Page 178 - Plant-Based Remediation Processes

9 Phyto-transport and Assimilation of Selenium 169

plant materials as selenate (Pilon-Smits et al. 1999). Se tolerance and accumulation
in APS transgenic plants was also significantly higher than untransformed plants
(Pilon-Smits et al. 1999). Similarly, the APS transgenics are able to contain 2.5-
fold higher shoot Se levels compared with the wild type of Indian mustards (Van
Huysen et al. 2004). However, there is no difference in cell growth or sensitivity to
selenate between transgenic (overexpression of APS2 genes encoding ATP
sulfurylase) and wild-type cells of tobacco (Hatzfeld et al. 1998). CGS mediates
the conversion of SeCysth from SeCys. Transgenic Indian mustards that
overexpress CGS were shown to have 2–3-fold higher Se volatilization rate,
20–40 % lower shoot Se level and 50–70 % root Se levels, and higher Se tolerance
than the wild type (Van Huysen et al. 2003). The higher Se volatilization rates of
the CGS transgenics suggest that CGS is rate limiting for Se volatilization as
DMSe (Van Huysen et al. 2004).
In Se hyperaccumulating plants, the amino acids methylselenocysteine
(MeSeCys) and methylcysteine (MeCys) are produced from the methylation of
SeCys and Cys in the presence of the enzyme selenomethyltransferase (SeMT)
using SMM as the methyl donor (Neuhier et al. 1999; Ellis and Salt 2003). Indeed,
Indian mustard transgenic plants accumulated more Se in the form of MeSeCys
than the wild type, using SeMT gene probe from Se hyperaccumulator Arabidopsis
(Leduc et al. 2004). Additionally, SeMT transgenic seedlings tolerated Se (particu-
larly selenite) better than untransformed plants, producing 3–7-fold greater biomass
and 3-fold longer root lengths (Leduc et al. 2004). A similar finding has been
reached that MeSeCys accumulation in transgenic broccoli closely correlated to the
SeMT gene expression (Lyi et al. 2005). Obviously, Se accumulation in genetically
engineered plants provided important information for maximizing MeSeCys pro-
duction in beneficial vegetable plants (Leduc et al. 2004; Lyi et al. 2005).
It is noted that overexpression of SeMT in plants would be expected to lead to
increased methylation of SeCys, resulting in decreasing production of SeMet (Ellis
and Salt 2003). Consequently, the resulting reduction in SeMet would decrease the
formation of DMse. However, SeMT overexpressing Indian mustards has signifi-
cantly increased Se volatilization compared with the wild type (Leduc et al. 2004).
This is because a different assimilation pathway exists in plants, in which the
MeSeCys produced from the methylation of SeCys is further converted into another
volatile species DMDSe rather than DMSe (Meija et al. 2002).
Other genes have also been isolated. The APS reductase (PaAPR) was isolated
from the bacterium P. aeruginosa and expressed in A. thaliana (Bruhl et al. 1996).
Plants supplied with selenate increase Se reduction by 50–80 %, suggesting the
capacity of reducing APSe (Bruhl et al. 1996; De Fillips 2010). SeCys lyase is the
enzyme involved in Se assimilation. In transgenic B. juncea originally sourced from
A. thaliana, overexpression of SeCys lyase is able to reduced selenate toxicity
(Banuelos et al. 2007), which attributes to a reduction in the incorporation of Se into
proteins. Banuelos et al. (2007) also cloned and expressed the gene of SeCys
transferase, which has little effect on selenate toxicity, but causes a small effect
on selenite toxicity.