Agriculture Reference
In-Depth Information
(Rao et al.
2011
). The toxic effects of heavy metals are speculated to be due to
direct interaction of arsenic with proteins and/or as deleterious effects of ROS. As
also causes cell death in plants on higher concentrations (Rao et al.
2011
; Requejo
and Tena
2005
).
3 Arsenic Uptake and Transport in Plants
Arsenic is known to form organic and inorganic complexes in environment (Zhao
et al.
2010
), which are absorbed by the plants in three different forms, namely ar-
senate, arsenite and methylated arsenic. While arsenate is the main species in aero-
bic soil, arsenite is predominant under anerobic conditions. Both these species are
interconvertible depending on the redox potential. Methylated species, which exist
as monomethylatsonic acid (MMA) and dimethylarsinic acid (DMA) are usually
minor arsenic species in the environment (Francesconi and Kuehnelt
2002
).
The aerobic soil species arsenate is recognized by phosphate transporter. This
has been demonstrated by several physiological and electrophysiological experi-
ments, where these transporters have shown higher affinity for phosphate than for
arsenate (Asher and Reay
1979
; Ullrich-Eberius et al.
1989
; Meharg et al.
1994
).
The uptake of one molecule each of arsenate and phosphate involves co-transport
with at least two molecules of proton (Ullrich-Eberius et al.
1989
). Several phos-
phate transporters are reported and characterized in plants with Phosphate Trans-
porter 1 (Pht 1) family having more than 100 members (Rausch and Bucher
2002
;
Bucher
2007
). Most of these phosphate transporters are expressed strongly in
roots and are responsible for the uptake of phosphate from the soil. Interestingly,
double mutant of phosphate transporters, Pht1;1 and Pht1;4 (
Pht1;1
∆
4
∆) in
Ara-
bidopsis thaliana
were found to be resistant to arsenate compared to the wild
type. This suggested that the arsenate is taken up by Pht1;1 and Pht1;4 phosphate
transporter (Shin et al.
2004
). The finding of Pht1;1 transporter assisting the up-
take of arsenate was further supported by González et al.
2005
, who reported that
A. thaliana
mutant defective in phosphate transport traffic facilitator 1 (PHF1),
a trafficking protein of Pht1;1, also makes plants resistant to arsenate. Similarly,
Pht1;3 transporter is also reported in
A. thaliana
as arsenate transporter (Catarecha
et al.
2007
).
The information about the uptake of arsenite has been deduced largely from the
studies in microbial system. It has been reported that in
E. coli
, yeast and even in
humans, arsenite can be taken up by aquaglycoporins, a subfamily of the aquapo-
rins superfamily having larger pores (Bhattacharjee and Rosen
2007
). However,
an alternative mechanism has also been predicted in yeast, whereas deletion of
hexosepermease gene results in 80 % inhibition in arsenite uptake (Liu et al.
2004
).
Recent research has shown growing evidence of involvement of nodulin 26-like
intrinsic proteins (NIPs), a subfamily of plant aquaporins family to be involved
in arsenite transport. Many genes from
Arabidopsis
(
AtNIP5;1 andAtNIP6;1
), rice
