To elucidate the physiological and genetic mechanisms underlying arsenic (As) absorption and metabolism in rice, we used a set of diverse rice germplasms, called As World Rice Collection (WRC). These cultivars consist of 69 accessions which cover the genetic diversity of almost 32,000 accessions of cultivated rice. The collection was cultivated in a paddy field with naturally abundant level of As. Approximately three times difference at the maximum in grain As concentration was found in WRC. The dominant As species in grains was arsenite, followed by dimethyarsinic acid (DMA) and arsenate, irrespective of cultivars. However, the proportions of As species were different among rice cultivars. These results suggest the possibility that As absorption and metabolim in rice could be genetically controlled.

Cadmium (Cd) concentration in eggplant (Solanum melongena) fruits can be drastically reduced by grafting them with Solanum torvum rootstock. However, mechanisms of Cd uptake in roots and translocation from roots to shoots in both plants are not understood well. We thus characterized symplastic Cd uptake in roots and translocation from roots to shoots via xylem loading process of both plants. Additionally, we also examined effect of other elements in roots uptake and translocation process with the increase of Cd concentration in the medium. Cd concentration in roots of S. torvum was almost similar to that of S. melongena for 7 days. On the other hand, Cd concentrations in the shoots and xylem sap were higher in S. melongena than in S. torvum. A concentration dependent experiment in roots indicated that Km values were almost the same for species, but the Vmax values were 1.5-fold higher in S. melongena than in S. torvum. A concentration dependent experiment in xylem loading process indicated that Km values were approximately 7-fold higher in S. torvum than in S. melongena. These results suggest that xylem loading process is a critical factor for determining Cd accumulation in the shoots of both plants. Several elements concentration in roots and xylem sap of both plants decreased with the increase of Cd concentration in the medium, suggesting that Cd uptake and translocation process are partly mediated by similar transporters involving these elements transport.

We analyzed the real-time translocation of 107Cd from root to shoot among 6 rice cultivars using a positron-emitting tracer imaging system (PETIS). 107Cd first accumulated at the basal region of the shoot within 2 h after the exposure and the signals were much stronger in high-Cd-accumulating cultivars (indica) than in low-Cd-accumulating ones (japonica). 107Cd accumulated increasingly at the upper portion of the shoot with time, and the signals were obviously strong in high-Cd-accumulating cultivars. These results suggest that the different ability of root-to-shoot translocation of Cd is closely related to the genotypic variation in shoot Cd accumulation in rice.