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

et al. 1998). The shoot/root ratio of total Se content in plants ranged from 0.6 to 1
for plants supplied with SeMet and was less than 0.5 for those supplied with
selenite, while this ratio can range from 1.4 to 17.2 when selenate is the only
form of Se supplied (Zayed et al. 1998). On the contrary, selenite is more mobile
than selenate after uptake by plant roots, although more selenate was eliminated by
willows from the plant growth medium than selenite (Yu and Gu 2008). Indeed, the
translocation efficiency of selenite was more than onefold higher than that of
selenate (Yu and Gu 2008).

9.4.2 Selenium Assimilation and Metabolism

The chemical and physical resemblance between Se and S establishes that both
elements share common metabolic pathways in plants (Sors et al. 2005). It has been
proposed that selenate is primarily transported into the chloroplasts, where it is
metabolized by enzymes of S assimilation (Leustek et al. 2000; Ellis and Salt 2003).
The same Se species was only detected in the roots of de-topped plants supplied
with selenate, supporting that the chloroplasts are the sites for ATP sulfurylase
activity and selenate reduction (Pilon-Smits et al. 1999; De Fillips 2010). In vitro
ATP sulfurylase has been shown to be able to activate sulfate (Leustek et al. 1994),
while the reduction of selenate to adenosine phosphoselenate (APSe) catalyzed by
ATP sulfurylase is also suggested (Fig. 9.1) (Leustek et al. 1994; Sors et al. 2005).
Indeed, overexpression of ATP sulfurylase in Indian mustard has confirmed that the
activation of selenate to APSe by ATP sulfurylase is one of the rate-limiting steps
for selenate assimilation in plants (Pilon-Smits et al. 1999; Ellis and Salt 2003).
Evidence is also available on that bound APSe can be further assimilated and/or
metabolized via either non-enzymatically or enzymatically pathways (Ng and
Anderson 1979; Terry et al. 2000). During the enzymatically catalyzed pathway,
APSe is converted into selenite by adenosine 5-phosphosulfate (APS) reductase
(Terry et al. 2000). Due to the possibility of the conversion of selenate into selenite
in the biosynthesis of organic Se compounds (De Souza et al. 1998), selenite is able
to subsequently non-enzymatically reduce into selenide in the presence of glutathi-
one in vitro (Ng and Anderson 1979). Therefore, we have a good reason to propose
that the existence of this nonenzymatic pathway for the reduction of selenite to
selenide explains why selenite is more readily assimilated by plants than selenate
(De Souza et al. 1998; Ellis and Salt 2003).
The sequential process is the production of SeCys due to the selenide assimila-
tion (Terry et al. 2000), in which SeCys is formed by the action of cysteine (Cys)
synthase, which couples selenide with O-acetylserine (Ng and Anderson 1978).
Ultimately, SeCys can enter the methionine (Met) biosynthetic pathway via
selenocystathionine (SeCysth) and selenohomocysteine (SeHoCys) to form
SeMet (Sors et al. 2005). Both SeCys and SeMet are nonspecifically incorporated
into proteins, which contribute to the phytotoxicity of Se (Brown and Shrift 1982;
EI Mehdawi and Pilon-Smits 2011). Clearly, the spinach cystathionine-γ-synthase