Arsenic Cycling in the Rice (Oryza sativa L.) Rhizosphere: Evidence for the Passive Transport of Arsenite at Lateral Root Junctions and Root Apices.

Arsenic (As) uptake by rice affects grain yield and may affect human health upon contaminated grain consumption; therefore, it is important to understand how As cycles in the rhizosphere and is taken up by rice roots. Four major chemical species of As have been detected in rice paddy porewaters and include the acutely toxic inorganic species arsenite (As(III)_i: H₃AsO₃⁰ at circumneutral pH) and arsenate (As(V)_i: H₂AsO₄^- or HAsO₄^2- at circumneutral pH) and the organic species monomethylarsonous acid (MMA) and dimethylarsinic acid (DMA). Of these species, As(III)_i is the predominant form of As in flooded paddy porewater. The prevailing view of arsenite (As(III)_i) uptake in rice is that it occurs principally at the Casparian band via Lsi1 and Lsi2 Si transporters. However, lsi1 and lsi2 rice mutants dosed with As(III)_i accumulate appreciable As in their shoots, indicating alternate As(III)_i uptake mechanisms exist. We hypothesized that areas devoid of Casparian bands – lateral root junctions and root apices –passively transport As(III)_i. We analyzed the elemental distribution and As concentration, speciation, and localization in rice roots from soil-grown and solution-grown plants. With solution-grown plants dosed with As(III)_i, we sectioned hydrated rice roots as a function of distance from the root apex and analyzed the cross-sections using confocal microscopy coupled to synchrotron X-ray fluorescence imaging and spectroscopy. We observed elevated As(III) associated with lateral root junctions and root apices in rice. As(III) entered the stele at lateral root junctions and radially permeated the root interior in cross-sections 130-140 µm from the root apex that are devoid of Casparian bands. Our findings suggest that lateral root junctions and rice root apices are hot-spots for passive As(III) transport in rice and may be important for up to 25% of As(III)_i transport to shoots.