Characterization of cadmium uptake and translocation in a cadmium-sensitive mutant of rice (Oryza sativa L. ssp. japonica).

To understand the physiological mechanism that confers Cd sensitivity/tolerance, Cd uptake and translocation by Cd-sensitive mutant and wild-type rice seedlings were characterized using radioactive tracer. Time-dependent Cd accumulation during 210 min was 1.8-fold higher in roots of the mutant than of wild-type rice. The kinetics of Cd uptake showed that two processes were involved: a first, fast, probably passive process and a second, slower, possibly physiological process. After 96 h, more Cd was translocated to the shoot of mutant compared with wild-type rice. Low temperature (2 degrees C), carbonyl cyanide m-chlorophenylhydrazone, and dicyclohexylcarbodiimide inhibited Cd accumulation to a greater extent in mutant than wild-type rice, suggesting that metabolism plays a more important role in Cd uptake for the rice mutant. Cd uptake into the symplasm is linked to Ca2+ transport, as revealed by the inhibition of Cd accumulation by La and high Ca by equal percentages in both mutant and wild-type rice. Cd uptake and root-to-shoot translocation in the mutant were inhibited more markedly by Zn and Mn than in wild-type rice. This indicates that transport sites other than entry into the root symplasm are also stimulated in the mutant. The results suggest that enhanced absorption into the root and further accumulation in the shoot is one of the mechanisms involved in Cd accumulation and sensitivity, and the difference in Cd uptake and root-to-shoot translocation between mutant and wild-type rice may be mediated partly by energy supply and transporters for Zn and Mn.