Direct and indirect selenium speciation in biofortified wheat: A tale of two techniques

Abstract

Wheat can be biofortified with different inorganic selenium (Se) forms, selenite or selenate. The choice of Se source influences the physiological response of the plant and the Se metabolites produced. We looked at selenium uptake, distribution and metabolization in wheat exposed to selenite, selenate and a 1:1 molar mixture of both to determine the impact of each treatment on the Se speciation in roots, shoots, and grains. To achieve a comprehensive quantification of the Se species, the complementarity of high‐performance liquid chromatography coupled with inductively coupled plasma mass spectrometry and X‐ray absorption spectroscopy was exploited. This approach allowed the identification of the six main selenium species: selenomethionine, selenocysteine, selenocystine, selenite, selenate, and elemental selenium. The three treatments resulted in similar total selenium concentration in grains, 90–150 mg Se kg−1, but produced different effects in the plant. Selenite enhanced root accumulation (66% of selenium) and induced the maximum toxicity, whereas selenate favored shoot translocation (46%). With the 1:1 mixture, selenium was distributed along the plant generating lower toxicity. Although all conditions resulted in >92% of organic selenium in the grain, selenate produced mainly C‐Se‐C forms, such as selenomethionine, while selenite (alone or in the mixture) enhanced the production of C‐Se‐Se‐C forms, such as selenocystine, modifying the selenoamino acid composition. These results provide a better understanding of the metabolization of selenium species which is key to minimize plant toxicity and any concomitant effect that may arise due to Se‐biofortification.

Phytochemical Profile, Bioactive Properties, and Se Speciation of Se-Biofortified Red Radish (Raphanus sativus), Green Pea (Pisum sativum), and Alfalfa (Medicago sativa) Microgreens

“…The Se treatment consisted of tap water with 20 μM Se based on a 1:1 molar mixture of sodium selenite and sodium selenate. This treatment was selected as the most suitable for this hydroponic cultivation following our previous findings , and the results of other works in the literature ,, in which similar concentrations did not induce any toxic effects to the plant. A control culture was obtained from a separated vertical farming system in which only tap water was used as the nutrient solution.…”

“…The E 0 of these organic references appears at lower energies compared to the inorganic ones, and the white-line is significantly wider. 15 By employing reference spectra, LCF analysis (Table S2) facilitated the determination of the species contributing to each sample spectrum. In red radish treated with Se, selenate (38.2 ± 0.2%) and C−Se−C species (37 ± 1%) were the dominant contributions followed by C−Se−Se−C species (22 ± 0.9%) and with a small amount of selenite species (2.6 ± 0.3%).…”

“…Selenate demonstrates a significantly higher diffusion coefficient, being 2−3 orders of magnitude greater than that of selenite in various media and conditions, whereas the diffusion coefficients of organic Se species fall in between these two extremes. 2,15 Furthermore, the metabolic pathways of the Se species exhibit variability. Selenate undergoes reduction to selenite before subsequent conversion to selenide and, eventually, to organic species.…”

“…Analogously, selenate follows a comparable pattern, accumulating without undergoing metabolism within the vacuoles of shoots, thereby eliciting no toxicity response. 15 It is worth mentioning that selenate transportation across the cell membrane is an energydependent process mediated by the sulfate transport system. 2 On the other hand, the low quantities of selenite in our results could be explained by the fact that this compound, which is absorbed by phosphate transporters and aquaporins (OsNIP2), is more concentrated in the root systems instead of being transported to the aerial parts due to the rapid transformation into organic forms of Se.…”

“…A similar effect was observed for C−Se−Se−C organic compounds since the Se mixture treatment yielded 38 ± 2%, whereas selenate and selenite produced 25 ± 2 and 44 ± 2%, respectively. 15 Hence, comprehending the conversion of inorganic Se into organic species in microgreens is essential to enhance the efficiency of Se-biofortified foods while minimizing the potential risks of undesired toxicity. To date, there is a lack of reported data concerning the impact of biofortification with a mixture of selenate and selenite on the Se species accumulated in microgreens.…”