Metabolic Differences Found in Seedlings of Rice Varieties That Produce Grains Low Versus High in Arsenic Concentration.

Arsenic (As) occurs naturally in air, water and soil and is also present to some degree in all edible and non-edible plant tissues.  Because As becomes more available for plant uptake when soils are flooded, there is more concern about As in rice than other grain crops.  Our research objective was to identify genetically controlled physiological/biochemical mechanisms breeders can select for to produce rice varieties that restrict accumulation of As in their grains.  Arsenic is toxic to plants as well as animals, and plants have evolved mechanisms to reduce toxicity or accumulation of As, such as reduced uptake into roots, reduced root-to-shoot transport, or increased vacuolar-sequestration of the As in roots, stems or leaves.  Plants also metabolize other As-induced toxic compounds to reduce injury from As exposure.  Many of these tolerance mechanisms could simultaneously lead to less As being transported into the grains of affected plants, therefore this study started with rice varieties known to be either high or low in grain As concentrations (a.k.a. As grain-accumulators or grain-excluders) and asked if seedlings exhibited differences in metabolic responses to arsenite [As(III)], the major chemical species of As present in anaerobic (flooded) rice paddies.  The study used three As grain-accumulators and three grain-excluders identified in a previous study of 1700 rice accessions from around the world.  Three-week old hydroponic seedlings exposed to 0 (control) or 100µM As(III) for 0 to 72 hours were analyzed for root and leaf concentrations of As plus metabolic compounds involved in heavy metal sequestration or injury.  While rates of As uptake and root-to-shoot transfer did not differ between As grain-accumulators and excluders, their survivability did; all three accumulators exhibited severe As-injury or death by 72 hours while the three excluders showed minimal to no reduction in growth compared to controls.  Notably, all three grain-excluders, but none of the grain-accumulators, showed As-induced enhancement of leaf glutathione (GSH) production.  Other studies have indicated that binding of As to either GSH or GSH-containing phytochelatins is required before the As can be transported to the vacuole for sequestration.  Therefore, the boost we observed in leaf GSH production suggests that grain-excluders are more efficient than grain-accumulators at sequestering As into leaf cell vacuoles.  Activation of As sequestration only among the grain-excluders suggests that leaf sequestration prevents larger amounts of free-As from being transported to grains in field-grown plants.