Cadmium fate and tolerance in rice cultivars

Cadmium (Cd) is present in all soils, usually as a trace constituent, but it can reach higher levels in agricultural soils. Cd can then be absorbed by plants and become a potential risk to human health. Once taken up by a plant, there are mechanisms for heavy metal detoxification in the plant. Here, a cadmium-tolerant and a cadmium-sensitive rice cultivars were grown hydroponically to investigate the effects of cadmium (Cd) applied at low levels on uptake and transport, subcellular distribution and binding forms of Cd in rice plants. Our results showed that increasing the Cd treatment from 1.0 M to 5.0 M Cd increased the shoot Cd content by 55% in the cadmium-tolerant cultivar, and by 108% in the cadmium-sensitive cultivar. For the cadmium-tolerant cultivar, increasing Cd treatment from 1.0 micromolar to 5.0 micromolar increased the root Cd content by 116%, whereas for the cadmium-sensitive cultivar, increasing Cd treatment from 1.0 micromolar to 5.0 micromolar increased the root Cd content by 80%. Further, the ratio of Cd accumulation in shoots over roots decreased from 0.19 to 0.14 in the cadmium-tolerant cultivar, while it increased from 0.20 to 0.26 in the cadmium-sensitive cultivar, showing that the transportation ability for Cd was different between the two tested rice cultivars. At the higher Cd level of 5.0 micromolar, most of the Cd in the plants was localized in cell walls and vacuoles in both cultivars, whereas small portions of Cd were distributed in the cytoplasm, suggesting that the important metabolic and physiological processes were not impaired under Cd stress. Furthermore, the major portions of Cd in the cells were combined with organic acids, proteins and polysaccharide, and were consequently detoxified. The difference in the distribution of cadmium in rice plants resulted in the difference in Cd tolerance between the two rice cultivars used. It can be concluded that the retention of Cd in root cell walls, compartmentation of Cd into vacuoles and the suppressed transportation of Cd from roots to shoots are the most important mechanisms involved in the detoxification of Cd in rice plants.