Laterally resolved speciation of arsenic in roots of wheat and rice using fluorescence‐XANES imaging

Abstract

SummaryAccumulation of arsenic (As) within plant tissues represents a human health risk, but there remains much to learn regarding the speciation of As within plants.We developed synchrotron-based fluorescence-X-ray absorption near-edge spectroscopy (fluorescence-XANES) imaging in hydrated and fresh plant tissues to provide laterally resolved data on the in situ speciation of As in roots of wheat (Triticum aestivum) and rice (Oryza sativa) exposed to 2 lM As(V) or As(III).When exposed to As(V), the As was rapidly reduced to As(III) within the root, with As(V) calculated to be present only in the rhizodermis. However, no uncomplexed As(III) was detected in any root tissues, because of the efficient formation of the As(III)-thiol complexthis As species was calculated to account for all of the As in the cortex and stele. The observation that uncomplexed As(III) was below the detection limit in all root tissues explains why the transport of As to the shoots is low, given that uncomplexed As(III) is the major As species transported within the xylem and phloem.Using fluorescence-XANES imaging, we have provided in situ data showing the accumulation and transformation of As within hydrated and fresh root tissues.

Synchrotron-based Techniques Shed Light on Mechanisms of Plant Sensitivity and Tolerance to High Manganese in the Root Environment

“…First, laterally resolved data on the speciation of Mn within plant tissues was obtained using the Maia detector system at the Australian Synchrotron. With fluorescence-XANES imaging, speciation data (i.e., XANES spectra) were collected for every pixel in the map as explained by Kopittke et al (2014). This procedure differs from the typical approach of obtaining micro-XANES spectra at specific individual locations.…”

“…7; Supplemental Fig. S15A) using the energy association module in GeoPIXE (Kopittke et al, 2014). Three Mn populations (other than the background pixels) were identified in which Mn concentration changed from the base to the tip ( Fig.…”

“…The XANES spectra were extracted from pixels within the selected areas in the stack series and subsequently background-and baseline-corrected using Athena version 0.9.2 (Ravel and Newville, 2005). As detailed by Kopittke et al (2014), LCF was performed using Athena, with the combination of standards yielding the lowest residual parameter chosen as the most likely set of components. No differentiation was made between Mn(II) malate and Mn(II) citrate due to the similarity of the XANES spectra.…”

Evidence for the Root-Uptake of Arsenite at Lateral Root Junctions and Root Apices in Rice (Oryza sativa L.)

“…Advanced imaging techniques used by different researchers have revealed elevated levels of As [18][19][20][21] or Se (as selenite) [22] associated with rice root apices where the Casparian bands are purportedly absent [12,13]. However, because some of these images [18,19] were generated from whole root mounts and not cross-sections, it was unclear if the elevated As signal in those studies was predominantly on the inside of the roots or on the outside of roots associated with Fe plaques (or both).…”

“…However, because some of these images [18,19] were generated from whole root mounts and not cross-sections, it was unclear if the elevated As signal in those studies was predominantly on the inside of the roots or on the outside of roots associated with Fe plaques (or both). Other researchers have successfully used computed tomography or modeling approaches to estimate the influx of As in root apices in rice, wheat, and cowpea roots in short-term studies [20,23]. Ma et al [24] demonstrated that lateral roots are important for Si transport in rice, but the importance of these zones for As(III) i root-uptake has only recently been investigated [25].…”