How Plants Control Arsenic Accumulation
Citation: Meadows R (
How Plants Control Arsenic Accumulation
Robin Meadows 0
0 Freelance Science Writer , Fairfield, California , United States of America
Figure 1. Arsenic is toxic, and its elimination from plants requires it to be converted into arsenite, a form of arsenic that can be released back into the soil from roots. When this fails, arsenic builds up to toxic levels inside the plant. doi:10.1371/journal.pbio.1002008.g001
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In many parts of the world,
groundwater contains so much arsenic that it builds
up in irrigated crops. Linked to cancer and
heart disease, this toxic element is
particularly worrisome in rice, which absorbs
arsenic more readily than other grains and
is a staple for billions of people. Countries
with the double whammy of arsenic-laced
groundwater and heavy rice consumption
include Bangladesh, India, and China.
While plants can detoxify arsenic, we
dont know precisely how they do it. In
this issue of PLOS Biology, the
collaborative team of Dai-Yin Chao, Fang-Jie Zhao,
and David E. Salt identify an
arsenicreducing enzyme in the plant Arabidopsis
thaliana and show that this protein is
critical to arsenic elimination (Figure 1).
Inorganic arsenic (arsenate) resembles
phosphate and, once taken up by roots,
likely loads via phosphate transporters into
the xylem, which delivers water and
nutrients to the shoots. Plants get rid of
arsenate by reducing it to arsenite, a form
that no longer mimics phosphate and is
readily extruded from the roots back into
the soil.
To find the enzyme that transforms
arsenate into arsenite in plants, the
researchers used genome-wide association
mapping, which links phenotypesin this
case, arsenic levels in leavesto genes.
They grew 349 types of A. thaliana
collected from around the world at an
environmentally relevant concentration of
arsenic, and found that leaf arsenic levels
varied more than 20-fold and that this
variation was associated with a region of
chromosome 2.
Comparison of strains with high and
average arsenic levels (Kr-0 and Col-0,
respectively) showed that the former has a
cytosine at a specific nucleotide in this
region, while the latter has a thymine in
the same spot. Crossing the two strains
showed that arsenic was high in about
25% of the offspring, suggesting that leaf
arsenic levels are controlled primarily by a
single gene. Named High Arsenic Content 1 (HAC1), this gene has a predicted amino
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acid domain characteristic of arsenate
reductases. To verify that this newly
discovered enzyme reduces arsenate to
the easily eliminated arsenite, Chao and
colleagues expressed HAC1 in an
Escherichia coli mutant that lacks its own
arsenate reductase. As expected, HAC1
restored arsenic elimination in this E. coli
mutant.
In addition, the team found that HAC1
is expressed in the roots and that root
expression rises in A. thaliana exposed to
arsenate. Moreover, in A. thaliana
mutants that lack HAC1, arsenic stunted both
root and overall plant growth. The latter is
important because it shows that HAC1
also keeps arsenic low in shoots, which are
often the edible part of a plant. Another
important finding is that arsenite extrusion
is dramatically reduced in an HAC1
mutant A. thaliana exposed to arsenate,
suggesting that this arsenate-reducing
enzyme may be coupled with the arsenite
efflux transporter.
Besides making a compelling case that
HAC1 is part of a major defense against
arsenic in plants, Chao and colleagues
cleared up a mystery over a previous
candidate for this job. Yeast reduces
arsenate with an enzyme called ACR2,
and initial studies had suggested that
plants use a similar enzyme to detoxify
arsenic. It turned out, however, that this
ACR2-like enzyme reduces arsenate only
in vitro and not in living plants. This
apparent discrepancy is resolved by the
fact that the two plant arsenate reductases
(ACR2 and the newly discovered HAC1)
share a similar DNA sequence, suggesting
that experiments meant to knock out
ACR2 actually knocked out HAC1.
To dispel any lingering doubts that
ACR2 may still play a role in plant arsenic
detoxification by interacting with HAC1,
the researchers compared arsenic in an A.
thaliana mutant that lacks HAC1 to a
double mutant that lacks both HAC1 and
ACR2. As expected, arsenic metabolism
Competing Interests: The author has declared that no competing interests exist.
was similar in the two mutants, confirming
that ACR2 has no impact on arsenic
detoxification and elimination in plants.
Using a different method, Eduardo
Sanchez-Bermejo and colleagues also
recently identified the same gene, which they
called ATQ1, showing that it encodes an
arsenate reductase enzyme involved in
plant tolerance to arsenate. However,
Chao and colleagues went further by
revealing the functional role of HAC1 in
arsenic accumulation and arsenate
resis (...truncated)