**6.2 Selenium metabolism in plants**

Selenium is transported in plants via the sulfate transporters in the plasma membrane of the root due to its chemical similarity to sulfur [13, 14]. It is further transported to the leaves and undergoes metabolism through the sulfur assimilation pathway either to a selenium methionine (SeMet) or a selenium cysteine (SeCys).

Inorganic selenium is first converted to selenite via the action of ATP sulfurylase (APS) and APS reductase (APR). The hydrolysis of adenosine triphosphate to adenosine phosphoselenate which is then reduced to selenite is catalyzed by APS and APR, respectively [13]. Sulfite reductase then converts selenite to selenide although glutaredoxins or glutathione can also reduce this step in plants [17]. The selenide couples with O-acetyl serine (OAS) to form selenium cysteine (SeCys) by cysteine synthase. The selenium cysteine can either undergo methylation to methyl-SeCys by selenocysteine methyltransferase or converted to selenium or selenomethionine by SeCys lyase or other enzymes, respectively. Sulfur analog of selenium can then be methylated and undergo vaporization to a non-toxic form in the atmosphere (**Figure 2**) [15].
