Abstract
The uptake of arsenite (As(III)i) at the Casparian band via Lsi1 and Lsi2 Si transporters is responsible for ~75% of shoot As(III)i uptake in rice and, therefore, ~25% of shoot As(III)i is taken up by other transport pathways. We hypothesized that areas devoid of Casparian bands—lateral root junctions and root apices—can transport As(III)i into roots. 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 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)i associated with lateral root junctions and root apices in rice. As(III)i 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 As(III)i transport into rice roots, but the contribution to shoot As requires further research.
Highlights
Arsenic (As) is phytotoxic and its uptake by rice roots affects rice yield [1,2]
Our findings suggest that lateral root junctions and rice root apices are hot-spots for As(III)i transport into rice roots, but the contribution to shoot As requires further research
Each of these species are transported into rice root cells due to their chemical similarities to plant nutrients: As(V)i is analogous to inorganic phosphate (Pi ) and shares the Pi transport pathway [5], whereas As(III)i and to some extent dimethylarsinic acid (DMA) and monomethylarsonous acid (MMA) are analogous to silicic acid [6,7]
Summary
Arsenic (As) is phytotoxic and its uptake by rice roots affects rice yield [1,2]. its storage in grain is a detriment to human health worldwide, for populations already exposed to As through drinking water [3,4]. Four major chemical species of As have been detected in rice paddy porewaters and include the acutely toxic inorganic species arsenite (As(III)i : H3 AsO3 0 at circumneutral pH) and arsenate (As(V)i : H2 AsO4 − or HAsO4 2− at circumneutral pH) and the organic species monomethylarsonous acid (MMA) and dimethylarsinic acid (DMA) Each of these species are transported into rice root cells due to their chemical similarities (i.e., hydrated radius and valence) to plant nutrients: As(V)i is analogous to inorganic phosphate (Pi ) and shares the Pi transport pathway [5], whereas As(III)i and to some extent DMA and MMA are analogous to silicic acid [6,7]. While management practices affect pore water speciation [8,9], the dominant As species in typical flooded paddy porewater is As(III)i [10] It is, imperative to understand the mechanisms and pathways by which As(III)i is transported into rice roots and stored in grains to effectively minimize the negative impacts of As on rice and humans
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