Rice Varietal Differences in Arsenite Metabolism.

Plants utilize an assortment of survival mechanisms to reduce arsenic toxicity, such as exclusion, translocation, and detoxification.  Detoxification is the primary method plants use to mitigate heavy metal stress through formation of reactive oxygen species (ROS) and methylglyoxal (MG) metabolism.  Both ascorbic acid (AsA) and glutathione (GSH), compounds generated in response to ROS, increase with application of As in some varieties of rice; this increase has been correlated with the rice variety’s ability to detoxify As.  Recently, key enzymes in MG metabolism that produce lactic acid have been shown to be active in the rice genome.   The goal of our study was to determine which key components of the heavy metal detoxification pathway are also associated with differences in the amount of arsenic that accumulates in the grain, addressing a potential food safety issue.  Six varieties of rice previously determined to accumulate especially high or low grain As levels were surveyed for differences in compounds associated with the As-detoxification pathway.  We grew these six varieties hydroponically for 14 days and then added As(III) to a subset for 72hrs, growing the other half in standard hydroponic solution.  Concentrations of AsA, GSH, D-lactic acid, and As levels were measured in both root and leaf tissue at various time points.  GSH and D-lactic acid levels differed between varieties and tissue types.  In that these differences coincided with grain As levels among the varieties, the data suggest that As detoxification in the leaves and roots can decrease the amount of As that is transported to and accumulated in rice grains.