Agriculture Reference
In-Depth Information
in-planta function of ACR2 warrants further investigations since T-DNA insertion
(knock-out) lines and RNAi (knock-down) lines show no deviation from wild type
phenotype under normal conditions (Dhankher et al.
2006
; Bleeker et al.
2006
). Ad-
ditionally, ACR2 knock-out mutants show dominance of arsenite suggesting func-
tional redundancy, presence of other arsenate reductase, non-enzymatic reduction
and/or existence of other enzymes in reduction of arsenate in plants (Zhao et al.
2009
,
2010
). A triosephosphate isomerase (TPI), an enzyme involved in glycolysis
and isolated from
P. vittata
.
PvTPI,
is shown to confer arsenate resistance in
E. coli
strain lacking
ArsC
. However, in-planta function of PvTPI in arsenate reduction is
not yet known (Rathinasabapathi et al.
2006
).
In addition to arsenite and arsenate, plants also show presence of methylated
arsenic species in the form of MMA, DMA trimethylarsinine oxide (TMAO) (Fran-
cesconi and Kuehnelt
2002
; Meharg and Hartley-Whitaker
2002
). Plants grown
in hydroponics with no methylated arsenic species have shown presence of DMA
and/or MMA in tissues and xylem sap albeit at very low concentrations (Raab et al.
2007a
; Xu et al.
2007
; Quaghebeur and Rengel
2003
). Different rice genotypes also
show various levels of inorganic arsenic (As
i
) and DMA in grains (Liu et al.
2006
).
Plants starved with nitrogen and phosphorus also show significant proportion of
methylation (Nissen and Benson
1982
). These studies indicate existence of in- planta
biomethylation activity. Arsenic-methylation activity of leaf but not root extract
of
Agrostis capillaris
has been demonstrated in an in vitro assay with
3
H-labelled
S-adenosyl-L-methionine (SAM) as the methyl donor (Wu et al.
2002
). The activity
is induced upon pre-exposure of plants to arsenic with MMA and DMA as meth-
ylation product. However, arsenic methylation pathways and enzymes are largely
unknown at present. Possibly in plants, arsenic methylation follows the Challenger
pathway which has been well studied in fungi and bacteria (Zhao et al.
2009
; Bent-
ley and Chasteen
2002
).
In soil bacterium
Rhodopseudomonas palustris
, arsenic methytransferases
(ArsM) have been identified and genes encoding the same are cloned (Qin et al.
2006
). Similarly in an algae,
Cyanidioschyson
sp., living in arsenic rich environ-
ment, ArsM have been identified which are able to methylate arsenite sequentially
to mono-di- and trimethyl arsenic with final product, a volatile TMA gas (Qin et al.
2009
). Interestingly, rice genome possesses methyltransferase genes as that of mi-
crobes (Norton et al.
2008a
), however role of these genes in arsenic methylation
remains to be investigated. Furthermore, it is not known whether plants can gener-
ate and volatilize TMA as seen in microorganisms.
5 Arsenic Detoxification
Arsenic once taken up by plants needs to be detoxified to avoid detrimental effects on
cellular processes. Detoxification of arsenic in plants occurs through complexation,
vacuolar compartmentalization and efflux of arsenite to the external environment.
