Arsenic Stress in Plants: An Inside Story - Crop Improvement: New Approaches and Modern Techniques - page 382

Table 13.1 Enzymes affected by arsenic toxicity in plants
Mode of as toxicity Target enzyme References

Phosphate replacement
with arsenic

F1-F0 ATP synthases Gresser ( 1981 )

GDPH (glycolytic enzyme) Orsit and Cleland ( 1972 )

Aspartate-β-semialdehyde
dehydrogenase

Kish and Viola ( 1999 )

Purine Nucleoside Phosphorylase
(PNP)

Park and Agrawal ( 1972 )

Binding with thiol groups Dihydrolipomide (Co-factor of pyru-
vate dehydrogenase complex (mt
PDC, pt PDC)

Bergquist et al. ( 2009 )

Gly decarboxylase complex (GDC) Peters et al. ( 1946 )

Branched Chain 2-oxoaciddecarboxyl-
ase complex (BCOADC)

Bergquist et al. ( 2009 )

teins and act as an effective inhibitor of enzymes requiring free sulfahydryl groups
(Webb et al. 2003 ). Thus phosphate replacement and inactivation of enzymes by
binding with their thiol groups are the main modes of arsenic toxicity (Table 13.1 ).
Arsenic interferes with various events of respiratory cycle. Arsenite act as an
inhibitor of α-ketoglutrate dehydrogenase enzyme in TCA cycle, causing the accu-
mulation of substrate α-ketoglutrate and no product formation (succinyl Co-A). The
nutrient uptake is also affected due to arsenic toxicity. Phosphorus uptake decreases
with increasing arsenic concentration due to similarity with arsenate. Also uptake of
nitrate is reduced on exposure to arsenic. The uptake of nitrate and further assimila-
tion to ammonium is also altered, possibly due to interference of arsenic with the
involving enzymes, nitrate reductase and nitrite reductase (Table 13.2 ).

Arsenic Induced Oxidative Stress in Plants

There is significant evidence that exposure to inorganic arsenic species (ROS)
results in the generation of reactive oxygen species caused oxidative damages to
plants (Sharma 2012 ). This probably occurs through the conversion of arsenate
to arsenite, a process which readily occurs in plants. After this reduction, arsenic
may be potentially further metabolized to methylated species leading to further
oxidative stress (Zaman and Pardini 1996 ) because methylation is thought to be
redox driven and such reactions could give rise to reactive oxygen species (ROS).
However there is no sufficient evidence for methylation in higher plants but it can
be seen in the cell suspension of Cathranthus roseus (Cullen and Hettipathirana
1994 ) and in phosphate starved tomato plants (Nissen and Benson 1982 ). Also Wu
et al. ( 2002 ) has been shown in vitro methylation of arsenic in cell extracts from
bent grass ( Agrostis tenuis ). These reactive oxygen species causes peroxidation of
lipid by reacting with lipid bilayer and thereby leading to membrane leakage. Lipid
peroxidation can be measured in terms of MDA content. An increase in MDA con-