Selenium Distribution and Speciation in the Hyperaccumulator Astragalus bisulcatus and Associated Ecological Partners    

Abstract

The goal of this study was to investigate how plant selenium (Se) hyperaccumulation may affect ecological interactions and whether associated partners may affect Se hyperaccumulation. The Se hyperaccumulator Astragalus bisulcatus was collected in its natural seleniferous habitat, and x-ray fluorescence mapping and x-ray absorption near-edge structure spectroscopy were used to characterize Se distribution and speciation in all organs as well as in encountered microbial symbionts and herbivores. Se was present at high levels (704–4,661 mg kg−1 dry weight) in all organs, mainly as organic C-Se-C compounds (i.e. Se bonded to two carbon atoms, e.g. methylselenocysteine). In nodule, root, and stem, up to 34% of Se was found as elemental Se, which was potentially due to microbial activity. In addition to a nitrogen-fixing symbiont, the plants harbored an endophytic fungus that produced elemental Se. Furthermore, two Se-resistant herbivorous moths were discovered on A. bisulcatus, one of which was parasitized by a wasp. Adult moths, larvae, and wasps all accumulated predominantly C-Se-C compounds. In conclusion, hyperaccumulators live in association with a variety of Se-resistant ecological partners. Among these partners, microbial endosymbionts may affect Se speciation in hyperaccumulators. Hyperaccumulators have been shown earlier to negatively affect Se-sensitive ecological partners while apparently offering a niche for Se-resistant partners. Through their positive and negative effects on different ecological partners, hyperaccumulators may influence species composition and Se cycling in seleniferous ecosystems.

Effects of selenium on the growth and photosynthetic characteristics of flue-cured tobacco (Nicotiana tabacum L.)

“…Previous studies have demonstrated that hyperaccumulators were characterized by a high leaf Se concentration, and a higher shoot:root Se concentration ratio [28,29]. Valdez et al [30] reported that as a Se hyperaccumulator, the pattern of total Se concentration in Astragalus bisulcatus follows the order root < leaf < stem. Under 24 mg kg −1 Se, the Se concentration in the roots (31.36 mg kg −1 ) was 3.17 and 7.57 times higher than those in leaves and stems, respectively (Tab.…”

Do selenium hyperaccumulators affect selenium speciation in neighboring plants and soil? An X-Ray Microprobe Analysis

“…It is feasible that the Se hyperaccumulators, given enough time, can influence Se speciation in surrounding soil in a radius of 10 m or more. Microbes may additionally affect soil Se speciation by converting inorganic soil Se to organic forms, and by converting organic Se deposited by hyperaccumulators into inorganic forms (Lindblom et al 2012). It has been reported for Symphyotrichum eatonii growing in Se-containing reclaimed mine soil, that rhizosphere soil and plant roots contained relatively more selenate (+6), while bulk soil contained more reduced Se (-2, 0) (Oram et al 2011).…”

“…The canopy of individual hyperaccumulators at Pine Ridge Natural Area reaches up to 0.8 m in diameter, and the plants often occur in clusters. Their litter may be spread by wind, and Se ingested from hyperaccumulators (live or litter) may be spread by various detrivores and herbivores Freeman et al 2006a;Valdez Barillas et al 2012). The soil sample collected next to non-hyperaccumulators was on average 10 m away from Se hyperaccumulators.…”

“…There is also evidence of rhizodeposition of Se by hyperaccumulators. Roots of hyperaccumulators A. bisulcatus and S. pinnata were shown to contain mainly C-Se-C, both when growing in the field and when treated with selenate in a controlled greenhouse study (Lindblom et al 2012). Roots of A. bisulcatus treated with selenate were found to exude fairly high levels of Se in the form of C-Se-C compounds ).…”

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

“…Similarly, in leaves of Stanleya pinnata , Se was present almost exclusively as MeSeCys (Freeman et al, 2006a). In the hyperaccumulator Astragalus bisulcatus , Se was present mainly as organic (C‐Se‐C) compounds in roots, stems, and leaves (Valdez Barillas et al, 2012). An important difference in Se speciation between hyperaccumulators and non‐accumulators is potentially the form of organoselenium present (e.g., LeDuc et al, 2004), although XAS techniques often lack the sensitivity to differentiate between such forms.…”