Agriculture Reference
In-Depth Information
(
OsNIP2;1
and
OsNIP3;2
), lotus (
LjNIP5;1
and
LjNIP6;1
) have been identified
as potential NIPs involved in arsenite transport by complementing in yeast system
(Bienert et al.
2008
). Isayenkov and Maathuis (
2008
) predicted the role
AtNIP7;1
also in arsenite transport. Interestingly, Ma et al. (
2008
) identified in rice OsNIP2;1
(also named as Lsi1 because of its role silicon (Si) transport) as major pathway
during the entry of arsenite in rice roots. This data postulated that arsenite and sili-
con transport follow the same route in rice. Apart from OsNIP2;1, three other NIP
proteins in rice, OsNIP1;1, OsNIP2;2 and OsNIp3;1 were also shown in arsenite
transport in an heterologous system. In addition to Lsi1, a different protein Lsi2 was
shown to mediate arsenite efflux in the direction of xylem (Ma et al.
2008
).
The mechanism or the uptake of methylated form of arsenic species is large-
ly unknown at present. The efficiency of taking methylated forms like MMA and
DMA is much lower compared to arsenite and arsenate forms (Raab et al.
2007a
).
The concentration dependent uptake of MMA in rice roots can be described by
Michaelis-Menten kinetics whereas DMA uptake did not follow Michaelis-Menten
kinetic (Abedin et al.
2002
). Interestingly, in
Zea mays
DMA uptake followed the
Michaelis-Menten kinetic (Abbas et al.
2008
).
Arsenic species once taken up by roots are transported to different parts of the
plants through xylem stream. However, the rate of transport of arsenic species is
always much slower than that of phosphorus species (Raab et al.
2007a
). This in-
efficient uptake of arsenic is generally determined by looking at shoot:root ratio
of arsenic accumulation. There are several studies in plants where the shoot:root
ratio of arsenic uptake was determined. The most significant of these studies was
one performed by Raab et al. (
2007a
) where 46 different species were analyzed.
They proposed the ratio between 0.01 and 0.9, with the median at 0.09. Interest-
ingly DMA, which is very poorly taken up by the roots is efficiently translocated
to shoots from the roots. The ratio of the DMA transport varied from 0.02-9.8 with
a median at 0.8 (Raab et al.
2007a
). The main limitation in transport of arsenic
species is because of the rapid reduction of arsenate species into arsenite species
in the roots. These arsenite following complexation with thiols gets sequestered
in the root vacuoles. This was shown by silencing
Arabidopsis
arsenate reductase
(
AtACR2
) by RNAi resulting in more accumulation of arsenic in shoot (Dhankher
et al.
2006
). However a T-DNA insertion line of AtACR2 showed the opposite ef-
fect, where the mutant was found to accumulate less arsenic in shoots (Bleeker et al.
2006
). Several studies confirmed that arsenite is the main form found in the xylem
sap, even when the arsenate was fed to the plants (Zhao et al.
2009
). DMA was
rarely found in the xylem sap (Mihucz et al.
2005
; Xu et al.
2007
). As compared to
xylem, little is known about the transport of arsenic species by phloem. Other heavy
metals like cadmium (Cd) can make complexes with phytochelatine (Cd-PC) and
glutathione synthatase (Cd-GS) and are known to be transported through phloem
(Mendoza-Cózatl et al.
2008
). However, existence of arsenic species in As-C and
As-GS forms is not known. Further, As-PC and As-GS are unstable at pH of more
than 7.5 and hence their stability in phloem sap, which is slightly alkaline, is also
a question. A simplified schematic diagram of arsenic uptake and metabolism is
shown in Fig.
16.1
.
