The sinking of tube wells, low-cost and shallow water wells in Southeast Asia as well as mining in various regions of China, Thailand, and the US often boost arsenic concentrations in water that frequently exceed the World Health Organization (WHO) limit of 10 micrograms per litre (mcg/l), the value above which health problems start to occur.
Tens of millions of people are exposed to the risks associated with high levels of arsenic by drinking contaminated water or by ingesting cereal crops cultivated in polluted soils. Long-lasting exposure to this highly toxic metalloid can have a disastrous affect on human organs including the gastrointestinal transit, kidneys, liver, lungs and skin, and it increases the risk of cancer. In Bangladesh alone, it is estimated that 25 million people drink water that contains more than 50 mcg/l of arsenic and that 2 million of them risk of dying from cancer caused by this toxic substance.
Scientists from laboratories in Switzerland, South Korea and the US, and from the Swiss National Centre of Competence in Research (NCCR) Plant Survival believe that by identifying the key genes responsible for the accumulation of arsenic in plant cells they have made the first step towards tackling these problems. They explained their findings in the journal Proceedings of the National Academy of Sciences (PNAS).
Plants offer a way for toxic metals to enter the food chain. For example, arsenic is stored within rice grains, which, in regions polluted with this toxic metalloid, constitutes a danger for the population whose diet depends to a great extent on this cereal. Arsenic or cadmium in soils is then transported to plant cells and stored in compartments called vacuoles.
Within the cell, the translocation of arsenic and its storage in vacuoles is ensured by a category of peptides - the phytochelatins, which are important for the detoxification of heavy metals - that bind to the toxic metalloid, and are transported into the vacuole for detoxification.
The researchers said the process was similar to hooking up a trailer to a truck with the 'truck and trailer' complex being stored in the vacuole.
'By identifying the genes responsible for the vacuolar phytochelatin transport and storage, we have found the missing link that the scientific community searched for the past 25 years,' said Enrico Martinoia, a professor in plant physiology at the University of Zurich in Switzerland. He and his team pointed out that controlling these genes will make it possible to develop plants capable of preventing the transfer of toxic metals and metalloids from the roots to the leaves and grains thereby limiting the entry of arsenic into the food chain.
'By focusing on these genes, we could avoid the accumulation of these heavy metals in edible portions of the plant such as grains or fruits,' said Youngsook Lee from the Pohang University of Science and Technology (POSTECH) in South Korea.
Citation: Song, W-Y., et al. (2010) Arsenic tolerance in Arabidopsis is mediated by two ABCC-type phytochelatin transporters. PNAS. DOI:10.1073/pnas.1013964107.
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