The physiological basis for variable cadmium accumulation in rice : interaction of environmental and genetic factors.

Abstract

Human exposure to elevated levels of Cd in the environment is known to lead to accumulative toxicity and rice is a key pathway of entry for communities exposed to elevated levels of Cd in Asia. Rice cultivars vary in the degree to which they accumulate Cd in their grain, and the mechanisms responsible for genotypic variation in Cd accumulation are not fully understood. What follows is a study of the physiological mechanisms involved in Cd accumulation in rice and the factors responsible for genotypic differences in Cd accumulation between rice cultivars. This research included a study of the timing of Cd loading into grain over the rice lifecycle, which demonstrated that post-flowering Cd uptake contributed 40% of grain Cd in hydroponically grown rice plants. Grain Cd is therefore not just the product of shoot accumulation of Cd prior to flowering. There was also an attempt to contrast naturally occurring variation in grain Cd accumulation with root expression of genes potentially involved in Cd uptake and translocation. A selection of germplasm from an international rice genebank, representing a large degree of the diversity in Cd accumulation in modern rice varieties, was used for this study. There was not a consistent pattern between expression of these candidate genes and Cd accumulation characteristics, although there were some general trends that received further study, including higher root expression of Fe/Cd transporters, including OsNRAMP1, in the high Cd accumulating indica varieties. Other nutritional factors were examined alongside this work for their role in influencing Cd uptake. Silicon supply decreased the accumulation of Cd in rice, most likely through the physical blocking of transpiration pathways in shoot tissue. Growing rice without Fe led to a large increase in the accumulation of Cd, showing evidence of the link between Cd uptake and Fe nutrition. However, Fe deficiency response, which in other plant species has been shown to have a large positive effect on Cd uptake, was found to have a smaller effect than competition with Fe²⁺ ions. In fact, Fe deficiency response only led to small increases in shoot Cd concentration, with no accompanying increase in root Cd concentration or overall plant Cd uptake. It has previously been postulated that Fe deficiency could play a role in Cd accumulation in field conditions, and this was tested with rice plants grown under variable flooding regimes in potted paddy soil. Changes in redox conditions and Cd availability were contrasted with Fe deficiency response during the growth of the plants. Upregulation of Fe-deficiency-responsive genes was observed in some plants grown in aerobic soils, especially indica varieties, but this was not directly associated with an increase in Cd accumulation. OsNRAMP1 and its effect on Cd translocation were studied further using transgenic rice lines with manipulated expression of this gene. Despite reports to the contrary, there was a lack of evidence for an important role for OsNRAMP1 in the accumulation of Cd by rice plants. Large differences in OsNRAMP1 expression did not correlate well with Cd accumulation. The main observable effect of OsNRAMP1 was increased shoot Fe content in over-expressing plants, but this only occurred with the co-upregulation of other genes during minus Fe conditions. OsNRAMP1 plays a significant role in the pathway of movement of Fe from root to shoot, but this was not demonstrated for Cd at physiologically relevant concentrations. This research is a step towards a better understanding of the physiological and molecular regulation of Cd uptake in rice, and higher plants in general.