Selenate triggers diverse oxidative responses in Astragalus species with diverse selenium tolerance and hyperaccumulation capacity

Abstract

Selenium (Se) hyperaccumulators are capable of uptake and tolerate high Se dosages. Excess Se-induced oxidative responses were compared in Astragalus bisulcatus and Astragalus cicer. Plants were grown on media supplemented with 0, 25 or 75 μM selenate for 14 days. Both A. bisulcatus and A. cicer accumulated >2000 μg/g dry weight Se to the shoot but the translocation factors of A. cicer were below 1 suggesting its non hyperaccumulator nature. A. cicer showed Se sensitivity indicated by reduced seedling fresh weight, root growth and root apical meristem viability, altered element homeostasis in the presence of Se. In Se-exposed A. bisulcatus, less toxic organic Se forms (mainly MetSeCys, γ-Glu-MetSeCys, and a selenosugar) dominated, while these were absent from A. cicer suggesting that the majority of the accumulated Se may be present as inorganic forms. The glutathione-dependent processes were more affected, while ascorbate levels were not notably influenced by Se in either species. Exogenous Se triggered more intense accumulation of malondialdehyde in the sensitive A. cicer compared with the tolerant A. bisulcatus. The extent of protein carbonylation in the roots of the 75 μM Se-exposed A. cicer exceeded that of A. bisulcatus indicating a correlation between selenate sensitivity and the degree of protein carbonylation. Overall, our results reveal connection between oxidative processes and Se sensitivity/tolerance/hyperaccumulation and contribute to the understanding of the molecular responses to excess Se.

Keywords: Astragalus bisulcatus; Astragalus cicer; Hyperaccumulation; Oxidative stress; Selenate; Tolerance.