Deciphering Controls On Arsenic Uptake and Mechanisms of Attenuation On Roots of Rice (Oryza sativa L.) with Variable Fe Plaque Coatings.

Arsenic present in rice grains arises because of arsenic uptake by roots. Thus, deciphering the controls on arsenic accumulation into grains first requires that we fully understand arsenic uptake into rice roots.  Under hydroponic conditions, As(V) has been shown to share the phosphate uptake mechanism whereas As(III) shares the silicic acid uptake mechanism into root cell membranes.  In soil-grown plants, however, understanding arsenic uptake into rice roots is complicated by the presence of competing soil anions (e.g. P and Si) and Fe plaque coatings.  Fe plaque attenuates both As(III) and As(V), but to different extents, and perhaps minimizes As transport across root cell membranes.  We have shown that Fe plaque forms differentially on the rice root in soil-grown plants, with most occurring near the water/air interface and near the root tips.  We use synchrotron-based X-ray imaging techniques, X-ray absorption spectroscopy, and X-ray diffraction to decipher Fe mineralogical phases and As species in root thin sections and on whole root portions with differential Fe plaque coatings.  Our findings suggest that As uptake may be significantly influenced by the extent of Fe plaque coatings on rice roots, particularly in the region of maximum solute uptake.