Adenosine 5′-phosphosulfate reductase (APR2) mutation in Arabidopsis implicates glutathione deficiency in selenate toxicity

APR2 is the dominant APR (adenosine 5'-phosphosulfate reductase) in the model plant Arabidopsis thaliana, and converts activated sulfate to sulfite, a key reaction in the sulfate reduction pathway. To determine whether APR2 has a role in selenium tolerance and metabolism, a mutant Arabidopsis line (apr2-I) was studied. apr2-I plants had decreased selenate tolerance and photosynthetic efficiency. Sulfur metabolism was perturbed in apr2-I plants grown on selenate, as observed by an increase in total sulfur and sulfate, and a 2-fold decrease in glutathione concentration. The altered sulfur metabolism in apr2-I grown on selenate did not reflect typical sulfate starvation, as cysteine and methionine levels were increased. Knockout of APR2 also increased the accumulation of total selenium and selenate. However, the accumulation of selenite and selenium incorporation in protein was lower in apr2-I mutants. Decreased incorporation of selenium in protein is typically associated with increased selenium tolerance in plants. However, because the apr2-I mutant exhibited decreased tolerance to selenate, we propose that selenium toxicity can also be caused by selenate's disruption of glutathione biosynthesis leading to enhanced levels of damaging ROS (reactive oxygen species).