The Chromium Detoxification Pathway in the Multimetal Accumulator Silene vulgaris

Abstract

Phytomanagement could be a viable alternative in areas polluted with wastes from chromium-using industries. This study investigated the ability of Silene vulgaris to take up Cr(III) and Cr(VI) with special attention on the mechanism used by this species to tolerate high doses of Cr(VI). Plants were grown semihydroponically with different concentrations of either Cr(III) or Cr(VI). A combination of synchrotron X-ray spectroscopic techniques, scanning electron and light microscopy and infrared spectroscopy were used to determine the distribution and speciation of Cr. S. vulgaris accumulated more Cr when grown with Cr(VI) resulting in an overall reduction in biomass. Starch accumulation in leaves may be attributed to an impartment between carbon utilization and assimilation resulted from stunted plant growth but not the complete inhibition of photosynthesis indicating that S. vulgaris possess tolerance mechanisms that allows it to survive in Cr(VI) rich environments. These primary tolerance mechanisms are (a) the total reduction of Cr(VI) to Cr(III) in the rhizosphere or just after uptake in the fine lateral root tips and (b) chelation of Cr(III) to the cell wall both of which reduce metal interference with critical cell functions. These mechanisms make S. vulgaris suitable for in situ remediation of Cr polluted soils.

Synchrotron‐based X‐Ray Approaches for Examining Toxic Trace Metal(loid)s in Soil–Plant Systems

“…Regardless of whether it is present as Cr(III) or Cr(VI) in the soil solution, Cr accumulates almost exclusively as Cr(III) within plant roots, this being demonstrated in both soil‐based systems and nutrient solutions, including for Indian mustard [ Brassica juncea (L.) Czern.] (Bluskov et al, 2005), subterranean clover ( Trifolium brachycalycinum L.) (Howe et al, 2003), Silene vulgaris (Moench) Garcke (Pradas del Real et al, 2014), Prosopis L. spp. (Aldrich et al, 2003), Convolvulus arvensis L. (Montes‐Holguin et al, 2006), and Parkinsonia aculeate (Zhao et al, 2009).…”

“…The first study to demonstrate this conversion to Cr(III) using bulk XANES was Zayed et al (1998a), who examined a wide range of vegetable crops. Whether the transformation from Cr(VI) to Cr(III) occurs within the rhizosphere or rapidly on uptake by the root is unclear (Pradas del Real et al, 2014), but regardless, concentrations of Cr(VI) in root tissues are often sufficiently low that they are not easily detectable using synchrotron‐based approaches. Within root tissues, this Cr(III) is often complexed with simple organic acids (Howe et al, 2003; Bluskov et al, 2005; Zhao et al, 2009), although Pradas del Real et al (2014) found that Cr accumulated in the apoplast of the root apical tissues.…”

“…Upon its translocation to the shoots, Cr(III) remains the dominant form, often remaining complexed by simple organic acids (Howe et al, 2003; Bluskov et al, 2005; Zhao et al, 2009). Within the leaves, Cr tends to accumulate in the leaf margins, as shown in Subterranean clover (Howe et al, 2003) and Silene vulgaris (Pradas del Real et al, 2014). Furthermore, Pradas del Real et al (2014) used μ‐XRF to identify that concentrations of Cr were highest in the mesophyll cells (both palisade and spongy) of the leaf tip.…”

“…Within the leaves, Cr tends to accumulate in the leaf margins, as shown in Subterranean clover (Howe et al, 2003) and Silene vulgaris (Pradas del Real et al, 2014). Furthermore, Pradas del Real et al (2014) used μ‐XRF to identify that concentrations of Cr were highest in the mesophyll cells (both palisade and spongy) of the leaf tip. These authors hypothesized that the accumulation of Cr in the apoplast is a tolerance mechanism.…”

“…Whether the transformation from Cr(VI) to Cr(III) occurs within the rhizosphere or rapidly on uptake by the root is unclear (Pradas del Real et al, 2014), but regardless, concentrations of Cr(VI) in root tissues are often sufficiently low that they are not easily detectable using synchrotron-based approaches. Within root tissues, this Cr(III) is often complexed with simple organic acids (Howe et al, 2003;Bluskov et al, 2005;Zhao et al, 2009) Within the leaves, Cr tends to accumulate in the leaf margins, as shown in Subterranean clover (Howe et al, 2003) and Silene vulgaris (Pradas del Real et al, 2014). Furthermore, Pradas del Real et al (2014) used m-XRF to identify that concentrations of Cr were highest in the mesophyll cells (both palisade and spongy) of the leaf tip.…”