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Rice: more silica in soil reduces arsenic uptake

Two proteins in rice are responsible for transporting arsenite from soil

Scientists in Japan have cracked the reason why rice is particularly efficient in assimilating arsenic from paddy soils. The findings reported in the latest issue of the Proceedings of the National Academy of Sciences journal have particular relevance to West Bengal and Bangladesh where arsenic poisoning is seen.

Arsenic is a carcinogen and is present in the ground water. It is also taken up by rice plants grown in paddy fields irrigated by the ground water. Apart from explaining the reason for rice being highly efficient in assimilating arsenic, the scientists have also put out a possible way of preventing rice plants from taking up arsenic in the first place.

Affinity for arsenic : The paper states that "Arsenic concentrations in rice grains are often high enough to cause concern even in uncontaminated soils containing background levels of arsenic." This is because rice has a particular affinity for and efficiency in arsenic assimilation compared with other cereal crops. The scientists have found two plant proteins that primarily transport arsenite from the soil. Both transporter proteins are expressed in the roots. While one protein (Lsi1) is the port of entry for arsenite from the soil to the roots, the other protein (Lsi2) controls the flow of arsenite from the roots to the stalk and grain. The scientists, who used mutant paddy, found that when the Lsi1 transporter was knocked out, the arsenite in the shoots and roots were 71 per cent and 53 per cent lower respectively when compared to wild-type rice.

Silica transporatation :Both the proteins that transport arsenite are primarily meant for transporting silica from the soil to the shoots via the roots. And the scientists found that in the presence of silica in the soil, uptake of arsenite by rice was reduced. "Since silicic acid and arsenite are transported by the same transporters, they have competition during uptake. When silica is rich, the uptake of arsenite will be decreased. We did not compare the preference," noted Dr. Jian Feng Ma in an email communication to this correspondent.

Dr. Ma is from the Research Institute for Bioresources, Okayama University, Japan, and is the led author. "We have not done soil experiment, but we did water culture. We found that when silica is present in the nutrient solution, the arsenite accumulation was significantly decreased," she added. After finding that silicic acid interferes with arsenite uptake, the authors suggest that "..ensuring sufficient silicon availability in soil is likely to suppress arsenite accumulation in rice." Silica is normally taken up by the roots in the form of silicic acid. It gets accumulated in the form of silicon in the leaf blades and sheaths. Its accumulation in the husk of the grains helps protect the husk from diseases and excess transpiration.

SOURCE: The Hindu Newspaper Dated: 17 July, 2008.
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