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.