A review of recent developments in the speciation and location of arsenic and selenium in rice grain
Rice is a staple food yet is a significant dietary source of inorganic arsenic, a class 1, nonthreshold carcinogen. Establishing the location and speciation of arsenic within the edible rice grain is essential for understanding the risk and for developing effective strategies to reduce grain arsenic concentrations. Conversely, selenium is an essential micronutrient and up to 1 billion people worldwide are selenium-deficient. Several studies have suggested that selenium supplementation can reduce the risk of some cancers, generating substantial interest in biofortifying rice. Knowledge of selenium location and speciation is important, because the anti-cancer effects of selenium depend on its speciation. Germanic acid is an arsenite/silicic acid analogue, and location of germanium may help elucidate the mechanisms of arsenite transport into grain. This review summarises recent discoveries in the location and speciation of arsenic, germanium, and selenium in rice grain using state-of-the-art mass spectrometry and synchrotron techniques, and illustrates both the importance of high-sensitivity and high-resolution techniques and the advantages of combining techniques in an integrated quantitative and spatial approach.
KeywordsArsenic Selenium Germanium Rice grain Speciation Location
This work was supported by a Biotechnology and Biological Sciences Research Council Doctoral Training Grant. Part of this research was undertaken on the X-ray fluorescence microscopy beamline at the Australian synchrotron, Victoria, Australia. The authors thank Drs David Paterson and Daryl Howard for their help in collecting the germanium microtomography images.
Glossary of terms
The aleurone cells house hormones and enzymes instrumental in the germination and development of the grain and are rich in phytic acid. Phytic acid, or phytate, a salt of phytic acid, is the main grain store of phosphorus and is known to chelate cations including those of many micronutrients, for example iron, zinc, and calcium. The aleurone is understood to specifically accumulate phytate and metals for germination and growth of the rice seed.
Anthesis refers to the period of flowering. The point of anthesis is the point at which the anthers have emerged, demonstrating that the flower has reached the reproductive stage.
The bran comprises the pericarp, testa, and aleurone cells and is rich in micro and macronutrients, B vitamins, protein, and fibre.
The caryopsis is the whole grain (not including the husk/hull).
The embryo, or germ, is the point at which the new plant develops and contains abundant proteins, lipids, and vitamins. In the processing of rice grain for consumption, the bran and embryo are removed together, leaving the sub-aleurone and endosperm to make up the polished (white) rice that is commonly consumed.
The inner starchy part of a grain which serves as food reserves for the seed’s development. In contrast with the embryo and bran layer, the endosperm contains relatively less protein, lipids, and nutrients and is mainly composed of starch and non-starch polysaccharides.
The husk, or hull, is the outermost layer of a rice grain, a silica-rich envelope that protects against predation and disease.
The node is the section of stem between two nodes.
Nodes are joints along the plant stem at which leaves and buds develop. They are also the points at which vascular contents are transferred to leaf and/or panicle.
The nucellar tissue is the tissue that initially surrounds the embryo. As the starchy endosperm develops and expands, the nucellar tissue becomes stretched, and is eventually crushed in the mature grain.
The panicle is the flower head of the rice plant. Technically, a panicle is an inflorescence in which each flower is on its own stalk.
The peduncle is the panicle stem, i.e. the stem supporting the inflorescence (group of flowers).
Pericarp is the ripened ovary wall. In the rice grain this thin, fibrous layer offers protection against moulds and oxidation.
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