Investigating the Role of Dimethyl-Arsenic in Inducing Straighthead Disease in Rice

Abstract

Arsenic contamination of food is a global concern, both from human health and agronomic perspectives. Compared to other cereals, rice can accumulate arsenic to 10-fold higher levels. Rice is inherently efficient at accumulating arsenic as a result of it being grown under anaerobic conditions which promotes mobilisation and uptake of inorganic arsenic. The accumulation of arsenic in the grain of the rice plant can pose a significant risk to human health. Inorganic arsenic (As(i)) is a class one, non-threshold carcinogenic, while organic arsenic (DMA and MA) species are relatively nontoxic to humans. This results in organic arsenic species being overlooked when evaluating risks for human health and agricultural practices. The objectives of the study were to determine how rice plants respond to different arsenic species and how the different arsenic species affected the plants' health. The study specifically aimed to determine the role arsenic plays in inducing straighthead disease.

In this project, rice was grown both hydroponically and in the field. For the hydroponic experiments, rice plants were grown under controlled conditions exposing the plants to individual arsenic species of either As(i) or DMA. The results from these hydroponic experiments showed that rice plants exhibit an ability to control the distribution of As(i) within the plant. Rice plants exposed to DMA, however, displayed no ability to control the distribution of DMA once taken up. Through exposing rice to DMA hydroponically, the plants with the most significant reduction in growth displayed higher DMA concentrations.

For the field trials, a range of rice varieties were cultivated utilising different agronomic practices (e.g. straw incorporation and nitrogen fertilisation) to change the bioavailability on arsenic in the soil. The incorporation of straw into the soil resulted in the formation of DMA and an increase of arsenic in the grain as DMA. DMA accumulated linearly in the grain to become the dominant species whereas, As(i) plateaus at 0.2 ug/g. DMA uptake also increased the severity of straighthead disease. The finding reinforced the finding from the hydroponic experiment that rice plants lack the ability to control the accumulation and distribution of DMA within the plant. This lack of ability to control DMA accumulation poses a threat to the health of the rice plants. The plants with straighthead disease in the field recorded higher DMA concentrations in the husks and leaves of the plants when compared to healthy plants. This demonstrated that DMA can cause stress to the plant. Favourable conditions, including straw incorporation, increase DMA formation and bioavailability can result in elevated DMA concentrations in the grain and the expression of straighthead disease.

This study has highlighted that DMA could pose a threat to crop productivity and potentially human health. Rice appears to have some capacity to control As(i) concentrations in the grain, whereas DMA can accumulate unregulated.