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168 X.-Z. Yu and J.-D. Gu

tolerance traits in Astragalus are not related to CGS kinetics (Dawson and
Anderson 1988; Sors et al. 2005).
SeMet synthesized by the Met biosynthetic pathway can be methylated and
converted into DMSe, which is the major species of volatile Se compounds
produced (Tagmount et al. 2002). The first committed step involved in the produc-
tion of DMSe is the methylation of SeMet to form methylselenomethionine
(MeSeMet), which is catalyzed by the enzyme S-adenosyl-L-methionine: L-methi-
onine S-methyltransferase (MMT) (Tagmount et al. 2002; Ellis and Salt 2003; Sors
et al. 2005). More than likely, the botanical conversion of Se-methylmethionine
(SeMM) to DMSe is catalyzed by S-methylmethionine hydrolase (Ellis and Salt
2003; Sors et al. 2005), which is widely observed in plants during the conversion of
S-methylmethionine (SMM) to dimethylsulfide (DMS) (Pimenta et al. 1998).
Another possible biochemical pathway is also suggested, in which DMSe is likely
produced by the conversion of SeMM to the intermediate dimethylselenoio-
propionate (DMSeP) in the chloroplast (De Souza et al. 1998). Indeed, Indian
mustards supplied with DMSeP are able to volatilize significantly higher Se than
plants supplied with SeMet (De Souza et al. 1998; Sors et al. 2005). It has been
suggested that dimethylsulfoniopropionate (DMSP) lyase is responsible for the
conversion of DMSeP to DMSe; however, DMSP lyase has not yet been identified
in plants (De Souza et al. 1998; Ellis and Salt 2003). Due to the leaf portion being
exposed to the air, plants are able to transpire the resultant methylated forms of Se
into the atmosphere through leaves. Actually, Se volatilization is the process by
which gaseous forms such as DMSe and DMDSe are produced from other inor-
ganic or organic forms of Se (Terry et al. 1992; Zayed et al. 1998; De Souza et al.
1998; Meija et al. 2002; Yu and Gu 2008). The rate of Se volatilization varies with
plants species. Willow cuttings likely transpired approximately 10 % applied Se in
forms of selenate and selenite (Yu and Gu 2008). As much as 10–30 % of the
Se can be removed by biological volatilization (Hansen et al. 1998), whereas
wetland plants showed a 50-fold variation in Se volatilization (Duckart et al.
1992). Additionally, plants supplied with selenite volatilized more Se than selenate
(Zayed et al. 1998). Because DMSe is less toxic than other species of Se (De Souza
et al. 1998), phyto-volatilization has drawn more attention as a possible method for
the phytoremediation of Se-contaminated soils (Terry et al. 1992).

9.4.3 Genetics Involved in Selenium Metabolism in Plants

Despite the existence of naturally occurring Se-accumulating plants, an interest has
been generated in using unusual plants as tools to improve and clarify our basic
understanding of Se biochemistry in plants (Ellis and Salt 2003). Indeed, different
genes related to Se tolerance, accumulation, and metabolism have been identified
or isolated. For example, overexpression of the Arabidopsis APS1 genes encoding
a plastidic ATP sulfurylase in Indian mustard has been found to be able to increase
the assimilation of selenate into SeMet, whereas the wild-type accumulated Se in