope imaging has become one of the important methods for human imaging in the medical field. In the case of plant studies, however, real-time radioisotope imaging is little-known among plant researchers. The author has developed radioisotope imaging systems using various radioisotopes to study living plant activity, both for elements and for water. Here we review the real-time radioisotope imaging methods we developed, and show new aspects of plant physiology discovered by live imaging.

Visualization of 14CO2 gas fixation by plants

Using the real-time radioisotope imaging system (RRIS), we present the carbon dioxide gas fixation process of a soybean plant applying the 14C-labeled gas. When 14CO2 gas was supplied to the selected mature leaf, the fixed carbon, photosynthate, was transferred and accumulated to the younger leaves preferentially within 24 h. When 14CO2 gas was supplied to the younger leaves, fixed carbon was hardly moved. In the case of the pods, fixed 14CO2 gas in the leaf was preferentially transferred to the closest pod.

Research with radiation and radioisotopes to better understand plant physiology and agricultural consequences of radioactive contamination from the Fukushima Daiichi nuclear accident

Research carried out by me and my group over the last almost four decades are summarized here. The main emphasis of my work was and continues to be on plant physiology using radiation and radioisotopes. Plants live on water and inorganic elements. In the case of water, we developed neutron imaging methods and produced 15O-labeled water (half-life 2 min) and applied them to understand water circulation pattern in the plant. In the case of elements, we developed neutron activation analysis methods to analyze a large number of plant tissues to follow element specific distribution. Then, we developed real-time imaging system using conventional radioisotopes for the macroscopic and microscopic observation of element movement. After the accident in Fukushima Daiichi nuclear power plant, we, the academic staff of Graduate School, have been studying agricultural effects of radioactive fallout; the main results are summarized in two books published by Springer.

Visualization of Uptake of Mineral Elements and the Dynamics of Photosynthates in Arabidopsis by a Newly Developed Real-Time Radioisotope Imaging System (RRIS)

Minerals and photosynthates are essential for many plant processes, but their imaging in live plants is difficult. We have developed a method for their live imaging in Arabidopsis using a real-time radioisotope imaging system. When each radioisotope,(22)Na,(28)Mg,(32)P-phosphate,(35)S-sulfate,(42)K,(45)Ca,(54)Mn and(137)Cs, was employed as an ion tracer, ion movement from root to shoot over 24 h was clearly observed. The movements of(22)Na,(42)K,(32)P,(35)S and(137)Cs were fast so that they spread to the tip of stems. In contrast, high accumulation of(28)Mg,(45)Ca and(54)Mn was found in the basal part of the main stem. Based on this time-course analysis, the velocity of ion movement in the main stem was calculated, and found to be fastest for S and K among the ions we tested in this study. Furthermore, application of a heat-girdling treatment allowed determination of individual ion movement via xylem flow alone, excluding phloem flow, within the main stem of 43-day-old Arabidopsis inflorescences. We also successfully developed a new system for visualizing photosynthates using labeled carbon dioxide,(14)CO2 Using this system, the switching of source/sink organs and phloem flow direction could be monitored in parts of whole shoots and over time. In roots,(14)C photosynthates accumulated intensively in the growing root tip area, 200-800 µm behind the meristem. These results show that this real-time radioisotope imaging system allows visualization of many nuclides over a long time-course and thus constitutes a powerful tool for the analysis of various physiological phenomena.

Tracer experiment using 42K+ and 137Cs+ revealed the different transport rates of potassium and caesium within rice roots

The differences in the transport characteristics in planta between potassium (K+) and caesium (Cs+) was investigated using their radionuclides, 42K+ and 137Cs+. A tracer experiment using nutrient solutions supplemented with 42K and 137Cs revealed that the ratio of the root's K+ uptake rate to its Cs+ uptake rate was 7-11 times higher than the K+:Cs+ concentration ratio in the solution, and the number was varied depending on the K concentration in the solution and also on the growth condition. After entering through the root tissues, the 42K+:137Cs+ ratio in the shoots was 4.28 times higher than the value in the roots. However, the 42K+:137Cs+ ratio in each leaf did not differ significantly, indicating that the primary transport of K+ and Cs+ in the shoots are similarly regulated. In contrast, among the radionuclides stored in the roots over 4h, 30% of the 42K+ was exported from the roots over the following hour, whereas only 8% of 137Cs+ was exported. In addition, within the xylem, K+ was shown to travel slowly, whereas Cs+ passed quickly through the roots into the shoots. In conclusion, our study demonstrated very different transport patterns for the two ions in the root tissues.

Imaging of radiocesium uptake dynamics in a plant body by using a newly developed high-resolution gamma camera

We developed a new gamma camera specifically for plant nutritional research and successfully performed live imaging of the uptake and partitioning of (137)Cs in intact plants. The gamma camera was specially designed for high-energy gamma photons from (137)Cs (662 keV). To obtain reliable images, a pinhole collimator made of tungsten heavy alloy was used to reduce penetration and scattering of gamma photons. A single-crystal scintillator, Ce-doped Gd3Al2Ga3O12, with high sensitivity, no natural radioactivity, and no hygroscopicity was used. The array block of the scintillator was coupled to a high-quantum efficiency position sensitive photomultiplier tube to obtain accurate images. The completed gamma camera had a sensitivity of 0.83 count s(-1) MBq(-1) for (137)Cs with an energy window from 600 keV to 730 keV, and a spatial resolution of 23.5 mm. We used this gamma camera to study soybean plants that were hydroponically grown and fed with 2.0 MBq of (137)Cs for 6 days to visualize and investigate the transport dynamics in aerial plant parts. (137)Cs gradually appeared in the shoot several hours after feeding, and then accumulated preferentially and intensively in growing pods and seeds; very little accumulation was observed in mature leaves. Our results also suggested that this gamma-camera method may serve as a practical analyzing tool for breeding crops and improving cultivation techniques resulting in low accumulation of radiocesium into the consumable parts of plants.