Superfund Research Program
Molecular Mechanisms of Heavy Metal Detoxification and Accumulation in Plants
Project Leader: Julian I. Schroeder
Grant Number: P42ES010337
Funding Period: 2000-2017
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Arsenic is one of the most toxic pollutants at contaminated Superfund sites, yet little is known about the mechanisms by which certain plants survive exposure to high arsenic levels. To gain insight into the mechanisms of arsenic tolerance, Dr. David Lee in the Schroeder lab developed a genetic screen to isolate Arabidopsis thaliana mutants with altered tolerance to arsenic. The Schroeder lab identified and characterized a new mutant, ars4, that shows increased tolerance to arsenate. A genetic screen was performed to isolate mutants showing increased arsenic tolerance using an Arabidopsis thaliana population of activation tagged lines. The most arsenic resistant mutant ars4 shows increased arsenate and arsenite tolerance at all stages of development, while at the same time accumulating higher levels of arsenic than wild type plants. Arsenic tolerance of ars4 is not due to arsenic extrusion and ars4 has wild type sensitivity to other toxic metals. X-ray absorption spectroscopy analysis indicates that ars4 accumulates higher levels of free arsenite and lower levels of arsenate than wild type plants. ars4 contains a single activation tag that co-segregates with arsenic tolerance and is inserted in the PHYA gene and expression of PHYA is disrupted. In agreement with this finding, when grown under far-red light conditions ars4 shows the same elongated hypocotyl phenotype as the previously described strong phyA-211 allele. Both the phyA-211 allele and a newly isolated T-DNA insertion allele of PHYA also show increased tolerance to arsenate, though to a lesser degree than ars4. This observation indicates that a second genetic component contributes to arsenic tolerance in ars4, and genetic analyses of ars4 suggest that the second locus is unlinked to the T-DNA. Supporting the observations that the PHYA mutation contributes to arsenic tolerance, wild type plants grown in the dark, which prevents PHYA from being activated, showed increased tolerance to arsenic compared to plants grown under normal lighting conditions. These results show that ars4 has an increased ability to tolerate free arsenic in planta and that PHYA negatively regulates a pathway conferring arsenic tolerance.