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Campus-Only Access for Five (5) Years

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Degree Program

Plant & Soil Sciences

Degree Type

Master of Science (M.S.)

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Arsenic is a highly toxic heavy metal that poses a significant risk to human health. Owing to the toxicity at low concentrations of its inorganic forms, its frequency and the potential for human exposure, arsenic is ranked number one in the priority list of the Agency for Toxic Substances and Disease Registry. A major avenue of exposure is arsenic entering the food system through the cultivation of rice (Oryza sativa L.) owing to its capacity to uptake and accumulate arsenic more readily than other crops. Rice is widely grown in areas of the world where high levels of arsenic in soil and groundwater have been recorded such as China and the Bengal Basin. Due to the aforementioned reasons, finding biological solutions to the arsenic problem in rice has been one of the top priorities in agricultural research for improving food safety.

Inorganic arsenic enters plants either through phosphate transporters in the form of arsenate (AsV) or through aquaporins and silicon transporters in the form of arsenite (AsIII). Once inside the plant cell, arsenate is reduced to arsenite by arsenate reductases such as HAC1 or AtACR2, allowing it to be conjugated with thiol-rich peptides such as glutathione (GSH) and phytochelatins (PCs). Once conjugated, arsenite is able to be sequestered in the vacuole of root cells, preventing translocation to aboveground tissues and therefore reducing its toxicity and accumulation in shoot and grains. Enhancing arsenate reductase expression could provide tolerance and reduced arsenic accumulation by increasing the available arsenic to be sequestered into roots, therefore preventing translocation and accumulation in rice straw and grains. The work included in this thesis sought to evaluate the capacity of overexpression of the arsenic reductase, AtACR2, in rice to provide arsenic tolerance and reduced accumulation. This was explored by transforming rice with gene constructs carrying the AtACR2 gene under the Actin-1 constitutive promoter and analyzing the effects of arsenate stress on AtACR2 overexpressing rice at the seedling stage in hydroponics assays and at full maturity in a soil assay.

Initial results at the seedling stage showed evidence of AtACR2 playing a role in arsenic accumulation in rice where AtACR2 overexpressing rice showed 2-4-fold less arsenic accumulation in shoot than wild type rice at the seedling stage. Tolerant phenotypes were also observed with transgenic lines showing significantly higher biomass and shoot length. In the soil assay, 18-41% significantly less arsenic accumulation was observed in grains, as well as significant reductions in flag leaf arsenic content for AtACR2 overexpressing rice lines, however no significant differences for arsenic contents were observed in the shoot tissues. Analysis of tolerance in the soil assay was not as conclusive owing to the transgenic rice lines growing significantly smaller than wildtype under untreated conditions. This might be due to the dual role of AtACR2 as arsenate reductase and CDC25 phosphatase that regulate the cell cycle. From this research, we can conclude that AtACR2 plays a major role in limiting arsenic accumulation in rice straw, flag leaf and grains and provides strong tolerance to arsenic. Further research is needed to decipher the underlying mechanism and the role of AtACR2 in arsenic tolerance and shoot accumulation in rice at maturity in arsenic contaminated soils. Nevertheless, this study has significant applications in enhancing crop productivity and reduced arsenic accumulation when grown on arsenic contaminated agricultural soils.

First Advisor

Om Parkash Dhankher

Available for download on Saturday, February 01, 2025