Influence of microbial associations on selenium localization and speciation in roots of Astragalus and Stanleya hyperaccumulators

► Selenium in Stanleya and Astragalus roots is most concentrated in the cortex. ► In field roots, 46–95% of Se was CSeC, with up to 35% elemental Se (Se0). ► Three fungi from field roots produced mainly Se0; these were used to inoculate. ► In greenhouse-grown plants Se0 was found only in nodules and some inoculated roots. ► Microbial symbionts appear to affect Se speciation in hyperaccumulator roots. Selenium (Se) hyperaccumulator plants can accumulate and tolerate Se up to 1% of their dry weight. Since little is known about below-ground processes of Se uptake and metabolism in hyperaccumulators, X-ray absorption spectromicroscopy was used to characterize the chemical composition and spatial distribution of Se in roots of Astragalus and Stanleya hyperaccumulators. Selenium was present throughout the roots, with the highest levels in the cortex. The main form of Se (48–95%) in both species collected from naturally seleniferous soil was an organic CSeC compound, likely methyl-selenocysteine. In addition, surprisingly high fractions (up to 35%) of elemental Se (Se0) were found, a form so far not reported in plants but commonly produced by Se-tolerant bacteria and fungi. Four fungi collected from hyperaccumulator roots were characterized with respect to their Se tolerance and ability to produce Se0, and then used to inoculate hyperaccumulators in a controlled greenhouse study. The roots of the greenhouse-grown Astragalus and Stanleya contained mainly CSeC; in most plants no Se0 was detected, with the exception of Astragalus nodules and roots of Astragalus inoculated with Alternaria astragali, an Se0-producing fungus. Apparently, Se0-producing endosymbionts including nitrogen-fixing bacteria and endophytic fungi or bacteria in the root can affect Se speciation in hyperaccumulator roots. Microbes that affect plant Se speciation may be applicable in phytoremediation and biofortification, especially if they are promiscuous and affect Se tolerance in crop species.