## 2 Selenium from Soil to Food Bowl
### Transfer and Absorption of Se from Soil to Plant  
Each plant variety has the ability to accumulate Se at the root and shoot level, depending on the concentration of Se in the soil and the ability of the plant species to absorb, accumulate and metabolize Se [30]. There are several types of plants, so-called hyper-accumulators that grow only in soils very rich in Se \"seleniferous\" and accumulate between 1000 and 15,000 mg kg\({}^{-1}\), including species of the genera Astragalus, Stanleya, Morinda, Neptunia, Oonopsis and Xylorhiza. Secondary accumulators such as cereals, some plants of the cruciferous family (rape, broccoli) and Allium species such as garlic and onion grow in soils with variable Se content and can accumulate 30 to 1000 mg kg\({}^{-1}\). Non-accumulating plants generally grown on \"selenium-poor\" soils and cannot accumulate high concentrations of Se in their tissues and contain less than 30 mg kg\({}^{-1}\). This is the case for most plants used for food and feed (forage, vegetables, fruits, etc.) [30].  
Absorption of organic selenium by plants is more efficient than of inorganic forms such as Se (IV) or Se (VI) [31]. Less than 5% of the Se present in the soil is used by plants [32]. Other forms of Se such as selenide and elemental Se are not absorbed by plants since they are insoluble in water [31] (Figure 1).  
The accumulation of Se from soil to plants requires several processes and depends on the speciation of Se in the soil. Sulfate (S) and phosphate (P), present in the soil could affect the absorption of Se by plants. As selenate is structurally and chemically similar to sulfate, both ions follow the same metabolic pathways during the translocation process in plants [34]. The high-affinity H+/sulfate supporters, homologous to sulfate transporter, AtSULTR1;1 and AtSULTR1;2, catalyze the influx of selenate into root cells from the rhizosphere [35; 36]. The preference for selenate uptake over sulfate varies from plant to plant and is influenced by various factors for example, soil salinity and pH [37], the two parameters affecting Se(IV) and Se(VI) adsorption on soil particles [31]. In other non-accumulating plants, other transporters behave similarly. In hyperaccumulative plants, the genes of AtSULTR1;1 and AtSULTR1;2 have a high expression [36].  
Selenite is absorbed by roots as HSeO\({}_{3}\)\({}^{-}\) by members of the phosphate transporter family [38] or as H\({}_{2}\)SeO\({}_{3}\) by aquaporins [39]. After being absorbed, selenate moves from the root symplast to the stele and is then transferred to the shoot, while selenite is converted into organosenelenated compounds [34; 39; 40]. In the xylem, selenate is the dominant form of Se, however minor amounts of  
Figure 1: Selenium speciation from soil to plant metabolism (Adapted from [PERSON] et al. [29] and [PERSON] et al. [33]). Se\({}^{(0)}\) = elemental selenium, Se\({}^{2-}\) = selenide, SeO\({}_{4}\)\({}^{2-}\) = selenate, SeO\({}_{3}\)\({}^{2-}\) = selenite, H\({}_{2}\)Se = hydrogen selenide, Se-Cys = selenocysteine, Se-Met = selenomethionine.  
Se-Met and selenomethionine Se-oxide (SeOMet) have been reported. After being delivered to the shoot by the xylem, selenate enters leaf cells by SULTR transporters [35; 40; 41].