CADMIUM EXCLUSION FROM RICE GRAINS:DEVELOPMENT OF MOLECULAR AND PHYSIOLOGICAL MARKERS

Among trace elements not essential for plant growth and metabolism, Cd is of particular concern as it may exert phytotoxic effects and have direct consequences on human health by accumulating in staple food crops which make up a large proportion of dietary intake. Cd is generally present in the soil medium either naturally and/or from anthropogenic sources. Concerning agricultural activities, the application of sewage sludge and phosphate fertilizers containing Cd as an impurity, as well as the use of Cd containing irrigation water, are of particular relevance. Compared to other heavy metals, Cd constitutes a big issue in terms of food safety as it tends to be more mobile and thus more available to be translocated to the edible portion of the plant, causing acute or chronic toxicity to humans even at low soil concentrations. The well established tendency of rice (Oryza sativa L.) to accumulate Cd to levels often exceeding the international limits for the cereal grain trade highlights the need to apply sound strategies aimed at reducing the risk of grain Cd accumulation. Compared to the sole use of agronomic techniques, the selection of rice cultivars that accumulate low Cd in the grains by taking advantage from the broad variability in the Cd accumulation trait observed in Indica and Japonica cultivars is far more promising. Therefore, the general purpose of this study was to deepen the knowledge of the physiological basis governing Cd distribution in rice, with particular concern on Cd root retention and Cd translocation, as they have been seen to be crucial in determining Cd accumulation. Specifically, the role of phytochelatins (PCs) in chelation and subcellular compartimentalization of Cd in the roots was investigated, both by characterizing Cd-PCs complexes with respect of the external Cd concentration and examining the molecular basis of their synthesis. As Cd chelation by PCs has seen to be a crucial but not the only determinant in limiting the amount of Cd potentially available to be translocated to the shoots, the focus moved on the identification of the genes encoding transporters putatively involved in Cd xylem loading. Particularly we looked at two transporters, OsHMA2 and OsHMA4, belonging to the P1B-type ATPase subfamily, acknowledging the major role of such class of transporters in Cd translocation. While characterizing these transporters both by molecular and physiological analysis, the occurrence of clear competition effects of Cd over Zn at the translocation level emerged. Such an outcome highlighted that Cd movement determining its allocation through the plant is not strictly associated to Zn, which is likely to result from the existence of Cd transport pathways that are Zn-independent. These results, obtained by exposing rice plants to relatively low Cd concentrations aiming at simulating the real conditions in moderately contaminated soils, contributed to advance the understanding of the complex network of processes governing Cd accumulation in rice grains which, despite the economical and agricultural relevance of this crop, is still lacking. With regard to this, our study could be intended as a step further towards the development of molecular and/or physiological markers to early select rice genotypes able to exclude Cd from the grains with the intent of ensuring the food safety of the consumers.