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IN BRIEF
The WKRY6 Transcription Factor Is a Key Player in a Multifaceted
Defense against Arsenate
Arsenic (As) is a highly toxic, nonessential
metalloid that may have played a large role
in the evolution of life (Oremland et al.,
2009). Arsenate [As(V)] is taken up and
moved through the plant by phosphate (Pi)
transporters (reviewed in Mendoza-Cózatl
et al., 2011). Previously, it was reported
that exposure of Arabidopsis thaliana to
As(V) led to specific repression of genes
responsible for Pi uptake, including that for
the high-affinity phosphate transporter
PHT1;1 (Catarecha et al., 2007). New work
from Castrillo et al. (2013) reveals that
this repression is just one aspect of a
rapid and sensitive As(V)-specific defense
response.
Castrillo et al. sought to elucidate the
mechanism responsible for As(V) repression of PHT1;1. PHT1;1 promoter-driven
expression of the luciferase (LUC) reporter
showed that Pi starvation induced LUC
expression and that treatment with As(V),
even at very low concentrations, reduced
LUC expression even more than did treatment with Pi, consistent with the previous
results that PHT1;1 is sensitive to As(V).
Analysis of PHT1;1-GFP (for green fluorescent protein) fusion lines revealed that As(V)
exposure led to PHT1;1 removal from the
plasma membrane via endocytosis (see
figure). The timing of the relocalization of
the transporter to the vacuole lumen corresponded with that of a decrease in the
cellular uptake of As(V), supporting the idea
that this relocalization represents a method
to terminate As(V) uptake.
The authors then treated Pi-starved
plants with As(V) or Pi and analyzed the
promoter regions of those genes that were
more responsive to As(V) than to Pi (similar
to PHT1;1). They discovered a cis As(V)
repression element (ARE), which is similar
to the WK-box motif bound by WRKY
PHT1;1-GFP is relocalized from the plasma membrane to the vacuole upon treatment with 30 mM As(V)
Arabidopsis root cells. Bars ¼ 10 mm. (Reprinted from Castrillo et al. [2013], Figure 2B.)
transcription factors in plants. Indeed,
WRKY6 gene expression was induced by
As(V), and overexpression of WRKY6
caused a reduction of PHT1;1 expression.
Furthermore, WRKY6 interacted with the
ARE in the PHT1;1 promoter, and a wrky6
null mutant showed no As(V)-induced repression of PHT1;1. Together, these results
suggest that WRKY6 acts as a transcriptional repressor of PHT1;1 in response to
As(V).
Transcriptomic analysis also revealed
that 869 transposon genes were induced
by As(V). This transposon burst was repressed by WRKY6 overexpression, and
many of the As(V)-induced transposons
had WRKY binding sites in their promoters.
The promoters of at least six of these
transposon genes could be directly bound
by WRKY6, suggesting that in addition to
repressing PHT1;1, WRKY6 also represses
transposon activation in response to
As(V) stress.
Together, this work demonstrates that
plants have evolved an As(V)-specific response wherein the high-affinity Pi transporter PHT1;1, which also transports
As(V), is removed from the plasma membrane and is also transcriptionally repressed upon exposure to As(V). This dual
regulation allows the plants to halt As(V)
uptake quickly. WRKY6 mediates the
transcriptional repression of PHT1;1 and
further protects the plant by suppressing
transposon activation in response to As(V)
stress via directly interacting with transposon promoters. This work thus establishes WRKY6 as a key mediator of arsenate
defenses in plants.
Nancy R. Hofmann
Science Editor
[email protected]
REFERENCES
Castrillo, G., et al. (2013). WRKY6 transcription
factor restricts arsenate uptake and transposon activation in Arabidopsis. Plant Cell
25: doi/10.1105/tpc.113.114009.
Catarecha, P., Segura, M.D., Franco-Zorrilla,
J.M., Garcia-Ponce, B., Lanza, M., Solano,
R., Paz-Ares, J., and Leyva, A. (2007). A
mutant of the Arabidopsis phosphate transporter PHT1;1 displays enhanced arsenic
accumulation. Plant Cell 19: 1123–1133.
Mendoza-Cózatl, D.G., Jobe, T.O., Hauser, F.,
and Schroeder, J.I. (2011). Long-distance
transport, vacuolar sequestration, tolerance,
and transcriptional responses induced by
cadmium and arsenic. Curr. Opin. Plant Biol.
14: 554–562.
Oremland, R.S., Saltkov, C.W., Wolfe-Simon,
F., and Stolz, J.F. (2009). Arsenic in the
evolution of earth and extraterrestrial ecosystems. Geomicrobiol. J. 26: 522–536.
www.plantcell.org/cgi/doi/10.1105/tpc.113.250811
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The WKRY6 Transcription Factor Is a Key Player in a Multifaceted Defense against Arsenate
Nancy R. Hofmann
Plant Cell; originally published online August 9, 2013;
DOI 10.1105/tpc.113.250811
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