Resumen de Selenium biofortification of wheat: distribution and spatially resolved selenium speciation by synchrotron-based techniques
Maria de los Ángeles Subirana Manzanares
Selenium, as an essential micronutrient for humans, has several roles in health and thus, its intake at optimum levels is highly beneficial. However, 500-1000 million of people worldwide suffer selenium deficiency, due to the low Se levels in soils of agricultural lands.
Biofortification of crops with Se-rich fertilizers is the most effective approach to counteract selenium deficiency. However, the selenium speciation is also fundamental: plants are able to transform the soil inorganic selenium, i.e. selenite and selenate ions, into organic selenium, such as selenoamino acids, which are less toxic and more bioavailable.
Wheat is the most consumed cereal worldwide and is able to tolerate and accumulate over 100 mg Se per kg of dry weight, thus being a suitable candidate for Se biofortification to produce an enriched functional food.
Selenium in wheat is found in the form of five major selenium species: selenite, selenate, selenomethionine, methylselenocysteine and selenocystine. In the present thesis, the content and distribution of these species in wheat was determined by the tamdem of high-performance liquid chromatography with inductively coupled plasma mass spectrometry (HPLC-ICP-MS) after appropriate enzymatic sample digestion, and by X-Ray Absorption Spectroscopy (XAS), using synchrotron radiation, among other techniques.
The speciation and concentration of Se, the plant growth conditions and the stage in which selenium is applied to the plant define the degree of selenium uptake, metabolization and distribution through the different plant organs. Selenite is readily reduced in wheat roots, and thus, it accumulates preferentially in underground tissues; on the other hand, selenate is highly mobile through the plant xylem and its translocation is faster than its reduction, therefore accumulating in shoots. The application of high Se concentrations may result in excessive tissue accumulation, and thus, plant stress and Se-induced toxicity, decreased plant biomass production and reduced grain yield. However, wheat phytotoxicity may be reduced by the application of selenium at florescence time, but still achieving similar enrichment of grain and Se metabolization.
Selenite was almost completely reduced into organic species, especially in roots, where the induced toxicity effects produced a strong oxidizing environment within the plant, thus producing a high accumulation of organic selenium in grain in the form of selenocystine. Oppositely, selenate showed slower metabolization and a significant accumulation of selenium in inorganic forms in shoots, although in grain selenium was found as organic species in the form of selenomethionine, which can be unspecifically incorporated into proteins.
On the other hand, the application of both anions simultaneously contributed to balance the Se enrichment due to their separate metabolic pathways. The mixture caused a more equilibrated distribution of Se in the plant tissues, reducing its phytotoxicity, but resulting in the same total selenium concentration in grain and an intermediate amount of selenomethionine and selenocystine.
Furthermore, the spatially resolved speciation analysis of wheat grains, showed high selenium accumulations in the germ, bran and pigment strand, and a low selenium concentration in the endosperm, which correlated positively with the concentration of proteins in the different parts of the grain.
Finally, the protective effect of selenium against mercury toxicity was shown and it seems that it was due to the formation of a protein-Se-Hg complex in roots. This complex reduced the translocation of mercury to shoots and grain, the selenate mobility and the selenite reduction in roots, but at the same time it enhanced the accumulation of C-Se-C amino acids, such as selenomethionine, in wheat grain. As a result, selenium counteracted mercury phytotoxicity and reduced the risk in crops exposed to mercury polluted soils.
