| Many studies have shown genotypic differences in Cadmium (Cd) accumulation among rice cultivars, and concentrations in shoots and grains are generally higher in indica rice cultivars than in japonica rice cultivars, but the mechanism is still unknown. The main objective of this study was to investigate differences in heavy metal accumulation between rice subspecies through the analysis of 46 indica cultivars and 30 japonica cultivars. At the same time, a real-time polymerase chain reaction analysis were also used to detect the expression levels of Cd transporter genes under the different conditions, and the correlations were also investigated between expression levels and Cd content in rice tissues. In this study, the differentiation of Cd accumulation in rice tissues between indica and japonica rice subspecies at different stages were well characterized, so is the function of the main Cd transporter genes, and all these would contribute to the understanding of the difference between indica and japonica subspecies. The main results were summarized as follow:At the seedling stage, the mean concentrations of Cd in the shoots of indica subspecies were significantly higher than those in japonica subspecies (1.22-fold), but this was not the case in roots; At the filling stage, the mean concentrations of Cd in the shoots and spikes of indica sutspecie,were 1.66-and 2.14-fold higher than the respective concentrations in japonica subspecies; At the harvest stage, the mean concentrations of Cd in the shoots and brown rice of indica subspecies were 1.61- and 2.27-fold higher than the respective concentrations in japonica subspecies. These results indicate that root-to-shoot and shoot-to-grain translocation, rather than Cd absorption in roots, may be the key processes that determine the differences in Cd accumulation between rice subspecies. Gene expression analysis revealed that overall, the expression levels of the Cd transporter gene OsNrampl was notably increased (22.46-fold), but the expression levels of OsHMA2,OsHMA3 and OsNRAMP5 were not significantly changed at the seedling stage in the 76 cultivars exposed to Cd and the expression levels of OsNrampl were positively correlated with Cd concentrations in spikes at the filling stage. In addition, significantly difference was observed in the expression levels of OsNrampl between the indica and japonica subspecies, which may explain the higher Cd concentrations in roots, but lower Cd concentrations in spikes and brown rice for the overall japonica subspecies. Together, these results demonstrate that OsNrampl may be the most important gene among the selected four genes in promote Cd uptake by roots and transfer Cd into spikes, and eventually into brown rice.Using six indica and six japonica cultivars that represent the genetic diversity of Cadmium accumulation aboveground, the growth, heavy metal accumulation and gene expression of related heavy metal transporter genes were studied with a hydroponic experiment under six Cadmium levels. Cadmium stress reduced plant height, shoot dry weight and chlorophyll content in rice leaves which showed inhibition on rice growth. Compared with the control, manganese content in rice roots and shoots of each cultivar under Cadmium treatment decreased significantly, and the iron content of the shoots as well. Significant variation of Cadmium concentration in rice roots and shoots among rice cultivars were found under different Cadmium treatment,and results showed that root-to-shoot translocation may be a key process that determines differences in Cd accumulation between rice cultivars and subspecies, rather than Cd absorption in roots. Under the treatment of Cadmium from 0.50 to 2.0 mg/kg, Cadmium content did not increase notably or even decreased in rice shoots of some cultivars;Besides, the expression of OsNrmap1,OsIRT2 and OsNAS2 were largely induced at such Cadmium treatment, which may correlated with the adjustment of high accumulation of Cd in rice plants. |
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