Biotechnological Approaches for Mitigating Arsenic Threat in the Environment and Developing Arsenic Free Food Crops.

Arsenic (As) contamination is widespread and being a potent carcinogen, As affects the health of more than 500 million people worldwide. Large amount of As has built-up in agricultural soils due to repetitive irrigation with contaminated ground water, pesticides, and other industrial activities. There is no efficient, cost-effective and environment friendly strategy for As remediation. Previously, we engineered Arabidopsis thaliana plants co-expressing the E. coli arsC gene (arsenate reductase) in leaves and the g-ECS (g-glutamylcysteine synthetase) genes, constitutively. These plants showed significantly greater arsenic tolerance and accumulation than control plants (Dhankher et al., 2002, Nature Biotech. 20:1140-45). To further enhance As movement from roots to the aboveground tissues, we examined the endogenous plant activity that affects the electrochemical state and binding of As in roots. We identified an endogenous arsenate reductase, AtACR2, from Arabidopsis that reduces As^V to As^III in plants. Inactivation of AtACR2 by RNAi caused the translocation of 10-16 fold more As from root to shoot tissues when these plants were exposed to As^V (Dhankher et al., 2006, PNAS 103: 5413-18). These results clearly shows that the synergistic activity of these genes could lead to more than a 50-fold increase in the levels of As accumulation in the above ground tissues for later harvest. To further characterize AtACR2, we overexpressed the AtACR2 in Arabidopsis and the AtACR2 transgenic lines were highly resistant to As^V, presumably due to binding of resulting As^III to cysteine-rich C-terminal domain. These AtACR2 transgenic lines accumulated 2-3 fold less arsenic in shoot tissues and attained 6 to 7-fold more biomass. In order to transfer this portable As phytoremediation strategy for remediation of contaminated soil and water, the ArsC and g-ECS genes were transferred to high biomass, non-food, fast growing Crambe abyssinica and Brassica juncea plants. Both C. abyssinica and B. juncea plants transformed with ArsC and g-ECSgenes, exhibited phenotypes and As accumulation similar to those achieved in Arabidopsis. These results are promising and showed the significant potential for utilization of Crambe and Brassica species in the field for toxic metals and metalloids remediation.

Apart from the contamination in drinking water, accumulation of As in food crops, particularly in rice, is a significant health concern. In order to develop As free rice, we engineered rice for enhanced tolerance and reduced arsenic uptake by overexpressing AtACR2 under a constitutive promoter. Our results showed that the AtACR2 overexpressing rice lines were highly tolerant to arsenate, attained 5-6 fold more biomass and accumulated 2- to 3-fold less arsenic in rice straw and seed grains. Additionally, we are exploring the rice, Arabidopsis, and Crambe genome and isolating genes to understand the molecular and biochemical mechanisms of arsenic uptake, transport, tolerance, and detoxification in plants for commercial phytoremediation as well reducing the arsenic uptake in plants to block arsenic contamination in food chain. Updated results will be presented at the conference.