Methylated
and inorganic thioarsenates have recently been reported
from paddy fields besides the better-known oxyarsenates. Methylated
thioarsenates are highly toxic for humans, yet their uptake, transformation,
and translocation in rice plants is unknown. Here, hydroponic experiments
with 20 day old rice plants showed that monomethylmonothioarsenate
(MMMTA), dimethylmonothioarsenate (DMMTA), and monothioarsenate (MTA)
were taken up by rice roots and could be detected in the xylem. Total
arsenic (As) translocation from roots to shoots was higher for plants
exposed to DMMTA, MTA, and dimethylarsenate (DMAV) compared
to MMMTA and monomethylarsenate (MMAV). All thioarsenates
were partially transformed in the presence of rice roots, but processes
and extents differed. MMMTA was subject to abiotic oxidation and largely
dethiolated to MMAV already outside the plant, probably
due to root oxygen loss. DMMTA and MTA were not oxidized abiotically.
Crude protein extracts showed rapid enzymatic reduction for MTA but
not for DMMTA. Our study implies that DMMTA has the highest potential
to contribute to total As accumulation in grains either as DMAV or partially as DMMTA. DMMTA has once been detected in rice
grains using enzymatic extraction. By routine acid extraction, DMMTA
is determined as DMAV and thus escapes regulation despite
its toxicity.
Inorganic
and methylated thioarsenates have recently been reported
to contribute substantially to arsenic (As) speciation in paddy-soil
pore waters. Here, we show that thioarsenates can also accumulate
in rice grains and rice products. For their detection, a method was
developed using a pepsin–pancreatin enzymatic extraction followed
by chromatographic separation at pH 13. From 54 analyzed commercial
samples, including white, parboiled and husked rice, puffed rice cakes,
and rice flakes, 50 contained dimethylmonothioarsenate (DMMTA) (maximum
25.6 μg kg–1), 18 monothioarsenate (MTA) (maximum
5.6 μg kg–1), 14 dimethyldithioarsenate (DMDTA)
(maximum 2.8 μg kg–1), and 5 dithioarsenate
(DTA) (maximum 2.3 μg kg–1). Additionally,
we show that the commonly used nitric acid extraction transforms MTA
to arsenite and DMMTA and DMDTA to dimethylarsenate (DMA). Current
food guidelines do not require an analysis of thioarsenates in rice
and only limit the contents of inorganic oxyarsenic species (including
acid-extraction-transformed MTA), but not DMA (including acid-extraction-transformed
DMMTA and DMDTA).
Arsenic
is one of the most relevant environmental pollutants and
human health threats. Several arsenic species occur in soil pore waters.
Recently, it was discovered that these include inorganic and organic
thioarsenates. Among the latter, dimethylmonothioarsenate (DMMTA)
is of particular concern because in mammalian cells, its toxicity
was found to exceed even that of arsenite. We investigated DMMTA toxicity
for plants in experiments with Arabidopsis thaliana and indeed observed stronger growth inhibition than with arsenite.
DMMTA caused a specific, localized deformation of root epidermal cells.
Toxicity mechanisms apparently differ from those of arsenite since
no accumulation of reactive oxygen species was observed in DMMTA-exposed
root tips. Also, there was no contribution of the phytochelatin pathway
to the DMMTA detoxification as indicated by exposure experiments with
respective mutants and thiol profiling. RNA-seq analysis found strong
transcriptome changes dominated by stress-responsive genes. DMMTA
was taken up more efficiently than the methylated oxyarsenate dimethylarsenate
and highly mobile within plants as revealed by speciation analysis.
Shoots showed clear indications of DMMTA toxicity such as anthocyanin
accumulation and a decrease in chlorophyll and carotenoid levels.
The toxicity and efficient translocation of DMMTA within plants raise
important food safety issues.
Arsenic (As) occurrence in rice is a serious human health
threat.
Worldwide, regulations typically limit only carcinogenic inorganic
As, but not possibly carcinogenic dimethylated oxyarsenate (DMA).
However, there is emerging evidence that “DMA”, determined
by routine acid-based extraction and analysis, hides a substantial
share of dimethylated thioarsenates that have similar or higher cytotoxicities
than arsenite. Risk assessments characterizing the in vivo toxicity
of rice-derived dimethylated thioarsenates are urgently needed. In
the meantime, either more sophisticated methods based on enzymatic
extraction and separation of dimethylated oxy- and thioarsenates have
to become mandatory or total As should be regulated.
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