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.
