Rice is a major source of inorganic As (iAs), a class one carcinogen, in the human diet. Arsenite can be the dominant form of iAs in flooded soils. It is a silicic acid analogue and is assimilated by roots via silicic acid transport systems. Silicon is accumulated by rice as it plays an important role in the defense against a range of biotic and abiotic stresses.
In addition, several methylated As species, predominantly dimethylarsinic acid (DMA), a weak carcinogen, are present in rice with variable proportions.
Globally, As speciation in rice varies widely. Regional patterns are summarized by Fang-Jie Zhao, Nanjing Agricultural University, China, and colleagues from China and the UK.
Rice produced in Asia shows a strong linear relationship between iAs and total As concentration with a slope of 0.78. A high percentage of iAs is of particular concern and suggests a high risk for food safety from soil and water contamination. Arsenic speciation in USA and European rice is more variable. It shows generally an increasing DMA percentage with increasing total As concentration. Environmental variation has a much stronger influence on As speciation than rice genotypes.
It is assumed that methylated As species in rice are derived from the soil, while rice plants lack the ability to methylate As. Soil flooding and additions of organic matter increase microbial methylation of As. However, the microbial community responsible is poorly understood. Methylated As species are taken up by rice roots less efficiently than iAs. However, thay are transported to the grain much more efficiently. The level of DMA ingestion from rice consumption is much lower than that of concern.
Many questions with regard to As speciation in rice remain unanswered and should be the focus of future research. These include the key microorganisms mediating As methylation in paddy soils, the key environmental factors controlling As methylation and if we can predict the As methylation potential.
- Methylated Arsenic Species in Rice: Geographical Variation, Origin, and Uptake Mechanisms,
Fang-Jie Zhao, Yong-Guan Zhu, Andrew A. Meharg,
Environm. Sci. Technol. 2013.