Page 263 - Plant-Based Remediation Processes

256 P. Kotrba

attributed mainly to the acquired advantage of an improved antioxidative defense
potential (Freeman and Salt 2007). In cysteine biosynthesis, inorganic sulfate after
uptake is activated by ATP sulfurylase to form adenosine phosphosulfate (APS),
which is subsequently reduced to free sulfide by APS reductase. In an extensive
study measuring the effect of ATP sulfurylase overproduction on the accumulation
of 12 metal and metalloid cations and oxyanions, Wangeline et al. (2004) observed
that the expression of the APS1 gene of A. thaliana in B. juncea markedly
contributed to the metallotolerance in seedlings. Compared to WT controls, shoots
of transgenic seedlings from the complex metalliferous media then contained
3 2 2
higher levels of Cd (1.9 times), VO 4 (2.5 times), CrO 4 (1.5 times), WO 4
2
(1.7 times), and MoO 4 (1.4 times). The higher tolerance and accumulation of
cations was attributed to the ATP sulfurylase-promoted increase in GSH levels
(Pilon-Smits et al. 1999). The authors also suggested that enhanced accumulation of
the metal oxyanions, as are sulfate analogues (Leustek 1996), was contributed by
upregulation of sulfate uptake function to complement virtual sulfate starvation
caused by the removal of free sulfate by the overproduced enzyme. Indeed,
constitutive expression in B. juncea of SHST1 gene encoding of a high-affinity
plasma membrane sulfate transporter from pencil flower Stylosanthes hamata was
later demonstrated promoting uptake of metal oxyanions in the same manner
(Lindblom et al. 2006). Current research on the use of nicotianamine to promote
metal uptake and translocation is largely focused on improving micronutrient
(Zn, Fe) contents in crops (Johnson et al. 2011; Lee et al. 2011; Zheng et al.
2010). In a study relevant to phytoremediation, Kim et al. (2005) showed that
introduction of barley nicotianamine synthase HvNAS1 gene into N. tabacum
rendered tobacco producing 5 times higher NA levels than control WT. Consistent
with the translocation-promoting role of NA in planta, transgenic plants
accumulated from serpentine soil by 1.3, 3.3, 2.1, and 4.0 times higher
concentrations of Ni, Fe, Zn, and Mn than WT tobacco.

12.4.4 Plants Engineered to Produce Heterologous
Metal-Binding Proteins

Overproduction of recombinant MTs to enhance metalloresistance and to support
metal accumulation in plants has been the first strategy considered for the construc-
tion of phytoremediation plants (Misra and Gedamu 1989; Evans et al. 1992; Pan
et al. 1994; Hasegawa et al. 1997; de Borne et al. 1998). This approach, applied in
several laboratories, has resulted in different phenotypes. Although the constitutive
expression of genes encoding mouse MT-1, human hMT-1A and h-MT-II, Chinese
hamster MT-II, and yeast CUP1 in tobacco cabbage Brassica oleracea and
A. thaliana markedly enhanced Cd resistance, the transgenic plants showed a
20–70 % reduction in metal accumulation in the shoots. On the other hand,
production of CUP1 in N. tabacum grown in soil with 1,645 mg kg 1 Cu resulted