Hyperaccumulation of arsenic in the shoots of Arabidopsis silenced for arsenate reductase (ACR2)

Abstract

Endogenous plant arsenate reductase (ACR) activity converts arsenate to arsenite in roots, immobilizing arsenic below ground. By blocking this activity, we hoped to construct plants that would mobilize more arsenate aboveground. We have identified a single gene in the Arabidopsis thaliana genome, ACR2, with moderate sequence homology to yeast arsenate reductase. Expression of ACR2 cDNA in Escherichia coli complemented the arsenate-resistant and arsenate-sensitive phenotypes of various bacterial ars operon mutants. RNA interference reduced ACR2 protein expression in Arabidopsis to as low as 2% of wild-type levels. The various knockdown plant lines were more sensitive to high concentrations of arsenate, but not arsenite, than wild type. The knockdown lines accumulated 10-to 16-fold more arsenic in shoots (350 -500 ppm) and retained less arsenic in roots than wild type, when grown on arsenate medium with <8 ppm arsenic. Reducing expression of ACR2 homologs in tree, shrub, and grass species should play a vital role in the phytoremediation of environmental arsenic contamination.Escherichia coli ArsC ͉ drinking water ͉ CDC25 ͉ toxicant ͉ arsenic pollution E nvironmental arsenic pollution is widely recognized as a global health problem (1) (www.epa.gov͞ogwdw͞ars͞arsenic.html). High levels of arsenic in soil and drinking water have been reported around the world, but the situation is worst in India and Bangladesh, where Ͼ400 million people are at risk of arsenic poisoning (2). The World Health Organization predicts that long-term exposure to arsenic could reach epidemic proportions, estimating that 1 in 10 people in the most contaminated areas may ultimately die from diseases related to arsenic poisoning (3). The high financial cost associated with repairing the environmental damage by using physical remediation methods such as excavation and reburial make these technologies unacceptable for cleaning up the vast areas of the planet that need arsenic remediation. As a result, the overwhelming majority of arsenic-contaminated sites are not being cleaned up.Phytoremediation is the use of plants to clean up environmental pollutants and is considered an important alternative to physical methods for cleaning up arsenic (4). Our objective is to develop a genetics-based arsenic phytoremediation strategy that can be used in any plant species. Plants that hyperaccumulate arsenic to high levels aboveground would be harvested and the arsenic further concentrated by incineration. In previous studies, we engineered model plants expressing a bacterial arsenate reductase (ArsC; EC 1.20.4.1) aboveground and constitutively expressing ␥-glutamylcysteine synthetase (5). By reducing arsenate to arsenite in leaves and trapping arsenite in thiol-peptide complexes, these plants accumulate 3-fold more arsenic aboveground than wild type and are also highly tolerant to toxic levels of arsenic. The research described herein extends these observations and attacks a particular problem limiting the engineered phytoremediation of arsenic: its transpor...

Natural variation in arsenate tolerance identifies an arsenate reductase in Arabidopsis thaliana

“…2c). Although the biological significance of AtACR2 was questioned in a study 25 , Atacr2 knockout line showed no arsenate reductase activity 26 but displayed a remarkable As(V)-sensitive phenotype 25,27 . Therefore we cannot exclude the possibility that under certain conditions AtACR2 has a relevant role in As(V) tolerance as an As(V) reductase.…”

“…In plants, arsenic tolerance is critical for adaptation to specific soils and has determined plant distribution 4,5 ; the molecular mechanisms are analyzed mainly by studying proteins presumably involved in arsenate (As(V)) tolerance based on sequence homology with bacterial and yeast proteins 6,7 . Genetic approaches are little used to study As(V) tolerance mechanisms [8][9][10][11][12] , and the genes implicated in natural intraspecific variation are unknown [13][14][15] .…”

A Vacuolar Arsenite Transporter Necessary for Arsenic Tolerance in the Arsenic Hyperaccumulating Fern Pteris vittata Is Missing in Flowering Plants

“…Arsenate is taken up by phosphate transporters in the root (Meharg and Macnair, 1992;Meharg and Hartley-Whitaker, 2002;Quaghebeur and Rengel, 2004;Catarecha et al, 2007) and then rapidly reduced to arsenite. This reduction is, at least in part, facilitated by a dual function CDC25 phosphatase/ACR2 arsenate reductase in Arabidopsis (Landrieu et al, 2004a(Landrieu et al, , 2004bSorrell et al, 2004;Bleeker et al, 2006;Dhankher et al, 2006). CDC25-like arsenate reductases have also been characterized in rice (Duan et al, 2007), velvetgrass (Holcus lanatus), and P. vittata (Bleeker et al, 2006;Ellis et al, 2006).…”

Regulation of sugar metabolism in rice (Oryza sativa L.) seedlings under arsenate toxicity and its improvement by phosphate

“…Arsenate, after accumulation in plants, is converted to arsenite in presence of arsenate reductase (Rosen, 2002;Duan et al, 2005;Ellis et al, 2006;Dhankher et al, 2006). Arsenite reacts with sulphydryl groups (-SH) of enzymes and tissue proteins, leading to the inhibition of cellular function and death of the plant (Ullrich- Eberius et al, 1989).…”