A microautoradiographic method for fresh-frozen sections to reveal the distribution of radionuclides at the cellular level in plants

Cadmium accumulation dynamics in the rice endosperm during grain filling revealed by autoradiography

Cadmium (Cd) is one of the environmental pollutants contaminated in our food. Several previous reports showed that rice polishing cannot be efficient to reduce Cd content in white rice, implying the characteristic Cd distribution in rice grain. However, Cd distribution has not been fully elucidated so far. Herein, 109Cd radiotracer experiment was performed using the rice seedlings at various time points after flowering to obtain autoradiographs of the brown rice to visually understand the Cd transport and distribution during the grain-filling process. It was shown that 109Cd accumulated in the outermost area of the brown rice, and also in the middle part of the starchy endosperm, resulting in the appearance of the double circle distribution pattern, which was not observed in the autoradiographs of 65Zn. The inner circle of 109Cd located around the center of the endosperm was developed particularly at around 8 and 10 days after flowering. After this period, 109Cd started to deposit at the outer part of the endosperm, which was also found in the autoradiograph of 14C-sucrose. Considering the physiology of grain development, the contribution of water transport and protein synthesis in the endosperm on the characteristic Cd distribution pattern was hypothesized.

Visualization of phosphorus re-translocation and phosphate transporter expression profiles in a shortened annual cycle system of poplar

Phosphorus (P) is an essential macronutrient for plant growth. In deciduous trees, P is remobilized from senescing leaves and stored in perennial tissues during winter for further growth. Annual internal recycling and accumulation of P are considered an important strategy to support the vigorous growth of trees. However, the pathways of seasonal re-translocation of P and the molecular mechanisms of this transport have not been clarified. Here we show the seasonal P re-translocation route visualized using real-time radioisotope imaging and the macro- and micro-autoradiography. We analysed the seasonal re-translocation P in poplar (Populus alba. L) cultivated under 'a shortened annual cycle system', which mimicked seasonal phenology in a laboratory. From growing to senescing season, sink tissues of ³² P and/or ³³ P shifted from young leaves and the apex to the lower stem and roots. The radioisotope P re-translocated from a leaf was stored in phloem and xylem parenchyma cells and redistributed to new shoots after dormancy. Seasonal expression profile of phosphate transporters (PHT1, PHT5 and PHO1 family) was obtained in the same system. Our results reveal the seasonal P re-translocation routes at the organ and tissue levels and provide a foothold for elucidating its molecular mechanisms.

Regulatory guidelines and preclinical tools to study the biodistribution of RNA therapeutics

The success of the messenger RNA-based COVID-19 vaccines of Moderna and Pfizer/BioNTech marks the beginning of a new chapter in modern medicine. However, the rapid rise of mRNA therapeutics has resulted in a regulatory framework that is somewhat lagging. The current guidelines either do not apply, do not mention RNA therapeutics, or do not have widely accepted definitions. This review describes the guidelines for preclinical biodistribution studies of mRNA/siRNA therapeutics and highlights the relevant differences for mRNA vaccines. We also discuss the role of in vivo RNA imaging techniques and other assays to fulfill and/or complement the regulatory requirements. Specifically, quantitative whole-body autoradiography, microautoradiography, mass spectrometry-based assays, hybridization techniques (FISH, bDNA), PCR-based methods, in vivo fluorescence imaging, and in vivo bioluminescence imaging, are discussed. We conclude that this new and rapidly evolving class of medicines demands a multi-layered approach to fully understand its biodistribution and in vivo characteristics.

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.

Seeing is better than believing: visualization of membrane transport in plants

Recently, the plant transport field has shifted their research focus toward a more integrative investigation of transport networks thought to provide the basis for long-range transport routes. Substantial progress was provided by of a series of elegant techniques that allow for a visualization or prediction of substrate movements in plant tissues in contrast to established quantitative methods offering low spatial resolution. These methods are critically evaluated in respect to their spatio-temporal resolution, invasiveness, dynamics and overall quality. Current limitations of transport route predictions-based on transporter locations and transport modeling are addressed. Finally, the potential of new tools that have not yet been fully implemented into plant research is indicated.

Agricultural aspects of radiocontamination induced by the Fukushima nuclear accident - A survey of studies by the Univ. of Tokyo Agricultural Dept. (2011-2016)

Immediately after the Fukushima nuclear power plant accident, a team of 40-50 researchers at the Graduate School of Agricultural and Life Sciences at the University of Tokyo began to analyze the behavior of radioactive materials in the fallout regions. The fallout has remained in situ and become strongly adsorbed within the soil over time. 137Cs was found to bind strongly to the fine clay, weathered biotite, and organic matter in the soil; therefore, it has not mobilized from mountainous regions, even after heavy rainfall. In farmland, the quantity of 137Cs in the soil absorbed by crop plants was small. The downward migration of 137Cs in soil is now estimated at 1-2 mm/year. The intake of 137Cs by trees occurred through the bark and not from the roots. This report summarizes the findings of research across a wide variety of agricultural specialties.

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.