JAEA-Review-2010-065.pdf:15.99MB - 日本原子力研究開発機構

3-49
Noninvasive Imaging of Zinc Dynamics in an Intact
Plant Using the Positron-emitting Tracer 65 Zn
N. Suzui a) , H. Yamazaki a),b) , N. Kawachi a) , S. Ishii a) , N. S. Ishioka a) and S. Fujimaki a)
a) Radiation-Applied Biology Division, QuBS, JAEA,
b) Faculty of Science and Technology, Tokyo University of Science
Introduction
Zinc is an essential element for all living organisms
including higher plants. Zinc deficiency of crops is one of
the most serious problems in food production in the world.
Therefore, it is important to understand how plants regulate
zinc uptake. In attempt to elucidate the mechanism of zinc
uptake, noninvasive imaging of zinc dynamics has been a
powerful tool. In the past decade, we have employed a
positron-emitting tracer imaging system (PETIS), which
provides serial time-course images of the two-dimensional
distribution of a radioisotope (e.g. 11 C, 13 N, 52 Fe, 64 Cu,
107 Cd) in an intact plant without contact 1, 2) . Noninvasive
imaging of zinc by PETIS has been conducted using 62 Zn
(half-life: 9.2 hours), which was produced with an energetic
proton beam delivered from AVF cyclotron at TIARA
(Takasaki Ion Accelerators for Advanced Radiation
Application) 3, 4) . Although 62 Zn is a weak positron emitter
(7%), its daughter 62 Cu (half-life: 9.7 min) decays with
97.8% by positron emission, and therefore it was thought
that the image of 62 Zn could be obtained by PETIS with high
efficiency. However, because it is possible that 62 Cu
migrates differently from 62 Zn in a plant body, there has
been the argument that the positron imaging using 62 Zn
correctly reflects zinc dynamics in plants.
On the other hand, one of other zinc radioisotopes, 65 Zn
(half-life: 244 days) is commercially available and
frequently used as a zinc tracer in plants. 65 Zn decays with
98.6% by electron capture and 1.4% by positron emission to
stable 65 Cu. Because of its weak positron emission, 65 Zn
was thought to be unsuitable for positron imaging, but there
JAEA-Review 2010-065
has been no verification of the possibility. Thus, in this
study, we examined whether positron imaging of zinc is
possible using 65 Zn and PETIS.
Results and Discussion
First, a filter paper (5 mm in diameter) containing 60 kBq
65 Zn (RIKEN) was monitored by PETIS, and obvious image
of 65 Zn was obtained with counting efficiency of 6.1 ×
10 -4 cps/Bq. Then, the tracer solution containing 400 kBq
65 Zn was fed to a rice plant (Oryza sativa L.) and the
dynamics of 65 Zn in plant was monitored by PETIS. As a
result, serial images of 65 Zn distribution were successfully
obtained every 5 minutes for 48 hours (Fig. 1). In addition,
the uptake kinetics (Km/Vmax) and the translocation
velocity of zinc in plant were determined from the image
data.
These results indicate that 65 Zn is a suitable radioisotope
for noninvasive imaging by PETIS. By taking advantage
of the long half-life, 65 Zn translocation can be visualized all
through the life of plants. Furthermore, the commercial
availability of 65 Zn makes it possible to conduct noninvasive
imaging of zinc in facilities without cyclotron, accelerating
the research of zinc dynamics in plants.
References
1) S. Fujimaki, ITE Let. 8 (2007) 404-413.
2) S. Fujimaki et al., Plant Physiol. 152 (2010) 1796-1806.
3) S. Watanabe et al., Radiochim. Acta 89 (2001) 853-858.
4) M. Suzuki et al., Plant Mol. Biol. 66 (2008) 609-617.
Fig. 1 Photograph of the tested rice plants (left) and the integrated PETIS image of 65 Zn in the same area (right).
Each image is integration of 36 original frames, corresponding to 3 hours.
- 105 -