A different cadmium accumulation in two rice (Oryza sativa L. ssp. japonica) genotypes may be related to a different root retention

Cadmium (Cd) is a toxic heavy metal and is one of the major environmental pollutants. In plants, Cd2+ is absorbed by the roots from the soil and transported to the shoot, negatively affecting nutrient uptake and homeostasis. It adversely impacts various biochemical and physiological processes, resulting in inhibited root and shoot growth and, ultimately, reduced yield. Moderate Cd contamination of arable soils can result in considerable Cd accumulation in edible parts of crops. Such levels of Cd in plants are not toxic to crops but can contribute to substantial Cd dietary intake by humans. Considering that a substantial contribution of Cd intake in human diet is due to the Cd present in rice grains, the reduction of rice grain Cd accumulation is one promising option for decreasing risks to human health. Physiological studies indicate four major transport processes for Cd accumulation in rice grains: (i) root Cd uptake, (ii) root-to-shoot translocation by xylem flow, (iii) redirection at nodes and (iv) remobilization from leaves. Then, Cd accumulation in rice grain is a multigenic trait, depending on the combination of many genes, proteins and metabolic pathways, and thus difficult to control and engineer. Exploiting natural variation occurring in worldwide rice genotypes may be a powerful approach to discover new traits to exclude Cd from rice grain. In this contest, a ionomic analysis has been previously carried out for two years on a rice worldwide germplasm collection grown under flooded conditions. Among the 297 considered rice cultivars, Beirao and Capataz showed the lowest and the highest Cd accumulation in grain, respectively, when grown under flooded conditions, suggesting a different ability of these two genotypes to exclude Cd from the grain. Further analyses were performed on these genotypes, in order to understand the physiological mechanism behind this difference. Beirao and Capataz plants were grown for 10 or 14 days in hydroponic solutions, then a treatment with different concentrations of Cd (0.01, 0.1, 1 μM) was imposed for 10 days. Cd amount has been measured in shoot, root and xylem sap and the root retention of the two cultivars was calculated. Moreover, the kinetic of Cd uptake of the two cultivars has been compared. Finally, the concentration of non-protein thiols (NPTs) in the roots of Beirao and Capataz was analyzed. The analyses showed a different Cd amount in root and shoot and a different root retention between the two cultivars. Interestingly, when plants were exposed to 1 μM of external Cd concentration, Beirao roots showed a NPT level two times higher than that observed in Capataz roots. These data suggested that the less amount of Cd in Beirao grains may be due, at least in part, to a higher ability of this cultivar to retain at the root level a high part of the adsorbed Cd, thus limiting its translocation to the shoot and then to the grain. Further analyses are in progress to confirm these data; among them, an expression analysis to discover possible differences in the expression of genes involved in root retention mechanisms.