Differences and similarities in selenium biopathways in Astragalus, Neptunia (Fabaceae) and Stanleya (Brassicaceae) hyperaccumulators

Background and aims: Selenium hyperaccumulator species are of primary interest for studying the evolution of hyperaccumulation and for use in biofortification because selenium is an essential element in human nutrition. In this study, we aimed to determine whether the distributions of selenium in the three most studied hyperaccumulating taxa (Astragalus bisulcatus, Stanleya pinnata and Neptunia amplexicaulis) are similar or contrasting, in order to infer the underlying physiological mechanisms. Methods: This study used synchrotron-based micro-X-ray fluorescence (µXRF) techniques to visualize the distribution of selenium and other elements in fresh hydrated plant tissues of A. racemosus, S. pinnata and N. amplexicaulis. Key results: Selenium distribution differed widely in the three species: in the leaves of A. racemosus and N. amplexicaulis selenium was mainly concentrated in the pulvini, whereas in S. pinnata it was primarilylocalized in the leaf margins. In the roots and stems of all three species, selenium was absent in xylem cells, whereas it was particularly concentrated in the pith rays of S. pinnata and in the phloem cells of A. racemosus and N. amplexicaulis. Conclusions: This study shows that Astragalus, Stanleya and Neptunia have different selenium-handling physiologies, with different mechanisms for translocation and storage of excess selenium. Important dissimilarities among the three analysed species suggest that selenium hyperaccumulation has probably evolved multiple times over under similar environmental pressures in the US and Australia.