Selenium assimilation and volatilization from dimethylselenoniopropionate by Indian mustard

Earlier work from our laboratory on Indian mustard (Brassica juncea L.) ide ntified the following rate-limiting steps for the assimilation and volatili zation of selenate to dimethyl selenide (DMSe): (a) uptake of selenate, (b) activation of selenate by ATP sulfurylase, and (b) conversion of selenomet hionine (SeMet) to DMSe. The present study showed that shoots of selenate-t reated plants accumulated very low concentrations of dimethylselenoniopropi onate (DMSeP). Selenonium compounds such as DMSeP are the most likely precu rsors of DMSe. DMSeP-supplied plants volatilized Se at a rate 113 times hig her than that measured from plants supplied with selenate, 38 times higher than from selenite, and six times higher than from SeMet. The conversion of SeMet to selenonium compounds such as DMSeP is likely to be rate-limiting for DMSe production, but not the formation of DMSe from DMSeP because DMSeP was the rate of Se volatilization from faster than from SeMet and SeMet (b ut no DMSeP) accumulated in selenite- or SeMet-supplied wild-type plants an d in selenate-supplied ATP-sulfurylase transgenic plants. DMSeP-supplied pl ants absorbed the most Se from the external medium compared with plants sup plied with SeMet, selenate, or selenite; they also accumulated more Se in s hoots than in roots as an unknown organic compound resembling a mixture of DMSeP and selenocysteine.